Site 1195 (proposed Site CS-10A) is located on the Marion Plateau, ~60 km northeast of the south-central Great Barrier Reef margin, at a water depth of 420 m (Figs. F1, F2). The main objective at this site was to recover a lower Miocene to Holocene sedimentary section that would provide a complete chronostratigraphy of the Neogene Marion Plateau depositional succession. Site 1195 is located at the intersection of regional seismic lines MAR15 (shotpoint 3505) and MAR04 (shotpoint 3928), 70 km east of the early middle Miocene MP2 platform and 60 km north of the Miocene MP2/MP3 platform. The sediments at Site 1195 record a distal shelf facies with the combined effects of changes in platform shedding, detrital input, and pelagic sedimentation, all of which can be linked to sea level and paleoceanographic changes.
A successfully drilled complete deep-water section, such as was recovered at Site 1195, provides an ideal sedimentary archive with which to accurately date seismic reflectors and seismic sequence boundaries. The ages for these horizons can be correlated with the other sites in the seismic grid to refine the age constraints in environments with lower age resolution.
In addition to calibrating the chronostratigraphy of the regional seismic sequences, Site 1195 will provide information on paleoceanographic changes recorded by variations in sedimentologic composition, physical properties, geochemistry, and stable isotope signatures. Results from Site 1195 will also provide insight into the age and sedimentologic response of the postrift drowning of the Marion Plateau.Operations
Operations at Site 1195, located 40 nmi south-southeast of Site 1194, began at 0700 hr on 26 January 2001. Hole 1195A was cored with the APC to 80.7 mbsf, where the wireline parted and the drill string had to be recovered. Average recovery was 98.2% (Table T2). The APC temperature tool was deployed at 33.2 and 71.2 mbsf. Cores were oriented starting with 194-1195A-4H (23.7 mbsf).
The vessel was offset 20 m east of Hole 1195A, and Hole 1195B was cored with the APC to refusal depth at 207.9 mbsf, with an average recovery of 101.1%. XCB coring deepened the hole to a total depth of 521.2 mbsf, with an average recovery of 60.4%. The recovery for the entire hole was 399.5 m (76.7%).
To evaluate the magnetic overprint imparted by the drill bit, a Russian-made 9.875-in APC/XCB PDC bit was used in Hole 1195B, instead of the standard 11.4375-in APC/XCB bit used in Hole 1195A. The Davis-Villinger Temperature Probe (DVTP) was deployed at 112.9 mbsf (following Core 194-1195B-12H) and 150.9 mbsf (following Core 16H). Cores were oriented starting with Core 194-1195B-3H.
Wireline logging operations were shortened because of deteriorating hole conditions. The triple combo tool string had to be pumped out of the pipe but subsequently descended to within 2 m of the bottom of the hole (~600 mbsf) without problems. For the second run, a successful check-shot survey with the WST was the highest priority. The WST also had to be pumped out of the pipe, but it failed to descend into the hole. In a few instances, the drill pipe became stuck and the WST had to be recovered. After the pipe was freed, the WST was run back in the hole but could not be lowered below 121 mbsf. Three levels were shot at 118, 95, and 86 mbsf using the air gun. No further logging was attempted.Principal Scientific Results
Drilling at Site 1195 recovered a 517-m-thick sedimentary succession ranging in age from the early Miocene to Pleistocene and showing little change in average sedimentation rate. In the lowest part of the sedimentary section (517.5-517.7 mbsf), 20 cm of well-indurated grainstone with nummulitids of possible Eocene age were recovered. The lithostratigraphy of Site 1195 comprises five main units that are defined by texture, color, biotic associations, and the presence or absence of glauconite, quartz, and pyrite. Induration increases from unlithified at the top to strongly lithified at the base of the succession. With the exception of Units I and V, which are mainly pale yellow and light yellowish brown, most of the succession displays subtle color variations from light to dark greenish gray or olive gray. A complete and well-established biostratigraphy provides an accurate framework to date this succession and to calibrate the entire sequence stratigraphy of the Neogene Marion Plateau. The basal sediments record a transgression with lithoclast-bearing sandstone and grainstone, containing abundant quartz and glauconite (~500-517.5 mbsf; lithologic Unit IV). Unit IV was deposited in a proximal periplatform setting in <100 m of water. The overlying 467 m of sediment (lithologic Units III-I) were deposited in upper bathyal depths (>200 m). This deeper-water section records changes in carbonate platform growth, detrital input, and current dynamics by changes in texture, neritic constituents, clay content and, glauconite content. The end of the MP2 growth phase (lithologic Unit II/III boundary), and the associated sea level drop near the middle to late Miocene boundary are recognized at Site 1195 as an upcore decrease in neritic components and the occurrence of a major glauconite-rich unit that coincides with the seismic Megasequence B/C boundary (~11 Ma).
