Site locations, water and drilling depths, core recovery, etc., are listed in Table T2. For stratigraphic and lithologic summaries of all sites see Figures F9A and F9B.
The main objective at Site 1192 (proposed Site CS-13A) was to perform a feasibility test of the Hydrate Autoclave Coring Equipment (HYACE), including the pressure core sampling tool (H-PCS) developed at the Technische Universit”t Berlin and the vibracore sampling tool (HF-VS) developed by Fugro Geotechnical Engineering. In July 2000, the HYACE team was awarded 72 hr of testing time at the beginning of Leg 194. The HYACE developer did not specify a particular test site, so ODP selected proposed Site CS-13A, a contingency site of the Leg 194 scientific program. The scientific objectives at this contingency site were to investigate the slope sediments ~40 km east of the paleomargin of the early middle Miocene MP2 carbonate platform margin. In particular, Site 1192 was drilled to provide information on the facies and age range of regional Megasequences B through D, which together form the carbonate platform to slope architecture of the Marion Plateau from early Miocene time till today. Drilling at this site was aimed to recover the marginal slope sediments shed from the MP2 carbonate platform, the lithologic signature of basinward unconformities adjacent to the MP2 platform, the age of the erosional top of MP2 by dating the correlative seismic horizon, and to calibrate regional seismic sequence stratigraphy.Operations
Leg 194 began on 3 January at 2200 hr, ~2 days earlier than scheduled. The ship departed Townsville at 0700 hr on 8 January, three days ahead of schedule, and operations at Site 1192 began at 1720 hr on 9 January. During the initial fit test of the HYACE tools in the bottom-hole assembly (BHA), it was discovered that the outside diameter of the top 1 m of the H-PCS tool was several thousandths of an inch too large to pass through the latch sleeve of the ODP APC (advanced hydraulic piston corer)/XCB (extended core barrel) BHA. The latch sleeve was therefore left out of the BHA in the hope that pump pressure would generate enough resistance to offset the rotation and torque caused by rotation of the small bit.
Hole 1192A was cored continuously with the APC to refusal at a depth of 241.5 meters below seafloor (mbsf). Twenty-five APC cores were retrieved with 101.7% average recovery. The HF-VS was deployed three times, at 28.5-29.5 mbsf, 86.5-87.5 mbsf, and 192.0-193.0 mbsf. The corer recovered 0.19, 0.91, and 0.77 m of sediment respectively. The shear pin parted in the first and second run, but not in the third run. The hammer could not be activated in the first run, presumably because the formation was too soft. The hammer did operate in the subsequent runs for several seconds. The flapper only closed in the second run and a sample chamber pressure of 609 psi (close to estimated in situ pressure) was maintained. The flapper did not close in runs one and three, presumably because of difficulties with the PVC liners.
The H-PCS was run two times in Hole 1192A, from 231.0 to 232.0 mbsf and from 241.5 to 242.5 mbsf. This tool is designed to operate in more indurated sediment than the HF-VS tool. The positive displacement motor was not activated in either run, and the end of stroke was not indicated. After the instrument was recovered, the inner barrel was still retracted in the autoclave and the flapper valve was not properly seated. It is possible that the lack of a latch sleeve in the BHA allowed the upper part of the tool to rotate instead of the lower section. Prior to the second deployment, the tool was marked with paint on the upper and lower sections in order to ascertain if slippage occurred and if the tool was seated. After deployment, the marking was reviewed. Abrasions of the paint suggested that the upper section had rotated and that the bottom section had not seated properly. It was decided to recover the drill string and add the latch sleeve. The rig mechanic machined the top 1 m of the tool to the same diameter as the tool body.
Hole 1192B was spudded at 1145 hr on 11 January. Following a single mudline core that established the seafloor depth at 387.1 m, the hole was drilled ahead to 179.9 mbsf. Five more APC cores were retrieved with 43.6 m recovery (87.3%), followed by 13 XCB cores with 60.43 m recovery (48.3%), to a total depth of 355.5 mbsf. Total recovery in Hole 1192B, including test cores, was 105.1 m, representing 59.1% of the cored interval.
