Turbidite sedimentology and history at DSDP Site 89-585

At Site 585 in the East Mariana Basin, a 900-m section of Aptian-Albian to Recent sediments was recovered. The upper 590 m are pelagic components (carbonate, siliceous, and clay); small-scale graded sequences and laminations are common. The underlying sediments are volcaniclastic sandstones with a large proportion of shallow-water carbonate debris; sedimentary structures including complete Bouma sequences, cross-laminae, and scouring are common. These structures indicate that the entire section was deposited by turbidity currents. The change in lithology upward in the section reflects the evolution of the surrounding seamounts, from their growth stages during the middle of the Cretaceous to the later subsidence phases. Several black layers containing pyritized organic debris and associated turbidite structures were cored near the Cenomanian/Turonian boundary; this material has been transported from the flanks of the seamounts where it was deposited within a shallow anoxic zone. Seismic data extends the stratigraphy across the entire Basin, showing the reflectors onlapping the seamounts, and indicating at least 1200 m of sediment at Site 585. The crust is placed at 6900 m after correcting for sediment loading, and the subsidence curve indicates that the Basin has been deeper than 5500 m since before the Aptian.

Benthic foraminifera of cores from ODP Leg 117

The relative abundances of benthic foraminifers from the Oman margin have been analyzed from ODP Sites 725 and 726 near the upper boundary of the oxygen-minimum zone (OMZ) and 728 near the lower boundary. The relative abundance pattern of the benthic foraminiferal species in the two shallow sites show synchronous changes, which, together with variations in the faunal composition, may be attributed to changes in the location of the upper boundary of the OMZ during the last 7 million years. At the deeper site, the relative abundance pattern shows considerable variation in the faunal composition during the last 8 million years. The strong dominance of the shallow-water species Ammonia beccarii during the early Pliocene at Site 728 suggests a water depth less than 400 m during the early Pliocene and subsequent subsidence during the middle and late Pliocene to the present > 1400 m water depth.

Stable isotope record of northeastern Australian Margin

Stable isotopic data obtained from planktonic and benthic foraminifers were used to study paleoceanographic changes along the northeastern Australian margin from late Miocene (10 Ma) to Holocene time, and to evaluate the influence of these changes on reef growth. The data indicate that variations in surface-water temperatures may have had an important effect on the reef complexes on the Queensland Plateau and possibly off the northeastern Australian margin. Three sites were studied: Leg 21, Site 209 on the eastern edge of the Queensland Plateau, and Leg 133, Site 811 on the western margin, and Site 817 on the lower southern slope of the plateau. Shallow-water bioclasts recovered from Holes 811A and 817A indicate extensive reef growth on the Queensland Plateau during the middle Miocene (before 12 Ma), signifying surface-water temperatures of 20°C or greater. The amount of reefal detritus produced during the late Miocene (10.0-5.2 Ma) decreased progressively, resulting in a reduction in area of the reef complexes. The isotopic data from planktonic foraminifers in these late Miocene age sediments indicate the presence of relatively cool surface waters (16°-19°C), which may have been a major factor contributing to the demise of the reefs on the Queensland Plateau. Surface waters remained cool until the middle Pleistocene (1.2-0.5 Ma), when the surface-water temperature apparently increased to approximately 25°C, recorded both in the isotopic data and by renewed reef growth. This increase occurred simultaneously (within the error of the age model) with the initiation of the Great Barrier Reef. We propose that cooling of surface waters during the early late Miocene contributed to reef decline on the Queensland Plateau, and that subsequent warming of surface waters during the middle Pleistocene promoted the initiation of reef growth on the northeastern Australian margin. Reef development on the Queensland Plateau never recovered to the middle Miocene extent because of a combination of tectonic (accelerated subsidence of the plateau) and paleoceanographic (the cooler surface waters present from the late Miocene throughout the Pliocene) factors. Variations in seafloor d18O appear to be controlled by regional factors, as indicated by the similarity of data from Sites 811 and 817 to those from Site 590 on Lord Howe Rise.