Lithologic Unit I (0-36.9 mbsf) is composed of skeletal grainstone with planktonic foraminifers and subordinate mollusks. Deposition was strongly current controlled. The base of Unit I consists of a 20-cm-thick rudstone with phosphatized lithoclasts, which is visible in the seismic sections as an unconformity within seismic Megasequence D. Unit II (36.9-255.9 mbsf) consists of bioturbated skeletal packstone with clay and minor wackestone, and grainstone intervals are present. The top 15 m of this unit is glauconite rich. Compared to Unit I, Unit II is darker and has greater biotic diversity. In the upper 200 m of Site 1195, rare to common larger benthic foraminifers indicate deposition in upper bathyal water depths. The relative proportion of coarser and finer planktonic foraminiferal fractions varies among samples, possibly indicating winnowing variations in a current-influenced environment. To a depth of 95 mbsf, sediments of Unit I and II, are characterized by high-frequency variability in NGR and MS, which contrast sharply with the underlying intervals to 220 mbsf, where values approach zero. The high NGR and MS amplitudes above 95 mbsf are likely caused by increased terrigenous clay deposition, an observation supported by carbonate measurements of 80-90 wt% with clay dominating the noncarbonate fraction. Between 220 mbsf and the base of lithologic Unit II, NGR and MS values are reduced and more consistent than in the uppermost section.
Three subunits separated by prominent glauconite-rich horizons can be distinguished within lithologic Unit II. Subunit IIA (36.9-93.9 mbsf) is a fine-grained, poorly bioturbated packstone and grainstone with clay defined by the absence of pyrite and textural recurrent variations. Subunit IIB (93.9-123.9 mbsf) is composed of silt-sized skeletal packstone with common burrows. Subunit IIC (123.9-255.9 mbsf) is characterized by a series of 2-m-thick fining-upward intervals, which display color changes presumably related to variations in clay content. Some of these intervals have a scoured base and display a gradual change into the overlying coarser interval. Subunit IIC coincides with seismic Megasequence C and thus is coeval with growth of the MP2/MP3 platform to the south of Site 1195. The base of Unit II is glauconite rich. Based on both foraminiferal assemblages and the presence of fine sand-sized fragments of neritic origin, the lower part of Unit II (below 200 mbsf) and all of lithologic Unit III (255.9-467.3 mbsf) were deposited in a distal periplatform environment at outer neritic to upper bathyal water depths. The Unit III to II boundary corresponds to the top of Megasequence B and thus marks the time at which the MP2 platform to the east ceased carbonate production. In addition to the increase in neritic components, Unit III sediments contain more organic matter and, especially in the lower part, more clay and quartz. The base of Unit III is glauconite rich.
Unit IV (467.3-517.5 mbsf) consists of grainstone to sandstone with quartz, rounded glauconite, carbonate lithoclasts, nannofossils, and rare broken foraminifers. The biotic assemblage and coarse fraction, mixed with neritic and pelagic components, indicate a proximal periplatform depositional environment in outer neritic water depths. This unit was deposited during flooding and initial transgression over acoustic basement. Similar to Unit I, Unit IV NGR and MS values display high-amplitude variability.
At the base of Site 1195, 20 cm (Unit V; 517.5-517.7 mbsf) of a well-cemented skeletal grainstone with coralline algae, larger benthic foraminifers, and mollusks was recovered. This light yellowish brown limestone contains abundant nummulitids that may be late Eocene in age. These rocks/sediments are overlain by or possibly are encased in sediment containing benthic foraminifers and nannofossils that indicate early Miocene deposition.
