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 comparable to those seen over the same age range at the other sites.
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.
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 components 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
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 currents. 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. Alternatively, some of the foraminifer-rich sands could result from current winnowing of turbidites.
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.