166 Preliminary Report


Site 1007, at the crossing of seismic lines 106 and 102B, is located on the toe of the western Great Bahama Bank slope in 647 m of water. The target depth was the base of the Neogene, which was estimated to be at approximately 1230 mbsf. The site is positioned to penetrate the thin basinal portions of 16 prograding sequences and, in the upper part, a thick onlapping wedge that could be either a drift deposit or an accumulation of mass-gravity flows. To the west, the distal portions of the prograding clinoforms interfinger with the drift deposits of Site 1006. Site 1007, therefore, was the link between this basinal site and the proximal slope sites to the east. In addition, a higher percentage of microfossils with good preservation was expected here than in the more proximal Sites 1005, 1004, and 1003. Therefore, the main objectives of this site in regard to sea level were to (1) precisely date the sequence boundaries, (2) determine the facies in the distal portions of carbonate sequences, and (3) assemble a data set suitable to compare the sedimentary record with the oxygen isotopic record of the Neogene to recent sea-level fluctuations. Similarly as for the sea-level objectives, Site 1007 was also an intermediate site for the fluid-flow objectives. Interstitial water chemistry and in situ temperature measurements should provide information about the fluid movement in the lower slope and, by comparison with the three previously drilled proximal sites, assess lateral changes in fluid chemistry. A logging suite, including a vertical seismic profile experiment, was performed for optimal correlation between the cores and the seismic data.

At Site 1007, the entire Neogene section and 20 m of Oligocene sediments were penetrated in a 1235.4 mbsf deep hole. Therefore, this site provides a complete sedimentary record of the Neogene sea-level changes. Its position at the base of the slope makes it the link between the proximal Sites 1003, 1004, and 1005 and the basinal Site 1006. Similar to site 1006, overall recovery was better than in the proximal sites and the planktonic foraminifers and nannofossils were more abundant and better preserved than in the proximal sites. As a result, a detailed biostratigraphy was established for the entire Neogene section. This enables the sequence boundaries at this site to be precisely dated and allows a comparison of the ages of the boundaries with the other sites. The comparison yields an excellent correlation between the sites, documenting the age consistency of the sequence boundaries and the chronostratigraphic significance of these seismic reflections. Furthermore, the biostratigraphic dating was of sufficient quality to record sedimentation rates within several individual seismic sequences, thus providing an independent record of the platform-derived sedimentation during sea-level highstands and the reduced pelagic sedimentation during sea level lowstands. Sixteen seismic sequences were distinguished prior to drilling Site 1007. The good core recovery provides the facies information of these sequences. The facies succession indicates reduced or absent platform-derived sedimentation during the formation of the sequence boundaries, probably as a result of platform exposure during sea-level lowstands. Redeposited carbonates are present in the lower portion of the Miocene sequences. For example, turbidites and slump packages, in a more pelagic background sediment (lowstands), and biowackestones with more platform-derived material in the upper portions (highstands). With the transition into a steep-sided platform in the Pliocene, redeposited carbonates become more abundant in the upper parts of the sequences. This difference documents the relationship between platform morphology and highstand vs. lowstand shedding in carbonates. Facies and diagenesis act in concert to produce petrophysical differences within the sedimentary section. As a result, the logs reflect closely the sedimentary and stratigraphic record. All sequence boundaries can be clearly identified on the log data. A core-to-log-to-seismic correlation is possible by the time-depth conversion using the vertical seismic profile that was acquired in the open hole after logging.

In regard to the fluid-flow objectives, Site 1007 is also a link between the basinal and the proximal sites. As at all other sites, it shows evidence of a flushed zone in the upper 20 mbsf, which is somewhat shallower than in the proximal sites. The lower portion of the profile is dominated by diffusion, with local reactions involving the oxidation of organic material, the formation of pyrite, the precipitation and dissolution of carbonate minerals, and diagenesis of clay minerals and cherts controlling the concentration of various pore-water constituents.