Overall, bulk density (gamma-ray attenuation and moisture and density data) shows a decreasing downhole trend to 60 mbsf (1.68 to 1.45 g/cm3) followed by a slow but consistent increase to 380 mbsf (1.45 to 2.08 g/cm3). Below this, density is approximately constant to the bottom of the hole. Porosity and thermal conductivity reflect these bulk-density trends; porosity ranges between 30% and 80%, whereas thermal conductivity ranges from 0.8 to 1.4 W/(m…K). In contrast, P-wave velocity increases from 1541 to 2241 m/s at 370 mbsf. Below this depth, the mean velocity remains constant to the bottom of the hole.
Calcareous nannofossils and planktonic foraminifers ranging from late Pleistocene to late Miocene age are generally well preserved, whereas early to middle Miocene microfossils vary from poorly to well preserved. Calcareous nannofossils are abundant for most of the sequence recovered at Site 1195. Unlike other sites, the sequence cored at Site 1195 is virtually complete, with no apparent hiatuses. Average interval (0.5-4 m.y.) sedimentation rates range from 20 to 65 m/m.y.
Magnetostratigraphy obtained at this site is in agreement with the biostratigraphic datums for the middle Miocene and early late Miocene intervals. However, the late Miocene to Pleistocene magnetostratigraphy suggests ages ~0.6-0.8 m.y. younger than the biostratigraphy suggests. Nevertheless, the interpreted magnetostratigraphy between Chrons C1n and C2An provides the best record of all Leg 194 holes. A comparison between APC cores taken with the reduced magnetic PDC drill bit in the BHA with those taken with the standard bit reveals that the core top intensity anomalies in the vertical component are much reduced with the non-magnetic bit. At this site, glauconite layers coincide with intervals of high magnetic intensity, suggesting the presence of magnetite. Magnetic intensity peaks could be correlated between Hole 1195A and 1195B, indicating a depth offset of ~6 m between the holes.
The low gas content at Site 1195 is likely a function of appreciable pore water SO42- concentrations limiting methanogenesis and the lack of mature organic matter that could provide a thermogenic component to the gas fraction. The pore water chemistry at Site 1195 can be subdivided into two zones. In the upper ~80 mbsf, the chemistry is similar to seawater and appears to be the result of pore fluid exchange between the sediments and overlying waters. Strong oceanographic currents in the area might play a role in driving fluid exchange. Between 80 mbsf and the basement, pore water chemistry varies little with depth and shows evidence for authigenic dolomite formation and organic matter remineralization during sulfate reduction.
Wireline logging in Hole 1195B provided a continuous geophysical record from 80 to 517 mbsf. Five logging units were distinguished. Logging Unit 1 (80-240 mbsf) has low gamma-ray, bulk-density, and resistivity values and displays little variability, which is consistent with the homogeneous lithology of lithologic Unit II. The boundary with logging Unit 2 at 240 mbsf coincides with the seismic sequence B/C boundary. This boundary is marked by an increase in bulk-density, resistivity, and natural gamma-ray values. The onset of cyclic alternations of light gray and greenish gray sediment correspond to a cyclic appearance in all logging data within logging Unit 2 (240-418 mbsf). At the top of logging Unit 3 (418-451 mbsf), bulk-density and resistivity values increase slightly and neutron porosity and NGR decrease. Toward the bottom of this unit, higher amplitudes in gamma-ray and resistivity logs were observed. This pattern of higher variability in the lower part and less variability in the upper part of the unit is similar to logging Unit 2, perhaps indicating a repetition of the sedimentation pattern, although with a reduced thickness. At the top of logging Unit 4 (451-468.5 mbsf), the various logs become relatively constant and values decrease slightly. The base of this unit, however, includes a dramatic gamma-ray peak that correlates to a glauconite-rich layer. The last significant change in log character was detected at 468.5 mbsf with a significant drop in the resistivity and an increase of gamma-ray values, dominated by an increase in uranium from 3 to 6 ppm. The lithologies corresponding to logging Unit 5 (468.5 mbsf to the bottom of the logged section) consist of glauconitic sand and packstone.
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