HOLE 900A
Date occupied: 10 May 1993
Date departed: 22 May 1993
Time on hole: 11 days, 20 hr, 15 min
Position: 46°40.994'N, 11°36.252'W
Bottom felt (rig floor; m, drill-pipe measurement): 5048.5
Distance between rig floor and sea level (m): 11.72
Water depth (drill-pipe measurement from sea level, m): 5036.8
Total depth (rig floor; m): 5853.50
Penetration (m): 805.00
Number of cores (including cores having no recovery): 86
Total length of cored section (m): 805.0
Total core recovered (m): 519.61
Core recovery (%): 65
Oldest sediment cored:
Depth (mbsf): 739.86
Nature: claystone
Age: late Paleocene Measured velocity (km/s): 2.40
Hard rock:
Depth (mbsf): 748.90
Nature: altered mafic igneous rocks
Measured velocity (km/s): 5.8
Principal results: Site 900 is situated in the Iberia Abyssal Plain within the ocean/continent transition (OCT) zone over an angular basement high that has some similarity to a tilted fault block. Geophysical modeling had indicated that this basement high lay within a part of the OCT having a very weak magnetization and thus was probably thinned continental crust. The site was one of a transect of sites across the OCT designed to study the petrologic changes in the basement rocks within the OCT to identify the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. Cores were obtained from a hole that penetrated 748.9 m of Pleistocene to Paleocene sediments and 56.1 m of basement composed of retrograde metamorphosed mafic igneous rocks. Coring was terminated when the rate of penetration came close to 1 m/hr and bit failure was imminent. A total of 380 m of sonic, resistivity, and FMS logs was acquired from three separate intervals in the sediments and basement.
Two lithostratigraphic units were identified at Site 900.
The sediments at this site reveal the history of development of the lower continental rise adjacent to the Iberia Abyssal Plain during Cenozoic time. The cores chronicle the deposition of silt and clay layers with laminated bases under the influence of bottom currents, which were probably con tour-following currents and part of the general oceanic circulation. Mud turbidites were occasionally seen, too. These sediments were succeeded by carbonate-rich turbidites and then by mud-dominated turbidites as the abyssal plain sediments built upward and sideways onto the rise.
The cores provide a discontinuous fossil record from Pleistocene through late Paleocene time. Calcareous nannofossils generally are abundant to very abundant. Planktonic foraminifers generally are common to abundant in the upper section of the hole, but samples from the deeper sections contained fewer specimens. Two major hiatuses representing the early late Miocene and the early Eocene, as well as several minor hiatuses, have been identified.
The Matuyama, Gauss, and Gilbert chrons tentatively were identified from paleomagnetic measurements of the sediments above 145 mbsf. Below that depth, the sediments are very weakly magnetized and no chrons have been identified. The metamorphic basement rocks do not provide any reliable magnetic results, mainly because of weak magnetization. Magnetic susceptibility values generally follow the pattern of remanent magnetization values in both sediments and crystalline rocks.
Fifty-six meters of fine- to coarse-grained metamorphosed mafic rocks were drilled in the basement. The rocks are highly deformed and brecciated and veined by later calcite, epidote, and clinozoisite. A porphyroclastic texture having large porphyroclasts of plagioclase and clinopyroxene in a recycled matrix of the same minerals can be seen in thin section. Chemical analyses suggest that the rocks are relatively depleted in large ionic litho phile elements. These rocks may be (1) Paleozoic mafic rocks that were accreted onto continental basement during the Hercynianorogeny, (2) Cumulate gabbro (of any pre-late-Paleocene age) either formed in, or possibly underplated at the base of, continental crust. The rocks subsequently experienced a series of deformation and metamorphic events. In any case, they were exposed at the seafloor prior to the late Paleocene, probably by the Early Cretaceous rifting.
The sonic, resistivity, and Formation Microscanner (FMS) logging strings were run over three separate parts of the hole, including 36 m of basement. Hole conditions made logging difficult and forced us to use the conical side-entry sub. Eventually, logging had to be abandoned because of persistent obstructions in the hole and damage to the logging cable and FMS tool.
Measurements of physical properties in the sediments indicate a small but steady increase in bulk density, seismic velocity, formation factor, shear strength, and thermal conductivity, and a concomitant decrease in porosity with depth. The clay-rich sediments were notable for their significant seismic anisotropy in places (more than 7%) and relatively strong vertical velocity gradient (1 s-1), compared to that at Sites 897, 898 and 899. The density of the basement rocks is about 2.6 to 2.9 g/cm3, and their velocity ranges from 3.7 to 7.5 km/s, with a cluster of observations at 5.7 km/s.
Interstitial-water samples were obtained from lithologic Units I and II (13-722 mbsf). The principal result is a steady downward decrease in concentrations of sulfate throughout the hole, from a value near that of sea-water concentration at 13 mbsf to a minimum of 1.9 mM at 702 mbsf. The profile is slightly convex upward, suggesting that some sulfate reduction has occurred within the sedimentary column. Concentrations of ammonia are consistent with this interpretation. A similarly shaped magnesium pro file may result from clay mineral alteration. Concentrations of calcium and strontium suggest carbonate recrystallization at about 300 mbsf and below 636 mbsf.
Profiles of carbonate content vs. depth reflect a history of generally low biological productivity and deposition of hemipelagic sediments be low the carbonate compensation depth (CCD), combined with delivery by turbidites of carbonate-rich material initially deposited above the CCD. An average 0.3% organic carbon was found in Unit I; this is much less than that found at Sites 897 and 898. Variable organic C/N ratios from Unit I indicate the fluctuating predominance of marine or terrigenous sources of organic matter. Concentrations of biogenic methane encountered in head space gas analyses of lithostratigraphic Unit I to Subunit IIB generally are low, as were those found at Site 899. Methanogenesis may have been inhibited by interstitial sulfate, as indicated by the generally high sulfate concentrations in the pore waters.
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Ms
149IR-106
Reproduced online: 15 October 2004
Site 900 (Fig. 1, "Site 897" chapter, this volume) was one of a series of sites drilled during Leg 149 to elucidate the nature of the top of the crust (acoustic basement) within the ocean/continent transition (OCT) beneath the Iberia Abyssal Plain. The regional background for this and the other Leg 149 sites is presented elsewhere (see "Introduction" chapter, this volume; Whitmarsh et al., 1990,1993). Site 900 is located about 23 nmi (43 km) east of Site 898, about 25 nmi (46 km) southwest of Vasco da Gama Seamount and 45 nmi (83 km) west-southwest of Site 398 (see Fig. 1 in "Introduction" chapter, this volume, and "Site Geophysics" section, this chapter). Site 900 was chosen to sample a basement high located near the western edge of a region of weakly magnetized, thinned continental crust that appears to extend from the OCT to the continental slope (see Fig. 4 in "Introduction" chapter, this volume). The basement high under the site is angular and only slightly asymmetric in profile, but appears as an isolated almost circular feature in plan view (Fig. 2 in "Site 897" chapter, this volume). We had interpreted the high as a fault block of continental crust. Should the acoustic basement under the site prove to be continental, then the almost 200-km-wide region of crust between the site and the base of the Portuguese continental slope might logically also be assigned a continental origin. Further, assuming symmetrical rifting, one could argue that a wide area of thinned continental crust might also exist under the conjugate Newfoundland margin, where conflicting views exist regarding the extent of thinned continental crust (Keen and de Voogd, 1988; Tucholke et al., 1989).
By analogy with Site 398, we expected to encounter ooze/chalk with turbidites over chalk, mudstone, and claystone (Sibuet, Ryan, et al., 1979). Seismic-reflection profiles traced back to Site 398 indicated that the basal sediments would be as old as Paleocene. The regional unconformity, which resulted from gentle folding that occurred during the Miocene northwest-southeast Rif-Betic compressional phase in southern Spain and North Africa, is not clearly seen in reflection profiles across the site. This may be because it is obscured by two unusual acoustic facies; the interval from 0 to 0.38 s two-way traveltime consists of hummocky sediment waves and the interval from 0.38 to 0.58 s two-way traveltime consists of a series of inclined reflectors (Fig. 1). The sediments thicken to about 1.7 s two-way traveltime (1.9 km) in the basin west of the site and to at least 2.3 s two-way traveltime (3.0 km) to the east. Although we anticipated that acoustic basement at Site 900 might contain continental rocks, the exact petrology of these rocks and the amount of any pre-rift sediments were completely unknown. Phyllites and meta-arkoses of un known age have been dredged about 20 nmi (37 km) north of the site (Dredge C56-09; Capdevila and Mougenot, 1988).
OPERATIONS
Hole 900A
After completing drilling at Site 899, we moved the ship to 40°41.00'N, 11°36.25'W, Site 900 (IAP-5), and deployed a Datasonics beacon. The precision depth recorder indicated a water depth of 5045.4 mbsl. After the ship was stabilized in dynamic positioning mode over the primary beacon, a second backup Datasonics beacon was deployed.
A rotary core barrel (RCB) bottom-hole assembly (BHA) having a mechanical bit release (MBR) was assembled and run to the seafloor. The bit was positioned at 5048.5 mbrf, and Hole 900A was spudded at 0605 hr Universal Time Coordinated (UTC), 11 May 1993, by advancing 1.5 m. From Core 149-900A-1R, 1.36 m was recovered; therefore, the seafloor was assigned a depth of 5048.5 mbrf. Cores 149-900A-1R to -86R were taken from 5048.5 to 5853.5 mbrf (0-805.0 mbsf; Table 1), with 805.0 m cored and 519.61 m recovered (65% recovery). The first three cores were taken without rotating or pumping seawater; the weight-on-bit, rotation speed, and pump rate gradually were increased in response to increasing formation hardness and for maintaining good hole conditions and core recovery. The water sampler and temperature probe (WSTP) was successfully run at 5170.9 mbrf (122.4 mbsf), 5219.1 mbrf (170.6 mbsf), and 5267.3 mbrf (218.8 mbsf).
We conducted two wiper trips in the sedimentary section. The first was after Core 149-900A-39R, when we moved the pipe between 5412.0 to 5196.6 mbrf (363.7-148.1 mbsf). A second wiper trip was made after Core 149-900A-75R, when we moved the pipe between 5758.8 and 5600.0 mbrf (710.3-551.5 mbsf). We encountered a bridge at 5747.0 mbrf, washed and rotated the bit past it to total depth, and found no fill in the bottom of the hole.
Metamorphic basement rocks were encountered at 748.9 mbsf. After Core 149-900A-86R (805.0 mbsf), suspecting imminent failure of the bit, we decided to stop coring and prepare for logging. The hole was circulated clean, and a short trip was made to 5187 mbrf. Minor overpull was observed while pulling up, and the tight sections were reamed out while moving the pipe down. We then circulated clean the 10 m of fill at the bottom of the hole. The bit was released, and the end of the pipe was pulled to 5185.4 mbrf (136.9 mbsf) for logging.
Two logging runs were made. During the first run (dual induction tool [DIT], caliper tool [MCD], dipole shear imager [DSI], and natural gamma-ray spectrometry tool [NGT-C]), the tool would not pass 5286 mbrf (237.5 mbsf). The drill pipe was repositioned at 5378.5 mbrf (330.0 mbsf). No obstructions or drag were noted while running the drill string farther into the hole. The first logging combination was rerun; however, it would not pass 5500 mbrf (451.5 mbsf). After the logs were run, we pulled the drill pipe up to 5244 mbrf (195.5 mbsf) and picked up the conical side-entry sub (CSES).
A tool combination that included the Formation Microscanner (FMS) and NGT-C was loaded into the CSES, and the drill string was run to 5802.2 mbrf (753.7 mbsf); however, the tool would not pass 5837 mbrf (788.5 mbsf). The pipe was pulled up to 5742 mbrf (693.5 mbsf) in an effort to get above the collapsed hole. The combination was rerun from 5837 to 5695 mbrf (788.5-646.5 mbsf). The tool became stuck at 5695 mbrf (646.5 mbsf). The drill string was washed down to 5698 mbrf (649.5 mbsf) over the stuck tool. The logging tool came free, and the drill string and tool were pulled to 5528.9 mbrf (480.4 mbsf). This combination was run again and became stuck at 5613 mbrf (564.5 mbsf). We washed the drill pipe over the tool from 5528 to 5614 mbrf (478.5-565.5 mbsf). At this point, we abandoned logging be cause of poor hole conditions and damage to the logging cable and FMS tool. We recovered the drill string, recalled the two beacons, and terminated operations at Site 900 at 1518 hr, 22 May 1993.
SITE GEOPHYSICS
Geophysical Data near Site 900
Two migrated multichannel seismic-reflection profiles pass through Site 900 (Fig. 2). They are the north-south Sonne Line 75-21 (Fig. 3), and the east-west Lusigal Line 12 (see Fig. 1). Site 900 is located over a basement high that is 14 km wide on Lusigal Line 12 (west to east) and 10 km wide on Sonne Line 75-21 (south to north). The high is triangular in west-east cross section (Fig. 1). The apparent dip of the basement on the west side of the high is 7° and on the east side of the high is 16°. At its shallowest known point, the top of the high is at 750 ms two-way traveltime below the seafloor or 720 mbsf. Site 900 is located slightly to the west of the top of the basement high, where the basement is predicted to be at 770 mbsf. Sediments thought to be Paleocene in age cover the basement high; older sediments onlap this high, and the basin section to the east is thicker than that to the west. The sedimentary section at Site 900 consists of three main seismic units (Fig. 1). The, topmost unit (0-380 ms two-way traveltime below seafloor, 0-360 mbsf) consists of discontinuous undulating reflectors. The second unit (380-580 ms two-way traveltime below seafloor, 360-540 mbsf) consists of parallel inclined reflectors that dip to the west or southwest. We mapped the regional distribution of these inclined reflectors during Leg 149 (Fig. 4, "Site 898" chapter, this volume). The third unit (580-800 ms two-way traveltime below seafloor, 540-770 mbsf) consists of parallel, continuous, high-amplitude reflectors.