Cenozoic planktonic foraminifera of the Falkland Plateau and Argentine Basin

Sites 511 and 512 (Falkland Plateau) and 513 (Argentine Basin) penetrated calcareous-siliceous oozes of the middle and upper Eocene and lower Oligocene with rather numerous planktonic foraminifers. Upper Oligocene, Miocene, Pliocene, and Quaternary sections are composed mostly of siliceous sediments (Sites 511-514) where planktonic foraminifers are rare or absent. High-latitude planktonic foraminifers of the Austral Province are characterized by impoverished assemblages - only representatives of Globigerina, Globigerinita, Globorotaloides, and Globorotalia with a rounded peripheral margin are found. In the Paleogene, these species are supplemented, in lesser amounts, by representatives of Globigerapsis, Acarinina, Pseudogloboquadrina, Pseudohastigerina, and Chiloguembelina. Assemblages of planktonic foraminifers have low stratigraphic resolution, especially in the upper Oligocene-Quaternary. This reflects the generally deteriorating Cenozoic climate, which evinced a sharp change in the upper Oligocene that is connected with initiation of the circum-Antarctic current near the Paleogene/Neogene boundary. Comparison of Paleogene and Neogene planktonic foraminifers of the South Atlantic (Falkland Plateau, Argentine Basin, 46-51°S) and the North Atlantic (Rockall Plateau, 55-56°N) indicates that the South Atlantic climate was much colder than that of the same latitudes of the North Atlantic. Paleogene oozes of the Falkland Plateau rest unconf ormably on Maestrichtian sediments and in their turn are overlain unconformably by Neogene-Quaternary oozes. Cenozoic sections are stratigraphically discontinuous: periods of intensive biogenic sedimentation resulting in a thick succession of sediments alternated with periods of nondeposition and strong erosion that resulted in hiatuses and unconformities. In the Argentine Basin, Oligocene calcareous-siliceous oozes rest on basalts of the oceanic basement; they are replaced upward in the section by Neogene-Quaternary siliceous oozes with some hiatuses. Planktonic foraminifers here clearly demonstrate the processes of oceanic subsidence and CCD fluctuations as well as Polar Front migrations during Cenozoic time. Fifty species of planktonic foraminifers are discussed and illustrated.

Preliminary results and documentation of sediment cores CRP2 and CRP-2A from the Ross Sea off Cape Roberts, Antarctica