In summary, with the completion of this deep site and a total depth (TD) within the Oligocene, Leg 166 provides a record of sea-level changes through the entire Neogene. The facies succession documents that the seismic sequences are indeed sea-level controlled. Furthermore, with the ability to precisely date the boundaries and several horizons within the sequences, the timing of these sea-level changes can be established. The well-preserved foraminifers at this site and Site 1006 will provide an oxygen isotopic record of the Neogene sea-level changes in the same transect. Interstitial-water geochemistry yields depth profiles that are indicative of stratiform fluid flow in the upper portions of the sections. Thus, a data set is collected that fulfills all the sea-level and fluid-flow objectives of the Leg 166 Bahamas Transect.

A nearly continuous section of Pleistocene to upper Oligocene sediments was recovered at Site 1007 at the toe-of-slope of present-day Great Bahama Bank. These deposits contain elements identified at more proximal Site 1003 and more distal Site 1006. This sedimentary succession is interpreted to record an interplay among: (1) change in platform-to-basin morphology; (2) sea-level variation; (3) pelagic input; and (4) ocean currents. In contrast to other sites drilled during Leg 166, the section at Site 1007 is characterized by an increased occurrence of turbidites and slumped intervals and by a relatively high amount of clay- and silt-sized siliciclastics in the late Pliocene and Pleistocene parts of the section. In addition, below 1120 mbsf sediments at Site 1007 are marked by the presence of compaction-dissolution features, namely anastomosing or wispy solution seams.

The Pleistocene section at Site 1007 is characterized by a meter-scale alternation between silt- and clay-rich sediment and silt- and clay-poor sediment. These alternations are similar to the clay cycles observed in the Pleistocene section at Site 1006, and are interpreted to reflect erosion of continent-derived siliciclastics during sea-level lowstands and incipient sea-level rise, and a subsequent increase in neritic and pelagic input following flooding. Seismic stratigraphy and biostratigraphic data indicate that the upper Pliocene section is thicker and characterized by a higher sedimentation rate relative to other sites. This thickness variation is caused by two processes: (1) an accumulation of mass-gravity flow deposits, which bypassed the upper slope and accumulated at Site 1007; and (2) an increase in the amount of current drift deposits. This latter increase could reflect a migration of the northward-flowing, Santaren Channel Current toward the platform during the late Pliocene and early Pleistocene.

As at other sites drilled on Leg 166, the sedimentary succession at Site 1007 records an important change in the mode of neritic carbonate production. At the base of Unit I (base of upper Pliocene), there is a sudden increase in the occurrence of platform-derived material, including aragonite needles and peloids. The turnover from skeletal- to aragonite needle-/peloid-dominated sediments probably reflects the coupled effects of climate change, and the transition of the Bahamas Bank from a carbonate ramp, characteristic of the Miocene, to the present-day, flat-topped carbonate platform.

The Miocene section at Site 1007 is nearly identical to the Miocene section recovered at Sites 1003, 1005, and 1006, and consists of alternating intervals of (1) well-cemented, decimeter- to meter-scale light-colored, clay-free intervals, which show no evidence of compaction; and (2) decimeter-scale dark-colored, relatively clay-rich intervals that show evidence of compaction. These alternations are interpreted to reflect changes in the rate of neritic carbonate input and the subsequent impact on the primary diagenetic potential of sediments. In this regard, light-colored, well-cemented intervals may reflect periods of higher neritic input (aragonite and high-Mg calcite) and the deposition of sediments with a higher diagenetic potential. In contrast, dark-colored, weakly cemented intervals may reflect periods of decreased neritic input and the deposition of sediments with higher amounts of low-Mg calcite.

Sediments at Site 1007 are divided into eight lithologic units on the basis of compositional and textural changes. Most unit boundaries correspond to (1) inferred periods of reduced sedimentation, (2) change in sediment composition, and (3) intervals of increased mass gravity flow deposits.

