Sites 1196 and 1199 were both drilled into the top of the MP2/MP3 carbonate platform on the Marion Plateau. Site 1196 (proposed Site CS-06A) was drilled in 304 m of water, 20 km east of the Great Barrier Reef margin (Figs. F1, F2) The site is positioned at the intersection of regional seismic line MAR07 (SP 2808) and local grid line MAR70 (SP 178). Two holes were drilled (Hole 1196A, 0-672.2 mbsf; Hole 1196B, 0-265.3 mbsf) through a 663 m sequence of carbonate platform sediments overlying a phosphate-rich substrate. One hole was drilled at Site 1199 (proposed Site CS-16A; 0-419.5 mbsf). This site is located in 316 m of water ~5 km northeast of Site 1196 and is positioned on regional seismic line MAR20 (SP 2690).
Sites 1196 and 1199 were drilled to provide information on the initiation and facies development of the MP2/MP3 carbonate platform, the nature of unconformities separating each platform phase, and the age and nature of the section equivalent to the MP2 platform drilled at Site 1193. This platform is an important element for the reconstruction of Miocene sea level history as its base was theorized to mark the first platform growth phase after the major middle to late Miocene sea level fall. Postcruise analysis on recovered samples will also enable the study of fluid flow processes within the carbonate platform as interpreted from the pore waters from adjacent slope sites (1197 and 1198).
Scattering of seismic energy within the well-indurated MP2/MP3 platform makes it difficult to determine the stratigraphy and nature of sediment deposition underneath the platform cap. Despite this, analysis of regional seismic data suggested that Sites 1196 and 1199 should penetrate a thick sequence of upper Miocene-Pliocene carbonate platform sediments representing the deposition of the MP3 carbonate platform. The base of this sequence was predicted to overlie basinal sediments shed from the MP2 platform drilled at Site 1193. Drilling at Sites 1196 and 1199 only partially confirmed this interpretation. Although not conclusive, evidence from these sites suggest that only the upper 130 to 150 m of the platform facies could have been accreted during the late Miocene, overturning the working hypothesis that this platform was completely of late Miocene to early Pliocene age. This discovery also necessitates a lower-amplitude sea level change for this time interval than was originally predicted.Operations
Site 1196 is located within the Great Barrier Reef Marine Park. All operations adhered to the guidelines established by the Great Barrier Reef Marine Park Authority. Hole 1196A was spudded with the RCB into the hard carbonate cap of the MP2/MP3 platform and advanced to 85.8 mbsf. Coring was interrupted for 10 hr because the drill string became stuck and had to be freed. To reduce the risk of the pipe becoming stuck, the hole was flushed with mud sweeps following the retrieval of each core and additional wiper trips were made at 285 and 429 mbsf. The pipe was stuck and freed again after a connection was made with the bit at 499 mbsf. Coring was terminated at 672.2 mbsf, well below the original target depth of 570 mbsf. Acoustic basement was not penetrated. A total of 86.34 m of core was recovered (12.9% average recovery) (Table T2).
The wireline triple combo was run with the NGT on the top and the LDEO temperature tool on the bottom for the first logging run in Hole 1196A. The NGT experienced electrical problems when it was lowered in the pipe and was subsequently removed. The shortened string was deployed but could not pass a tight spot at ~520 mbsf, and the hole was logged up from that depth. The tool string was recovered with loosened connections and without the brass bottom nosepiece (~4 in long) of the LDEO temperature tool, most likely a result of current-induced vibration of the drill pipe. For the second wireline run, the FMS-sonic combination was deployed but an obstacle was encountered in the pipe-possibly the missing piece of the temperature tool-that had to be pushed out by a core barrel. The FMS-sonic tool combination was subsequently run starting from a tight spot at 524 mbsf. The third run was a check-shot survey with the WST with 13 stations successfully shot between 94.4 and 523.5 mbsf.
Hole 1196B was spudded with the RCB to prepare a pilot hole for ADCB coring. Following the recovery of Core 194-1196B-6R, the vessel was forced to wait on weather for 11 hr as the heave was routinely exceeding the shallow-water guideline threshold of 2 m. Coring then resumed and advanced to the target depth of 110 mbsf. Average recovery for that interval was 10%.