A magnetic anomaly map (Fig. 7, "Site 897" chapter, this volume; P.R. Miles, J. Verhoef, and R. MacNab, pers. comm., 1993) shows that Site 900 is located on the western side of a roughly circular, about 20-km diameter, magnetic anomaly low. Beslier et al. (1993) and Whitmarsh, Miles, and Mauffret (1990) interpreted seismic and magnetic data, respectively, to indicate that Site 900 is located over extended continental crust.
LITHOSTRATIGRAPHY
Introduction
Hole 900A penetrated a 749-m-thick sedimentary succession consisting of a lower carbonate-rich contourite-turbidite-pelagite sequence (Unit II) and an upper turbidite-pelagite sequence (Unit I). The two sequences contrast sharply in terms of evidence for reworking by contour currents (which is present only in the lower sequence) and in the abundance of siliceous allochems (which are virtually absent in the upper sequence). In the light of the results at Site 900, the shipboard sedimentologists realized that they could recognize the same twofold division at Sites 897, 898, and 899 as summarized in Figure 4.
Rotary coring (RCB) was employed in Hole 900A. Coring began at the seafloor in Unit I and penetrated basement rocks at 748.9 mbsf at the base of Unit II. Figure 5 summarizes the core recovery, lithologies, and ages of the lithostratigraphic units recognized in Hole 900A. The ages, averaged lithologic compositions, overall colors, facies and depositional environments, boundary depths, and cored intervals of Units I and II are summarized in Table 2; Table 3 and Table 4 show the color variations exhibited by the lithologies in Units I and II, respectively.
Unit I, though containing turbiditic and hemipelagic/pelagic facies, contains little siliciclastic sand in comparison to its counterparts at Sites 897, 898, and 899. The unit is divided into three subunits. Subunits IA and IC contain turbiditic and hemipelagic/pelagic sediments and are separated by hemipelagic/pelagic nannofossil clays and oozes that comprise Subunit IB.
Unit II is more indurated than Unit I and is dominated by clay stone, claystone with silt/silty claystone, nannofossil claystone, and nannofossil chalk. Foraminifer-rich sandstones and calcarenites occur as a distinctive minor lithology in Subunit IIA, and form up to 20% to 30% of some cores in Subunit IIB, where they are often calcite-cemented . Both subunits contain upward-darkening sequences of nannofossil claystone and claystone/calcareous claystone with silt, some times with siliciclastic/bioclastic sandy bases. Upward-lightening sequences also are present in Subunit IIA. Both subunits are interpreted as the deposits of turbidity flows and contour currents.
Figure 6 is a plot of ages vs. depths presented in Table 5 for the sedimentary sequence penetrated at Site 900. Sediment accumulation rates, generalized from Figure 6, show less variation between Units I and II than rates observed for the corresponding lithostratigraphic units and ages at Sites 897, 898, and 899. The rate for Unit I ranges from 12.9 m/m.y. over the lower part of Subunit IC, to 22.5 to 24.0 m/m.y. for the remainder of the unit; this is significantly lower than the rates for Unit I at Sites 897 (60 m/m.y.), 898 (90 m/m.y.), and 899 (35 m/m.y.). The sediment accumulation rates in Unit II (13-27 m/m.y.) are com parable to those seen over the same age range at the other sites.
Unit I
Cores 149-900A-1R through 149-900A-21R-1, 125 cm
Depth: 0-181.5 mbsf
Age: Pleistocene to late early Miocene
General Description
Core recovery averaged 65% in Unit I with 108.56 m being described. Core disturbance in the unit is soupy in Core 149-900A-1R, severe to moderate down to Core 149-900A-5R, and slight to moderate in the remaining cores.
Much of the unit consists of siliciclastic muddy turbidites (a few are carbonate-rich) containing some thin basal silty or sandy intervals and associated pelagic/hemipelagic sediments. The turbidites range in thickness from about 10 cm to more than 1 m and often are capped by pelagic nannofossil oozes up to 60 cm thick. Major lithologies are olive gray, yellowish brown, pale orange, and light gray nannofossil clay and nannofossil ooze. Minor lithologies include olive black, dark gray, olive gray, and greenish gray clay, clayey silt/silty clay, sandy silt/silty sand, and sand. Many occurrences of the sandy lithologies are foraminifer-rich and light gray.
Three types of turbidite sequence are present in Unit I:
Three subunits were recognized in Unit I on the basis of the pro portions of different facies. In Subunit IA, the three types of turbidite sequence described above occur throughout and most are overlain by pelagic nannofossil ooze. The base of this subunit was placed at the last turbidite sequence with a basal sand interval at 149-900A-9R-2, 122 cm. Subunit IB consists entirely of pale orange and yellowish brown nannofossil claystones and oozes mixed to varying degrees by bioturbation. Its lower boundary is defined at the top of the first turbidite sequence at Sample 149-900A-12R-2, 110 cm. Subunit IC shows a gradual change in color from the pale orange and yellowish brown typical of Subunit IB, to olive and greenish gray. All three types of turbidite sequence occur throughout this subunit, and most of them are overlain by pelagic nannofossil oozes. The top of Subunit IC is dominated by nannofossil clays (in Cores 149-900A-12R and -14R), but the proportion of nannofossil ooze increases down the subunit.
Petrography
Applying the classification of Folk (1980), sands and silts of Unit I are subarkoses to arkoses. Grain types in Unit I indicate derivation from a source area that exposed mostly sedimentary, metamorphic, and, possibly, granitic rocks. The detailed petrography of the sediments in Unit I is identical to that described for Site 898 (see "Lithostratigraphy" section, "Site 898" chapter, this volume).
Depositional Processes
The location of Site 900, at the foot of the gently sloping continental rise, provides an explanation for the absence of significant amounts of siliciclastic sand at the base of the turbidite sequences in Unit I. The velocity of the turbidity flows would have been higher on the continental rise than on the nearly flat abyssal plain. The flows would also have had higher mud contents as they flowed over the rise, which would have increased their competency to transport sand. Therefore, most of their sand component would have remained in turbulent suspension and so would have bypassed the continental rise to be deposited later on the abyssal plain. This would have resulted in the fine-grained sediment being carried in the "tails" of the flows, with the nepheloid layer being the dominant part of the turbidite sequences.
The presence of both siliciclastic and carbonate bases to the turbidites suggests two distinct provenances for the sediments trans ported by the turbidity flows. The carbonate sediment was probably derived by the reworking of nannofossil oozes deposited higher on the continental rise or slope, whereas the siliciclastic sand, silt, and mud originated from more distant sources on the shelf or on land. The thick intervals of nannofossil clay and ooze characteristic of Subunit IB indicate the absence of siliciclastic sand- or mud-laden turbidity flows, although carbonate-rich flows and/or nepheloid layers could have supplemented the pelagic influx.
Unit II
Cores 149-900A-21R-1, 125 cm to 149-900A-79R-CC
Depth: 181.50-748.9 mbsf
Age: early Miocene to Paleocene
General Description
Core recovery within Unit II averaged 68% and ranged from 0% to 101%; it remained above 70% down to Core 149-900A-53R, whence it began to deteriorate, to rise only once above 40% below Core 149-900A-61R. From the top of the unit, the sediments are significantly more indurated, and so the cores were cut using a saw.
Unit II was divided into two subunits. Both are dominated by olive gray and light greenish gray colors, but from Core 149-900A-69R to the base of Subunit IIB, brown colors dominate nearly all the sections. Figure 8 shows downhole variations in the proportions of calcareous lithologies, sand and silt, and claystones. This figure indicates that downhole changes in composition are gradational, so that further sub division of Unit II would be difficult.
The major lithologies in Unit II are claystone, silty claystone/clay stone with silt, and nannofossil claystone (Table 2). Foraminifer-rich siliciclastic sandstones and calcarenites form up to 20% to 30% of some cores in Subunit IIB, where they are often calcite-cemented; these lithologies are a minor component of Subunit IIA. Nannofossil chalk forms about a quarter of the sediment in Subunit IIA, but in Subunit IIB it occurs only rarely in intervals up to 10 cm thick into which overlying darker lithologies usually have been mixed by bioturbation. In Subunit IIA and the upper part of Subunit IIB, a significant biogenic silica component is present in the sandstones and silty claystones (Fig. 9), but it is not present below Core 149-900A-47R (440.6 mbsf), except for a trace in Core 149-900A-51R.
The transition from Unit I to Unit II coincides with a change from greenish-gray turbidites to an interval containing upward-darkening and upward-lightening sequences. The division of Unit II into two subunits is based on changes in color, composition, and the nature of repetitive lithologic sequences. Subunit IIA consists mostly of lighter greenish-gray nannofossil claystones and claystones, which occur in both upward-darkening and upward-lightening sequences. Subunit IIB contains a greater variety of lithologies and colors, mostly arranged in upward-darkening sequences, and slumped sediments occur at the top of it. The boundary between the two subunits was placed at 149-900A-26R-4, 135 cm. A fault of unknown displacement occurs 2 cm below this boundary. The sediments beneath the boundary are fractured, and those in Sections 149-900A-27R-1 and -2 show maxi mum bedding dips of 35°, whereas the remaining sections contain almost horizontal bedding. These features suggest that the boundary occurs at a fault or fault zone having a significant but unknown displacement (see "Structural Geology" section, this chapter).
Subunit IIA
Core recovery from Subunit IIA averaged 92%. It is composed predominantly of greenish gray to light gray nannofossil claystone, claystone, and silty claystone. Minor lithologies include calcareous and siliciclastic silty sandstone/sandy siltstone, locally foraminifer-rich, and nannofossil chalk. The proportion of silty claystone increases, and nannofossil chalk decreases, downhole (Fig. 8), with the latter lithology forming 50% of Core 149-900A-22R. Bioturbation is pervasive and commonly mixes different lithologies; Planolites, Chondrites, and Zoophycos are common.
Subunit IIA consists of both upward-darkening and upward-lightening sequences; bioturbation is concentrated at the top of both types of sequence. Individual upward-lightening sequences are 5 to 30 cm thick and begin with a claystone overlain by a nannofossil claystone and/or nannofossil chalk. Fine siliciclastic sandstone or siltstone intervals, generally less than 5 cm thick, sometimes underlie the basal claystone. Upward-darkening sequences 5 to 15 cm thick are similar to those described in Subunit IIB at Sites 898 and 899 (see "Lithostratigraphy" sections, "Site 898" and "Site 899" chapters, this volume). They consist of a nannofossil claystone to chalk gradationally over-lain by darker claystone to silty claystone. The base of these couplets frequently is composed of an uncemented to poorly cemented siliciclastic to foraminifer-rich sandstone or siltstone.
Sandstone and siltstone layers range up to 3 cm thick and show parallel to cross laminations, sharp bases, and sharp to gradational tops (Fig. 10). Bioturbation occurs in some of these layers. Sand stones and siltstones do not exceed 10% of any core. Boundaries of drilling biscuits coincide with the sandstone/siltstone intervals, indicating that some of these lithologies may have been washed out. Consequently, the sandstone/siltstone proportions shown in Figure 8 may be underestimated.
Subunit IIB
Subunit IIB was cored over an interval of 505.61 m., with an average recovery of 65%. The unit consists predominantly of light-colored nannofossil claystone and dark greenish to brownish clay-stone with silt and claystone (see Table 2 and Table 4). Minor lithologies include nannofossil chalk and calcite-cemented to poorly indurated siliciclastic to bioclastic sandstones and calcarenites or silty sand stone. Sandstones and calcarenites form 20% to 30% of the cores in the lower part of the subunit below Core 149-900A-60R (Fig. 8).
Downhole variations in color, degree of sandstone cementation, bioturbation, and bedding dips occur in this subunit. Brownish colors first appear in Core 149-900A-50R; above this level, the darker colors consist mostly of greenish gray and olive gray. Calcite-cemented sand stones appear in Core 149-900A-33R and are present in most cores below this level. They become thicker (up to 12 cm) and more abundant (up to 35% of a core) below Core 149-900A-53R.
Trace fossils become more prominent below the top of Core 149-900A-52R, and in this core, well-preserved and abundant Zoophycos burrows occur. Below this core, structural dips of about 15° are visible which, together with the existence of a hiatus at this level (Fig. 6), suggest the presence of an unconformity.
Upward-darkening sequences are common in Subunit IIB (Fig. 11) and are similar to those described in Subunit IIB at Sites 898 and 899 (see "Lithostratigraphy" sections, "Site 898" and "Site 899" chapters, this volume). They begin with an irregular, basal, thin bed (up to 5 cm thick, but more commonly 0.5 to 3 cm thick) of mixed biogenic and terrigenous fine sandstone to siltstone. The basal silt- and sand-rich intervals of the upward-darkening sequences have been overlain by claystones, claystones with silt, or nannofossil claystones. Sometimes, intervals in these lithologies contain zones up to 1 to 2 cm thick of sand-filled burrows 1 to 3 mm in diameter. The boundary with the overlying darker-colored silty claystones and claystones is transitional as a result of mixing by bioturbation. Clearly identifiable ichno-fauna (Zoophycos, Planolites, and Chondrites) are concentrated (or more clearly visible) in the light-colored carbonate-rich lithologies in the lower part of each individual sequence. Usually, the burrowing extends down to the basal calcareous sandstone/calcarenite bed.
The sharp-based siliciclastic sandstones and bioclastic calcarenites are thicker (1-20 cm) than those in Subunit IIA and occur both as normally graded and sharp-topped beds. The sandstones and calcarenites are common in Cores 149-900A-35R to -51R, and from Core 149-900A-58R to the bottom of the hole. They often show parallel- and cross-lamination (Fig. 12) and frequently contain a few burrows filled with overlying lithologies. Where the tops of the sandstones or calcarenites are gradational, the transition to the upper nannofossil claystone is sometimes characterized by a wavy, parallel, or lenticular lamination between both lithologies, as well as by burrow mottling between them.