Unit I (0-231 mbsf; Pleistocene to late Pliocene) consists of a succession of nannofossil ooze and variable lithified wackestones to packstones. Aragonite needles and peloids are an important but variable component in the silt- and fine sand-sized fraction. The disappearance of peloids marks the lower boundary of this unit. Unit I is subdivided into four subunits. Subunit IA (0-10.9 mbsf) is a sequence of nannofossil ooze, unlithified wackestone and packstone, which is interrupted by gravity-flow deposits. Subunit IB (10.9-43.5 mbsf) is characterized by meter-scale alternations between intervals of silt- and clay-rich sediments and intervals of silt- and clay-free sediments. The contact between Subunit IB and Subunit IC occurs at the top of the lowermost of two hardgrounds. Subunit IC (43.5-165 mbsf) encompasses a sequence of faintly laminated, unlithified peloidal wackestone to packstone. This succession is interrupted by an interval of partially lithified foraminifer wackestone and a slumped interval containing interbeds of unlithified packstone to floatstone. The contact between Subunit IC and ID is defined by a change in degree of lithification that coincides with a firmground. Subunit ID (165.8-203.1 mbsf) consists of slightly dolomitized wackestone and packstone.

Unit II (203.1-302.0 mbsf; early Pliocene) consists entirely of bioturbated light gray to pale yellow foraminifer nannofossil chalk. Foraminifers are the dominant allochem, whereas other bioclastic debris are minor constituents.

Unit III (302.0-362.6 mbsf; late Miocene) displays variable degrees of lithification of foraminifer wackestone, nannofossil chalk, and nannofossil limestone. In the upper part of the unit, a series of coarse-grained turbidites are intercalated. The boundary of the underlying Unit IV is placed above a series of fine-grained, laminated packstones.

Unit IV (362.6-696.4 mbsf; middle to upper Miocene) is made up of 5-10 cm thick layers of fine-grained packstone and an alternation between decimeter- to meter-scale intervals of densely cemented sediment with intervals of weakly cemented intervals that show evidence of compaction. These alternations probably reflect the rate of neritic input that is related to sea-level changes (see above).

Unit V (696.4-832.7 mbsf; middle Miocene) consists primarily of a succession of bioturbated foraminifer wackestone with minor intervals of packstone and mudstone. Turbidites are concentrated in the upper part of Unit V, whereas the lower part of the unit contains similar cyclic alternations of better and less cemented wackestones described above.

Unit VI (832.7-986.2 mbsf; early to middle Miocene) also contains bioturbated biowackestones with a small amount of packstones. In addition, several firmgrounds and thin clay-rich layers are observed within the unit.

Unit VII (986.2-1187.3 mbsf; early Miocene), like Units IV, V, and VI, is characterized by an alternation between well-cemented decimeter- to meter-scale intervals and decimeter-scale dark-colored intervals that show evidence of compaction. In addition, it also contains numerous calcareous turbidite deposits and clay-rich layers.

Unit VIII (1187.3-1235.4 mbsf; late Oligocene to early Miocene) consists of foraminifer wackestone and bioclastic wackestone arranged in alternations of decimeter- to meter-scale light and dark-colored intervals that are generally similar to those described in the overlying units. Black chert nodules and compaction/dissolution seams differentiate this unit from the overlying ones.

Sedimentation rates were determined on the basis of nannofossil and planktonic foraminiferal biostratigraphy. The average sedimentation rate for the Pleistocene interval is 5 cm/k.y. However, the top of Hole 1007B is older than 0.95 Ma. Obviously, current action along this part of the slope increased during the last 1 m.y. and caused non-deposition and/or erosion. A hiatus just below the Pliocene/Pleistocene boundary that lasted for approximately 0.4 m.y. might also be the result of current activity. The upper Pliocene interval is highly expanded with an average sedimentation rate of 21 cm/k.y. This is much higher than in time equivalent intervals at the other transect sites, where sedimentation rates of about 2-3 cm/k.y. were found. The increased sedimentation rate is the combined result of deposition of platform-derived material that bypassed the upper slope and was deposited at the toe of the slope and the accumulation of drift sediments at Site 1007. The lower Pliocene package is bounded by a hiatus. The duration of the hiatus spanning the early/late Pliocene boundary is approximately 1 m.y. (3.2-4.2 Ma). The sedimentation rate below this hiatus is 17.5 cm/k.y. and is similar to the more proximal slope sites (Sites 1003, 1005) and in the Florida Straits (Site 1006). Below 304 mbsf, an interval of poor preservation, represents either an interval of very low sedimentation (<1 cm/k.y.) or contains a hiatus that straddles the Miocene/Pliocene boundary.