Once the bit was on deck, the weather remained unfavorable for shallow water operations and the seas were deemed too rough for ADCB coring. In response, drilling operations were moved to Site 1197, with a planned return to Site 1196 when the environmental conditions might provide better success for ADCB reentry and coring. The vessel departed Site 1196 at 0415 hr on 6 February, leaving the beacon on the seafloor. After initial operations at Site 1197 and completion of Site 1198, the weather abated and ADCB coring in Hole 1196B appeared feasible. Reoccupation of Hole 1196B began at 0545 hr on 11 February.
Hole 1196B was reentered with the RCB, without the use of a FFF, and washed ahead to 110 mbsf to insure a clean and stable hole for the ADCB reentry. The drill string was recovered and the ADCB drilling assembly was reentered. ADCB coring advanced from 110 to 265.3 mbsf (Cores 194-1196B-13Z through 51Z). Coring was terminated before reaching the target depth of 360 mbsf because of diminishing core recovery and to conserve time for an additional site. Total recovery from the 155.3-m interval cored was 17.92 m, or an average of 11.5%. The RCB recovery for a comparable interval (17 cores from 105.0 to 268.4 mbsf) from Hole 1196A was 10.4%. The heave rarely exceeded 1 m while coring in Hole 1196B. Although still in the development stage, the ADCB system operated continuously for 35 hr without hardware problems, broken parts, improper seating of the core barrel, or downtime of any kind.Site 1199
After a revisit of nearby Site 1197 (13-17 February), the vessel proceeded to Site 1199 (proposed Site CS-16A) in dynamic positioning mode. After waiting on weather for 3.5 hr, Hole 1199A was spudded with the RCB at 1235 hr on 18 February. Rotary coring advanced to a final depth of 419.5 mbsf (Table T2). The recovery from the top 159.8 m of the hole was 83.6 m (52.3%). The average penetration rate (ROP) (4.7 m/hr) was slow until the hole deepened and the increased bit weight gradually helped to increase the ROP. Below 159.8 mbsf, the recovery dropped off dramatically and the ROP increased substantially. The average recovery from 159.8 to 419.5 mbsf was 3.3%. The total recovery for the hole was 92.16 m, or 22.0%.
After completion of coring due to time constraints, the hole and drill string were prepared for logging and the triple combo tool string was lowered to a depth of 418 mbsf, ~2 m above total depth of the hole. A successful logging run was completed. The FMS-sonic velocity tool suite was subsequently lowered in the hole but would not move past an obstruction at 129.2 mbsf, presumably because of a karst cave. The logging tool indicated a hole inclination of up to 7°. After countless attempts to pass the tight spot, including lowering of the pipe and adding sections to the tool string, logging was initiated above this horizon and obtained good data over the short interval. Logging operations were terminated at 1000 hr on 19 February. The drill string was recovered and the beacon retrieved for the last time during Leg 194. The BHA was disassembled and the vessel was secured for the extended sea voyage to Guam, leaving the site at 1415 hr on 22 February.Principal Scientific Results
The MP2/MP3 carbonate platform drilled at Site 1196 was established on an upper Oligocene (to early Miocene?) siliciclastic, phosphate-rich substrate. Above this substrate, an ~660-m-thick edifice of shallow-water carbonates was established from the early (to late?) Miocene that underwent several phases of growth, exposure, and dolomitization. The oldest platform, undated but probably mostly early Miocene in age, is ~300 m thick and is pervasively dolomitized, nearly obliterating initial depositional facies. This dolomitized platform is the foundation for an (early to?) middle Miocene platform growth phase. Within these two packages, platform development starts with a reefal unit, which is overlain by a sequence of shallow-water sediments with highly variable facies, ranging from high-energy grainstone to seagrass roots and blades and larger benthic foraminiferal assemblages. The lack of mud in the 200-m-thick middle Miocene succession suggests a high-energy environment of deposition throughout the sequence. This platform is capped by a 25-m-thick dolomitic interval. Overlying this dolomitic interval is a nondolomitized reef with evidence of exposure on its surface. The youngest phase of platform growth is represented by 125.5 m of dolomitic floatstone/mudstone.
