166 Preliminary Report


Site 1003 (BT-2/F-2), is one of five sites of a transect that connects the shallow bank with the deeper water areas. Site 1003 is located on the middle slope of the prograding western margin of Great Bahama Bank approximately 4 km from the platform edge and 12.6 km from the borehole, Clino, drilled on the banktop from a self-propelled jack-up barge.

A thick Neogene section (1300 m) was cored at Site 1003. The strata consist of a series of mixed pelagic and bank-derived carbonates with a carbonate content of 92-97%. In the lower Miocene section (Unit VII), a small amount of fine-grained siliciclastics are admixed in the pelagic intervals. The bank-derived sediments occur in pulses, indicating variations of sediment production on the adjacent platform. Foraminifer and nannofossil biostratigraphy yielded precise ages for the lithologic units. Within the Miocene section there are four hiatuses, ranging from 4 to 1 m.y. in duration. Comparison of the lithostratigraphic unit boundaries and the biostratigraphic breaks correlate well with seismic sequence boundaries previously identified in the prograding slope section. Furthermore, the biostratigraphic data confirm the ages of the late Miocene to Pleistocene sequences that were determined in the two core borings, Unda and Clino, on the shallow bank. The good core recovery obtained throughout the entire section at Site 1003 (overall <40%) and the abundance of biostratigraphic markers have made it possible to accomplish two of the primary sea-level objectives. That is, it is possible to define the ages of the Neogene sequence boundaries and to test their consistency with ages determined in the Unda and Clino boreholes, and to retrieve the sedimentary record of the middle- to lower-slope portions of the prograding sequences. Diagenetic alteration in the lower portions of the hole, however, will preclude the establishment of a stable-isotope stratigraphy for the entire Neogene at this site.

Chemistry from interstitial waters yielded interesting depth profiles with abrupt changes across stratigraphic boundaries. These data in conjunction with the temperature profiles support fluid movement in stratigraphically defined conduits.

Seven major lithologic units are distinguished. Each of these units displays a general trend from bioturbated mudstones and wackestones at the bottom to an increase of packstones, grainstones, and floatstones toward the top. Within the units, subdivisions are defined based on the facies types and associations, textures, and the major components. Unit I (0-162.1 mbsf; latest Pliocene to Holocene) consists of unlithified to partially lithified mudstones, wackestones, packstones, and floatstones. Its base is defined by the sudden appearance of peloids, which are a major component throughout the unit. Two subunits are recognized. In Subunit IA (0-59.9 mbsf), two intervals of unlithified packstones to floatstones, containing Halimeda debris and lithoclasts, are intercalated in unlithified mudstones. In Subunit IB (59.9-162.1 mbsf), an upper aragonite-rich part with some laminated redeposited intervals overlays a poorly stratified and bioturbated mud- to wackestone.

Unit II (162.1-368.2 mbsf; early Pliocene) consists of partially lithified, intensely bioturbated wackestones with some variation in bioturbation and skeletal fragments. The unit is slightly dolomitized (20-25%) at the top, decreasing downhole to about 10%.

Unit III (368.2-492.7 mbsf, early Pliocene) has a characteristic succession of facies that is repeated in the underlying units. The top part of this and the following units are dominated by packstones to grainstones that often display sedimentary structures indicative of turbidites and other mass-gravity flow deposits. The background sediment of these packstones and grainstones are bioturbated wackestones. The bottom part of the units are composed of packstone to wackestone alternations of light gray, lithified beds with an average thickness of 30 to 40 cm and darker, greenish to brownish gray, less lithified intervals of 15 to 25 cm thickness. The alternations are moderate to heavily bioturbated and contain planktonic and benthic foraminifers and other bioclasts.

Unit IV (492.7-643 mbsf; middle to late Miocene) is subdivided into two subunits. Subunit IVA (492.7-591.2 mbsf) is dominated by packstones and grainstones, and is characterized by interbedded light gray to grayish laminated, fining-upward beds and greenish to brownish gray, strongly bioturbated beds. Subunit IVB (591.2-643 mbsf) consists of moderately to heavily bioturbated light gray to dark gray, slightly dolomitized bioclastic packstones to grainstones. Components in the packstones and grainstones include planktonic and benthic foraminifers, bioclasts (including rare Halimeda debris), and lithoclasts. Subunit IVB is characterized by a cyclic alternation between well-lithified packstones with large well developed burrows and finer-grained zones with flattened burrows.