The site for CRP-2, 14 km east of Cape Roberts (77.006°S; 163.719°E), was selected to overlap the early Miocene strata cored in nearby CRP-1, and to sample deeper into the east-dipping strata near the western margin ofe he Victoria Land Basin to investigate Palaeogene climatic and tectonic history. CRP-2 was cored from 5 to 57 mbsf (metres below the sea floor) (core recovery 91 %), with a deviation resulting in CRP-2A being cored at the same site. CRP-2A reached down to 624mbsf (recovery 95%), and to strata with an age of c. 33-35 Ma. Drilling took place from 16 October to 25 November 1998, on 2.0-2.2 m of sea ice and through 178 m of water. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were run from 63 to 167 mbsf and subsequently from 200 to 623 mbsf, although density and velocity data could be obtained only to 440 mbsf because of hole collapse. Sonic velocity averages c. 2.0 km S-1 for the upper part of the hole, but there is an sharp increase to c. 3.0 km s-1 and also a slight angular unconformity, at 306 mbsf, corresponding most likely to the early/late Oligocene boundary (c. 28-30 Ma). Velocity then increases irregularly to around 3.6 km s-1 at the bottom of the hole, which is estimated to lie 120 m above the V4/V5 boundary. The higher velocities below 306 mbsf probably reflect more extensive carbonate and common pyrite cementation, in patches, nodules, bedding-parallel masses and as vein infills. Dip of the strata also increases down-hole from 3° in the upper 300 in to over 10° at the bottom. Temperature gradient is 21° k-1. Over 2 000 fractures were logged through the hole. Borehole televiewer imagery was obtained for the interval from 200 to 440 mbsf to orient the fractures for stress field analysis. Lithostratigraphical descriptions on a scale of 1:20 are presented for the full length of the core, along with core box images, as a 200 page supplement to this issue. The hole initially passed through a layer of muddy gravel to 5.5 mbsf (Lithological Sub-Unit or LSU 1.1), and then into a Quaternary diatom-bearing clast-rich diamicton to 21 mbsf (LSU 2. l), with an interval of alternating compact diamicton and loose sand, and containing a rich Pliocene foraminiferal fauna, to 27 mbsf (LSU 2.2). The unit beneath this (LSU 3.1) has similar physical properties (sonic velocity, porosity, magnetic susceptibility) and includes diamictites of similar character to those of LSU 2.1 and 2.2, but an early Miocene (c. 19 Ma) diatom assemblage at 28 mbsf (top of LSU 3.1) shows that this sub-unit is part of the older section. The strata beneath 27 mbsf, primary target for the project, extend from early Miocene to perhaps latest Eocene age, and are largely cyclic glacimarine nearshore to offshore sediments. They are described as 41 lithological sub-units and interpreted in terms of 12 recurrent lithofacies. These are 1) mudstone, 2) inter-stratified mudstone and sandstone, 3) muddy very fine to coarse sandstone, 4) well-sorted stratified fine sandstone, 5) moderately to well-sorted, medium-grained sandstone, 6) stratified diamictite, 7) massive diamictite, 8) rhythmically inter-stratified sandstone and mudstone, 9) clast-supported conglomerate, 10) matrix-supported conglomerate, 11) mudstone breccia and 12) volcaniclastic sediment. Sequence stratigraphical analysis has identified 22 unconformity-bounded depositional sequences in pre- Pliocene strata. They typically comprise a four-part architecture involving, in ascending order, 1) a sharp-based coarse-grained unit (Facies 6,7,9 or 10), 2) a fining-upward succession of sandstones (Facies 3 and 4), 3) a mudstone interval (Facies l), in some cases coarsening upward to muddy sandstones (Facies 3), and 4) a sharp-based sandstone dominated succession (mainly Facies 4). The cyclicity recorded by the strata is interpreted in terms of a glacier ice margin retreating and advancing from land to the west, and of rises and falls in sea level. Analysis of sequence periodicity awaits afirmer chronology. However, apreliminary spectral analysis of magnetic susceptibility for a deepwater mudstone within one of the sequences (from 339 to 347 mbsf) reveals ratios between hierarchical levels that are similar to those of the three Milankovitch orbital forcing periodicities. The strata contain a wide range of fossils, the most abundant being marine diatoms. These commonly form up to 5% of the sediment, though in places the core is barren (notably between 300 and 412 mbsf). Fifty samples out of 250 reviewed were studied in detail. The assemblages define ten biostratigraphical zones, some of them based on local or as yet undescribed forms. The assemblages are neritic, and largely planktonic, suggesting that the sea floor was mostly below the photic zone throughout deposition of the corcd sequence. Calcareous nannofossils, representing incursions of ocean surface waters, are much less common (72 out of 183 samples examined) and restricted to mudstone intervals a few tens of metres thick, but are important for dating. Foraminifera are also sparse (73 out of 135 samples) and represented only by calcareous benthic species. Changing assemblages indicate a shift from inshore environments in the early Oligocenc to outer shelf in the late Oligocenc, returning to inshore in the early Miocene. Marine palynomorplis yielded large numbers of well-preserved forms from most of the 116 samples examined. The new in situ assemblagc found last year in CRP-1 is extended