Site 1198 (proposed Site CS-05A) is located in 319 m water depth at the intersection of regional seismic lines MAR-07 (shotpoint 2262) and MAR-54 (shotpoint 900). This site lies ~10 km northeast of the modern south-central Great Barrier Reef and 13.5 km northwest of Site 1196 (Figs. F1, F2). Site 1198 is located adjacent to the current-swept margin of the Miocene MP2/MP3 carbonate platform ~5 km from the Miocene platform escarpment.
The main goal of this site was to recover platform proximal sediments that record the growth phases of the adjacent MP2/MP3 carbonate platform. Deposition of material shed from the MP2/MP3 carbonate platform on the Marion Plateau is predominantly controlled by current flow in a similar fashion to the Nicaraguan Rise, as opposed to the generally wind-dominated system of the Great Bahama Bank. To investigate sediment and subsequent deposition resulting from current processes, Site 1197 was located off the southeast margin of the MP2/MP3 platform, which, with Sites 1196 and 1198, forms a transect from the leeward, depositional side to the top of the platform and to the current-dominated margin. A comparison of the two marginal sites shows that the lower energy site has significantly thicker sediment sequences.
In their offbank setting, the sediments at Site 1198 were expected to contain both platform and open marine sedimentary signals. In addition to the primary objective listed above, other objectives were to recover the proximal sequences adjacent to the MP2/MP3 carbonate platform and to determine the age of both the initiation and demise of the MP3 platform. Also, the Megasequence D sediment drift, which overlies the unconformity marking the end of the MP3 carbonate platform growth phase, is expected to provide information on the Pliocene-Holocene paleoceanography of the Marion Plateau region. A further objective was to investigate the potential of fluid flow in a platform proximal setting.Operations
Operations at Site 1198 began at 1636 hr on 7 February 2001. Hole 1198A was APC cored to refusal at 203.0 mbsf and average recovery for this interval was 102.2%. Cores were oriented starting with Core 194-1198A-3H. Four downhole temperature measurements with the APC temperature tool at 33.5, 62.0, 119.0, and 147.5 mbsf and three measurements with the DVTP at 176.0 and 203.0 mbsf, were unsuccessful.
Subsequent coring with the XCB deepened the hole to 251.5 m. The first XCB core had 29% recovery, and the subsequent four cores had zero recovery. Based on a similar zero-recovery experience with XCB coring at Site 1197 in the same stratigraphic unit (seismic Megasequence C), the hole was abandoned in favor of a new hole to be cored with the RCB.
The vessel was offset 20 m east of Hole 1198A. After drilling with the RCB to 195.7 mbsf, coring resumed to a total depth of 522.6 mbsf (Table T2). During the process, the drill string became stuck and was subsequently worked free. To reduce the potential for further hole problems, coring was interrupted twice for wiper trips and the annulus was frequently flushed with sepiolite. Average recovery for the intervals from 195.7 to 359.1 and from 359.1 to 522.6 mbsf was 6.1% and 65.2%, respectively. The average recovery for the entire hole was 35.7%.
Once coring was completed, the bit was released on bottom, the hole was flushed and displaced with sepiolite, and the pipe was pulled back to 91.8 mbsf for logging. The triple combo tool was unable to descend below 235.5 mbsf because of a collapse of the upper part of the borehole (corresponding to the interval poorly recovered in the cores). The tool became stuck and was subsequently freed. The hole was logged with one pass from 231 mbsf to the seafloor. Above 192 mbsf the hole was found to be significantly enlarged, adversely affecting log quality. It was decided not to make any additional logging runs because of the deteriorating hole conditions. The vessel departed at 0430 hr on 11 February, ending operations at Site 1198 to return to Site 1196.Principal Scientific Results
Site 1198 penetrated a 522.6-m-thick lower Miocene to upper Pleistocene sedimentary sequence that was nearly all deposited at water depths >200 m in a proximal periplatform to hemipelagic environment. At the bottom of the hole, a thin veneer of deep-euphotic foraminifer species and rhodolith floatstone (estimated paleowater depth of 100-150 m) was deposited under the influence of seafloor currents directly on top of the basaltic basement. This carbonate unit is capped by a thin hardground surface and overlain by 100 m of hemipelagic clay-rich carbonates (seismic Megasequence B). Above is a ~200-m-thick periplatform section that consists partly of talus that contains a record of variations in platform growth from the late middle Miocene until carbonate production was terminated in the late Miocene (~7.7 Ma). This unit may be hydrologically connected to the MP2/MP3 platform edifice, because pore water chemistry indicates circulation of almost unaltered seawater within lithologic Unit II (seismic Megasequence C). After the termination of MP3 platform growth, a submarine hardground formed on the slope. Subsequently, a 200-m-thick Pliocene-Pleistocene sediment drift (seismic Megasequence D) was deposited on top of the proximal periplatform sediments.