In the lower part of the subunit (Core 149-900A-66R and below), well-cemented calcareous sandstones occur that contain features not encountered in the sandstones at higher levels such as ball and pillow structures (Fig. 13), isolated ripples, flaser and wavy bedding, and sole marks. These sandstones also show parallel- and cross-lamination, rippled tops, siltstone and claystone laminae, and are lenticular to discontinuous and commonly bioturbated (Fig. 14).
Unit II Petrography
Sand- and silt-sized detritus in Unit II includes detrital compo nents similar to the assemblage observed in Unit I. In addition, a substantial diagenetic component first appearing downhole in Core 149-900A-38R in the form of calcite cement in the sandstones and silt-stones is present at this site. The cement is equant, sparry calcite that fills primary porosity (including intraskeletal porosity in carbonate allochems), but shows scant evidence of grain replacement.
Unit II also contains clay-rich lithologies that are similar to those in Unit I, except that the Type 2 (oriented clays in carbonate-poor lithologies) clay-rich lithologies (defined in the "Lithostratigraphy" section, "Site 897" chapter, this volume) are a more persistent and volumetrically significant part of the lithologic assemblage.
Clay minerals examined in Units I and II at Site 897 contained an expandable clay, discrete illite, and kaolinite (see "Lithostratigraphy" section, "Site 897" chapter, this volume). Preliminary examination of XRD data for two samples at Site 900 revealed the presence of mixed-layer illite/smectite in the deeper, more lithified portions of Subunit IIB (Samples 149-900A-67R-2, 44-16 cm and -69R-1, 33-34 cm). These mixed-layer clays contain around 40% illite layers. We are not sure if the appearance of these clays is related to provenance variation or to diagenetic effects. The increased degree of lithification in the claystones is at least circumstantial evidence that the mixed-layer clay is diagenetic in origin. Other phases within the clay mineral assemblage at this site are discrete illite and kaolinite.
Another possible diagenetic effect observed at this site is the possible alteration of the siliceous allochems. Samples from Cores 149-900A-44R to -47R contain sponge spicules showing pitted surfaces suggestive of dissolution. However, there is no evidence of replacement of spicules by crystalline silica or of precipitation of other authigenic silica mobilized through dissolution of the spicules.
Depositional Processes
Subunit IIA
Sedimentation during deposition of Subunit IIA resulted from turbidity and contour current deposition near the foot of the continental rise. Upward-lightening sequences are similar to those described in the turbiditic parts of Unit I, suggesting that turbidity currents periodically deposited sediment. Upward-darkening sequences contain siliciclastic to bioclastic sandstone/siltstone layers that show evidence (sharp bases and tops, cross- and planar-laminations) of trans port by traction currents, which suggests deposition from contour cur rents. High biogenic carbonate contents in hemipelagic intervals plus pervasive bioturbation suggest that deposition occurred in a dysaerobic environment above the carbonate compensation depth (CCD). Shifts from bioclastic to siliciclastic sands may result from changes in sediment source areas, with the siliciclastic sediments derived from the continental shelf, and the bioclastic sands from local highs. Alter natively, some of the foraminifer-rich sands could result from current winnowing of turbidites.
Subunit IIB
Repetitive upward-darkening sequences, which occur through most of the upper part of Subunit IIB (above Core 149-900A-53R), do not show sequences that can unequivocally be associated with deposition from turbidity currents. Thin, silty sandstone and siltstone intervals at the bases of many of the upward-darkening sequences lack clear normal grading. Furthermore, these silty sandstones and siltstones contain small-scale parallel and cross-lamination, and sharp bases and some sharp tops, indicating bottom current activity. These features point to reworking by contour currents as described by Stow and Piper (1984). The homogeneous, carbonate-poor terrigenous silty claystone and claystone couplets may be mud turbidites or contourites.
Continuous to lenticular calcite-cemented sandstones in the lower half of Subunit IIB may also reflect a combination of turbidity and contour current deposition. Scour and fill structures, cross, wavy, and parallel laminations, are more representative of turbidity than contour current deposition (Fig. 15). The occurrence of isolated ripples of sandstone enclosed in claystone that occur near the base of the sub-unit may have resulted from a combination of low clastic influx and bottom-current reworking. These features occur in thicker sandstones than those present in the upper part of the subunit, and so it is possible that they were deposited in a lobe fringe setting where turbidity flows and contour currents were operative at the same time.
BIOSTRATIGRAPHY
Sediments recovered from Hole 900A provide a discontinuous record for Pleistocene through the upper Paleocene. Calcareous nannofossils generally are abundant to very abundant, and preservation varies throughout the recovered successions. Planktonic foraminifers generally are common to abundant in the upper sections of Hole 900A, but samples from the deeper sections contained fewer specimens.
Calcareous Nannofossils
Site 900 is located near the eastern edge of the Iberia Abyssal Plain at a water depth of 5037 m. The calcareous nannofossils define four major stratigraphic successions within the Cenozoic sediments; one succession from the upper Pleistocene (Zone NN21 of Martini, 1971) to the upper lower Pliocene (Zone NN15), a second from the upper-most Miocene (base Zone NN12) to the upper Miocene (Zone NN11), a third from the middle Miocene (NN7) to the middle Eocene (NP14), and a fourth restricted to the upper Paleocene (NP9). Two major hiatuses, representing most of the lower Pliocene and the lower upper Miocene, correspond to those observed at Sites 897, 898, and 899.
In the interval from the upper Pleistocene to the upper lower Pliocene, calcareous nannofossils are abundant and well preserved. Nannofossils are generally abundant and moderately well preserved from the Miocene to the Oligocene. The Eocene succession is characterized by fewer, moderately preserved calcareous nannofossils, or by barren samples. From Core 149-900A-78R, upper Paleocene sediments were recovered that contain abundant and well-preserved nannofossils. No sediments were recovered below Core 149-900A-79R. Ages of calcareous nannofossils are summarized in Figure 16.
Pleistocene
Section 149-900A-1R contains rare to few Emiliania huxleyi and very abundant small Gephyrocapsa spp. (<2.5 µm), indicating the lower part of Zone NN21. Samples 149-900A-2R-1, 28 cm, and -2R-CC were assigned to Zone NN20 by the absence of Emiliania huxleyi and Pseudoemiliania lacunosa. Intervals 149-900A-3R-1, 83 cm, highest occurrence (HO) of Pseudoemiliania lacunosa, to -6R-4, 15 cm, lowest occurrence (LO) of Gephyrocapsa caribbeanica (>4.0 µm), were placed in Zone NN19. Mediterranean subzones of Rio, Fornaciari et al. (1990) were used within this succession. Samples 149-900A-3R-1, 83 cm, and -3R-CC include Pseudoemiliania lacunosa and Reticulofenestra sp. A (>6.5 µm), indicating Zone NN19F. The LO of small Gephyrocapsa omega (>3.8 µm) in Sample 149-900A-3R-CC indicates the lower part of Zone NN19F. The presence of very abundant small Gephyrocapsa spp. (<2.5 µm) and common Reticulofenestra sp. A (>6.5 µm) place Sample 149-900A-4R-3, 80 cm, in Zone NN19E. Sample 149-900A-5R-2, 17 cm, contains few Helicosphaera sellii and common Gephyrocapsa spp. (>5.5 µm), indicating the upper part of Subzone NN19C. The absence of large Gephyrocapsa spp. (>5.5 µm) in Sample 149-900A-5R-CC indicates the lower part of Zone NN19C. Sample 149-900A-6R-4, 15 cm, contains the HO of Calcidiscus macintyrei (cir.; >10.0 µm), few Gephyrocapsa oceanica (>4.0 µm) and Gephyrocapsa caribbeanica (>4.0 µm). This assemblage indicates Zone NN19B.
Pliocene
The Pliocene/Pleistocene boundary was placed between Samples 149-900A-6R-4, 15 cm (LO of Gephyrocapsa oceanica [>4.0 µm]) and 149-900A-6R-6, 6 cm (HOs of Discoaster brouweri and Discoaster triradiatus). A few Discoaster brouweri and Discoaster triradiatus co-occur in Samples 149-900A-6R-6, 6 cm, and -6R-CC, indicating the upper part of Zone NN18. Intervals 149-900A-7R-CC to -8R-CC contain only rare Discoaster brouweri and Discoaster triradiatus. The co-occurrence of Discoaster pentaradiatus and Discoaster surculus in Sample 149-900A-9R-1, 99 cm, defines the top of Zone NN16. Discoaster tamalis appears in Sample 149-900A-9R-CC . The HO of Reticulofenestra pseudoumbilica was observed in Sample 149-900A-10R-CC, together with abundant Discoaster asymmetricus, indicating Zone NN15. The upper/lower Pliocene boundary, equivalent to the zonal boundary NN15/NN16, lies between Samples 149-900A-10R-4, 128 cm, and -10R-CC. Most of the lower Pliocene succession is missing in Hole 900A.
Miocene
Sample 149-900A-11R-4, 93 cm, contains Amaurolithus delicatus, Triquetrorhabdulus extensus, but lacks Discoaster quinqueramus. This assemblage indicates the lower part of Zone NN12, which has been placed in the uppermost Miocene. An unconformity, representing most of the lower Pliocene sequence, lies between Samples 149-900A-10R-CC (acme in Discoaster asymmetricus) and -11R-4, 93 cm (presence of Triquetrorhabdulus extensus). Samples 149-900A-11R-CC to -16R-5, 86 cm, contain Discoaster quinqueramus, the total range of which defines Zone NN11. The LO of Amaurolithus delicatus was observed in Sample 149-900A-13R-CC and defines the NN11a/ NN11b boundary. The LO of Discoaster berggrenii can be observed in Sample 149-900A-16R-CC, indicating the lower part of Zone NN11 or the upper part of Zone NN10. The absence of Discoaster bollii, Discoaster hamatus, and Catynaster calyculus between this sample and Sample 149-900A-17R-3, 54 cm (HO of Cyclicargolithus floridanus ) indicates an unconformity representing Zones NN8 to NN10. Samples 149-900A-17R-3, 54 cm, and -17R-CC were placed in Zone NN7 on the basis of the presence of rare Cyclicargolithus floridanus and common Coccolithus miopelagicus and Triquetrorhabdulus rugosus. The NN6/NN7 zonal boundary was difficult to define because of only a rare occurrence of Discoaster kugleri; however, it was placed between Samples 149-900A-17R-CC and -18R-3, 97 cm, on the basis of the LO of Triquetrorhabdulus rugosus. The interval 149-900A-19R-CC to -20R-CC contains rare to few Discoaster deflandrei, few Calcidiscus premacintyrei, and rare Reticulofenestra umbilica, which indicate Zone NN5. The HO of Sphenolithus heteromorphus occurs in Sample 149-900A-20R-2, 89 cm. In the absence of Helicosphaera ampliaperta, the NN4/NN5 boundary may be approximated by the end of the Discoaster deflandrei acme (Rio, Raffi, et al., 1990) (Sample 149-900A-22R-1, 122 cm), by the LOs of Discoaster formosus and Discoaster moorei (Sample 149-900A-21R-1, 94 cm), and by the LO of Discoaster exilis (Sample 149-900A-20R-CC). This boundary was placed above the LO of Discoaster exilis. The LO of Sphenolithus heteromorphus occurs in Sample 149-900A-25R-CC and marks the NN4/NN3 boundary. The HO Triquetrorhabdulus carinatus was ob served in Sample 149-900A-26R-3, 53 cm, and indicates the NN3/ NN2 boundary. From Samples 149-900A-26R-3, 53 cm, to -31R-CC, the nannofossil assemblages include Sphenolithus dissimilis and the S. dissimilis-S. belemnos Intergrade. The LO of the rare Discoaster druggii was difficult to place because of the occurrence of overgrowth specimens, which were slightly smaller than 15 µm, between Sample 149-900A-32R-CC and -34R-CC. Therefore, the NN2/NN1 boundary was placed by the LO of Helicosphaera elongata (Sample 149-900A-34R-CC). The interval from Samples 149-900A-35R-CC to -38R-CC was assigned to Zone NN1. The calcareous nannofossil assemblages in this interval are characterized by rare Triquetrorhabdulus carinatus and few Reticulofenestra bisecta, Clausicoccus fenestratus, Clausicoccus tasmaniae, Helicosphaera perch-nielseniae, Helicosphaera intermedia, Helicosphaera euphratis, Discoaster woodringii, and Zygrhablithus bijugatus. Sphenolithus conicus was present from Samples 149-900A-36R-CC to -37R-CC.
Oligocene
The Oligocene/Miocene boundary, situated in the middle of Zone NN1, lies between Samples 149-900A-36R-CC and -37R-CC and was identified by the occurrence of common Reticulofenestra bisecta, Zygrhablithus bijugatus, and by the HO of Pontosphaera enormis in Sample 149-900A-37R-CC. The HO of Sphenolithus ciperoensis in Sample 149-900A-39R-3, 147 cm, defines the top of Zone NP25. The interval from this sample to Sample 149-900A-44R-CC was placed in Zone NP25. Sample 149-900A-39R-CC contains common Sphenolithus ciperoensis and very rare Sphenolithus distentus; the absolute HO of Sphenolithus distentus was not used at Site 900 to define the top of Zone NP24, because of its very rare, sporadic occurrence through out Zone NP25. Instead, the zonal NP24/NP25 boundary was defined by the highest common occurrence of Sphenolithus distentus in Sample 149-900A-44R-CC. The NP23/NP24 boundary was placed between Samples 149-900A-47R-CC and -48R-CC on the basis of the change in dominance from common Sphenolithus distentus to common Sphenolithus predistentus. This event was substituted for the LO of Sphenolithus ciperoensis (Sample 149-900A-44R-CC), the common marker for this boundary. The LO of Sphenolithus distentus was observed in Sample 149-900A-48R-CC. The interval below, down to Sample 149-900A-51R-4, 143 cm, was assigned to Zone NP23 on the presence of a few, large Helicosphaera compacta, common Sphenolithus predistentus, and on the absence of Reticulofenestra umbilica. The LO of Coronocyclus nitescens occurs in Sample 149-900A-49R-CC . Zones NP22 and NP21 from the lower Oligocene were not ob served, and an unconformity was interpreted as existing between Samples 149-900A-51R-4, 143 cm (Zone NP23), and -51R-CC at 7 cm (Zone NP19/20).