down into the late Oligocene and a further new assemblage is found in the early Oligoccnc. Many taxa are new, and cannot us yet contribute to an improved understanding of chronology or ecology. Marine invertebrate macrofossils, mostly molluscs and serpulid tubes, are scattered throughout the core. Preservation is good in mudstones but poor in other lithologies. Climate on land is reflected in the content of terrestrial palynomorphs, which are extremely scarce down to c. 300 mbsf. Some forms are reworked, and others represent a low growing sparse tundra with at least one species of Nothofagus. Beneath this level, a significantly greater diversity and abundance suggests a milder climate and a low diversity woody vegetation in the early Oligocene, but still far short of the richness found in known Eocene strata of the region. Sedimentary facies in the oldest strata also suggest a milder climate in the oldest strata cored, with indications of substantial glacial melt-water discharges, but are typical of a coldcr climate in late Oligocene and early Miocene times. Clast analyses from diamictites reveal weak to random fabrics, suggesting either lack of ice-contact deposition or post-depositional modification, but periods when ice grounded at the drill site are inferred from thin zones of in-situ brecciated rock and soft-sediment folding. These are more common above c. 300 mbsf, perhaps reflecting more extensive glacial advances during deposition of those strata. Erosion of the adjacent Transantarctic Mountains through Jurassic basalt and dolerite-intruded Beacon strata into basement rocks beneath is recorded by petrographical studies of clast and sand grain assemblages. Core below 310 mbsf contains a dominance of fine-grained Jurassic dolerite and basalt fragments along with Beacon-derived coal debris and rounded quartz grains, whereas the strata above this level have a much higher proportion of basement derived granitoids, implying that the large areas of the adjacent mountains had been eroded to basement by the end of the early Oligocene. There is little indication of rift-related volcanism below 310 mbsf. Above this, however, basaltic and trachytic tephras are common, especially from 280 to 200 mbsf, from 150 to 46 mbsf, and in Pliocene LSU 2.2 from 21 to 27 mbsf. The largest volcanic eruptions generated layers of coarse (up to 1 cm) trachytic pumice lapilli between 97 and 114 mbsf. The thickest of these (1.2 m at 112 mbsf) may have produced an eruptive column extending tens of km into the stratosphere. A source within a few tens of km of the drill site is considered most likely. Present age estimates for the pre-Pliocene sequence are based mainly on biostratigraphy (using mainly marine diatoms and to a lesser extent calcareous nannofossils), with the age of the tephra from 112 to 114 mbsf (21.44k0.05 Ma from 84 crystals by Ar-Ar) as a key reference point. Although there are varied and well-preserved microfossil assemblages through most of the sequence (notably of diatoms and marine palynomorphs), they comprise largely taxa either known only locally or as yet undescribed. In addition, sequence stratigraphical analysis and features in the core itself indicate numerous disconformities. The present estimate from diatom assemblages is that the interval from 27 to 130 mbsf is early Miocene in age (c. 19 to 23.5 Ma), consistent with the Ar-Ar age from 112 to 114 mbsf. Diatom assemblages also indicate that the late Oligocene epoch extends from c. 130 to 307 mbsf, which is supported by late Oligocene nannofossils from 130 to 185 mbsf. Strata from 307 to 412 mbsf have no age-diagnostic assemblages, but below this early Oligocene diatoms and nannofossils have been recovered. A nannoflora at the bottom of the hole is consistent with an earliest Oligocene or latest Eocene age. Magnetostratigraphical studies based on about 1000 samples, 700 of which have so far undergone demagnetisation treatment, have provided a polarity stratigraphy of 12 pre-Pliocene magnetozones. Samples above 270 mbsf are of consistently high quality. Below this, magnetic behaviour is more variable. A preliminary age-depth plot using the Magnetic Polarity Time Scale (MPTS) and constrained by biostratigraphical data suggests that episodes of relatively rapid sedimentation took place at CRP-2 during Oligocene times (c. 100 m/My), but that more than half of the record was lost in a few major and many minor disconformities. Age estimates from Sr isotopes in shell debris and further tephra dating are expected to lead to a better comparison with the MPTS. CRP-2/2A has recorded a history of subsidence of the Victoria Land Basin margin that is similar to that found in CIROS-170 km to the south, reflecting stability in both basin and the adjacent mountains in late Cenozoic times, but with slow net accumulation in the middle Cenozoic. The climatic indicators from both drill holes show a similar correspondence, indicating polar conditions for the Quaternary but with sub-polar conditions in the early Miocene-late Oligocene and indications of warmer conditions still in the early Oligocene. Correlation between the CRP-2A core and seismic records shows that seismic units V3 and V4, both widespread in the Victoria Land Basin, represent a period of fluctuating ice margins and glacimarine sedimentation. The next drill hole, CRP-3, is expected to core deep into V5 and extend this record of climate and tectonics still further back in time.