The lithostratigraphy at Site 1198 consists of five main units that are divided according to variations in sedimentary structures, texture, grain size, terrigenous content, and biotic assemblages. Lithologic Unit I (0-200.6 mbsf; upper Pliocene to Pleistocene) is characterized by a thick sequence of alternating mudstone and skeletal grainstone intervals with varying amounts of terrigenous constituents as indicated by cyclic changes in texture and carbonate content. The sediments of Unit I, coinciding with seismic Megasequence D, were deposited in a hemipelagic environment in water depths >200 m. Lithologic Unit I shows an unusual downhole trend of nearly constant velocity (1625 m/s), bulk-density (1.75 g/cm3), and porosity values (60%). The uniformly high porosity values throughout this interval indicate a lack of compaction that may result from elevated fluid pressures generated during deposition or induced by fluid migration. MS and NGR increase, on average, throughout lithologic Unit I. MS shows variations at the scale of 25-50 m that are superimposed on the average trend. The average MS and NGR trends correlate with sediment lightness, which decreases downhole from ~70% to ~50%.
The lithologic Unit I/II boundary occurs at 200.6 mbsf at a 10-cm-thick brown phosphate nodule layer that is interpreted to be a reworked hardground surface. The middle to upper Miocene lithologic Unit II (200.6-397.6 mbsf) was divided into three subunits. The upper and lower of these subunits consist of fine-grained, neritic-rich skeletal packstone to grainstone deposited in a periplatform environment (Subunit IIA, 200.6-224.8 mbsf; Subunit IIC, 330.3-397.6 mbsf). Subunit IIB (234.0-330.3 mbsf) consists of coarse-grained skeletal grainstone to rudstone with abundant larger benthic foraminifers, rhodoliths, and hermatypic corals. Based on seismic interpretation, Subunits IIA, IIB, and part of IIC were deposited in a proximal slope environment or reef talus at the base of a nondepositional escarpment and correspond to seismic Megasequence C, which thickens toward the MP2/MP3 platform, suggesting that the MP2/MP3 platform is the source of the neritic carbonate sediment. Lithologic Unit II is separated from Unit III by a change from silt-sized packstone with minor identifiable neritic debris above and a unit of mud/wackestone deposits with no identifiable neritic detritus below 397.6 mbsf. Few velocity measurements were obtained between 200 and 400 mbsf (lithologic Unit II) because of poor recovery. Of the obtained data, the average velocity is 2225 m/s with two high velocity measurements (4250 and 3750 m/s) corresponding to the hardground near 200 mbsf (boundary of lithologic Units I and II), which is also characterized by high MS and NGR values.
The lower middle Miocene lithologic Unit III (397.6-503.6 mbsf) is characterized by meter-scale, gradational alternations of packstone and wackestone textures that coincide with color changes. Within lithologic Unit III, carbonate content increases overall from ~50 wt% at the top to ~95 wt% at the base. This unit was subdivided on the basis of small-scale fining-upward sequences at the base of wackestone intervals and an increased amplitude of variability in MS in Subunit IIIA.
At the top of lithologic Unit IV (503.6-513.2 mbsf), thin phosphatic laminae suggestive of a hardground surface are present. Below this hardground and just above the basement, a skeletal floatstone to rudstone facies with thin red algal crusts and very large and flat benthic foraminifers indicate a paleowater depth of ~100-150 m.
Throughout lithologic Units III and IV, bulk density (2.1-2.4 g/cm3) and velocity (2.0-2.8 km/s) increase with depth and porosity (40%-25%) decreases. Below 400 mbsf, MS gradually decreases, which is mirrored by a gradual lightness increase (45%-55%). NGR is highly variable throughout this interval.
At the base of Site 1198, 3.9 m of olivine basalt represents the acoustic basement (Unit V; 513.2-518.1 mbsf), which is characterized by a darkish green matrix consisting of smectite, magnetite, hematite, and feldspars (plagioclase) and abundant mineral-filled fractures. Abrupt increases in MS, NGR, and velocity and a decrease in porosity characterize the basement lithology. A strong increase in NRM marks the transition from sediment to basement with average intensities of 0.5 A/m, a value ten times less than the average intensities of both subaerial and submarine basalt.