Eocene
The interval 149-900A-51R-CC, 7 cm, to -54R-CC was assigned to Zone NP19/20, as indicated by the highest co-occurrence of rare Discoaster saipanensis, very rare Discoaster barbadiensis, Cyclococcolithus formosus, Reticulofenestra hillae, Reticulofenestra umbilica, and by the acme of Clausicoccus subdistichus. The HOs of Discoaster saipanensis and Discoaster barbadiensis were used to define the Eocene/Oligocene boundary. Zones NP19 and NP20 were not separated because the LO of Sphenolithus pseudoradians, the zonal marker for the base of NP20, could not be used at Hole 900A because it was also observed in Zone NP17. Sphenolithus pseudoradians was not observed at other sites during Leg 149. The LO of Isthmolithus recurvus occurs in Sample 149-900A-54R-CC and marks the NP18/NP19 boundary. The NP17/NP18 boundary was defined by the HO of Sphenolithus obtusus in Core 149-900A-54R-CC. The LO of large Reticulofenestra bisecta occurs in Sample 149-900A-57R-CC . Chiasmolithus solitus, Sphenolithus spiniger, and Clausicoccus vanheckae appear in Sample 149-900A-59R-CC and indicate the top of Zone NP16. Sample 149-900A-60R-CC contains the HOs of Neococcolithes dubius, Pseudotriquetrorhabdulus inversus, Pyrocyclus inversus, and numerous species of genera Micrantholithus, Pemma, and Braarudospohaera. The HOs of Nannotetrina alata and Discoaster wemmelensis were observed in Sample 149-900A-67R-CC. Sample 149-900A-68R-CC was assigned to Zone NP15, as indicated by the absence of Reticulofenestra umbilica. Chiasmolithus gigas occurs in Sample 149-900A-69R-CC. The HO of Discoaster lodoensis in Sample 149-900A-75R-CC defines the top of Zone NP14.
The early Eocene Zones NP13 to NP10 have not yet been observed at Site 900. The sediments in Cores 149-900A-76R and-77R are composed mostly of noncalcareous brown claystone layers separated by rare calcareous claystones that contain few-to-common calcareous nannofossils.
Paleocene
Upper Paleocene sediments from Zone NP9 occur in Sample 149-900A-78R-CC and represent the oldest Cenozoic sediments containing calcareous nannofossils that were recovered during Leg 149. The assemblage contains abundant and well-preserved calcareous nannofossils. The presence of common Discoaster multiradiatus, few Fasciculithus bobii, F. hayii, Prinsius martinii, and Campylo sphaera eodela (>7.0 µm) indicates the middle of Zone NP9.
Foraminifers
Planktonic foraminifers in core-catcher samples from Hole 900A were examined to establish preliminary ages for the sediments (Fig. 16). The number of planktonic foraminifers was generally high in the upper sections of the hole; most of the samples yielded a high-diversity planktonic foraminiferal assemblage having good to moderate preservation (Table 6). Fewer, and more poorly preserved, planktonic foraminifers were present in the lower sections of Hole 900A. Similar patterns were observed in the benthic foraminiferal assemblages.
Hole 900A
Samples 149-900A-1R-CC and -2R-CC are characterized by the presence of Globorotalia truncatulinoides and can be assigned to Zone N23, which is of Pleistocene to Holocene age. The co-occurrence of Globorotalia truncatulinoides and Globorotalia tosaensis in the interval 149-900A-3R-CC to -6R-CC enables us to place it in Zone N22 or the lower part of Zone N23, which is of latest Pliocene to early Pleistocene age. The interval 149-900A-7R-CC to -9R-CC is characterized by the presence of Globorotalia inflata and the absence of Globorotalia tosaensis. It can be assigned to the interval from the top of Zone N19 to Zone N21, which is of late Pliocene age. Sample 149-900A-10R-CC is characterized by the presence of Globorotalia crassaformis crassaformis and Sphaeroidinellopsis paenedehiscens and the absence of Globorotalia inflata and Globigerina nepenthes; this sample can be assigned to the upper part of Zone N19, which is of late early to early late Pliocene age. Abyssal and reworked shallow-water benthic foraminifers were present in all these samples.
The interval 149-900A-11R-CC to -16R-CC contains Globigerina nepenthes and Neogloboquadrina pachyderma, while every species variant of the Globorotalia crassaformis group is absent. This interval can be assigned to Zones N16 to N17, which are of late Miocene age. A hiatus was interpreted between Samples 149-900A-10R-CC and -11R-CC, where the latest Miocene to early Pliocene age sediments (Zone N18 and the early part of Zone N19) are missing.
Sample
149-900A-17R-CC contains only the marker species Globigerina nepenthes, which ranges from Zone N14 to the lower part of
N19. As Zone N17 had already been reached in Sample 149-900A-11R-CC, Sample 149-900A-17R-CC was
assigned to Zones N14 to
N17, which are of late middle to
late Miocene age. Sample 149-900A-18R-CC is characterized by the presence of Globorotalia acrostoma,
Globorotalia siakensis, and Globorotalia mayeri and
the absence of
Praeorbulina spp. It can be assigned to Zones N9-N11, which are of
middle Miocene age. Although Zones N12
and N13 were not identified
in core-catcher samples,
calcareous nannofossil evidence does not sup
port an unconformity between Samples 149-900A-17R-CC and -18R-CC. Sample
149-900A-19R-CC is characterized by the presence of
Praeorbulina glomerosa curva
and P. glomerosa
glomerosa and can
be assigned to Zone N8, which is
of early middle Miocene age.
The co-occurrence of Praeorbulina
sicana and Catasydrax stainforthi and the absence of Praeorbulina glomerosa glomerosa in
Sam
ple 149-900A-20R-CC allow us to
assign this sample to the upper part
of Zone N7, which is of early
Miocene age. Samples 149-900A-21R-CC and -22R-CC do not contain
Praeorbulina sicana, thus these samples were placed in the lower part of Zone N7. The interval 149-900A-23R-CC to -27R-CC is marked by the presence of Catapsydrax dissimilis and Catapsydrax unicavus, thereby allowing us to assign
it to
Zones N5 to N6, which are of early
Miocene age. Within this interval,
Sample 149-900A-24R-CC is barren of planktonic foraminifers. The interval
149-900A-28R-CC to -32R-CC contains Globorotalia kugleri.
This species defines Zone "N4," which is of
latest Oligocene to early Miocene age. The
interval 149-900A-33R-CC to
-38R-CC contains primarily very
small planktonic foraminifers, none
of
which is a marker species. Within this interval, Sample 149-900A-34R-CC contains Globigerinoides spp., and Sample 149-900A-38R-CC is barren of planktonic foraminifers. The interval
149-900A-39R-CC to -41R-CC is characterized by the
absence of both Globorotalia
kugleri and Globigerinoides spp. and
has been assigned to the upper part of Zone P22, which is of late
Oligocene age. The top of the interval
149-900A-42R-CC to -45R-CC
is marked by the presence of
Globorotalia opima nana and Globigerina ciperoensis
angulisuturalis;
this interval
can be placed in the lower
part of Zone P22, which is of
late Oligocene age. Sample 149-900A-46R-CC contains Globigerina
cryptomphala and Globigerina ciperoensis angulisuturalis
and was assigned to Zone P21,
which is of late
early to early late Oligocene
age. Samples 149-900A-47R-CC, -49R-CC, and -51 R-CC contain no zonal
markers. Samples 149-900A-48R-CC and -50R-CC are barren of planktonic foraminifers. The interval
149-900A-52R-CC to -54R-CC contains an assemblage of Globorotalia
increbescens, Globigerina yeguaensis, Globigerina
tripartita, Globigerina corpulenta,
and relatively few Globigerina eocaena.
The presence of Globorotalia
increbescens allows
us to assign this interval to somewhere within Zone P15 to the lower
part of Zones P19/20, which are of latest
middle Eocene to early early
Oligocene age.
The presence of Globorotalia
cerroazulensis cocoaensis and Globigerinatheka index
s.1. in Sample
149-900A-55R-CC restricts it to
Zones P15 to P17, which are of latest middle to late Eocene age. The
first downhole occurrence of species
belonging to the Globorotalia
cerroazulensis group, which are markers for the top of the Eocene,
may be depressed in this section. The interval 149-900A-56R-CC to
-58R-CC is marked by the presence of
Truncorotaloides rohri and
Truncorotaloides topilensis
at the top, Globigerina
tripartita near the
base, and Globorotalia cerroazulensis cerroazulensis at the
base.
This interval can be assigned to Zone P14, which is of late middle
Eocene age. Samples 149-900A-59R-CC, -60R-CC, and -61R-CC
are barren of planktonic foraminifers.
The interval 149-900A-62R-CC to -69R-CC contains Truncorotaloides rohri and Truncorotaloides topilensis,
but no Globorotalia cerroazulensis
cerroazulensis.
Morozovella aragonensis
has not yet been observed. This interval
can
be assigned to Zones P12 and P13,
which are of middle Eocene age.
Within this interval, Samples 149-900A-63R-CC and -64R-CC are
barren of planktonic foraminifers. Sample
149-900A-70R-CC is barren of planktonic and calcareous
benthic foraminifers. Relatively rare arenaceous benthic foraminifers
(Glomospira
spp. and Bathysiphon spp.) and
ichthyoliths are present. The interval
149-900A-71R-CC to -72R-CC is marked by the
presence of Morozovella aragonensis at the top and Morozovella
lehneri at the base; this interval can be placed in Zone P11, which is
of middle Eocene age. Acarinina pentacamerata was first observed
in Sample 149-900A-72R-CC. The interval 149-900A-73R-CC to
-74R-CC is characterized by the presence of Morozovella aragonen
sis
and the absence of Morozovella lehneri. Acarinina soldadoensis was
not recorded. This interval can be assigned to Zone P10, which
is of early middle Eocene age.
Samples 149-900A-75R-CC and -76R-CC are barren of planktonic
foraminifers. Sample 149-900A-77R-CC
is a hard sandstone and could
not be disaggregated. Sample 149-900A-78R-CC contains Planorotalites
pseudomenardii, which defines Zone P4 and is of late Paleocene age. Sample
149-900A-79R-CC contains relatively few arenaceous benthic foraminifers (Glomospira charoides and
Ammodiscus spp.), one specimen
of a calcareous benthic
foraminifer (Cibicidoides spp.), and relatively
rare radiolarians and
ichthyoliths. The presence of younger planktonic
foraminiferal species in this core-catcher sample is the result of down
hole contamination; thus, no age was assigned to this sample. PALEOMAGNETISM We drilled about 740 m of sediment at Site 900.
However, only
from 30.4 to 145.3 mbsf, which was
composed of the nannofossil ooze
and clay of Unit I (see
"Lithostratigraphy" section, this chapter), were
the recovered cores sufficiently magnetized to permit alternating field
(AF) demagnetization and measurements using the cryogenic magnetometer. Below 151.3 mbsf (Cores 149-900A-18R through -79R),
sediments are very weakly magnetized
(generally between 1 × 10-4 and
2 × 10-4 A/m).
Consequently, only natural remanent magnetization was
measured in these cores. We measured sections of the basement rocks
at 2- to 5-cm intervals, using the pass-through cryogenic magnetometer. Thus far, a total of nine
discrete samples has been progressively AF
demagnetized to verify the reversed magnetizations indicated by the
cryogenic magnetometer measurements. Magnetic
Properties The quality of paleomagnetic data depends strongly
on the lithology of the recovered material; thus, the
discussion here of magnetic properties at Site
900 is organized on the basis of lithology. From
Cores 149-900A-1R to -4R, we
recovered mud-dominated turbidites.
These cores were too mechanically disturbed by drilling to allow for
meaningful measurements.
Cores 149-900A-5R
through -17R are mainly nannofossil clay and
ooze and have a strong magnetic
signal (NRM intensities typically in
the range 1 to 10 mA/m) that was easily measured. The whole-core
pass-through measurements agree well
with those from the discrete
samples. Figure 17 shows typical examples of AF demagnetization
diagrams for the discrete samples. The
magnetic behavior of these
sediments is relatively
straightforward, with easily identified characteristic magnetizations in the orthogonal demagnetization diagrams. Except for two short intervals, the sediments from
181.5 to 748.9
mbsf in Unit II (see "Lithostratigraphy"
section, this chapter) were
almost entirely characterized by weak magnetic remanence (<0.5
mA/m). We could not measure these sediments accurately on board
the ship. We hope that land-based studies
can successfully analyze the
weak remanence in these
sediments. Below 748.9
mbsf, the recovered cores are fine-grained, metamorphosed' mafic rocks (see
"Igneous and Metamorphic Petrology and
Geochemistry" section, this chapter) and
have not yet provided any reliable magnetic results, mainly because of their weak magnetizations. Magnetostratigraphy The best
evidence for polarity reversals occurs in the interval from
the upper part of Core
149-900A-5R to the top section of Core 149-900A-17R (30.4-145.3 mbsf). The combined shipboard paleomag
netic and biostratigraphic data (see "Biostratigraphy" section, this
chapter) have enabled us to construct a
tentative magnetostratigraphy
for the sediments in this
interval (Fig. 18). The Brunhes/Matuyama
boundary was not observed in Hole 900A. However, because biostratigraphic samples between 12.0 and 65.5 mbsf have been tentatively assigned ages in the range from 0.8 to 2.6 Ma, the observed
predominantly negative
inclinations in this interval suggest that these
sediments were deposited within
the Matuyama Chron. Thus, the shift
in polarity from reversed to
normal at about 65.0 mbsf may represent
the
Matuyama/Gauss boundary (2.6 Ma). In addition, the preliminary
biostratigraphic data suggest
that sediments at 46.9 mbsf may be of
late Pliocene age. This information suggests that the shift of
polarity
from reversed to normal at about 49.0 mbsf corresponds to the upper
Olduvai boundary (1.88 Ma). Finally, the
upper boundaries of both
nannofossil Zone NN15 and foraminiferal Zone N19 have been tentatively placed at about 80 mbsf,
which would suggest that the Gauss/
Gilbert boundary (3.5 Ma) should occur in the vicinity of this depth.