Sedimentology of the Vietnam shelf

On the Vietnam Shelf more than 1000 miles of shallow high-resolution seismics were analyzed to unravel post-glacial evolution in a tropical, siliciclastic environment together with 25 sediment cores from water depths between 21 and 169 m to determine stratigraphy, distribution and style of sedimentation. Fourty-seven samples were dated with the AMS-14C technique. The shelf was grouped into three regions: a southern part, a central part, and a northern part. On the broad Southern Shelf, sedimentation is influenced by the Mekong River, which drains into the SCS in this area. Here, incised valley fills are abundant that were cut into the late Pleistocene land surface by the Paleo-Mekong River during times of sea level lowstand. Those valleys are filled with transgressive deposits. The Holocene sedimentation rate in this low gradient accommodation-dominated depositional system is in the range of 5-10 and 25-40 cm/ky at locations sheltered from currents. The Central Shelf is narrow and the sedimentary strata are conformable. Here, numerous small mountainous rivers reach the SCS and transport large amounts of detrital sediment onto the shelf. Therefore, the Holocene sedimentation rate is high with values of 50-100 cm/ky in this supply-dominated depositional system. The broad Northern Shelf in the vicinity of the Red River Delta shows, as on the Southern Shelf, incised valleys cut into the Pleistocene land surface by paleo river channels. In this accommodation-dominated shelf area, the sedimentation rate is low with values of 5-10 cm/ky. Where applicable, we assigned the sampled deposits to different paleo-facies. The latter are related to certain intervals of water depths at their time of deposition. Comparison with the sea-level curve of (Hanebuth et al., 2000, doi:10.1126/science.288.5468.1033) indicates subsidence on the Central Shelf, which is in agreement with the high sedimentation rates in this area. In contrast, data from the Northern Shelf suggest tectonic uplift that might be related to recent tectonic movements along the Ailao Shan-Red River Fault zone. Data from the Southern Shelf are generally in agreement with the sea-level curve mentioned above.

Stable isotope composition of carbonates in the Tyrrhenian Sea

The Messinian evaporitic succession recovered at ODP Sites 652, 653, and 654 in the Tyrrhenian Sea was generated under various environmental conditions which ranged from brackish to hypersaline, as deduced from the sedimentary facies and stable isotope compositions of the carbonate and sulfate deposits. Water in the basins had to be shallow to undergo such rapid and large geochemical variations. The marine influence was omnipresent in the basin at least during the deposition of sulfate evaporites; seawater or marine brines might have been supplied either by direct input into evaporitic lagoons as at Sites 653 and 654, or by subterraneous infiltration in marginal areas as at Site 652. Episodes of severe dilution by continental waters occurred frequently throughout Messinian times in the more basinal areas at Sites 653 and 654, while a fresh water body was standing permanently at Site 652. The high heat flow present at Site 652 was responsible for a major late authigenesis of iron-rich dolomites, which was initiated during the subsidence of the basin and ended before Pliocene.

Geochemistry at DSDP Leg 74 Holes

Clay mineralogical and inorganic geochemical data from the Campanian to the Pleistocene provide information bearing on the evolution of both continental and marine paleoenvironments in the Walvis Ridge area. (1) Alteration processes of basalts occurred under subaerial conditions during the Campanian and Maestrichtian and were virtually absent in deeper marine environments. (2) Strong tectonic effects were present during the Campanian and persisted until the early Eocene. (3) Subsidence of this part of the Walvis Ridge became important during the late Maestrichtian and continued into the Paleocene and Eocene. (4) The influence of global climatic cooling was evident from the late Eocene on. (5) Modification of oceanic circulation and the increasing influence of surface and deep water masses on the sedimentation characterized the Cenozoic.