The HF-VS tool was deployed once more from 179.9 to 180.9 mbsf. The results of this test were similar to the last test in Hole 1192A. The shear pin did not part, hammering was detected for a few seconds, and when the tool was recovered it showed that the latching pawls were jammed, which prevented full retraction of the liner into the inner barrel. The flapper was not closed. The corer recovered 0.84 m of sediment.
Immediately following this last HF-VS deployment, the H-PCS tool was run for the third time from 180.9 to 181.9 mbsf. This time, the inner barrel did stroke out and rotate but no sediment sample was obtained.
During the fourth and final run of the H-PCS (335.2 to 336.2 mbsf), the inner barrel stroked out, rotated, and recovered ~0.27 m of sediment. The formation was too soft for the type of core catcher employed (the only type available) and it was felt that some sediment was lost in the recovery process. The retraction mechanism was not released so the core was not under pressure.
The allocated testing time expired and operations at Site 1192 ended at 1440 hr on 12 January.Principal Scientific Results
The sediments recovered at Site 1192 provide a record of the middle to late Miocene evolution of distal carbonate platform sediments on the Marion Plateau. The recovered upper Miocene to Pleistocene, hemipelagic carbonates document the interfingering of subtropical neritic carbonates with pelagic carbonates and terrigenous siliciclastics. These constituents were deposited in extensive sediment drifts on the Marion Plateau. Breaks in the production of shallow-water carbonates, due to exposure and/or drowning, coincide with the sudden appearance of glauconite-rich and phosphate-rich lithologies. A significant increase in the sediment fraction of the sediment beginning 4 m.y. ago, together with drift geometries as seen on the seismic data, indicate increased current activity or increased continental erosion and sediment transport.
Site 1192 penetrated the regional seismic Megasequences D and C and part of Megasequence A/B. The site is located midway between the carbonate platforms MP2 and MP3 (Figs. F1, F2). The biostratigraphic results and zonal assignments of Holes 1192A and 1192B indicate a Pleistocene to upper Miocene succession that was subdivided into five main lithologic units according to texture, color, and the presence or absence of quartz, glauconite, and phosphate grains. The entire sequence is generally heavily bioturbated and in many places reveals distinctive ichnofossils. Color variations are generally subtle, and most of the succession is light to dark green-gray or olive green in color.
Unit I (0-2.5 mbsf) includes the modern seafloor, a current-swept hardground with various stages of lithification as indicated by intraclasts with brown iron oxide-stained sediment. Carbonate content is over 90 wt%.
Unit II (2.4-258.1 mbsf) is an upper Miocene to Pleistocene heavily bioturbated, planktonic foraminifer mudstone to packstone with clay. Sediments of Subunit IIA (2.4-103.1) show highly variable grain density and magnetic susceptibility (MS), indicating a high and variable amount of terrigenous grains. An increase in bulk density at 85 mbsf may correlate with an impedance contrast imaged as a high-amplitude reflection on the seismic data. In Subunit IIB (103.1-258.1), sediment physical properties are nearly constant, reflecting an overall more homogeneous sedimentary composition. Carbonate concentrations vary between 73 and 82 wt% in Subunit IIA and between 80 and 92 wt% in Subunit IIB, reflecting the increased siliciclastic content in Subunit IIA.