As mentioned above, there are two
exceptions within Unit II
where the sediments are not too
weakly magnetized. These occur in
Cores 149-900A-51R (469.9 mbsf) and -77R (720.0 mbsf). Interestingly, pass-through measurements revealed that sediments at both
these two depths are reversely
magnetized. Although biostratigraphic
evidence suggests that they may be of early Oligocene and early
Eocene age, respectively, so far we have
been unable to relate the
reversed polarities to the geomagnetic reversal time scale. Magnetic
Susceptibility The magnetic susceptibility of all cores for Hole
900A was measured at intervals of 3 cm (Fig. 19). In
general, the whole-core susceptibility measurements vary in similar fashion to the NRM signals. For
Unit I, the magnetic susceptibilities have average values
consistently
above 4 × 10-4 SI units, similar to those observed at earlier Leg
149
sites. Average susceptibilities of the
weakly magnetized sediments of
Unit II and the metamorphosed mafic rocks in the basement are only
about 2 × 10-4 SI units. The two peaks of susceptibility at about
470
and 720 mbsf correspond to Cores 149-900A-51R and -77R, respectively, which also displayed relatively
high magnetizations. IGNEOUS AND
METAMORPHIC PETROLOGY
AND GEOCHEMISTRY Introduction Drilling at
Site 900 encountered the top of a series of metamorphosed and brecciated mafic rocks at 748.9 mbsf, beneath sediments
of Pleistocene to late Paleocene age. A total of 56.1 m of these mafic rocks was drilled and 27.71 m of material was recovered
before the
hole was abandoned at a depth of 805 mbsf, giving a recovery in the
basement section of the hole of 49.4%.
This succession is
composed of mainly fine-grained metamorphosed mafic rocks showing intense deformational features. Many
sections are now highly brecciated, separated from nonbrecciated
parts by sharp contacts (Fig. 20). The
age of the rocks is uncertain
beyond the obvious fact that they are pre-late Paleocene. Macroscopic
Core Descriptions Core recovery
in Core 149-900A-79R was poor, and the nature of
the sediments and their contact relationships to the underlying mafic
rocks in Core 149-900A-80R are unknown. The first hard-rock sample recovered in Section 149-900A-80R-1 is a fragment of amphibolite
(drilling dropstone?). Below this, extending downward to the base
of the drilled section, is a succession of fine- and coarse-grained mafic
rocks that have been cut by veins and 1-cm to 4-m (Core 149-900-84R) zones of brecciation. The color of
the whole section varies from
a light to a more frequently dark
greenish gray grading toward brownish gray at the bottom. Intervals
149-900A-82R-5, 0-38 cm, and -83R-2, 50-90 cm, display a marked planar fabric that
is the result of alternating discontinuous mafic and felsic bands, each
having a maximum thickness of 8
mm. This banding gives these intervals a distinctive "flasered" appearance (Fig. 21). Though less obvious,
this same planar fabric
characterizes the fine-grained massive-appearing material, in bands
less than 1 mm thick (Fig. 22). The
coarser-grained intervals grade rapidly over a few millimeters
into finer-grained banded zones, or are limited by brecciated zones or a sharp contact
with a matrix that displays differently oriented dipping
layers (Interval 149-900A-83R-2, 105-115 cm). These grain-size
variations might result from varying
degrees of deformation.
Late-stage
brecciated zones disrupt approximately 37% of the re
covered core. Their distribution is not homogeneous and Cores 149-900A-80R, -81R, and -84R are the most intensely brecciated with,
respectively, 68%, 53%, and 70% of their
length brecciated (vs. 16%
to 38% for the remaining cores).
This brecciation may be progressive:
the first stage shows little
displacement of blocks (Interval 149-900A-84R, 45-80 cm) and corresponds to an increase in veining frequency.
Veining The cores are cut by a large number of veins and
fractures that in
places represent more than 20% of the
rock. Although some variation
is visible, one can observe the
same paragenetic sequence of veins
throughout this section of the
core. This sequence is listed here, starting with the earliest veins: The most recent
veining and brecciation associated with calcite
(Type 4 above) is preferentially developed in the upper part and base
of the basement section, but extends
more rarely through the entire
mafic interval. The other vein-types are more uniformly developed
throughout this section. The breccia is
frequently deformed, particularly near the contact
of blocks. In that case, the epidote and some chlorite veining
clearly
has been deformed, sheared, and broken.
Rarely, even the calcite
veining has been sheared (Interval 149-900A-82R-3, 74-78 cm). No
intrusive or crosscutting relationships were observed, other than the
late zones of brecciation and
fracturing. Petrography Thirty-one thin sections were made of the
metamorphosed mafic
rocks from Hole 900A. The recovered
rocks include granoblastic and
cataclastic microgabbro, small
amounts of cataclastic norite, plagioclasite, and a
single (dropstone?) of garnet-bearing amphibolite. Most
of the massive rocks have a
porphyroclastic texture and discontinuous
foliation bands (Fig. 21).
Porphyroclasts of plagioclase (up to 4 mm
in size) have been strongly stretched and bent (Fig. 24), and porphyroclasts of clinopyroxene (up to 1.8
mm), and sometimes orthopyroxene, have been bent and sometimes kinked. Smaller (0.1-0.2 mm)
granular crystals of the same minerals
mark foliation planes. The fine-grained minerals show typical
recrystallization textures (i.e., triple
junction boundaries and little or no strain). This kind of structure is
developed in coarser-banded intervals (149-900A-83R-2, 67-70 cm),
as well as in finer-grained intervals (149-900A-82R-3, 11-15 cm). In
the latter, porphyroclasts may be absent, giving the rock a granoblastic texture (Sample 149-900A-85R-5, 30-32 cm).
A rough estimate of the ratio of plagioclase to
pyroxene in these
rocks is 60:40, but may reach 40:60. In most rocks, this mineralogy
has been partially overprinted by
amphibolite or greenschist facies
metamorphic minerals, mainly fibrous tremolite and/or hornblende,
chlorite, epidote, and zoisite or
clinozoisite. The greenish cast of most
of these rocks reflects this
metamorphism. However, a striking feature of nearly
all these rocks is the freshness of the recrystallized
plagioclase, which is unaltered except in brecciated zones and in the
bottom of the hole (Sections
149-900A-85R-6 and -86R-1). Green hornblende has partially or totally replaced
clinopyroxene
crystals with a clear obliquity of the cleavage to the compositional
banding. Because no granulation or
deformation of the amphiboles is
seen, this replacement post-dates the strain recrystallization (Interval
149-900A-82R-5, 0-15 cm). At the base of the recovered section,
fibrous green and colorless amphiboles
become major constituents of
the rock (Intervals 149-900A-85R-6, 135-139cm, and-86R-1, 1-4 cm).
Only in these intervals does the amphibole show some deformation. Chlorite often
has replaced what may have been orthopyroxene,
surrounds the plagioclase grains, and occurs in veinlets that
crosscut
these layers. It clearly post-dates the green hornblende. Epidote and
clinozoisite or zoisite are also
ubiquitous, but more often restricted to
veins. Prehnite occurs in the interval at 149-900A-84R-1, 131-132
cm. Calcite is the latest mineral to
crystallize, occurring mainly in
veins. It may have been preceded
by zeolite (phillipsite), as in Interval
149-900A-85R-2, 60-64 cm.
Geochemistry Major element determinations for nine metamorphosed
mafic
rocks are given in Table 7 and confirm
the mafic nature of these rocks.
However, the abundances of TiO2 and P2O5
are unusually low and
abundances of Al2O3
are high, with four samples containing more than
20% Al2O3.
The loss-on-ignition values reflect the generally hydrated
nature of the rocks and the presence of some carbonate veining. Results of trace
element analyses for 21 samples, given in Table 8,
are particularly revealing in
that they show the rocks contain very low
concentrations of incompatible elements zirconium and yttrium. Basalts having comparable low concentrations of these elements are
very rare. For example, typical
mid-ocean ridge basalts contain approximately 74 ppm zirconium,
whereas these rocks contain an aver
age of 16 ppm zirconium. The relative
consistency of trace elements within the suite of
analyzed samples suggests that large-scale mineralogical layering is
generally absent and elemental mobility
during metamorphism has
been limited. The only significant vertical variation in the composition of the rocks is in the
concentrations of yttrium, vanadium, and
TiO2. These elements and TiO2 have higher
abundances in Core
149-900A-81R (Fig. 25) that corresponds
to an enrichment in opaque
minerals noted in thin section. Discussion Petrographic,
mineralogical, and geochemical studies suggest that
these rocks are mafic in nature and have had a complex history. The
earliest minerals observed are
porphyroclasts of clinopyroxene, ortho
pyroxene, and plagioclase,
without spinel or magnetite (the rocks are
very weakly magnetized, see
"Paleomagnetism" section, this chapter). The earliest pyroxene/plagioclase mineralogy,
preserved in some
porphyroclasts, has suffered
high-temperature ductile deformation
accompanied by
dynamic recrystallization of these two minerals.
This ductile deformation was followed by a mainly static retrograde
metamorphism in the amphibolite or greenschist facies with amphibole or chlorite replacing pyroxenes. A late brittle deformation event
brecciated these rocks and filled voids
and fractures with chlorite,
epidote and clinozoisite, zeolite, then calcite. At present, and without radiometric dates to
constrain the events
indicated, it is not possible to
differentiate between an oceanic or
continental origin for these rocks. They might be:
Microprobe analyses and isotopic
studies are obviously required
to constrain pressure, temperature, and ages to differentiate among
these models. STRUCTURAL
GEOLOGY
Introduction Site 900 lies about 20 km east of Site 899, above a
basement high
at the eastern edge of the Iberia Abyssal Plain. The basement was
encountered at 748.9 mbsf, under a
sedimentary sequence of Pleistocene to Paleocene age. Basement rocks were expected to be of continental origin, potentially
reflecting both pre-rift and synrift deformation histories. It was also expected that the overlying sediments
might display evidence for post-rift deformation along the western
Iberia margin. However, few deformation
structures were observed in
the sedimentary sequence.
Basement had been ductily deformed at
high temperature and then sheared and fractured at low temperature. Sediment Deformation Sediments at
Site 900 display a few discrete deformation structures, primarily in concentrated zones containing slumped bedding
and
microfaults. However, a systematic increase is seen in bedding
dips through the deepest part of the sediment column. Few structural
features were observed in Hole 900A above 200
mbsf, where sediments were too soft and disturbed by drilling to
preserve structures. The first evidence for structural disruption was
observed between 234.1 and 240.0 mbsf (Cores 149-900A-26R and
-27R), within an approximately 6-m-thick zone containing faulted
and folded beds (Fig. 26) and locally
high bedding dips (up to 28°).
In places, microfaults offset color-banding in a normal sense; one
offsets a Zoophycus of 0.8
cm (Fig. 27). The consolidation state of the
sediments involved in this
deformation is relatively uniform, suggesting that these features developed
prior to sediment burial. Slight
differences in lithology above and below this zone of structural disturbance provide the
basis for the subdivision of lithostratigraphic Unit II, posing the possibility that this zone of deformation
marks a structural discontinuity (see "Lithostratigraphy" section,
this
chapter). Such a feature is not evident
in seismic profiles that cross
the site. Moreover, the excellent recovery through this zone (85%-90 %) suggests that significant deformation features associated with
a post-depositional structural
discontinuity are not present. The ob
served deformation, therefore, is most likely to have occurred near
the seafloor. Sediments
between 240 and 400 mbsf are relatively flat-lying and
undisturbed. Below about 412 mbsf (Core 149-900A-45R), bedding
dips steadily increase with depth to
about 30° at 731 mbsf (Core
149-900A-78R) almost immediately above the basement. Although
at that depth logging data indicate hole deviations of about 8°. The
apparent dips are larger and thus must be real. Drilling biscuits
from
the deepest sediments break preferentially along the dipping bedding
planes. Rare
microfaults occur in Intervals 149-900A-48R-1, 47-52
cm, to -48R-3, 130-136 cm, and display normal offsets of less than 1
cm and dips of about 47° (Fig. 28). As a result of the low magnetization of these sediments, it was not possible to orient these structures
with respect to true north.
Seismic profiles that cross Site 900 do not show the
high bedding
dips observed in the deeper cores. These dips may reflect local structures associated with the basement high,
but this cannot be established
at this time. Basement Rocks The basement at Site 900A is made of fine- to
locally coarse-grained metamorphic mafic rocks. Two main events of deformation
can be distinguished in these rocks: (1) a high-temperature ductile
deformation characterized by a well-marked foliation and (2) a later,
low-temperature event expressed as narrow shear zones, with intense
fracturing that evolves locally into
brecciation.
