Site 1194 (proposed Site CS-02A) lies on regional seismic line MAR13 (shotpoint 3951) and at the crossing of lines MAR24 (shotpoint 355) and MAR27 (shotpoint 1123). This site is located ~20 km east of Site 1193 (Figs. F1, F2). Drilling at Site 1194 provided information on the age and facies of Megasequences A, B, and D. The purpose of this site was to investigate the growth history of the MP2 platform as recorded in the adjacent marginal slope sediments. A prominent seismic horizon overlying these slope sediments can be traced to the surface unconformity on the MP2 platform. Thus, this horizon and the underlying section provide information concerning the causes for the demise of the MP2 platform. This site also provided information on the age and duration of unconformities within the prograding, proximal slope sediments of MP2 that are part of Megasequence B, the age of Megasequence A that represents the initial marine transgression over the Marion Plateau basement, and the nature of this basement.Operations
Hole 1194A was APC cored to refusal at 117.4 mbsf, with an average recovery of 104.1% (Table T2). APC temperature measurements were obtained at four depths. The hole was deepened with the XCB system to 169.9 mbsf with an average recovery of 7.6%. Hole 1194A was abandoned at that depth because of poor recovery. At 145.3 mbsf, the drill string became temporarily stuck and circulation was lost. The drill string was freed after working it for 30 min. Inspection of the bit at the end of operations in this hole revealed that all four bit nozzles were plugged with sediment.
The vessel moved 20 m east of Hole 1194A, and Hole 1194B was drilled ahead with the RCB to near the bottom of Hole 1194A. RCB coring advanced without incidents to the target depth at 427.1 mbsf, with an average recovery of 28.1%. Afterward, the hole was flushed with a sepiolite sweep, the bit was released on bottom, the hole was displaced with sepiolite mud, and the drill pipe was pulled up to logging depth.
With the LDEO temperature tool at the bottom and the LDEO gamma-ray tool (NGT) at the top, the triple combination (triple combo) tool (natural gamma ray, density, porosity, and resistivity) was the first wireline logging run in Hole 1194B. Hole conditions were good and the tools reached the base of the hole at 423 mbsf. The Formation MicroScanner (FMS) and dipole sonic instrument (DSI) were deployed for the second logging run. Electronic problems precluded the collection of DSI sonic data. One successful pass was completed with the FMS before the DSI was replaced with the Long-Spaced Sonic tool (LSS) for a second logging run. The second pass detected a narrow spot in the hole at ~230 mbsf, which was passed after several attempts.
The last run was the check-shot survey with the well seismic tool (WST). The air gun developed a leak, and therefore only the water gun was used. Downhole measurements were complicated by a strong current, which generated considerable drill string noise due to vibration. The end of the drill pipe was placed at 78.2 mbsf, and the deepest station was at 156.2 mbsf. However, the unclear source signature of the water gun resulted in an arrival waveform that could not be picked accurately by the software.Principal Scientific Results
The lowest portion of the sedimentary succession at Site 1194 is characterized by with an early Miocene-aged shallow-water transgressive deposit (Unit V) overlying a basement high (Unit VI). This interval was followed by an early middle Miocene high sea level episode characterized by abraded carbonate particles indicative of transport (Unit IV and Subunit VA). A late middle Miocene sea level fall led to the formation of shallow water facies (Subunit IIIA) followed by ~4 m.y. of sediment starvation. The late Miocene sea level rise led to deposition of hemipelagic sediments (Unit II) that are capped by highly winnowed Pleistocene deposits (Unit I) indicating strong bottom currents.
The sediments at Site 1194 are divided into six lithologic units. Unit I (0-3.8 mbsf; late Pleistocene in age) consists of a foraminiferal packstone without mud that reflects current-influenced deposition on the seafloor.