Unit V (634-915.8 mbsf; middle Miocene) can be separated into two subunits. Subunit VA (646-738.8 mbsf) consists of laminated, bioclastic packstone and grainstone beds with sharp or scoured bases. Dominant allochems include planktonic and benthic foraminifers. In Subunit VB (738.8-915.8 mbsf) wackestones and packstones grade downhole into mudstones and wackestones that show a distinct cyclicity of light gray, well-cemented, moderately bioturbated intervals, and strongly bioturbated, dark gray to green layers. These darker intervals also contain a small amount of clay- and silt-sized quartz, plagioclase, and clay. Thickness of a total cycle ranges from 50 to 200 cm.

Unit VI (915.8-1151.63 mbsf; early Miocene to middle Miocene) is separated into two subunits. The top of Subunit VIA (915.8-973.38) is characterized by a well-defined, irregular surface (hardground). The subunit has intercalations of packstones and floatstones within background sediment composed of bioturbated, greenish-grayish wacke- and packstones. The background deposits show the same type of cyclicity as Subunit VB. The underlying Subunit VIB (973.89-1151.38) is three times thicker than Subunit VIA and consists entirely of cyclic alternations of lighter and darker intervals as previously described.

In the lower Miocene Unit VII (1151.63-1296.41 mbsf), the packstone dominated Subunit VIIA (1151.63-1194.77) is even thinner (43 m) and its packstones are thin-bedded (5-10 cm). In contrast, the underlying cyclic Subunit VIIB (1194.77-1296.41) is over 100 m thick. In this subunit, the siliciclastic admixture is approximately 5-7%. Units IV-VII, therefore, show an overall thickening and coarsening-upward trend in their packstone-dominated subunits, indicating a pulsed, but progressive progradation of Great Bahama Bank throughout the Miocene.

Dating the Neogene sedimentary packages deposited in an upper bathyal environment on the slope of western Great Bahama Bank was possible, despite problems of diagenesis and dilution. Most of the conventional Neogene nannofossil and planktonic foraminifers datum levels used in the deep-sea pelagic environment were also found here, and a fine nannofossil planktonic foraminiferal stratigraphy was established. The sediments recovered at Site 1003 yielded a large number of biohorizons, both from calcareous nannofossils and planktonic foraminifers appearing in proper succession, which is remarkable given the nature of the sedimentary sequence close to the bank. The onset of modern platform production is documented by a high-sedimentation rate during the last 0.9 to 1.0 Ma (10 cm/k.y.). The lower Pleistocene to upper Pliocene interval shows a much slower sedimentation rate of about 2.5 cm/k.y., indicating a decreased input from the banktop. The lower Pliocene section (Units II and III) is an expanded section. During this period of time, the platform produced a lot of material that was shed onto the upper slope. In contrast, the early late Miocene is characterized by pelagic sedimentation rates of about 3 cm/k.y. This was a time of global cooling and falling sea level that ultimately caused an unconformity, separating the lowermost upper Miocene from the lower Pliocene. This unconformity coincides with the lithologic break between sedimentological Units III and IV. In general, the middle Miocene shows a very high sedimentation rate of up to 15 cm/k.y. One unconformity was identified within this interval; planktonic foraminiferal Zones N9 and N10 are missing. The unconformity coincides with the hardground that marks the top of Unit VIA. The lowermost unconformity separating lithological Unit VIIA from Unit VIIB occurs within nannofossil zone NN2 and straddles the planktonic foraminiferal Zones N4 and N5. The Miocene unconformities punctuate periods of high sedimentation rates during which the input from the platform must have been high.

