Site 1193 (proposed Site CS-01A) is located on the Marion Plateau, ~80 km east of the south-central Great Barrier Reef, in 348 m of water (Figs. F1, F2).
The main objective at this site was to recover the sedimentary rocks of the MP2 carbonate platform. The platform is an important element for the reconstruction of Miocene sea level, because its top marks the last platform growth phase before the middle to late Miocene sea level fall. Regional seismic line MAR13, along which Site 1193 is located, indicates that an irregular, karstic platform top is buried under a relatively thin cover of hemipelagic drift deposits. Further, seismic geometry suggests that the MP2 platform was established on gently dipping slope deposits, as indicated by inclined reflections underneath the platform.
The northern edge of the MP2 platform was drilled during ODP Leg 133 (Davies, McKenzie, Palmer-Julson, et al., 1991). Sites 816 and 826 showed that the top of MP2 at these locations consists of a tropical reefal assemblage deposited in water depths <20 m. This depth defines the approximate point from which sea level began to fall at approximately the middle/late Miocene boundary. Results from Leg 133 also indicated that the top of MP2 had been subjected to subaerial exposure.
Site 1193 provides a record of the total thickness of MP2 near the platform edge, the history of its growth phases and episodes of exposure, and by investigating the hemipelagic sediments overlying MP2, the timing of its burial. Biostratigraphic data from Site 1193 also provide crucial information for dating the regional seismic sequence boundaries necessary to calibrate the established sequence stratigraphic framework for the region. In addition, Site 1193 sediments will yield a record of fluid flow and diagenetic processes in this carbonate platform, parts of which are extensively dolomitized. The age and nature of the underlying deeper water sediments, together with the facies of the basal sediments directly overlying the basement, document the timing of and processes linked to the initial basement transgression. Finally, Site 1193 provides a lithologic record of the age and nature of the acoustic basement.
The total sediment thickness at Site 1193 is 531 m. Holes 1193A through 1193C penetrated to total depths of 515, 138, and 548.5 mbsf, respectively. Acoustic basement was encountered in Hole C. Site 1193 was the first ODP site at which the advanced diamond core barrel (ADCB) was used to drill a sedimentary section. Recovery with the ADCB in the platform carbonates was significantly better than XCB recovery over a comparable interval until the ADCB bit was accidentally damaged.Operations
Operations began at 2200 hr on 12 January 2001. Hole 1193A was APC cored to 37.1 mbsf (101% recovery) (Table T2), where the carbonate platform was encountered. XCB coring continued to 450.4 mbsf with an average recovery of 28%. The pipe had to be pulled as a result of the "bent pipe event," and coring resumed 8 hr later. From 450 to 515 mbsf, average recovery was 17%. At 515 mbsf, the pipe became stuck and was freed after 5 hr of working the hole; however, the BHA had to be retrieved soon after because of a stuck barrel caused by a broken latch.
On 13 January, the rendezvous boat from Mackay arrived with three scientists and two ODP technicians who had remained on shore so that the HYACE engineers could be accommodated during the first 3 days. In addition to the HYACE team, the personnel leaving the JOIDES Resolution included Lamont-Doherty Earth Observatory (LDEO) and ODP representatives and an ODP technician. On 17 January a helicopter from Mackay landed on the vessel and removed a roughneck who was diagnosed with probable kidney stones.
The vessel was offset 20 m and Hole 1193B was spudded at 1730 hr on 17 January with the rotary core barrel (RCB). After the hole was washed ahead to 35.0 mbsf, rotary coring penetrated the hard cover of the carbonate platform and advanced 53.8 mbsf with 19% recovery. A free-fall funnel (FFF) was deployed and the pipe was tripped to make up the ADCB system with a 7.25-in polycrystalline diamond compact (PDC) drill bit. The developmental ADCB was deployed in anticipation of improved recovery of reefal limestone. The ADCB reentered Hole 1193B and coring started at 53.8 mbsf. After Core 194-1193B-4Z was cut, the pipe had to be pulled to retrieve a stuck barrel and it was found that the inner tube had collapsed. Hole 1193B was reentered with the ADCB on 19 January, and coring resumed at 63.2 mbsf. Total recovery over the interval from 53.8 to 91 mbsf was 16% compared to 2.6% for the XCB over a similar interval. Below that interval, bit problems, perhaps associated with a clayey formation, reduced recovery dramatically. After cutting Core 194-1193B-11Z, an improvised bit deplugger was run in an attempt to clear the core blockage. Several blows of the wireline jars succeeded in landing the barrel, but subsequent cores had dismal recovery. The drill string was pulled, and it was found that ~50% of the cast matrix pilot portion of the bit that trims the core had been broken off by the earlier jarring with the bit deplugger. Hole 1193B was thus terminated.