Geochemistry of felsic rocks and minerals of ODP Hole 135-841B

A felsic volcanic series (605-825 mbsf) overlain by upper Eocene shallow-water sediments (500-605 mbsf) and basalticandesitic sills that intruded into sediments of Holocene to Miocene age (0-500 mbsf) was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The volcanic sequence at Site 841 includes altered and mineralized calc-alkaline rhyolites and dacites, dacitic tuffs, lapilli tuffs, flow breccias, and welded tuffs. These rocks formed subaerially or in a very shallow-water environment suffering a subsidence of >5000 m since Eocene times. Calculations of gains and losses of the major components during alteration show most pronounced changes in the uppermost 70 m of the volcanic sequence. Here, Al, Fe, Mg, and K are enriched, whereas Si and Na are strongly depleted. Illite, vermiculite, chlorite, and hematite predominate in this part of the hole. Throughout the section, quartz, plagioclase, kaolinite, and calcite are present. Sulfide mineralization (up to 10 vol%) consisting mainly of disseminated pyrite (with minor pyrrhotite inclusions) and marcasite together with minor amounts of chalcopyrite is pervasive throughout. Locally, a few sulfide-bearing quartz-carbonate veins as well as Ti-amphibole replacement by rutile and then by pyrite were observed. Strong variations in the As content of sulfides (from 0 to 0.69 wt%) from the same depth interval and local enrichments of Co, Ni, and Cu in pyrite are interpreted to result from fluctuations in fluid composition. Calculations of oxygen and sulfur fugacities indicate that fO2 and fS2 were high at the top and lower at the bottom of the sequence. Sulfur isotope determinations on separated pyrite grains from two samples give d34S values of +6.4ë and +8.4ë, which are close to those reported from Kuroko and Okinawa Trough massive sulfide deposits and calc-alkaline volcanic rocks of the Japanese Ryukyu Island Arc. Calculated chlorite formation temperatures of 265°-290°C at the top of the sequence are consistent with minimum formation temperatures of fluid inclusions in secondary quartz, revealing a narrow range of 270°-297°C. Chlorite formation temperatures are constant downhole and do not exceed 300°C. The presence of marcasite and 4C-type pyrrhotite indicates a formation temperature of <= 250°C. At a later stage, illite was formed at the top of the volcanic series at temperatures well below 200°C.

Chemical composition, characteristics and age of glauconites at DSDP Leg 66 Holes

During the analysis of "glaucony" recovered during Leg 66, off Mexico, we reviewed the data on previously studied glaucony layers in active margin areas. We found the depth of Leg 66 glaucony sediments to be significantly greater than conventionally assumed appropriate to their genesis (100-500 m). Accordingly, we hypothesize their occurrence at unusual depth to be due to (1) transport of shallow sediments and redeposition at greater depths, (2) margin subsidence, or (3) genesis at greater depth than is generally assumed. For the area off Mexico, we reject (1). (2) has already been verified in Japan and is possible as an explanation for the present phenomenon without excluding (3), which we investigate in this chapter.