High-temperature Ductile Deformation This deformation
is characterized macroscopically by a clear foliation marked by alternating
narrow dark green layers and gray to pale
orange elongated lenses. A thin section of a fresh, fine-grained
facies
(Sample 149-900A-85R-1, 22 cm) shows that
the dark layers are made
of plagioclase feldspar and the
gray ones of pyroxene crystals. The
rock texture is porphyroclastic to granoblastic. The plagioclase has
been recrystallized in
0.5-mm-sized crystals, some having undulatory
extinctions, which locally
surround some relict-strained porphyroclasts. A few strained pyroxene
porphyroclasts have been preserved in
clusters, where they are more or less recrystallized. Such a deformation
is classically described in flaser-gabbros. In some places, the rock
displays a larger grain size, with
porphyroclasts of 5 mm that have been isolated in
elongated bands of smaller pyroxene crystals that alternate
with bands of fine-grained recrystallized plagioclase (e.g., Interval
149-900A-83R-2, 65-100 cm; Fig. 29).
However, the heterogeneity of
the grain-size distribution is also illustrated in such intervals,
where
layers of fine-grained facies are
parallel to the foliation. This foliation
is present in the overall basement
section, even though it is hardly visible in some extremely fine-grained facies. It generally dips from 15°
to 45°, but is locally higher and even
becomes subvertical in Interval
149-900A-81R-2, 0-28 cm. However, the latter orientation might be a
consequence of extensive
fracturing and brecciation, which locally can
be seen to deviate the foliation
clearly.
The presence of
amphibole and chlorite, both mainly derived from
pyroxene, demonstrates a retrograde metamorphism and re-equilibration in the low amphibolite to greenschist facies (see "Igneous and
Metamorphic Petrology and Geochemistry"
section, this chapter).
Textural evidence suggests that the high-temperature deformation
occurred prior to the amphibole development.
Low-temperature Deformation The low-temperature deformation clearly overprints
the high-temperature deformation. It is expressed by three types of structural
features: Discussion The first
tectonic event recognized in the rocks is an intense ductile
deformation that developed a well-marked foliation. Textural evidence
suggests that this deformation
occurred at high temperature under dry
conditions, likely during a shear event. Some intriguing thin fractures
appear to have diverted or even
bent the foliation. Such brittle features
associated with "soft"
deformation features might indicate either that
(1) the conditions of deformation were close to the brittle/ductile transition during the formation of
these fractures or (2) the foliation is progressively diverted or bent by closely spaced microfractures. As the
amphibole derived from primary pyroxene does not appear
to be synkinematic, the retrograde metamorphism in low-amphibolite
to greenschist facies seems to post-date the high-temperature ductile
deformation event. Next, a
complicated structural evolution at low temperature affected the basement rocks. The
most striking features associated with
this late deformation are the successive types of vein and narrow
shear zones unevenly distributed throughout the section. Most of the
fractures, and particularly sets of
conjugate 45 ° -dipping
veins and
brecciated zones having a 60°-dip,
are in accordance with a horizontal
extensional regime. The presence
of shear zones in microbreccias and
epidote-chlorite veins
demonstrates that a shear event occurred during or after the formation of these veins and microbreccias. The cal
cite veining and fracturing clearly is
the latest event to have occurred.
The brecciation thus took place
during or after the epidote veining and
ended during the calcite veining. Further petrostructural, geochemical, and
geochronological studies may help to define the origin of
these rocks, the sequence of the
different tectonic and metamorphic events, and the kinematics of the
deformations, and thus to constrain the evolution and emplacement
mechanism of this mafic body. Although the
magnetization of these
rocks is very low, possible
reorientation of the structures in the geo
graphic reference frame may be obtained from shore-based paleo-magnetic data. Site 900 is
located between mantle rock outcrops (Site 897) and
rocks having mantle affinities
(Site 899) to the west and the continental crust of the passive margin
farther to the east. The most likely
hypotheses for the origin of these metamorphic basic rocks might be
either to associate them with the
Mesozoic rifting of the margin
(underplated gabbros or atypical oceanic crust), or to consider them
as older Hercynian units that have been incorporated in the passive
margin. A preliminary comparison
with the mantle rocks recovered at
Sites 897 and 899 suggests that the tectonometamorphic
evolution of
the ultramafic and mafic rocks may be
comparable at the three sites
(high-temperature ductile deformation in the peridotite and the mafic
rocks, low-grade retro-metamorphism,
late shear deformation and
fracturing, similarities in the texture of some breccias, traces of
fluid
circulation; see "Structural Geology"
sections, "Site 898" and "Site
899" chapters, this volume). As regards the synrift emplacement
hypothesis, the Site 900 results show that the spatial transition between continental and oceanic crusts may occupy a large area made
of transitional crust and serpentinized mantle. However, at present
any conclusion would be premature.
Further petrostructural studies
and geochronological studies of the freshest basement samples will
help to understand the presence of such rocks beneath the Iberia
Abyssal Plain.
ORGANIC GEOCHEMISTRY Concentrations of calcium carbonate were measured,
on average,
from three samples selected from each
core in Hole 900A. Concentrations of organic carbon, C/N ratios, and Rock-Eval pyrolysis were
employed to determine the type of
organic matter contained within
the upper sediment units. Routine monitoring of headspace gas con
tents, done for drilling safety, yielded
information that is interesting
to compare with Sites 897, 898, and 899.
Concentrations of Inorganic and Organic Carbon Concentrations
of carbonate carbon vary between 9.5% and essentially 0% in sediments from Site 900 (Table 9). These concentrations of carbonate carbon are equivalent to 78% to 0% CaCO3 in
the
sediments, assuming that all of the
carbonate is present as pure calcite.
The variability in carbonate content reflects a history of generally
low biological productivity and deposition of hemipelagic sediments
below the CCD, combined with delivery of carbonate-rich turbiditic
sediments initially deposited in
shallower waters. Concentrations
of organic carbon were measured in a subset of
sediment samples from the upper part of Hole 900A. The absence of
significant amounts of headspace methane
in Site 900 sediments
suggested that organic carbon concentrations would be low, and
therefore few organic carbon
measurements were performed. Indeed,
concentrations were low in nearly all of the Site 900 samples (Table
10). Unit I, a sequence dominated by
Pleistocene to lower Pliocene
turbidites, averages 0.3% organic carbon. This average is approximately the same as the average of 0.2% calculated from DSDP Legs
1 through 31 by McIver (1975). The equivalent lithological unit at
Sites 897 and 898 contained 0.5% to 0.6% organic carbon (see "Site
897" and "Site 898" chapters, this
volume). The two principal sources
of organic matter in oceanic
sediments are marine algal production
and land plant detritus supplied by rivers and winds. Algal organic
matter is typically oxidized and largely recycled during and shortly
after settling to the seafloor (e.g., Suess, 1980; Emerson and
Hedges,
1988). The land-derived organic matter
that is delivered to deep-sea
sediments is generally the less-reactive material that survives trans
port to the ocean. The organic carbon found in Unit I at Site 900
evidently has been substantially oxidized prior to sedimentation and,
consequently, is not very reactive.
Characterization of Sources of Organic Matter Organic C/N
ratios were measured for selected Site 900 samples to
determine the source of the
organic matter. Algal organic matter generally has C/N ratios of between 5 and 10, whereas organic matter
derived from land plants has values
between 20 and 100 (e.g., Emerson
and Hedges, 1988; Meyers, in press). Variable C/N ratios of samples
from Unit I (Table 10) indicate that some samples have
a predominantly marine source for their organic
matter, whereas other samples
contain mostly terrigenous organic matter. The C/N ratios of some
samples are low (<5). These values are
probably an artifact of the low
carbon contents, combined with the tendency of clay minerals to
absorb ammonium ions generated during the degradation of organic
matter (Müller, 1977). The C/N ratios in samples that are especially
low in organic carbon consequently are
not accurate indicators of the
source of organic matter. Headspace
Gases Concentrations
of headspace methane were monotonously low
throughout Site 900 (Table 11). These low concentrations contrast
with the high levels of biogenic
methane found in the upper, turbiditic
units at Sites 897 and 898, but are similar to the low amounts
present
at Site 899 (see "Site 897," "Site 898,"
and "Site 899" chapters, this
volume). The generally low amounts, and inferred inert character, of
organic matter in sediments from Site 900 evidently preclude methanogenesis. In addition, Claypool and
Kvenvolden (1983) observed
that the presence of interstitial sulfate inhibits methanogenesis in
marine sediments, and concentrations of sulfate are high throughout
the recovered Site 900 sediments (see
"Inorganic Geochemistry"
section, this chapter). Comparisons
of Organic Matter Type
from Rock-Eval Pyrolysis Rock-Eval pyrolysis of organic matter from selected
Unit I samples from Sites 897, 898, 899, and 900
provided insights into the
differences in concentrations of headspace gas at these locations. Two
Rock-Eval parameters are
especially useful for characterizing sedimentary organic matter. The hydrogen index (HI) is the quantity of
hydrocarbons generated from thermal decomposition of the organic
matter, expressed as milligrams of hydrocarbons per gram of total
organic carbon. Marine organic matter typically has high HI values
(Espitalié et al., 1977). The
oxygen index (OI) is the quantity of CO2
generated during pyrolysis and is given in the same units.
Cellulose
containing land plants produce organic
matter having high OI and low
HI values (Espitalié et al.,
1977). Organic matter in Unit I samples
from all four sites has high OI and low HI values (Table 12), which
would normally indicate a land-derived
origin. Relatively low C/N
ratios, however, contradict this interpretation. It is likely that
the organic matter in this turbiditic
sedimentary unit has experienced considerable post-depositional
oxidation, which would depress HI values
while enhancing OI values. This
evidence of alteration in the Rock-Eval source character implies that considerable microbial degradation of the marine organic matter in Unit I has occurred, which is
consistent with an inferred history of downslope relocation of the
Unit I sediments from a shallower site of initial accumulation. Sediments from Sites 899 and 900 have lower S1, S2, and HI
values than
those from Sites 897 and 898 and have been degraded to greater
degrees. This difference in organic
matter character in closely spaced
locations on the Iberia Abyssal
Plain implies different deliveries of
turbidite components at the sites. Furthermore, the greater amount of
degradation evident at Sites 899 and 900
appears to render the organic
matter less supportive of
methanogenic bacteria. The generally low
Tmax
values (Table 10) suggest that thermal
degradation of the organic
matter can be excluded; the large
range of Tmax (309°-441°C) probably reflects heterogeneous
mixtures of relatively fresh marine and
detrital organic matter in the turbidites. INORGANIC
GEOCHEMISTRY
Twenty-seven interstitial-water samples
were collected at Site 900
between 12.6 and 721.5 mbsf. Whole-round samples were collected
from each core in the interval from 12.6 to 34.9 mbsf (Cores 149-9OOA-3R to -5R) and from every third core
thereafter (when recovery
allowed). Interstitial-water
samples spanned lithostratigraphic Units
I and II. Results from shipboard interstitial-water analyses are presented in Table 13. Concentrations of sulfate decreased from 27.9 mM in
the first
sample, at 12.6 mbsf, to a minimum of
1.9 mM at 702.2 mbsf (Fig.
35A). The sulfate profile is slightly convex upward, suggesting that
some sulfate reduction occurred within
the sediment sequence. Values of alkalinity decrease slightly from 12.6 to
54.1 mbsf and
then increase to a maximum of 11.9 mM at
289.9 mbsf (Fig. 35B).
The increase in alkalinity results from anaerobic organic carbon degradation (Gieskes, 1974, 1983). Alkalinity decreases between 289.9
and 465.7 mbsf and then increases to a peak of 11.7 mM at 465.7
mbsf. Alkalinity decreases below 407.7
mbsf to a minimum of 2.5
mM at 606.8 mbsf. Sample size limitations prevented the analysis of
alkalinity in the deeper samples. Concentrations of ammonia generally increase with
depth from a
concentration of 321 µM at 12.6 mbsf to
a maximum of 698 µM at
435.5 mbsf and decrease slightly
thereafter (Fig. 35C). The maximum
is consistent with a zone of active sulfate reduction within the interval
between the surface and about 450
mbsf. Fluctuations in the profile
indicate alternating zones of production and removal (the two anomalously low ammonia values at 116 and
702 mbsf probably reflect
analytical problems). The dissolved
manganese profile shows three distinct zones of
release and removal that produced maxima at 12.6, 115.7, and 465.7
mbsf (Fig. 35D). The high concentrations
of manganese in the first
sample probably result from the reduction of manganese oxides during the oxidation of organic carbon. The
two deeper maxima may
result from the reduction of
manganese oxides and/or dissolution of
some manganese carbonate phase. Concentrations
of calcium are slightly depleted with respect to
typical concentrations of bottom water in the first four samples from
12.6 to 54.1 mbsf and then
increase linearly to 31.5 mM at 289.9 mbsf
(Fig. 36A). Below 289.9 mbsf,
concentrations of calcium increase
slightly to reach 36.0 mM by 636.3 mbsf and then increase rapidly to
a maximum value of 46.5 mM by 721.5
mbsf. The profile suggests
zones of calcium release at about
300 and below 636 mbsf. The
release of calcium probably
reflects regions of carbonate recrystalization (Gieskes, 1983). Concentrations
of magnesium generally decrease with depth from
near typical bottom water to a
minimum of 17.3 mM at 702.2 mbsf
(Fig. 36B). The profile shows two zones where the gradient is steep,
below 115.7 and below 606.8 mbsf. The steep negative gradient
suggests that magnesium removal is
greatest in these zones. The
removal of magnesium probably results from clay mineral alteration
(Gieskes, 1983). Concentrations
of strontium increase from a minimum of 125 µM
in the first sample at 12.6 mbsf
to a maximum of 846 µM at 465.7
mbsf and remain fairly constant below that depth (Fig. 36C). The
strontium profile is slightly convex up between the surface and the
maximum at 465.7 mbsf, indicating release of strontium from the
solids through this sequence.