Lithologic Unit II (3.8-117.4 mbsf) is composed of late Miocene to Pliocene-aged hemipelagic mudstone/wackestone. Potential hiatuses and intermittently high sedimentation rates inferred from the shipboard biostratigraphy suggest that these sediments were deposited by small-scale gravity flows. Carbonate content in this unit varies from 60 to 90 wt% with an increasing trend downcore. The sediments in lithologic Units I and II have similar physical properties with slightly increasing bulk density (averaging ~1.75 g/cm3) and velocity (averaging ~1600 m/s) and decreasing porosity (~70%-50%) downcore. MS and NGR show cyclic variations within these two units. Lithologic Units I and II correspond with the seismically defined Megasequence D.
The boundary between lithologic Units II and III (equivalent to seismic boundary B/D) consists of a sharp contact between poorly lithified wackestones and underlying lightly cemented, neritic-rich grainstones. This surface is coated with a reddish brown crust of laminated phosphate interpreted as a hardground. Micropaleontological data suggest that this surface may represent a hiatus from 11.8 to 7.7 Ma. A second strong reflection below seismic boundary B/D correlates with a wireline signature similar to the hardground above, but this layer apparently was not recovered. Carbonate content in this unit varies between 68 and 98 wt%.
Lithologic Unit III (117.4-177 mbsf), deposited in the middle Miocene, was divided into two subunits. Subunit IIIA (117.4-148 mbsf) consists of bryozoan-dominated packstones and floatstones in the upper part and by small benthic foraminifers in the lower part. This subunit was likely to have been deposited on a middle neritic ramp in 30-50 m of water during the late middle Miocene (~13 to ~11 Ma) lowstand. Here, dolomite content is as high as 40 wt% but decreases toward the base of the unit. The near absence of bryozoans and the predominance of benthic foraminifers differentiate Subunit IIIB from Subunit IIIA.
The top of the early middle Miocene Unit IV consists of a 1-cm-thick layer of grainstone with a sharp contact to Unit III. This horizon is interpreted to be a firmground and is recorded as a prominent reflection in Megasequence B, a peak in physical properties data sets and low calcium carbonate values (~46 wt%). The lower to middle Miocene sediments of Unit IV (177.3-331.1 mbsf) are characterized by silt-sized packstones with physically abraded, unidentifiable skeletal components indicating high-energy long-distance transport. The likely source for these skeletal components was the MP2 carbonate platform. Paleoenvironmental information and seismic sequence geometry indicate that lithologic Unit IV was deposited at outer neritic depths and displays an overall shallowing-upward trend. Carbonate content in Unit IV is variable (62-97 wt%). Unit IV was subdivided into two subunits. Subunit IVA has higher dolomite content (up to 20 wt%) and generally contains more clay. The common occurrence of angular quartz sand and the absence of clay differentiates Subunit IVB. Physical properties data from this interval show overall normal compaction trends with increasing bulk density and velocity and decreasing porosity.
The lower Miocene lithologic Unit V (331.1-421.1 mbsf) is characterized by partially burrowed dark layers. No clay was observed in this unit. Lithologic Unit V is interpreted as a shallow marine transgressive deposit over a topographic high on the Marion Plateau basement. Within lithologic Unit V calcium carbonate content varies between 76 and 99 wt% and generally increases downhole. Subunit VA is dominated by packstones with unidentifiable silt-sized skeletal components. The distinguishing characteristic of Subunit VB is that the skeletal components are coarse, sand sized, and dominated by planktonic foraminifers. Near the base of the unit, the larger benthic foraminifer Lepidocyclina is abundant, and glauconite infillings indicate reworking. Dolomite concentrations within lithologic Subunit V are ~10 wt% near the top and decrease to zero near the base. Porosity values show greater variability than in the overlying units superimposed onto the general decreasing trend resulting from compaction. Velocity and bulk density increase downhole within Unit V. MS and NGR show evidence of cyclicity. Unit V unconformably overlies the reddish brown olivine basaltic basement of Unit VI (421.1-421.7 mbsf).
Benthic foraminiferal assemblages suggest that water depths at Site 1194 increased from <100 m near the bottom of Subunit VB to >200 m near the base of Subunit IVA (264.5 mbsf). Above this interval, water depths progressively decreased to 30-50 m at the base of Subunit IIIA and increased to >200 m for the top 117.4 m of the section at Site 1194.