Late Pliocene carbonate mineralogy of ODP Hole 133-818B

Shedding of shallow carbonate material toward the deep slopes and basin floors is clearly tied to the position of the carbonate bank tops relative to the photic zone. The onset of bank shedding in periplatform sediments can record either the flooding of the bank tops within the photic zone during a rise in sea level following a period of exposure, referred to in the literature as the "highstand shedding" scenario, or the reentry of the bank tops into the photic zone during a lowering of sea level following a period of drowning, referred to as the "lowstand shedding" scenario. Results from Leg 133 post-cruise research on the Pliocene sequences, drilled in six sites within different slope settings of the Queensland Plateau, seem to point out that the latter "lowstand shedding" scenario can be applied to this particular carbonate system. At the Queensland Plateau sites, the early Pliocene (5.2-3.5 Ma) and the earliest part of the late Pliocene (3.5-2.9 Ma) age sequences were characterized, especially in the ôdeepö Sites 811 and 817, by pelagic sediments (foraminifers and coccoliths) and by typically pelagic sedimentation rates not exceeding 20 mm/k.y. The earliest part of the late Pliocene age section was characterized by well-developed hardgrounds in the "shallow" Sites 812 and 814 and by normal pelagic sediments mixed with reworked phosphatized planktonic foraminifers in Site 813. Finally, the early part of the late Pliocene (2.9-2.4 Ma) section was characterized by high sedimentation rates, related to the shedding and admixture into the pelagic sediments of bank-derived materials. These bank-derived materials consist of either diagenetically unaltered fine aragonite with traces of dolomite in Site 818 or micritic calcite resulting from seafloor and/or shallow burial alteration in the deepest Sites 817 and 811. The highest sedimentation rates (163 mm/k.y.) were recorded in Site 818, drilled nearest the modern carbonate bank of Tregrosse Reef. The sedimentation rates decrease with increasing distance from Tregrosse Reef - 120 mm/k.y. in Site 817 and 47.5 mm/k.y. in Site 811. The initial appearance of fine aragonite in Site 818, corresponding to the transition from pelagic to periplatform sedimentation rates, has been dated at 2.9 Ma. This Pliocene sediment pattern on the Queensland Plateau is different from the pattern observed in sediments from two earlier ODP legs (i.e., Leg 101 in the Bahamas and in Leg 115 in the Maldives), where aragonite-rich sediments, characterized by high periplatform sedimentation rates, were observed in the lower Pliocene section (5.2-3.5 Ma), whereas the upper Pliocene (3.5-1.6 Ma) sediments are more pelagic in nature and are characterized by low sedimentation rates or major hiatuses. These Pliocene periplatform sequences in the Bahamas and in the Maldives and late Quaternary age periplatform sequences worldwide have pointed out that "highstand shedding" was the typical response of carbonate platforms to fluctuations in sea level, just opposite to a "lowstand shedding" response to sea-level fluctuations, typical of siliciclastic shelves. Assuming that the envelope of Haq et al.'s (1987) sea-level curve, showing a well-defined lowering of sea level between 3.5 and 2.9 Ma, can also be applied to the southwest Pacific Ocean, based on a high-resolution Pliocene d18O record from the Ontong Java Plateau recently published by Jansen et al. (1993, doi:10.2973/odp.proc.sr.130.028.1993), the Pliocene periplatform sequences on the Queensland Plateau would have recorded the reentry of the bank tops into the photic zone during a general lowering of sea level, following an interval characterized by high sea level, during which the shallow carbonate system on the Queensland Plateau was drowned. The early Pliocene age (5.2-3.5 Ma) sediments deposited on the Queensland Plateau, an established interval of eustatic sea-level highstand, are typically pelagic in character. In addition, relatively cold surface temperatures (estimated to have ranged from 18° to 20°C by Isern et al. [this volume]) might have also stressed the reefs during early Pliocene time and contributed to the drowning of the Queensland Plateau carbonate system during the late Miocene and early Pliocene. Differential and relatively high subsidence rates, inferred by variations in paleodepth of water (based upon benthic foraminifer assemblages; Katz and Miller, this volume) may also have influenced the drowning of the carbonate bank tops on the Queensland Plateau during the late Miocene and early Pliocene. The sediments of early late Pliocene age (2.9-2.4 Ma), a well-established interval of lowering of sea level, are clearly periplatform and cyclic in nature. High-frequency (~40 k.y.) aragonite cycles, well-developed between 2.9 and 2.45 Ma, correlate with the planktonic high-resolution Pliocene d18O record from the Ontong Java Plateau, a good sea-level proxy (Jansen et al., in press). Contrary to late Quaternary age aragonite cycles from the Bahamas, the Nicaragua Rise, the Maldives, and the Queensland Plateau, the late Pliocene aragonite cycles in Hole 818B display high levels of aragonite during glacial stages and, therefore, lowstands of sea level. In addition, sediments deposited during the earliest part of the late Pliocene (3.5-2.9 Ma), transition between the early Pliocene highstand and the late Pliocene lowering in sea level, have recorded the first evidence of a fall in sea level, by (1) the occurrence of synchronous submarine hardgrounds in the two shallowest sites (Sites 812 and 814), (2) the deposition of reworked material from the shallower part of the slope into the intermediate Sites 813 and 818, and (3) the deposition of pelagic sediments in the deepest Sites 817 and 817. In summary, contrary to previous findings, the Pliocene periplatform sediments on the Queensland Plateau appear to have recorded a regional shedding of shallow carbonate bank tops during an interval of sea-level lowering, a good illustration of the "carbonate lowstand shedding" scenario, occurring during the reentry of previously drowned carbonate bank tops into the photic zone related to a decrease in sea level.