Sedimentation from the lowermost Miocene to the Holocene seems to be controlled by changing sea level. Thick and expanded upper and middle Miocene intervals coincide with the generally high sea level of this time. The late Miocene, a time of global cooling and sea level fall, is recorded in the sediments as a reduced interval and a subsequent hiatus. The Pliocene sea-level rise resulted in renewed flooding of Great Bahama Bank and sedimentation on the slope. The sedimentary successions of more pelagic deposits alternating with more neritic sediments provide evidence of the numerous sea-level changes during the Pliocene and Pleistocene. These sea-level controlled sedimentologic variations are recorded on the seismic data as well. With one exception, all the sequence boundaries determined prior to drilling coincide either with unit or subunit boundaries. Two unit boundaries occur within seismic sequences indicating a higher resolution within the sedimentary record. In addition, small-scale sedimentary cycles provide evidence for high-frequency sea-level changes.

The carbonate mineralogy is dominated by aragonite with lesser amounts of high-magnesium calcite (HMC) and dolomite throughout the upper 110 mbsf. Below this depth, corresponding approximately with the Pliocene/Pleistocene boundary, aragonite decreases markedly, HMC disappears, and dolomite becomes more important. Variable amounts of dolomite are present throughout the remainder of the core, commonly occurring below nondepositional surfaces. Small amounts of feldspar are also present in the lower portion of Hole 1003B. Diagenetic minerals such as celestite, elemental sulfur, and chert are found sporadically throughout, particularly in fractures. Carbonate contents are generally high throughout the samples measured, and organic contents are generally low. In the Miocene section, however, carbonate content gradually decreases to 85% and drops to 50% or less in the intervals above sequence boundaries, providing further evidence of reduced carbonate input in these intervals. The packstones to wackestones between 700 to approximately 1000 mbsf in Hole 1003C, possess a strong petroleum odor and have experienced some oil migration.

The pore-water chemistry shows very unusual trends, which are probably related to lateral movement along more permeable sedimentary units. Although there are gradual changes in most chemical constituents with increasing depth, these changes accelerate across major permeability barriers. The shallowest of these changes occurs at around 85 mbsf. Below this depth, the salinity starts to increase, reaching values as high as 62 at 820 mbsf. Alkalinity and H2S also increase across this barrier while sulfate decreases. Calcium continues to increase throughout the hole while the relative concentration of Mg decreases. Nonsteady-state profiles exist for all of the measured constituents (salinity, SO4, Ca, Sr, NH4, Li, F, alkalinity, and Mg). The C1/C2+ ratio was considered to be anomalous throughout the hole indicating the presence of migrated hydrocarbons. Alkanes higher than C2 first appeared below a major change in velocity at 750 mbsf, which, in addition to being a seismic reflector, was also a seal. It is probable that the hydrocarbons and bitumen found in the core were generated deep in the section and migrated upward and laterally. As these compounds moved upward they were oxidized by sulfate-reducing bacteria to produce H2S and consequently lower pH. Decreases in the pH promote the extensive carbonate alteration seen throughout the core.

The log data obtained from Hole 1003D correlate well with the sedimentary succession and can be used to fill in gaps in low recovery zones. With log-to-core correlation, the changes recorded in the logs can be traced along the seismic sequence boundaries, providing a tie of the sedimentation patterns at Site 1003 to other drill sites across the Great Bahama Bank platform margin and on the platform top. A general downhole trend of increasing density, resistivity, and sonic velocity, and of decreasing porosity probably results from sediment compaction. However, there is a pronounced change in this trend at 738 mbsf, where the downhole gradient in density, resistivity, and velocity is offset toward lower values while the porosity increased. This anomalous pattern may be the result of changing sediment composition as well as a diagenetic overprint. Below 738 mbsf, this major lithology change is apparent in the natural gamma-ray and geochemical (Ca, Al, and Si) logs. The increased frequency and magnitude of the uranium signal is particularly notable. The salinity indicator ratio in the geochemical logs shows a sharp increase below 738 mbsf, implying a more saline formation fluid, which is consistent with the high chlorinity measured in pore waters from these depths. Also below 738 mbsf, a notable cyclicity in the gamma-ray and FMS logs correlates to the sedimentation patterns noted throughout much of the Miocene portion of the cores.

Site 1004 Results

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