Site 1197 (proposed Site CS-08A) is located on the southern Marion Plateau, 40 km east of the Great Barrier Reef margin and ~5 km southeast of the downcurrent, windward slope of the Miocene MP2/MP3 platform (Figs. F1, F2) The site is located on regional multichannel seismic lines MAR07 (shotpoint 3721), MAR65 (shotpoint 645), and MAR 66 (shotpoint 907) in 348 m of water. The main purpose of this site was to retrieve a proximal platform margin record of the MP2/MP3 sedimentation; to document and date platform growth, shedding, and subsequent current reworking; to study the fluid flow within the platform and adjacent slope sequences; and to determine the age and nature of acoustic basement.Operations
Operations at Site 1197 began at 0711 hr on 6 February 2001 (Table T2). Hole 1197A was spudded with the APC. Piston coring advanced to 54.6 mbsf, where a hardground was struck. The average recovery for this interval was 101.6%. Coring continued with the XCB to 203.8 m, where operations were terminated because of very low recovery (0.48% average recovery). Eleven of the 16 XCB cores had zero recovery. An inspection failed to identify any mechanical reason for the lack of recovery, so operations at Site 1197 were terminated with the option to return at a later time. Before the vessel departed for Site 1198, it remained in standby mode on location to wait for the third helicopter arrival of Leg 194, to evacuate an injured ODP technician.
After completion of Site 1198 and a return to Site 1196, Site 1197 was reoccupied to drill the hole to basement. The second operation at Site 1197 began at 1900 hr on 13 February. Hole 1197B was spudded with the RCB, drilled to 50 mbsf, and cored to a total depth of 674.9 mbsf. Erratic torque was experienced at 175.0 mbsf (depth of a major unconformity at the bottom of the elusive Megasequence C), and a total of three wiper trips were made to maintain the integrity of the hole. The final depth of 674.9 mbsf was 25 m below the Pollution Prevention and Safety Panel imposed limit of 650.0 mbsf; approval to exceed this depth was received from ODP-Texas A&M University headquarters. The interval 54.6 to 203.8 mbsf, which had an average recovery of 0.48% with the XCB in Hole 1197A, had 1.56% recovery in Hole 1197B. The total average recovery for the entire hole was 213.64 m (34.2%).
On 15 February, a planned rendezvous with the Wyllaway from Mackay took place. The vessel brought various supplies and returned Operations Engineer Leon Holloway to the mainland. In addition, a Transocean Sedco Forex employee required evacuation for medical reasons.
The hole was flushed and replaced with sepiolite mud, and the bit was released in preparation for the logging program. The tool string could not be advanced past the end of the open pipe and was recovered. The pipe was repositioned and the tool was redeployed and advanced to 81.6 mbsf, where the tool contacted an obstruction. When the tool was recovered, the compensating section of the DITE was missing, suggesting a mechanical failure of the bit release top connector at the end of the pipe. Logging was cancelled after attempts to clear the blockage with the drill string were unsuccessful. The drill string was recovered, both beacons were retrieved by 2335 hr on 17 February, and the vessel was slowly offset with the dynamic positioning system to Site 1199 (proposed Site CS-16A).Principal Scientific Results
The initial, early Miocene, transgression over acoustic basement at Site 1197 was characterized by an interval of skeletal floatstone rich in oysters deposited on a shallow platform in water depths <30 m. Overlying this interval are the first periplatform sediments deposited at this location. This thick middle Miocene sequence was deposited in water depths >200 m and is dominated by fine-grained skeletal material shed from the earliest growth phase of the adjacent platform. These sediments are overlain by two units of coarser neritic sediments reworked from the platform and also deposited at depths >200 m. A major hardground surface overlies these deposits and marks the cessation of sediment shedding from the MP3 platform and the onset of hemipelagic drift sedimentation, which continues to the present day.