Five lithologic units were defined at Site 1196 (here, depth references are only given for Hole 1196A). Sediments recovered in lithologic Unit I (0.0-182.2 mbsf; middle [to late?] Miocene [to Pliocene?] in age) were deposited in inner to middle neritic water depths. This unit was subdivided based on biotic assemblages. Subunits IA, IB, and ID are mostly dolomitic, whereas Subunit IC is predominantly composed of calcite. Subunit IA (0.0 to 117.2 mbsf) consists of dolomitic floatstone/rudstone characterized by the occurrence of centimeter-sized rhodoliths in a moderately sorted sandy matrix containing larger benthic foraminifers, coralline algae, mollusks, and rare bryozoan fragments. Both intergranular and moldic porosity occurs, and some pores are filled by a complex succession of cements and internal sediments. Lithologic Subunit IB (117.2 to 125.9 mbsf) is ~8 m thick and consists of a well-lithified dolomitic skeletal floatstone with a recrystallized grainstone matrix. The coarse (>2 mm) fraction is >30% of the rock volume and includes elongated fragments of branching coralline algae and centimeter-sized mollusk shells. Numerous molds of flat, larger benthic foraminifers also occur. The base of this subunit corresponds to a peak in NGR in both core and downhole logging records. Lithologic Subunit IC (125.9 to 162.8 mbsf) is composed of skeletal rudstone, floatstone, and boundstone rich in centimeter-sized hermatypic coral debris, mollusk shell fragments, and rhodoliths. Coralline algal and bryozoan clasts, broken echinoid spines, and larger benthic foraminifers occur also in the finer-grained matrix. Both intergranular and moldic porosity is frequent throughout this predominantly calcitic interval. Lithologic Subunit ID (162.8-182.2) is mostly a dolomitic floatstone with a grainstone matrix. The main constituents include elongated fragments of branching coralline algae, small (<1 cm) rhodoliths, mollusk shells, rare bryozoan detritus, and molds of larger benthic foraminifers. Overall sorting is poor; however, distinct laminae are present in some intervals.
The top of lithologic Unit II (182.2 to 345.8 mbsf; middle Miocene in age) is composed of floatstone/rudstone containing abundant molds of branching coral and rare fragments of branching coralline algae. This lithology corresponds to peaks in resistivity and velocity in the downhole logging data. Unit II can be divided into two subunits, both of which were deposited in shallow water (<10 m). Lithologic Subunit IIA (182.2 to 335.9 mbsf) consists of well-lithified, poorly sorted skeletal floatstone with a silt-sized grainstone matrix and common moldic porosity. The predominant constituents include porcellaneous small (miliolids) and larger benthic foraminifers, and centimeter-sized mollusk shells. Coral, bryozoans, and rare Halimeda debris also occur. Thread-like, dark patches up to 1 cm long are interpreted as preserved seagrass roots. The lower part of Subunit IIA contains thin horizons with brown micrite mottles. This interval also contains exceptionally high values of NGR, shallow and deep resistivity, and sonic velocity in the downhole logging data. Lithologic Subunit IIB (335.9-345.8 mbsf) is characterized by a thin unit of well-lithified, poorly sorted skeletal rudstone/boundstone. The coarse (>1 cm) fraction includes colonial hermatypic corals, red algae, and mollusk fragments in a micritic matrix. Lithophaga borings are common, as is moldic porosity. The boundary with underlying Unit III is marked by a change from pure calcite to a mixed calcite and dolomite mineralogy.
Lithologic Unit III (345.8 to 617.1 mbsf, [early to middle?] Miocene in age) consists of coarsely crystalline dolostone lacking primary textures. This unit is divided into four subunits on the basis of color, presence or absence of molds, and identification of primary rock components. Lithologic Subunit IIIA (345.8 to 355.3 mbsf) consists of a mottled, recrystallized dolomitic rudstone with a grainstone matrix. Ghosts of rhodoliths, branching and massive corals, and dissolved mollusk fragments were also identified. The majority of the sediments within this subunit were deposited in water depths <30 m. Lithologic Subunit IIIB (412.7 to 470.4 mbsf) contains a brilliant white sucrosic dolostone with rare relicts of centimeter-sized rhodoliths and branching coralline algae and millimeter-sized lenticular molds of larger benthic foraminifers. The recovered portion of this subunit is visually homogenous. Downhole logging data, however, shows several peaks in sonic velocity suggesting a more heterogeneous composition in the unrecovered horizons. The lower boundary of Subunit IIIB corresponds to a reddish interval at 470.4 mbsf.