On 20 January, a second medical evacuation by helicopter was required for a scientist diagnosed with a fractured femur. The diagnosis was confirmed on shore, and the patient underwent surgery in a local hospital. The roughneck evacuated earlier returned to work on the same helicopter.
The vessel was offset 20 m, and Hole 1193C was spudded with the RCB at 1800 hr on 20 January with the primary objective of providing a dedicated logging hole. After drilling through the overlying hemipelagic section, the top of the carbonate platform was cored for a third time, from 35 to 70.1 mbsf, with an average recovery of 16.2%. A center bit was deployed and continuous drilling advanced to 510 mbsf. Four additional cores were taken from 510 to 548.5 mbsf (39.9% recovery), where acoustic basement was penetrated. The hole was then displaced with 172 barrels of sepiolite mud, and the drill string was pulled in preparation for logging. The pipe got stuck at ~297 mbsf and circulation or rotation could not be established despite a 3-hr attempt. The pipe was therefore severed with explosives at 434 m drill string length (75 mbsf). The fragmented end of pipe was on deck by 2100 hr, ending operations at Site 1193. Logging plans had to be abandoned. As the drill string was being recovered, several attempts to retrieve the beacon failed.Principal Scientific Results
Site 1193 penetrated to a depth of 548.5 mbsf and at the base recovered an immature fine marine sandstone consisting of volcanic detritus mixed with carbonate skeletal fragments. This acoustic basement is overlain by a 531-m-thick sedimentary succession ranging in age from early Miocene to late Pleistocene. The lowest part of the sedimentary sequence is a basal transgressive succession that deepens upward as indicated by early Miocene-aged upper-slope sediments. The 302-m-thick transgressive and slope sequence is overlain by a 194-m-thick, middle Miocene carbonate platform, MP2. This interval represents one of the major Miocene platform growth phases on the Marion Plateau. The platform rocks consist mostly of a bryozoan and larger foraminifer assemblage that is almost barren of corals. This assemblage is indicative of a cool subtropical depositional environment at paleowater depths within the euphotic zone (i.e., <100 m). In some intervals the rocks are heavily dolomitized. In the upper part of the platform, a series of exposure surfaces are observed. In addition, a couple of submarine hardground formations occur close to its top. Cessation of carbonate production on the MP2 platform could have occurred as early as early middle Miocene. Unconsolidated uppermost Miocene through late Pleistocene hemipelagic carbonates cover the platform, indicating that platform growth was not reestablished at Site 1193.
The lithostratigraphy of Site 1193 is divided into seven principle units. Unit I consists of a 4-m-thick skeletal grainstone, composed mostly of planktonic foraminifers, that reflects the modern current-swept conditions at the seafloor. Unit II (4-35 mbsf) is an unconsolidated Pleistocene to uppermost Miocene hemipelagic foraminiferal packstone. The heavily bioturbated sediment consists mostly of very fine sand-sized planktonic foraminifers in a clay-rich carbonate matrix. Carbonate content ranges between 70 and 90 wt%. These first two units are equivalent to seismic Megasequence D.
The boundary between lithologic Units II and III marks the top of the 194-m-thick carbonate platform MP2. This boundary is characterized by a major hiatus that, according to nannofossil and planktonic foraminifer datums, spans most of the late Miocene and possibly of the middle Miocene. If the broad age constraint imposed by the benthic foraminifer assemblage (12-24 Ma) is included, the hiatus spans from 5.6 to at least 12 Ma, and possibly to ~15 Ma. The hiatus at 35 mbsf corresponds seismically to the top of Megasequence B and is marked by distinct changes in all shipboard data sets. The platform rocks consist of coarse skeletal grainstone to rudstone and subordinate dolostone. In general, the sediments of lithologic Unit III have a diverse biotic assemblage of bryozoans and larger benthic foraminifers, with subordinate mollusks and coralline algae but rare coral. This assemblage indicates cool subtropical environmental conditions. The platform sequence is divided into two subunits based on a 10-m-thick clay-rich mudstone to floatstone interval rich in larger benthic foraminifers. This interval was deposited in a more protected, possibly lagoonal environment. Several irregular and reddish colored horizons occur within the uppermost part of Unit III and likely represent exposure horizons. The shallowest of these has a thin veneer, probably phosphatic coating, that indicates submarine hardground formation during a depositional hiatus. Analyses of benthic foraminiferal abundance and morphology verify that the paleowater depth was always within the euphotic zone (i.e., <100 m and more likely <60 m), with some intervals that were deposited in <10 m of water. Dolomite content is highly variable and generally higher in the upper part of the platform. Some intervals of the platform rocks are completely dolomitized and display an intense orange to red color. Physical properties of the platform rocks show large scatter. For example, bulk density varies from 1.95 to 2.65 g/cm3, porosity from 10% to 45%, and P-wave velocity from 2700 to 5200 m/s.