The 674.9-m-thick sedimentary succession and underlying basement drilled at Site 1197 were subdivided into six lithologic units. Sediments at Site 1197 consist of predominantly periplatform and hemipelagic deposits of wackestone to grainstone texture, formed in water depths >150 m with the exception of the lowermost sedimentary unit, which was deposited in depths of <100 m.
Sediments recovered in lithologic Unit I (0-59.6 mbsf; late Pliocene to Pleistocene in age) consist of grainstone rich in planktonic foraminifers and sponge spicules, with minor bivalve fragments, echinoderms, rare scaphopods, and abundant pteropods. This unit coincides with seismic Megasequence D. The low proportion of neritic skeletal components and clay suggests that the drift deposits of lithologic Unit I were accumulated in a distal setting removed from platform input. Upper bathyal benthic foraminiferal taxa are common and diverse through lithologic Unit I. In addition, bioclastic debris from deep hard substrate communities are common in this interval. At ~55 mbsf there is a marked transition from predominantly pelagic components to neritic components. Mineralogical analyses show that aragonite is present in Unit I from ~0 to 43 mbsf with decreasing abundance downcore, reflecting aragonite production of pteropods. Once deposited, this aragonite underwent dissolution resulting in decreased downcore concentrations. Sediment bulk density within lithologic Unit I is nearly constant at ~1.7 g/cm3, whereas porosity decreases from ~68% to 60%. MS shows a peak near 9 mbsf and then decreases downcore. NGR shows cyclic variations on an increasing downcore trend.
Core recovery in lithologic Unit II (59.6-175.0 mbsf; late Miocene [to early Pliocene?] in age) was very low (0.6% in Hole 1197A and 1% in Hole 1197B). The recovered sediments are characterized by dolomitized skeletal grainstone rich in planktonic and larger benthic foraminifers and coralline algae fragments. Lithologic Unit II contains abundant neritic debris including well-preserved specimens of larger benthic foraminifers indicating a middle neritic origin. The prevalence of outer neritic and upper bathyal benthic foraminifers indicates that the neritic material was transported to a proximal periplatform setting at outer neritic to upper bathyal depths. The prolific shedding of platform sediments ceased in the late Miocene or earliest Pliocene as relative sea level rose and deepened the site, allowing hemipelagic sedimentation to prevail. This change in sedimentation is marked by a major hardground surface at the upper boundary of lithologic Unit II, corresponding to the seismic Megasequence C/D boundary. This surface consists of a prominent, reddened hardground with a thin phosphate crust. Evidence for endolithic boring, the presence of gastropod molds, and distinct color changes from red stained to yellow to white downcore indicate a gradual decline in lithification. Dolomite is present within lithologic Unit II and increases in abundance downcore, as indicated by X-ray diffraction analysis. The abundance of the benthic foraminifer Amphistegina sp., along with fragments of coralline algae, is reminiscent of lithologic Subunits IA and IB from Sites 1196 and 1199 on the MP3 platform. Planktonic foraminifers of late Miocene age in lithologic Unit II indicate that MP3 platform growth and shedding may have ended in the late Miocene or early Pliocene. The base of lithologic Unit II is defined by a hiatus and correlates with the Megasequence B/C boundary and possibly with the boundary between the MP2 and MP3 platform growth phases at Site 1196 and 1199.
Core recovery in lithologic Unit III (175.0-261.5 mbsf; late middle Miocene in age), as with Unit II, was low (~3%). The recovered sediments have dolomite contents similar to lithologic Unit II and are characterized as skeletal grainstone dominated by larger benthic foraminifers (Lepidocyclina sp.) with planktonic foraminifers and smaller benthic foraminifers as minor to abundant components. Glauconite is present and becomes common toward the base of this unit. The abundance of larger benthic foraminifers filled by glauconite within lithologic Unit III is a result of sediment reworking from the nearby platform. Unit III represents the upper part of a progradational seismic package within seismic Megasequence B. The base of lithologic Unit III is defined by an abrupt decrease in the presence of coarse neritic detritus. The bioclastic sediments of Unit III were deposited in a distal periplatform setting in outer neritic to upper bathyal water depths as indicated by benthic foraminiferal assemblages. These sediments are a mixture of fine neritic and planktonic debris, with occasional pulses of slightly coarser sediments that originated from middle to inner neritic depths.