Sediments in Units III through V (258.1-355.5 mbsf; early to middle Miocene) record the distal influence of a platform (periplatform/hemipelagic) and the adjacent continent. Unit III (258.1-325.6 mbsf) contains abundant quartz, glauconite, and phosphate grains within a foraminifer packstone with clay that contains more shallow-water benthic foraminifers than the units above. Unit IV (325.6-336.2 mbsf) consists of alternating intervals of planktonic foraminifer mudstone and packstone with clay. Quartz and glauconite grains as well as dolomite rhombs are present. Unit V (336.2-355.5 mbsf) is a silt-sized dolomitic grainstone with clay that coarsens toward the bottom of the hole. Unlike Unit IIB, Units III through V are characterized by more variable grain density and MS, indicating changing concentrations of siliciclastic sediment components. This variability is supported by a large range of carbonate concentrations (66-94%). For the entire site, bulk-density, porosity, P-wave velocity, and thermal conductivity data show consistent downhole trends.
Nannofossil datums provided modest biostratigraphic resolution for the Pleistocene through middle early Miocene. Holes 1192A and 1192B in general reveal good to moderate planktonic foraminifer preservation from the Pleistocene to upper Miocene. Preservation is poor at the base of the hemipelagic sequence, which affects age assignments in the middle to lower Miocene samples from Hole 1192B. The average sedimentation rate of the entire sequence at Site 1192 is ~20m/m.y.
The presence of small, flat specimens of the larger benthic foraminifers, Amphistegina and Operculina, of which modern equivalents host algal endosymbionts, along with several porcellaneous taxa at the top of Hole 1192B, indicate a deep euphotic (~50-120 m) habitat (middle to outer neritic), suggesting reworking and lateral transport within the Unit I. Further downcore, benthic foraminifers are relatively rare in all samples examined from Hole 1192A, although the relatively diverse assemblages of rotaliid, buliminid, nodosarid and agglutinated taxa are characteristic of upper bathyal habitats.
Magnetostratigraphic interpretation of the uppermost 150 m of Site 1192 resulted in an average sedimentation rate of 20 m/m.y. However, the age model derived from the paleomagnetic data is offset from the biostratigraphic age model. The data are compromised by unremoved, possibly high-coercivity magnetic overprint. In addition, potential hiatuses may prevent an accurate interpretation of observed polarity intervals. Below 150 mbsf magnetostratigraphic interpretation is not possible as a result of incomplete core recovery, low intensities, and magnetic overprint.
The sediment and pore water chemistry of Site 1192 is fairly typical of hemipelagic to pelagic, carbonate-rich sediments with low sedimentation rates. Sedimentary organic carbon content is low (0.1-0.3 wt%) and is predominantly of marine origin. As a result of the low organic carbon content, carbon reoxidation rates are also low. The primary products of diagenesis are iron sulfides, celestite, and dolomite, all of which appear as accessory minerals in the sediments. The low gas content at Site 1192 is a function of low organic matter contents that prevent complete sulfate reduction and immature organic matter that does not provide a thermogenic gas fraction.
A seismic traveltime to depth conversion was achieved using shipboard velocity data measured with the P-wave sensor (PWS) tool. Seismic Megasequence D incorporates lithologic Unit I and part of Unit II, ranging in age from late Miocene to Pleistocene. The absence of Holocene sediments at this site confirms the assumption that modern sedimentation is strongly reduced or absent and the seafloor represents an unconformity, which is also supported by relatively low porosity in the uppermost sediments. The time-depth correlation places the Megasequence C/D boundary at 120 mbsf, which results in an age of ~7.2 Ma. In the area of Site 1192, this sequence boundary is conformable, and no large stratigraphic hiatus was observed in the drill cores across this boundary. The sediments of the underlying Megasequence C consist mostly of the late Miocene-aged lower part of lithologic Unit II, which is characterized by a rather uniform lithology, resulting in a seismic facies mostly characterized by low-amplitude reflections. The Megasequence B/C boundary at 240 mbsf correlates approximately with the boundary between lithologic Units II and III (259 mbsf). The sedimentological and geochemical signatures of lithologic Unit III point toward a relative increase of noncarbonate sediments (quartz, glauconite, and phosphate) into the depositional system during a period that can be seismically correlated with the exposure of platform MP2. Using the compiled age model, an age of ~11.9 Ma can be assigned to seismic Megasequence B/C boundary.
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