Lithologic Subunit IIIC (470.4 to 537.7 mbsf) is a mottled, recrystallized, dense dolostone. Ghosts of rhodoliths, mollusk shells, and coralline algal fragments are visible. Intercrystalline, shelter, moldic, and possibly, fenestral porosity were observed. The lower portion of Subunit IIIC contains several millimeter- to centimeter-thick, dense, brown or red layers interpreted to be exposure surfaces. Lithologic Subunit IIID (537.7.6 to 617.1 mbsf) is a well-lithified dolostone, with larger benthic foraminifers. Other constituents include rhodoliths, coralline algae, and mollusk fragments. Most original texture has been lost.
Lithologic Unit IV (617.1 to 662.6 mbsf; early Miocene in age) includes various dolostone facies that occur in meter-thick sequences separated by iron-stained zones interpreted to be exposure surfaces. The lower limit of this unit occurs at 653 mbsf. The deepest core at Site 1196 contains lithologic Unit V, which consists of a dark, phosphate-rich sandstone and claystone representing the Oligocene substrate of the carbonate platform.
Nearly all core catcher samples from this site contained lithified platform facies and thus were not processed for nannofossil or planktonic foraminifers once a number of samples were shown to be barren. One thin section from a few centimeters below the seafloor contained age-diagnostic planktonic foraminifers that provided a maximum age of 3.2 Ma. Few planktonic and no outer neritic/upper bathyal foraminifers were recovered within the platform sediments. Calcareous nannofossils found within lithologic Subunit IIA provide an age range of 13.6 to 18.2 Ma. This age was further constrained between 183 and 327 mbsf by the occurrence of the benthic foraminifer Flosculinella botangensis, with an age range of 13.3 to 15.2 Ma. Nannofossils were also found in the last core directly below the carbonate-platform sequence, providing an age of 24.2 to 24.6 Ma.
Benthic foraminifers characteristic of subtropical carbonate-platform environments at inner to middle neritic depths (<100 m) are abundant throughout most of Site 1196. Within the platform sequence, several shallowing and deepening sequences occur, although highly dolomitized intervals and poor core recovery seriously limited resolution, particularly below 335 mbsf. The interval from 183 to 336 mbsf indicates a shallow (<10 m) depositional setting, probably on a seagrass-dominated open platform. A complex interval from 117 to 183 mbsf includes exposure surfaces that bracket a skeletal boundstone interpreted to be a reefal facies. The upper 117 m of Site 1196 are open platform sediments deposited at middle neritic depths, shallowing to inner neritic depths with possible carbonate beach sands just below the hiatus surface at the top of the platform. The discovery of the larger benthic foraminifer Lepidocyclina in limestone at the top of the MP2/MP3 platform appears to support recent evidence that this genus survived into the late Miocene in the western Pacific. Most interpretations from northern Australia concluded that the youngest unreworked occurrence of this genus was in the late middle Miocene.
At Site 1196, bulk density ranges between 1.9 and 2.75 g/cm3 and generally increases downhole. Bulk density shows a high degree of scatter throughout the drilled interval and no apparent relation exists between density variations and lithostratigraphic units. Grain-density values average 2.80 g/cm3 and range between 2.55 and 2.95 g/cm3. The downhole trend correlates well with dolomite abundance. Porosity mirrors bulk density and decreases with depth (51 to 2%). The highest porosity values are found in dolomite-rich intervals due to the abundant intercrystalline porosity. P-wave velocity varies between 2300 and 6600 m/s. Anisotropy of P-wave velocity ranges from -9% to 14%. Most of the anisotropy values (71%) have a positive sign, meaning that the velocity in the z-direction is lower than average velocity in the x- and y-directions. MS ranges from -8 to 5 x 10-6 SI. A diamagnetic interval from 20 to 105 mbsf correlates with Subunit IA. NGR ranges from 0 to 65 counts per second (cps). Maximum NGR values follow trends observed in the downhole logs.