Lithologic Unit IV (261.5-601.7 mbsf; early to middle Miocene in age) was deposited in a distal periplatform environment at water depths >200 m. The sediments within this unit are predominantly silt-sized skeletal packstone/grainstone. Coarse neritic detritus is absent. This unit was divided into two subunits on the basis of carbonate mineralogy, the degree of bioturbation, and differences in the presence or absence of cyclic intervals within the succession. Subunit IVA (261.5-367.4 mbsf) is characterized by partially dolomitized, skeletal grainstone with moderate to high bioturbation. Dolomite, mainly contained in cements, decreases downcore in this subunit. On seismic data, Subunit IVA forms the basal part of an inclined, prograding slope package that interfingers with deeper drift deposits within seismic Megasequence B. Subunit IVB (367.4-601.7 mbsf) consists of fine sand- to silt-sized skeletal packstone, grainstone, and wackestone with alternating light and dark intervals. Dark portions of these intervals have increased clay contents. Grain size and skeletal composition are generally similar within an interval. Most of the fine skeletal particles are fragmented neritic grains, such as coralline algae and benthic foraminifers. These alternating intervals probably result from high-frequency sea level changes. In addition to the color alternations discussed above, small-scale fining-upward intervals with well-preserved parallel lamination, characteristic of turbidites, were also observed. These calciturbidites are enriched in coarse skeletal detritus, such as larger benthic foraminifers. Low recovery in lithologic Subunit IVA precluded the measurement of physical properties in this interval, but measurements from Subunit IVB show decreasing porosity (39%-22%) and increasing bulk density (2.1-2.3 g/cm3) downcore. In addition, the cyclic variability observed in the sediments is also seen in NGR and MS data.
The sediments of Unit V (601.7-656.1 mbsf; early Miocene in age) consist of coarse skeletal packstone to fine rudstone exceptionally rich in larger benthic foraminifers and coralline algae with common bryozoans and bivalve remains. Glauconite is abundant, both as pellets and infills of intraskeletal porosity. Quartz grains are present and become particularly abundant at the base of the unit. The top of Unit V is defined by the first occurrence of coarse periplatform sediments. Initial flooding of the basement is recorded by an interval of skeletal floatstone rich in articulated oysters, implying limited reworking across a shallow platform in water depths <30 m. Small- to medium-scale graded bedding and the strong laminar fabric of many of the deposits suggest deposition from unidirectional gravity flows, probably slope-proximal turbidites related to periplatform shedding. The base of lithologic Unit V is defined by a sharp transition to volcaniclastic breccia and basalt.
Unit VI (656.1-666.7 mbsf; bottom of curated interval) is characterized by a matrix-supported volcaniclastic breccia with coarse (up to 12 cm) angular clasts of basalt derived from the underlying basement. This unit was divided into two subunits. Subunit VIA (656.1-666.6 mbsf) consists either of a pyroclastic/ignimbrite flow or reworked, previously deposited, volcanic rocks. Native sulfur mineralization is observed. The dark breccia is overlain by grainstone rich in larger benthic foraminiferal tests that were originally produced in middle neritic water depths, possibly in an open shelf setting where they were rounded and mixed.
Subunit VIB (666.6-666.7 mbsf) consists of a highly altered dark olivine basalt with plagioclase phenocrysts, small plagioclase laths, and pyroxenes comprising the groundmass. This rock was recovered in the core catcher of the final core at Site 1197. It is unclear whether this basalt was recovered in situ or whether it is a large breccia clast. The age and origin of this olivine basalt is uncertain, but is interpreted to have originated during synrift tectonic activity. The surface was most likely exposed for a significant period of time and then covered by a veneer of volcaniclastic breccia (Subunit VIA) that accumulated from a series of landslides. This continental surface likely maintained significant topography as indicated by the nature of the clasts in the breccia. Unit VI represents the acoustic basement at Site 1197.