Well- to moderately preserved nannofossils and planktonic foraminifers are generally abundant above 117.3 mbsf at Site 1194 and are rare to few and poorly preserved below this depth. Correspondingly, biostratigraphic resolution is relatively good above 117.3 mbsf and poor below. A hiatus of ~4 m.y. duration occurs at the upper Miocene hardground at 117.3 mbsf. Average sedimentation rates for Site 1194 are estimated to be 30 m/m.y. for the early to middle Miocene and 25 m/m.y. for the late Miocene to Pliocene.
Paleomagnetic measurements on long cores over the upper 100 mbsf were pervasively corrupted by core-top magnetic anomalies. Susceptibility peaks, indicating that new magnetic material had been introduced at the top of cores and then magnetized in the direction of the downhole overprint often accompanied these anomalies in magnetization intensity. To correct for these difficulties, the data were filtered, high intensities were removed, and the z-component was used in an attempt to obtain a magnetostratigraphy. Ultimately this effort provided only limited agreement with late Miocene biostratigraphic data. Apart from the hardgrounds, where there was evidence for hematite, the predominant magnetic material was magnetite. The pore water chemistry of Site 1194 suggests two different chemical regimes divided by the contact between lithologic Units II and III. In the hemipelagic sediments of Units I and II, a strong correlation of downhole decreasing magnesium and sulfate concentrations indicates the formation of authigenic dolomite. The reoxidation of organic matter via sulfate reduction supplies carbonate ions for dolomitization, with the required calcium and remaining carbonate supplied from biogenic calcite. Pore water profiles in lithologic Unit III and below show little or no change for most constituents. Continued decreases in magnesium and increases in calcium concentrations in this interval are considered to be driven by reactions occurring in the mafic basement coupled with diffusional exchange with the sediment pore fluids. Taken as a whole, lithologic Units III-V do not appear to be undergoing further low-temperature diagenesis, but are acting as a passive aquifer.
TOC content at Site 1194 is <0.5 wt%. Hydrogen index values at Site 1194 range from 29 to 300 mgHC/g TOC, but the low TOC values in some intervals limit the reliability of the HI values. S content in Site 1194 sediments ranges from 0 to ~0.67 wt%; the sulfur profile is similar to that of TOC. Sediments from ~118 to 300 mbsf at Site 1194 can be organized into three geochemically defined units, which consist of a conspicuous horizon of increased TOC weight percent values, low HI values, and relatively elevated weight percent total S content, overlying a broader interval of relatively high CaCO3, low TOC, and higher HI values. The horizons appear to coincide with hardgrounds and firmgrounds. The low gas content at this site is likely a function of appreciable pore water SO42- concentrations to total depth limiting methanogenesis. The immaturity of the organic matter meant that there was no thermogenic component to the gas fraction.
Downhole logging measurements from Hole 1194B produced a continuous geophysical record extending from the basement (425 mbsf) to sediments at 84 mbsf. The measurements can be grouped into three logging facies (logging Units 1-3) that correlate well with the seismic facies of the drift, slope, and shelf packages. Logging Unit 1 (84-114.5 mbsf), with generally low values of density, resistivity, and natural gamma ray, correlates to the onlapping sediments of MegasequenceŻB. Logging Unit 2 (114.5 -260 mbsf) is characterized by increased variability and distinct peaks in all logging data sets and roughly coincides with the high-amplitude inclined slope reflections of the MP2 slope. Lithologically, neritic outer-ramp and neritic upper-slope deposits above shallowing-upward deeper slope deposits characterize this interval. Thus, the variability and especially the peaks in the logs are most likely the combined result of variations in sedimentation rates, clastic content, and cementation, as might be expected in proximal slope sediments. For example, peaks of uranium and velocity appear to correlate with hardground surfaces. Logging Unit 3 (260-425 mbsf) contains low values in all logs but has regular variability, suggestive of distal cyclic shelf sedimentation. Changes in log signatures in Hole 1194B correlate well with lithologic unit boundaries, indicating that facies changes across unit boundaries produce distinct petrophysical signals.
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