Based on conventional downhole logging data and FMS images, four logging units can be distinguished. Each of these units can be interpreted as a platform growth phase, where logging Unit 1 (76-128 mbsf) appears to correlate with the upper Miocene MP3 platform, logging Unit 2 (128-163 mbsf) is a reef growth phase of undetermined age, and logging Units 3 and 4 (163-524 mbsf) middle and possibly early Miocene platform phases. Within the logging units, subunits are recognized that may be related to facies changes during the progressive development of these platforms.
Results from sporadic paleomagnetic measurements indicate that the sediments are relatively weakly magnetized with average intensities of 10-4 A/m increasing downcore. Magnetostratigraphic interpretation of the sedimentary section was not possible because of poor recovery. Rock magnetic analyses performed throughout the recovered section to 600 mbsf indicate that magnetite is the predominant mineral phase recording magnetization. Below 600 mbsf, however, both magnetite and hematite are present. The varying role of the magnetite and hematite in magnetization and in associated color changes may provide a record of oxidizing fluid migration in the lower portion of this platform.
Carbonate (CaCO3) content at Site 1196 ranges from ~0.5 wt% in the siliciclastic interval at the bottom of the hole to near 100 wt% in the carbonate-dominant intervals. TOC content is mostly close to zero except from ~180 to 260 mbsf and from ~250 mbsf to the bottom of the section, where values of 0.3 and >0.8 wt%, respectively, were measured. The upper organic carbon-containing interval corresponds to samples containing seagrass, whereas in the lower interval, HI values and C/N ratios are indicative of terrigenous organic matter inputs. These relative TOC enrichments coincide with decreased carbonate contents, particularly in the lower horizon. S content in Site 1196 sediments ranges from 0.0 to ~5.38 wt%, and its distribution is similar to that of TOC. No volatile gases were detected at Site 1196. The dolomitized and highly indurated lithologies, as well as the low core recovery, prevented pore water sampling at this site.Site 1199
The geologic setting and lithologic units recovered at Site 1199 were similar to those at 1196, although the thicknesses of the units are different in some intervals. The 419.5-m-thick stratigraphic succession retrieved from Site 1199 consists of diverse carbonate facies, reflecting variations of accommodation space during the middle, and probably late, Miocene. This succession was subdivided into three major units on the basis of carbonate mineralogy and nature of constituent particles. Unit I (0.0-159.8 mbsf; middle? to late? Miocene in age) was divided into four subunits. Subunit IA (0.0-106.6 mbsf) is a rhodolith-bearing, dolomitic floatstone deposited in a neritic, open-platform environment at water depths estimated between 30 and 80 m. Varying hydrodynamic energy during the deposition of this subunit, possibly due to storm action, is recorded by the presence of distinctive facies and crude coarse-tail grading in some intervals. Dense pink-colored zones in this subunit correspond either to exposure horizons or, more likely, to submarine hardgrounds.
Subunit IB (106.6-114.1 mbsf) consists of well-lithified, dolomitized skeletal floatstone with a recrystallized grainstone matrix rich in coralline algae fragments. These sediments were deposited in an open-platform setting, possibly at slightly deeper water depths than the overlying and underlying subunits.
Subunit IC (114.1-121.9 mbsf) is characterized by rudstone/boundstone textures with centimeter-sized hermatypic coral debris, branching coralline algae fragments, and larger benthic foraminifers representative of shallow reefal facies. Unlike the corresponding interval at Site 1196, this subunit is pervasively dolomitized. These sediments overlie, and are overlain, by reddish crusts interpreted as exposure surfaces. Widespread vuggy porosity and reddish silt in large vugs support this interpretation. Downhole logging data, as well as these diagenetic features, suggest that Subunit IC has been pervasively karstified. Subunit IC corresponds to low NGR, density, and resistivity values in both the core and downhole physical properties data.
Subunit ID (121.9-159.8 mbsf) was deposited in a low-energy open-platform setting. Sediments are composed of dolomitic floatstone with a grainstone matrix with elongated fragments of branching coralline algae, small (<1 cm) rhodoliths, mollusk shells, rare bryozoan detritus, larger benthic foraminifers (as molds) and rare coral debris.