Shipboard analyses of calcareous nannofossils and planktonic foraminifers provided a good age-depth resolution within lithologic Unit I of latest Pliocene to Pleistocene age. Below 60 mbsf, poor core recovery and likely downhole contamination hindered detailed age assignments. A broad late Miocene to early Pliocene age can be assigned to lithostratigraphic Unit II. Unconformities are present both at the top (59.6 mbsf) and the bottom (175.0 mbsf) of Unit II, which corresponds to Megasequence C. An early to middle Miocene age can be assigned to lithologic Units III-V. The majority of samples from Hole 1197B are poorly preserved, have low microfossil abundance, and often contain no age-diagnostic fauna.
Paleomagnetic analyses performed on APC cores collected with a nonmagnetic drill bit provided a good magnetic record for Hole 1197A, whereas cores from Hole 1197B obtained with the RCB showed the usual downward magnetic overprint. In Hole 1197A, natural remanent magnetization (NRM) intensity generally varies between 10-3 and 10-2 A/m with an average intensity of 10-2.5 A/m. An NRM intensity increase at 11 mbsf was interpreted as a hiatus. The NRM intensity for Hole 1197B is low (~10-4.5 A/m). Intervals of negative and positive inclination corresponding to the Brunhes and Matuyama Chrons are evident in Hole 1197A. All early Miocene polarity intervals are identified in Hole 1197B although it was difficult to locate where polarity transitions occur. The NRM, anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM), and IRM demagnetizations were measured on discrete samples collected from 4 to 50 mbsf in Hole 1197A. These data show that the predominant minerals are magnetite and pyrrhotite. R values of 0.4 to 0.5 have been obtained, indicating a possible hemipelagic environment. The concentration of magnetite decreases downcore, suggesting dissolution of fine-grained primary magnetite.
The low gas content at Site 1197 is likely a function of appreciable pore water SO42- concentrations to total depth, limiting methanogenesis, and the lack of mature organic matter that could provide a thermogenic component to the gas fraction.
Carbonate (CaCO3) content at Site 1197 ranges from ~4 to nearly 100 wt%. CaCO3 content covaries inversely with TOC content, which ranges from 0.0 to 0.54 wt%. S content in Site 1197 sediments ranges from 0.0 to 1.56 wt%, and its distribution is similar to that of TOC. Variations in the generally high carbonate content and the inversely covariant relation between carbonate and TOC content at Site 1197 reflect variations in carbonate mineralogy (i.e., calcite or dolomite) and the ratio of biogenic carbonate and terrigenous sedimentation through time.
In lithologic Subunit IVB, TOC and carbonate content display a relationship of decimeter-scale inverse covariations. The amplitude of these covariations increases downsection and corresponds to alternations of white to yellow skeletal grainstone and packstone, and olive-gray packstone. These alternations are also reflected in variations of NGR. The highest S value (~1.56%) measured at Site 1197 was also obtained within this interval (at ~544 mbsf). HI values suggest that lithologic Subunit IVB contains either terrestrial or oxidized organic matter. The presence of highly oxidized organic matter within an interval characterized by high S content is unlikely; therefore, organic matter deposition may have been dominated by terrestrial input. Pore water sampling at Site 1197 was restricted to the upper 50 mbsf and 410-530 mbsf because of poor core recovery between these intervals. In the upper 50 mbsf, the concentrations of most ions are similar to seawater values, and little change in pore water chemistry is observed. Only calcium shows marked changes, increasing in concentration with depth to a value of 20 mM at 50 mbsf. In the lower sampled interval, the pore water profiles are more typical of those observed in near surface sediments. Sulfate, magnesium and potassium decrease, and ammonium and strontium increase in concentration. Calcium remains near 20 mM. Although the data set is limited, the results suggest active seawater circulation through seismic Megasequence C.
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