Shipboard paleontological analyses show that lithologic Unit I is composed of planktonic microfossils with a minor component of reworked material. Upper bathyal benthic foraminifers are common throughout this unit. Calcareous nannofossils and planktonic foraminifers indicate a hiatus of ~4 m.y. (~3.8-7.7 Ma) at the lithostratigraphic Unit I/II boundary. Subunit IIB contains abundant and well-preserved Lepidocyclina spp. that, based on seismic geometries, were (at least in part) shed from the MP3 platform. Planktonic foraminifers and nannofossils indicate a middle to late Miocene age (~12-8 Ma) for that interval. However, the biostratigraphic limit of Lepidocyclina spp. (last occurrence) reported in the literature is Zone N12 (12.1 Ma). The abundance and quality of Lepidocyclina spp. in these periplatform sediments provides strong evidence that these foraminifers did indeed survive into the late Miocene. Near the bottom of the cored sediment sequence, below the phosphatic hardground that marks the top of lithologic Unit IV (489.9 mbsf), nannofossil assemblages indicate an age of 18.2-18.5 Ma. Despite the expanded Pliocene-Pleistocene drift deposit of lithologic Unit I, the magnetic stratigraphy does not show the expected pattern of polarity reversals in part due to measurement difficulties. However, a tentative magnetostratigraphy was developed for the Brunhes and Matuyama Chrons. Below 205 mbsf, poor recovery and residual viscous remanent magnetization hindered interpretations. Rock magnetic analyses performed on representative discrete samples indicate that the dominant magnetic phase is magnetite, with some minor, indeterminate, high coercive magnetic mineral, possibly pyrrhotite. The low gas content (1.7 to 6.6 ppmv) at Site 1198, is likely a function of appreciable pore water SO42- concentrations limiting methanogenesis to total depth and the lack of mature organic matter that could provide a thermogenic component to the gas fraction.
Carbonate (CaCO3) content at Site 1198 ranges from ~47 to 99 wt% and mostly covaries inversely with the TOC content, which ranges from 0.0 to 0.38 wt%. S content in Site 1198 sediment ranges from 0.0 to ~0.78 wt% and its distribution is similar to that of TOC. Lithologic Subunits IB, IC, and IIIA are notable for broad ranges in inversely covarying CaCO3 and TOC values.
The pore water profiles for dissolved constituents at Site 1198 show distinctive patterns. In the hemipelagic sediments of lithologic Unit I, pore water profiles display typical deviations from seawater concentrations in the upper ~100 mbsf (e.g., bacterial sulfate reduction begins with a concomitant rise in ammonium concentration, and strontium concentration rises due to carbonate recrystallization). In the lower half of lithologic Unit I, however, these trends reverse. Sulfate concentration returns to a seawater value of 29 mM, whereas ammonium and strontium concentrations decrease to near seawater concentrations. No pore fluids were recovered from lithologic Unit II. Pore waters from the top of lithologic Unit III at 350 mbsf have a chemistry remarkably close to seawater. Through lithologic Unit III, sulfate again decreases, strontium concentration rises, and calcium rises to very high values of nearly 140 mM. The pore water chemistry immediately above and below lithologic Unit II suggests active circulation of seawater through the unconsolidated sediments of Unit II. The mechanism causing this flow and the flow properties in the third dimension are not known, but an active hydraulic connection to the adjacent carbonate platform is hypothesized.
Downhole logging operations in Hole 1198B were limited to the interval from 231 mbsf to the seafloor. The natural gamma-ray data are the most useful logs at Site 1198 because they are corrected for variations in the large borehole size. In the 135-m open-hole interval measured, two major changes in log response can be distinguished. Within logging Unit 1 (74-110.5 mbsf) natural gamma-ray values are low at (~20 gAPI) and show small-scale variability. This interval can be roughly correlated to a carbonate-rich packstone to grainstone described in the recovered cores. Logging Unit 2 (110.5-199 mbsf) shows an increase in the natural gamma-ray log to 30 gAPI with small-scale variations superimposed on a nearly linear downhole trend. From the top of the hole until ~200 mbsf, the sediments record a gradual downhole increase in clay content that is observed in the logs by a change to higher potassium and thorium values below 110.5 mbsf. At 200 mbsf a peak in natural gamma-ray marks the hardground between the skeletal packstone with clay of lithologic Unit I, and the coarse-grained rudstone and floatstone of Unit II.
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