Immediately below these high-energy, relatively shallow water sediments is a 20-m-thick interval of mudstone with up to 50% clay (Unit IV). This unit passes downcore into 136 m of fine-grained, bioturbated skeletal packstone (Unit V), representing the progradational platform slope. The slope sediments can be correlated to seismically imaged clinoforms underlying the hard platform cap. Carbonate content in Units IV and V average ~60 and ~80 wt%, respectively. Dolomite content in Unit V is relatively constant, averaging ~10%. The paleowater depth indicated by the benthic foraminiferal assemblages of Units IV and V sediments is consistently deeper than 100 m. The sediments below the MP2 platform can be recognized by a consistent and nearly linear downhole trend in bulk density, grain density, porosity, P-wave velocity, and thermal conductivity.
The lowest part of the drilled sedimentary section consists of 146 m of interbedded grainstone and subordinate quartzose sandstones (Unit VI). The siliciclastic fraction varies widely as a result of the presence of the sandstone beds. The depositional setting was probably inner to middle neritic, as indicated by abundant larger benthic foraminifers.
The deepest core from Site 1194 (540-549 mbsf) recovered what was possibly the top of the acoustic basement (Unit VII) consisting of a marine sandstone with occasional organic-rich layers and quartz grains showing undulose extinction. These quartz grains likely originate from deformed terrains on the continental margin.
At Site 1193, well-preserved calcareous nannofossils and planktonic foraminifers provided 15 latest Miocene to Pleistocene age-depth control points in the top 35 m of the cored section and six early Miocene control points below 230 mbsf. The two biostratigraphic data sets agree well within these intervals, which broadly constrain the age of growth and times of nondeposition and flooding of the MP2 carbonate platform (35-229 mbsf) to ~5.6-~16 Ma. No age diagnostic assemblages were found within the platform sequence. Larger benthic foraminiferal assemblages were observed throughout the platform interval (35-229 mbsf) and further constrain the time of platform buildup to 12-24 Ma. However, the 12-Ma time constraint does not necessarily exclude younger platform growth that could have been eroded.
Preliminary interpretation of the sequence of observed polarity reversals above the platform (lithostratigraphic Units I and II; 0-35 mbsf) provides ages offset by up to 0.5 m.y. from the depth-equivalent biostratigraphic ages. Within the platform sequence (35-220 mbsf), recovery was insufficient to allow development of a reliable magnetostratigraphic record. Below the platform (225-375 mbsf), discontinuous polarity sequences were identified, but their interpretation is not reliable without further investigation. Preliminary rock magnetic studies suggested that magnetic remanence may be carried by magnetite, hematite, goethite, and pyrrhotite.
Because of the difficulties with the magnetostratigraphy, biostratigraphic control points were used to construct the shipboard age model for Site 1193. Average sedimentation rates are 10 m/m.y. for the middle early Miocene, 100-150 m/m.y. for the late early Miocene and 0-25 m/m.y. for the Pliocene-Pleistocene. The significant late early Miocene increase (one order of magnitude) in sedimentation rate can be interpreted as the onset of shedding from the carbonate platform, augmented by clay from the continent and modified by current activity. This high rate of sedimentation evolves directly into carbonate platform growth at Site 1193 near the early/middle Miocene boundary. The period of platform growth, erosion, and nondeposition probably contains several depositional hiatuses. The Pliocene-Pleistocene sequence shows one distinct hiatus at 3.3-6.3 mbsf, which is constrained by nannofossil datums to at least 2.8-1.7 Ma. This hiatus correlates with peaks in the natural gamma radiation (NGR) and MS to 5.7 mbsf, coinciding with a change in sediment texture from packstone to wackestone, an iron-stained interval, and a seismic sequence boundary within Megasequence D.
Poor core recovery in the cemented shallow-water carbonate intervals at Site 1193 limited pore water sampling to finer-grained, more clastic rich sediments above, below, or within the platform carbonates. Overall, however, a general picture of the pore water chemistry was still possible. The interstitial water chemistry suggests that the fluids within sedimentary Units I-III, extending to the base of the carbonate platform facies, have probably not evolved from seawater. Even the strontium concentration, which might be expected to increase as a result of ongoing carbonate recrystallization, shows no increase until below the platform facies. The best explanation for the lack of change in fluid chemistry is that the platform sediments were completely altered to stable carbonate mineralogies, dolomite, and low-magnesium calcite before the platform was reflooded.
In the platform and underlying sediments, horizons defined by decreased CaCO3 content, increased total organic carbon (TOC) weight percent values, low hydrogen index (HI) values, and relatively elevated total sulfur (S) concentrations correlate with intervals containing increased clay content and transported and transport-abraded benthic and planktonic foraminifers.
The low gas content at Site 1193, as at Site 1192, is likely a function of appreciable pore water SO42- concentrations limiting methanogenesis to the deepest strata cored at the site. Also, the organic matter that does exist here is immature relative to petroleum generation, so no thermogenic component to the gas fraction is expected.
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