Lithologic Unit II (159.8-410.0 mbsf; early? to middle Miocene in age) is divided into two subunits. Hardground horizons observed at the top of Unit II at Site 1196, were not seen at Site 1199. This suggests that the uppermost part of the sequence was not recovered or was of a different composition because of lateral facies variability. Lithologic Subunit IIA (159.8-285.0 mbsf) consists of a well-lithified, slightly dolomitized skeletal floatstone with a silt-sized grainstone matrix rich in porcellaneous benthic foraminifers (alveolinids), mollusk shells, and rare coral fragments. This subunit corresponds to an interval of relatively high NGR values in the downhole logs. Lithologic Subunit IIA was most likely deposited in an inner platform setting at very shallow (<30 m) water depths, although in contrast to its lateral equivalent position at Site 1196, no evidence of seagrass roots or blades was found.
Lithologic Subunit IIB (285.0-410.0 mbsf) is characterized by a highly porous, friable limestone comprised of two facies that form meter-scale alternations. The first facies consists of well-sorted, skeletal grainstones with leached grains. The second facies is slightly darker and includes centimeter-sized rhodoliths. The sediments of Subunit IIB were deposited in shallow water depths (<60 m).
The core catcher of the last core recovered at Site 1199 contains 10 cm of sucrosic dolostone similar to that found at the same depth in Hole 1196A. Because of the similarity with rocks at Site 1196, this interval has been classified as lithologic Unit III (410.0-419.5 mbsf; early to middle? Miocene in age), although it is possible that it represents a dolomitized interval in lithologic Subunit IIB.
No age constraints were obtained from nannofossils or planktonic foraminifers. Benthic foraminifers characteristic of shallow-water carbonate platform environments are abundant throughout most of the sequence recovered at Site 1199. The larger benthic foraminifer assemblages indicate a Miocene age throughout, and distinctive benthic foraminifers in several samples from 170.3 to 265.9 mbsf indicate an age of 13.3-15.2 Ma.
Paleowater depths during deposition of the platform were consistently within the euphotic zone (0-150 m), as indicated by the dominance of red algae and larger benthic foraminifers. Shapes and sizes of the larger benthic foraminifers in lithologic Unit I indicate depths consistently <100 m and probably much shallower near the top of the platform. Miliolid foraminifers are abundant throughout lithologic Subunit IIA, whereas the larger porcellaneous taxa (alveolinids and soritids) were only observed occasionally. Paleowater depths are conservatively interpreted as <30 m throughout lithologic Subunit IIA. They were likely much shallower, including exposure surfaces and aeolian episodes. Lithologic Subunit IIB is dominated by larger foraminifer-rich grainstone and floatstone with varying proportions of red algae. The abundance, robustness, and sorting of Lepidocyclina indicate paleowater depths of <60 m and possibly shallower. At Site 1199, bulk density ranges between 1.4 and 2.7 g/cm3 showing high scatter throughout the entire interval with no apparent relation between density and the lithostratigraphic units. Grain density averages 2.79 g/cm3 and is nearly constant throughout the drilled interval. Porosity values range from 3% to 48% with values near ~30% at the top of the hole, decreasing to 3% at 90-110 mbsf and increasing to 20% at 170 mbsf. Below 170 mbsf, porosity values are scattered between 14% and 48%. Velocity mirrors the porosity profile and varies between 3000 and 5500 m/s with values near 4000 m/s at the top of the hole, increasing to 5500 m/s between 100 and 110 mbsf and decreasing to 3950 m/s at 170 mbsf. MS ranges from -2 to 20 x10-6 SI with little variability except for a spike to 22 x 10-6 SI at 141 mbsf. NGR values increase between 0 and 40 mbsf (~10 to ~24 cps) and between 40 and 114 mbsf (~10 to ~83 cps). A peak in NGR is visible at 170 mbsf.
The average sediment magnetization at Site 1199 is 10-3.5 A/m ranging between 10-4.5 to 10-2.5 A/m. An intensity increase of two orders of magnitude occurs at ~103 mbsf at the Subunit IA/IB boundary. The predominant magnetic mineral carrying remanence is magnetite.
Downhole logging data from Hole 1199A provide important information on the lithology and architecture of the MP3/MP2 platform, especially in the lower part of the site, where core recovery was low. Two karst holes are inferred from the logs, each ~10 m thick, at 118-129.2 and 155-162 mbsf, respectively. The logs show significant differences between this site and Site 1196. However, as with Site 1196. the logging data can again be subdivided into four logging units related to the growth stages of the MP2/MP3 platform.
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