Strontium release is probably associated
with recrystalization of
carbonate phases (Gieskes, 1974). Concentrations
of potassium generally decrease downhole, with
some narrow zones of release and removal indicated by fluctuations
in the profile (Fig. 36D).
Potassium values decrease from a maximum
of 10.3 mM in the first sample at 12.6 mbsf to a minimum of 0.7 mM
at 702.2 mbsf. The profile is
fairly linear, suggesting removal of
potassium by means of interaction with basement rock. Concentrations of silica remain near typical
bottom-water values
from 12.6 to 77.1 mbsf and then increase to a broad maximum,
reaching 1188 µM between 143.0 and 435.5 mbsf (Fig. 37). Concentrations of
silica decrease below 435.5 mbsf to a minimum of 130 µM
at 665.9 mbsf. Concentrations of chloride remain fairly constant
with respect to
typical bottom water through most of the sediment column, with the
exception of the last six samples, which
are lower (Table 13). In con
trast to chloride, sodium is generally depleted with respect to
typical
bottom water throughout the sediment
column (Table 13). PHYSICAL
PROPERTIES
Introduction Whole-core measurements taken at Site 900 included
magnetic
susceptibility, Gamma-Ray Attenuation
Porosity Evaluator (GRAPE)
bulk density, P-wave logger (PWL)
compressional-wave velocity, and
thermal conductivity. Discrete velocity measurements were obtained
in unlithified sediments using the
Digital Sound Velocimeter (DSV) on
split cores and within the more
consolidated units and hard rock using
the Hamilton Frame Velocimeter.
Undrained shear strength was measured on split sediment cores, and electrical resistivity was measured on
split sediment cores and drilled
"minicores" from crystalline rock.
Index properties were calculated
from the wet and dry masses and wet
and dry volumes of samples taken from each section of core. Index Properties Index properties were determined using gravimetric
methods
(Table 14; Fig. 38). Based on the
nominal uncertainties of the raw
mass and volume measurements, the estimated uncertainties for density and porosity are ±0.02 g/cm3 and ±2%, respectively. The
sedimentary section at Site 900 yielded fairly smooth
downhole trends in
bulk density, grain density, and
porosity. Minor offsets of these trends
may be attributed to changes in lithology and degree of
lithification.
Gravimetrically determined bulk density increases nearly linearly
from 1.7 g/cm3 at the
seafloor to about 2.2 g/cm3 at a depth of 620
mbsf, while porosity decreases from about 65% to 32% (Fig. 38).
Minor undulations in the trends were observed near 67, 180, and
between 370 and 420 mbsf. The
first of these corresponds to the boundary between lithostratigraphic
Subunits IA and IB, and an associated
downhole decrease in carbonate content (see "Lithostratigraphy" section, this chapter). The second corresponds to the boundary between
Subunits IC and IIA, which show little
difference in composition.
Below 180 mbsf, the sediments gradually
become more lithified
without major abrupt changes in bulk density and porosity. Minor
changes in the downhole trend may be attributed to variations in the
lithification state of the sediment.
Below about 620 mbsf, the data
exhibit considerable scatter with lower bulk density and higher porosity values. The highest bulk density and lowest porosity measurements (several near 0%) are associated
with highly cemented calcareous sandstones interbedded with the silty claystones. Grain densities decrease from about 2.8 g/cm3 near
the seafloor to about 2.75
g/cm3 at about 200 mbsf. Grain densities increase gradually
between
200 and 420 mbsf, and maintain a
relatively uniform value of 2.8
g/cm3 below 420 mbsf.
Index properties
below 748.9 mbsf reflect sampling of crystalline
basement. Bulk density approaches 3.0 g/cm3 (close to the grain
den
sity), and porosity is effectively 0%. Grain density increases
sharply
downward near the top of the crystalline
basement, increasing from 2.8
g/cm3 at 748.9 mbsf to
about 3.0 g/cm3 at 796.4 mbsf. This increase
can be attributed to the downhole decrease in calcite veining and
degree of alteration, with the deeper
values reflecting the higher grain
densities of the fine-grained metamorphosed gabbro (see "Igneous
and Metamorphic Petrology and
Geochemistry" section, this chapter). GRAPE Measurements Bulk densities
were also estimated from whole-core GRAPE
measurements taken in all
sections recovered from Hole 900A (see
"Explanatory Notes" chapter, this volume). In the sedimentary section, the curve defined by the maximum GRAPE density measurements best fit the corresponding
gravimetrically determined bulk density (Boyce, 1973; Gealy, 1971). The maximum
GRAPE density measurements are indicated by the
curve in Figure 38. In the sedimentary section, above approximately
300 mbsf, the GRAPE density estimates increase from 1.75 to 1.9
g/cm3. The bulk density increases more rapidly with depth, from
about 1.9 g/cm3 near 300 mbsf
to 2.2 g/cm3 at 500 mbsf. The high
degree of fracturing observed in
the lower sedimentary section results
in large scatter in the data below
500 mbsf, but the maximum densities
are nearly constant at 2.2 g/cm3.
In basement cores, GRAPE bulk
density increases from about 2.4 to 2.7 g/cm3 at the base
of the hole
(796.4 mbsf). Electrical
Resistivity Electrical
resistivity was measured at intervals of 0.5 to 0.75 m in
split cores from the sedimentary
section above 655.5 mbsf (Cores 149-900A-2R to -71R), and in drilled
minicores in the more lithified rocks
in Cores 149-900A-80R to -86R. Formation factors were calculated
for the interval down to 620 mbsf (see "Explanatory Notes" chapter,
this volume). In the upper 320 mbsf, the
formation factor increase linearly with depth from 2.5 to 7.0,
although locally between 200 and 250
mbsf larger scattered values up
to 12 were observed (Fig. 38). At 320
mbsf, an abrupt increase from 7 to 10 can be seen. Below this depth,
the average formation factor increases downhole to 20 at 620 mbsf.
Electrical resistivity in the basement rocks (Fig. 39) ranges from
50 to 500 Ωm . Values lower than 100 Ωm were observed in altered
breccias (748-755 mbsf) and microbreccias (778 and 782 mbsf; see
"Igneous and Metamorphic Petrology and Geochemistry" section,
this chapter). Electrical resistivity values higher than 100 Ωm generally were observed in microgabbros and strongly correlate inversely
with the degree of veining and directly with the degree of metamorphism. The maximum resistivity (500 Ωm at
787 mbsf) was measured
in a metamorphosed microgabbro
without any veining. Undrained
Shear Strength Undrained shear
strength was measured in Cores 149-900A-3R to
-20R using the shear vane apparatus (see "Explanatory Notes" chapter, this volume; Fig. 40). Peak shear strength increases downhole
from 18 kPa at 12 mbsf to about 205 kPa
at 172 mbsf. Between 12
and 80 mbsf, the measured peak strength oscillates between 15 and
45 kPa, showing only a slight net
increase with depth. Below 80 mbsf,
the measured values exhibit a
much wider variation (possibly reflecting different degrees of lithification), but there appears to be a
more
pronounced trend of increasing peak
strength with depth. Acoustic Velocity Discrete
acoustic velocity was measured in Cores 149-900A-3R
to -85R (Table 15). The DSV was
used on Cores 149-900A-3R to -18R
to measure velocity in sediment
from depths shallower than 152 mbsf.
The Hamilton Frame Velocimeter was used to measure velocities in
more cohesive or indurated sediments and
basement samples in Cores
149-900A-18R to -85R. The sedimentary samples were trimmed into
cubes, and velocity was measured in three mutually orthogonal directions (see "Explanatory Notes" chapter, this volume). Compressional-wave velocity in basement rock was measured in the horizontal direction in minicores. Repeated measurements of selected samples and
calibration standards suggest an accuracy
of 2% to 3% for the velocity
measurements. Discrete acoustic velocity measurements in the
sedimentary section show a general increase with depth, from about 1490 m/s at 10
mbsf to 2400 m/s at 730 mbsf (Fig. 41). Velocity measured in the
vertical direction in the clays, silty
clays, and claystones shows a linear
trend with a slope of 1.06 s-1 and a correlation coefficient of
0.92. The
horizontal velocities show
similar downhole variations. Acoustic anisotropy in the intervals 180 to 240 mbsf and 340 to 460 mbsf is significantly higher than the
estimated 4% uncertainty in the anisotropy
calculation and can generally be
attributed to slower propagation in the
vertical direction
(Table 15). Acoustic anisotropy was not calculated
for samples from cores below 550
mbsf (Cores 149-900A-62R to
-78R) because horizontal
fractures developed during sampling, resulting in an artificially slow
velocity in the vertical direction. Velocities
measured in the horizontal
directions in these samples were not affected by the fracturing and are
thought to be representative of the true
horizontal velocity of the
sample. Velocities greater than 3000 m/s in
Figure 41 are from cemented siltstone (the sample at about 400 mbsf)
or well-indurated silty claystone (the samples below 630 mbsf).
Compressional-wave
velocity also was measured with the PWL in
unsplit sections from Cores 149-900A-1R to -18R. This corresponds
to the interval in which discrete velocity measurements were taken
with
the DSV. From 0 to 65 mbsf (Cores 140-900A-1R to -9R), the
PWL velocities show a trend consistent
with the linear increase in
velocity with depth observed in the discrete velocity measurements
of the clays and claystones (Fig. 42). The clay velocity gradient
derived from the DSV measurements
provides an upper bound on the
PWL velocities below 110 mbsf
(Cores 149-900A-14R to -19R).
Average PWL velocities measured in cores from depths between 74
and 103 mbsf (Cores 149-900A-10R to -12R) are significantly lower
than the discrete velocity measurements.
Acoustic velocities in the basement rocks show wide
scatter that
ranges from 3750 to 7600 m/s (Table 15). The velocities show some
clustering about 5700 m/s (Fig. 43). No
systematic variation of velocity with depth was observed in
the basement rocks (Table 15). Magnetic
Susceptibility Magnetic susceptibility was measured at intervals of
3 to 5 cm in
all cores collected at Site 900. The
results are discussed in the "Paleo-magnetism" section (this
chapter). Thermal
Conductivity Thermal
conductivity for Site 900 was measured in every other
section of Cores 149-900A-1R to -60R within the sediments and in
the basement Cores 149-900A-80R to -85R (Fig. 44; Table 16). The
mean uncertainty associated with these measurements was estimated
as ±0.2 W/(m·K). In the sedimentary
section, the thermal conductivity
values show only a slight
increase with depth. Between 0 and 115
mbsf, the average thermal
conductivity is 1.2 W/(m·K) (Fig. 44; Table
16). In the interval between 115 and 370 mbsf, the mean value is 1.4
W/(m· K) and a slight increase
with depth can be observed. The data
points show larger scatter around a mean value of 1.5 W/(m· K)
from
370 to 563 mbsf, whereas the crystalline
rocks exhibit much higher
thermal conductivity values that range between 1.7 and 2.9 W/(m· K)
in the depth interval from 749 to 794
mbsf. DOWNHOLE
LOGGING
Logging Operations Three logging
runs were made in Hole 900A using two different
tool strings that obtained data from only part of the total depth of
the
hole. Total penetration in Hole 900A was 805.0 mbsf (5853.5 mbrf).
The wiper trip made in preparation for logging encountered drag
during the upward trip in the intervals
770-763, 753, and 600-543
mbsf.
The subsequent wiper trip downward had to ream through the
interval from 563 to 568 mbsf and a tight spot at 588 mbsf. The geophysical
combination was run first. Data were recorded on
two passes, the first upward from a bridge encountered in the hole at
238 mbsf to the drill pipe at 137 mbsf.
The pipe then was lowered
through this bridge to 330 mbsf, and a second pass run upward from
a lower bridge at 451 mbsf to the drill pipe at 330 mbsf. The Formation Microscanner (FMS) combination was
used for
the third and final run. We also used the
conical side-entry sub (CSES)
for this run because of the
bridges previously encountered in the hole.
The CSES was assembled on the drill string so as to allow logging up
to 220 mbsf. The pipe was then worked down to 753 mbsf. FMS data
were acquired upward from 785 to
712 mbsf. A bridge at 731 mbsf
prevented lowering the tool for a repeat run. A second pass from the
bridge at 731 mbsf upward ended at 646 mbsf, when the tool became
stuck. We freed the tool by lowering the
drill string down over it. We
raised the drill string above
this bad spot in the hole and attempted
again to obtain FMS data, but it became apparent that the cable had
been damaged when we freed the tool at 646 mbsf. The tool became
stuck again during these attempts and,
again, was freed by lowering
the pipe over the tool. Cable problems and deteriorating hole conditions forced us to abandon further
efforts to log this hole. The
Lamont-Doherty temperature tool was not used in this hole.
A "hole finder" was substituted at the bottom of the geophysical
combination to try to increase
the possibility of passing hole constrictions. The temperature tool was omitted from the FMS combination
because the CSES was deployed. Following is a summary of the
logging runs. Run 1 Pass 1: Geophysical combination; drill-pipe depth,
137 mbsf
(5185.4 mbrf). Logged interval: 137-238 mbsf; speed, 600 ft/hr (190
m/hr). Tools: natural
gamma-ray/shear sonic/resistivity. Pass 2: Geophysical combination; drill-pipe depth,
330 mbsf
(5378.7 mbrf). Logged interval:
330-451 mbsf; speed, 600 ft/hr (190 m/hr). Tools: natural
gamma-ray/shear sonic/resistivity. Run 2 FMS
combination; drill-pipe depth, variable (using CSES).
Logged intervals: 646-731 and
712-785 mbsf; speed, 600 ft/hr
(190 m/hr). Tools: natural
gamma-ray /FMS.