A late Miocene hiatus was detected at approximately 328 mbsf and spanned a period of approximately 2 m.y. Just above this unconformity, approximately 20 m of uppermost Miocene sediments were found. The sedimentation rate for this short section cannot accurately be determined, but it might be 1 cm/k.y. This may provide evidence for some platform production during the Messinian, or else it represents a lowstand wedge. The late Miocene is characterized by a high sedimentation rate (13 cm/k.y.). Sedimentation rates at the proximal Sites 1003 and 1005 were very similar, whereas Site 1006, located farther out in the basin, shows a much lower sedimentation rate (3 cm/k.y.; i.e., pelagic in nature). This sedimentation-rate pattern shows that the GBB was shedding material to the gently dipping slope during the late Miocene. Sedimentation rates in the middle and early Miocene can be divided into four cycles of alternating low (<2 cm/k.y.) and high (>5 cm/k.y) sedimentation. Slow sedimentation intervals occurred from 9.5 to 11.5 Ma, 13 to 15 Ma, 16.5 to 17.5 Ma, and 20.5 to 23 Ma and correlate to hiatuses of similar duration at Site 1003. Periods of faster deposition occurred from 11.5 to 13 Ma, 15 to 16.5 Ma, 17.5 to 20.5 Ma, and older than 23 Ma, corresponding to intervals at Site 1003 with similarly high sedimentation rates. These cycles represent increases and decreases in platform production in response to relative sea level changes.

Geophysical and geochemical data acquired during logging of Hole 1007C provide detailed information on the sedimentary properties and structure of the strata. Log-to-core correlation permits significant interpretations about variations in sedimentation patterns on the toe-of slope of GBB from the earliest early Miocene to Holocene. The general compatibility of the discrete data points from shipboard petrophysical measurements with the log data supports the integrity of the data sets. In general there is a downhole trend of increasing resistivity and sonic velocity. Small-scale and long-term excursions are superimposed over this general trend. These variations reflect the lithological and diagenetic variations in the core. Higher resistivity and velocity can be correlated approximately with sediments described as firmgrounds, hardgrounds, or turbidites. In addition, logs display little variation through monotonous sedimentary units. Furthermore, because the higher resistivity and velocity values in the log data coincide with sequence-stratigraphically important sedimentary units, they display all sequence boundaries precisely.

Sixteen seismic sequences were distinguished prior to drilling. Ages of the sequence boundaries were postulated from the biostratigraphic information of the adjacent sites. These predictions proved to be very accurate. In addition, the better preservation of the biostratigraphic markers at Site 1007 compared to the more proximal sites permit a refinement of some of the ages. For example, the early Pliocene sequence was not datable in the more proximal Sites 1003 and 1005 because of the absence of age diagnostic marker species. At this more distal site, a precise stratigraphy through this interval is possible and the basal sequence boundary is now dated as the top of the Messinian. For a comparison of the ages of the seismic reflection that mark the sequence boundaries, a vertical seismic profile was acquired for an accurate time-depth conversion. All the seismic sequence boundaries yield the same ages as in the adjacent sites. The age consistency of seismic reflections validates the basic assumption of sequence stratigraphy, which assumes that a chronostratigraphic significance to seismic reflections is correct. The good age control at this site and the entire transect allows for precise age dating of the sequence boundary and, thus, for the sea-level changes throughout the Neogene.