Quality of Logs
Brecciation of the rocks develops in zones where a
high degree of
fracturing and veining has occurred.
About 35% of the recovered
basement rocks are brecciated; in
particular, in Cores 149-900A-80R,
-81R, and -84R. Two main types of breccia can be distinguished. The
first type of breccia results
from an intense fracturing by thin, contorted, and anastomosing veins of pale green material, in dipping
zones a few centimeters thick (Interval 149-900A-81R-2, 15-20 cm;
Fig. 32). With increasing vein density, the green color of the breccia
suggests a pervasive fracturing and alteration possibly aided by fluid
circulation (Fig. 33; Intervals
149-900A-81R-3, 0-15 cm, and -84R-2, 60-85 cm). This brecciation post-dates the zoisite veining, but
pre-dates the calcite veins. The second
type of breccia is made of
angular blocks of fine-grained metamorphic mafic rocks embedded
in a bright green chlorite and/or
calcite matrix (e.g., Intervals 149-900A-83R-3, 0-50 cm; and -84R-1, 0-135 cm). In some places, the
calcite appears to have replaced the
chlorite after the formation of the
breccia. In areas where the blocks are of reduced size, the texture
of
these breccias is mostly comparable to those of the Upper Breccia
Unit described at Site 899 (e.g., Intervals 149-900A-80R-1, 47-70
cm, and -82R-5, 75-87 cm; see "Lithostratigraphy and Petrology"
section, "Site 899" chapter, this
volume). These breccias occur locally
in 10-cm-thick bands that dip
about 60° and may represent fault zones
(Interval 149-900A-83R-2, 35-62
cm; Fig. 34). Finally, the proportion
of epidote can be so high locally that angular clasts can be embedded
in a primarily epidote matrix (Interval 149-900A-86R-1, 0-15 cm).
The wire-line heave compensator was used during all logging runs; sea conditions were calm with minimal swell.
The quality of the sonic waveforms from the monopole transmitters for both passes generally was good, except for the shear waves. Real-time slowness-time coherence processing, however, proved problematic for all waveforms, resulting in unreliable P-wave velocity computations in both sections. This was observed as the "skip ping" in delta T (velocities) in Figures 45 and 46; shore-based processing will be required to improve these data. The amplitude of the recorded signals from the dipole transmitter in both logged sections was very low, thereby rendering the shear-wave data in the upper, "softer," logged interval useless. Shear-wave data from the lower section will require careful processing to obtain useful shear-wave velocities. Resistivity data from the induction phaser tool are good, and the correlation among shallow, medium, and deep resistivity over the interval also is good.
The quality of FMS data was poor in the lower pass, because the tool had problems with fully opening its arms and contacting the borehole wall. Good data were obtained from the second run between 646 and 731 mbsf. Good natural gamma-ray data were acquired from both passes and have been combined in Figure 47.
Depth Shifting
No intervals of the hole were logged with more than one tool string, so depth shifting of the logs is not necessary.
Logging Results
Interval at 137-238 mbsf
This section has natural gamma-ray values that range from 35 to 70 API units. The section has a uniform resistivity that averages 1.5 Ωm and tentative estimates of P-wave interval transit time are on the order of 160 to 175 µs/ft (1.7-1.9 km/s), all of which are typical of a sequence of relatively unconsolidated sediments. A slight peak in the natural gamma-ray log at 180 mbsf corresponds to the boundary between Units I and II in Core 149-900A-21R. The DSI sonic tool indicates a poor shear-wave response from the dipole source, which indicates both weak dipole source energy from the DSI and a poorly consolidated sedimentary formation.
Interval at 330-451 mbsf
This interval has constant values of natural gamma-ray and resistivity (a mean of 70-75 API and 1.5 Ωm, respectively). Tentative first estimates of P-wave interval transit time are on the order of 150 µs/ft (2.0 km/s). The log data suggest that the lithology in this section is similar to that of the upper section. A slightly higher natural gamma-ray value suggests a small change in lithology, although resistivity appears unchanged.
Interval at 646-785 mbsf
Natural gamma-ray data over this interval show a marked decrease from a high average of 80 API units above 710 mbsf, through a transitional zone (710-740 mbsf) to a low average of 15 API units below 740 mbsf. This corresponds to the change from the basal sedimentary Subunit IIB to acoustic basement; the contact is located some where between 740 and 750 mbsf. Given the nature of the acoustic basement, this sediment/basement contact probably is a sharp contact at the very base of the transitional zone. The transitional zone is likely to be the result of the change from the dominant clayey lithology of Subunit IIB to the sandy basal sediments, as observed in the cores. FMS data show that the hole is deviated (from vertical) by 12° through the basement section (azimuth 272°). In the section immediately above, the basement deviation is seen to decrease relatively steadily from 12° to 8° (azimuth 272°-273°) at the top of the logged interval.
INTEGRATION OF SEISMIC PROFILES WITH OBSERVATIONS FROM THE SITE
Two multichannel seismic-reflection profiles were obtained across the site before the cruise. Lusigal Line 12 crosses the site in an east-west direction (Fig. 1) and Sonne Line 75-21 crosses the site in a north-south direction (Fig. 3). The profiles indicate a number of sedimentary reflectors that have been recognized on a regional scale in the vicinity of the Iberia Abyssal Plain; these have been dated by tracing them back to Leg 103 sites west of Galicia Bank (Mauffret and Montadert, 1988) and to Site 398 near Vigo Seamount (Groupe Galice, 1979). One of these reflectors, which separates acoustic formations 1B and 2 (Groupe Galice, 1979; Fig. 1), crosses the site. The reflection profile across the site is also characterized by two unusual acoustic facies in the interval between 0.58 s two-way traveltime and the seabed. The interval from 0 to 0.38 s two-way traveltime consists of hummocky sediment waves, and the interval from 0.38 to 0.58 s two-way traveltime consists of a series of inclined reflectors (Fig. 1).
Downhole sonic logs were obtained at Site 900 over just two intervals: between 137 and 238 and from 330 to 451 mbsf. This was insufficient to compute the depths of seismic reflectors. However, the results of two sonobuoy lines shot over the Iberia Abyssal Plain (Whitmarsh, Miles, and Mauffret, 1990) were used to convert from two-way traveltime to depth (Fig. 66, "Site 897" chapter, this volume). Thus, we were able to estimate the downhole depths of the 1B/2 and basement reflectors seen in the Lusigal 12 seismic-reflection profile and the depths downhole of the two acoustic facies described above. These are summarized in Table 17.
IN-SITU TEMPERATURE MEASUREMENTS
A WSTP tool was deployed to collect in-situ temperature data at 122.4, 170.6, and 218.7 mbsf in Hole 900A (WSTP depth is based on the depth of the bottom of the previous cored interval). The general shape of the temperature vs. time curves for each deployment suggests that the tools were inserted in a single movement and were stationary during the measurements. The individual temperature measurements showed much more scatter about the typical cooling curve than is usual (Figs. 48B-50B). We suspect that this was a problem with the tool, but do not think that it renders the measurements invalid.
Analyses of the WSTP measurements at 122.4 mbsf, obtained immediately prior to Core 149-900A-15R, yielded a bottom-water temperature of 3.8 ± 0.1°C and an in-situ temperature of 10.2 ± 0.1°C (Fig. 48). The bottom-water temperature was obtained by averaging temperature readings between 2400 and 2900 s, when the tool had stopped just above the seafloor. The in-situ temperature was estimated by using 3171 s as the insertion time and by modeling the temperature decay over the interval from 3202 to 3770 s.
Analyses of the WSTP measurements at 170.6 mbsf, obtained immediately prior to Core 149-900A-20R, yielded a bottom-water temperature of 3.8 ± 0.1°C and an in-situ temperature of 12.2 ± 0.1°C (Fig. 49). The bottom-water temperature was obtained by averaging temperature readings between 2000 and 2200 s, when the tool had stopped just above the seafloor. The in-situ temperature was estimated by using 3250 s as the insertion time and by modeling the temperature decay over the interval from 3281 to 4150 s.
Analyses of the WSTP measurements at 218.7 mbsf, obtained immediately prior to Core 149-900A-25R, yielded a bottom-water temperature of 3.8 ± 0.1°C and an in-situ temperature of 14.4 ± 0.1°C (Fig. 50). The bottom-water temperature was obtained by averaging temperature readings between 2200 and 2400 s, when the tool had stopped just above the seafloor. The in-situ temperature was estimated by using 3198 s as the insertion time and by modeling the temperature decay over the interval from 3289 to 4023 s.
The slope of a linear least-squares fit of the temperature to depth (Table 18) yields an estimate of 49 ± 3 mK/m (95% confidence level) for the temperature gradient in the upper 218 mbsf at Site 900. The slope of a linear least-squares fit of the temperature to vertically integrated thermal resistivity (Table 18), yields an estimate of 59 ± 8 mW/m2 (95% confidence level) for the heat flow (see "Explanatory Notes" chapter, this volume).
SUMMARY AND CONCLUSIONS
Site 900 is situated in the Iberia Abyssal Plain over a roughly circular basement high (Fig. 2 in "Site 897," this volume) within the presumed ocean/continent transition (OCT) zone off western Iberia (see "Introduction" chapter, this volume). The site is one of a transect of drill sites across the OCT designed to study the petrologic changes in the basement rocks within the OCT as a means of identifying the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. The site is in a region of weakly magnetized, presumed thinned continental crust (see Fig. 4 in "Introduction" chapter, this volume). An RCB hole was drilled with the primary scientific objective of penetrating basement to a depth sufficient to firmly establish its character. The hole was drilled and cored to 805 mbsf and passed through Pleistocene to late Paleocene age sediments and 56 m of mafic igneous rock. Coring was terminated when the rate of penetration slowed to 1 m/hr and bit failure was imminent. A total of 380 m of sonic, resistivity, and FMS logs was acquired from three separate intervals in the sediments and basement.
The first event in the history of Site 900, which we can deduce from the cores, is the formation of the mafic igneous rock that now forms the basement here. At present, we can only speculate about the origin of this rock. It may be (1) cumulate gabbro (of any pre-late Paleocene age) either formed in, or possibly underplated at the base of, continental crust, or (2) pre-Mesozoic mafic rock involved in the Hercynian orogeny and later incorporated in the passive margin. The rock subsequently experienced a series of deformation and metamorphic events. The rock has a well-marked foliation, which attests to high-temperature shear deformation. This was followed by retrograde metamorphism to low-grade amphibolite or greenschist facies. An ensuing lower-temperature deformation is expressed in narrow shear zones, fractures and veins, and brecciation. The density of the basement rocks is about 2.6 to 2.9 g/cm3; their velocity ranges from 3.7 to 7.5 km/s, with a cluster of observations at 5.7 km/s. One effect of the metamorphism seems to have been the virtual destruction of any remanent magnetization.
Preliminary comparison with basement rocks from Sites 897 and 899 suggests that this tectono-metamorphic evolution may be common to each site, although the primary lithologies are different. Finally, the metamorphosed mafic igneous rocks were exposed at the seafloor prior to the late Paleocene, probably by the Early Cretaceous rifting.
The sediments cored at this site reveal a late Paleocene to Pleistocene history of sedimentation on an evolving continental margin, starting with predominantly contour current reworking of turbidites, followed by the dominance of turbidite deposition from the middle Miocene onward. Sedimentation over this interval ranged between 13 and 27 m/m.y.
From the late Paleocene until the early Miocene, a sequence of clay or silt with nannofossil clay was deposited. Sedimentation was interrupted for about 7 m.y. during the early Eocene. An important characteristic of these sediments is the presence of upward-darkening sequences. These range in thickness from 10 to 30 cm, usually have sharp bases and tops, and are intensely bioturbated. These features and small-scale structures in the cores point to reworking by contour cur rents. At the same time, evidence of downslope sediment movement can be seen in the form of scattered mud turbidites. A series of calcite-cemented sandstones, deposited until the earliest Miocene, may represent a combination of turbidity and contour-current deposition. Above a fault of unknown displacement in the early Miocene sequence, possibly the result of synsedimentary deformation, upward-lightening sequences appear that are indicative of turbidite deposition. Pervasive bioturbation indicates deposition above the CCD.
The sequence from the early Miocene to the Pleistocene is principally nannofossil clay and ooze. The turbidites were dominated by mud, and sandy bases are uncommon. The turbidite bases are mostly siliciclastic, but occasionally carbonate-rich, suggesting two distinct provenances. Turbidite deposition ceased from late Miocene to late Pliocene. Bioturbation is pervasive. A 4.0-m.y. hiatus, beginning in the middle Miocene (around 12 Ma), correlates with a regional angular unconformity in seismic-reflection profiles; this may be related to northwest-southeast compression on this margin during a compressional phase in the Betic Mountains in southern Spain and structural inversion in the Lusitanian Basin of Portugal.
Until more work is done on the basement rocks from this site, it is not possible to recognize the extent to which they will contribute to our understanding of the ocean/continent transition on this margin. Post-cruise studies may be expected to produce new understanding of these complex rocks.
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* Abbreviations for names of organizations and publication titles in ODP reference lists follow the style given in Chemical Abstracts Service Source Index (published by American Chemical Society).
Ms 149IR-107 NOTE: For all sites drilled, core-description forms ("barrel sheets") and core photographs have been reproduced on coated paper and can be found in Section 3, beginning on page 271. Forms containing smear-slide data can be found in Section 4, beginning on page 657. Thin-section data are given in Section 5, beginning on page 679. GRAPE, Index Property and MAGSUS data are presented on CD-ROM (back pocket).
Hole 900A: Resistivity-Natural Gamma Ray Log Summary
Note: These data were reprocessed post-cruise by the Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia University.
Hole 900A: Resistivity-Natural Gamma Ray Log Summary (continued)
Hole 900A: Natural Gamma Ray Log Summary (continued)