Three basic geochemical zones, which relate to zones observed previously, are present at Site 1007. First, there is an upper flushed zone wherein there are no changes in the conservative elements. There are only minimal changes in the non-conservative elements, such as Sr2+, Ca2+, and alkalinity. This zone is similar to that found at the upper slope sites (Sites 1003, 1004, and 1005), but differs in that minimal changes were observed in the concentration of Sr2+ at Site 1007. This contrasts with the slope sites, which exhibited no change in non conservative parameters such as Sr2+ over this interval. Site 1007 is similar to the most distal site (Site 1006) in this regard. This flushed zone is also thinner than in the proximal sites, with a thickness of only 20 m compared to 40 m. Below the flushed zone, the concentration of the conservative elements, (Cl- and Na+) shows a diffusional relationship to the bottom of the hole. The concentration of K+ decreases dramatically between 900 and 1000 mbsf related to diagenesis of clay minerals. In geochemical zone 2, concentrations of the non-conservative elements (Sr2+, SO42-, Ca2+, NH4+, and alkalinity) show changes which are related to local reactions such as the degradation of organic material and the recrystallization of carbonate minerals. Immediately below the flushed zone, the SO42- concentration decreases slightly as a result of the local oxidation of organic material. This gas is accompanied by an increase in the concentration of methane. The concentration of SO42- again rises to seawater values at 400 mbsf and then decreases to zero below 600 mbsf. The removal of SO42- marks the start of the third geochemical zone and corresponds to the appearance of concentrations of up to 10 ppm isobutane in the headspace samples. This is postulated to be of thermogenic origin. The concentration of Sr2+ in this interval is strongly controlled by the solubility of celestite. Once the SO42- has been completely removed from the pore waters, the Sr2+ increases to over 5 mM at a depth of 950 mbsf. A maximum in the Ca2+ concentration between 350 and 400 mbsf indicates a probable zone of carbonate dissolution between 350 and 400 mbsf. Below this depth, Ca2+ concentrations decrease. When normalized to seawater Cl- concentrations, this zone shows a net removal of Ca2+ from the pore waters.

Over the upper 900 mbsf the sediments consist of over 90% carbonate. Below this depth, concentration of carbonate decreases to between 60% and 80% with higher amounts of organic carbon (up to 1 wt%). Organic carbon is also higher in Unit IVB. The hole is dominated by LMC, although Unit I consists mainly of aragonite with small amounts of dolomite and HMC. Throughout the remainder of the hole, dolomite is a minor constituent with concentrations of up to 40% in small, isolated intervals. Quartz is a ubiquitous component throughout the core and there are documented occurrences of pyrite and clay minerals.

In summary, Site 1007 shows evidence of a flushed zone in the upper 20 mbsf, although profiles indicate that the flushing is slower than at the proximal sites and that it does not extend to as great a depth. The lower portion of the profile is dominated by diffusion, with local reactions involving the oxidation of organic material, the formation of pyrite, the precipitation and dissolution of carbonate minerals, and diagenesis of clay minerals and cherts controlling the concentration of various pore-water constituents.

Site 1007 completed the data set needed to address all the questions in regard to sea-level changes and fluid flow. A precise foraminifer and nannofossil biostratigraphy provides the age control on the depositional changes that are related to sea-level changes and the resulting sequence boundaries. This sedimentary record will add to the database that is needed to eventually establish the global synchrony of sea-level changes. Furthermore, the abundant, well-preserved foraminifers in the Bahamas Transect provide us with the unique opportunity to compare the sedimentary record with the oxygen isotopic proxy for sea-level changes. This comparison potentially will assess the causal relationship between the stratal pattern and sea-level changes.

The geochemical profiles at Site 1007 indicate, as in the other sites, that there is active fluid flow in the upper portion of the strata. The occurrence of the change of gradients at similar depth along the transect provides strong evidence that flow is at least partially horizontal and related to stratigraphic boundaries. With increasing depth, diffusive processes become the dominant transport mechanism.

Sites 1008/1009 Results

166 Table of Contents

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