Middle Eocene to early Oligocene planktonic and benthic foraminifers from ODP Hole 125-786A

Drilling at Site 786, located in the center of the Izu-Bonin forearc basin, penetrated an apparently continuous section of middle Eocene/lower Oligocene volcaniclastic breccias and nannofossil oozes. Planktonic foraminiferal faunas underwent a gradual transition from relatively high-diversity middle Eocene through late Eocene tropical or warm-water assemblages to a cooler-water, less diverse assemblage during the early Oligocene. In the cosmopolitan benthic foraminiferal faunas, the major transition occurred during the early late Eocene. Middle Eocene benthic assemblages resembling the bathyal 'Lenticulina' fauna (characterized by Osangularia mexicana, Cibicidoides eocaenus, and several buliminid species) changed to an upper Eocene abyssal 'Globocassidulina subglobosa' fauna (characterized by Cibicidoides praemundulus, Globocassidulina subglobosa, Gyroidinoides girardanus, Oridorsalis umbonatus, and Siphonodosaria aculeata). Even though no large, abrupt faunal changes appear to have been associated with the assumed Eocene/Oligocene boundary, benthic species turnover continued through the late Eocene and into the early Oligocene. This resulted in a slightly lower diversity early Oligocene fauna dominated by three species: Laevidentalina sp., Bulimina jarvisi, and Gyroidinoides girardanus. The progression from a middle Eocene bathyal 'Lenticulina' fauna, rather than an abyssal 'Nuttallides truempyi' fauna, to an abyssal 'Globocassidulina subglobosa' fauna during the early late Eocene, suggests that a bathymetric deepening occurred at Site 786. Increased water depths may have resulted from tectonic subsidence.

Physiography of the Ampère Seamount in the Horseshoe Seamount chain off Gibraltar

The Ampère Seamount, 600 km west of Gibraltar, is one of nine inactive volcanoes along a bent chain, the so called Horseshoe Seamounts. All of them ascend from an abyssal plain of 4000 to 4800 m depth up to a few hundred meters below the sea surface, except two, which nearly reach the surface: the Ampère massif on the southern flank of the group and the summit of the Gorringe bank in the north. The horseshoe, serrated like a crown, opens towards Gibraltar and stands in the way of its outflow. These seamounts are part of the Azores-Gibraltar structure, which marks the boundary between two major tectonic plates: the Eurasian and the African plate. The submarine volcanism which formed the Horseshoe Seamounts belongs to the sea floor spread area of the Mid-Atlantic Ridge. The maximum activity was between 17 and 10 Million years ago and terminated thereafter. The volcanoes consist of basalts and tuffs. Most of their flanks and the abyssal plain around are covered by sediments of micro-organic origin. These sediments, in particular their partial absence on the upper flanks are a circumstantial proof and a kind of diary of the initial rise and subsequent subsidence of about 6oo m of these seamounts. The horizons of erosion where the basalt substrate is laid bare indicate the rise above sea level in the past. Since the Ampère summit is 60 m deep today, this volcano must have been an island 500 m high. The stratification of the sediments covering the surrounding abyssal plain reveals discrete events of downslope suspension flows, called turbidites, separated by tens of thousands of years and perhaps induced by changes in climate conditions. The Ampère sea mount of 4800 m height and a base diameter of 50 km exceeds the size of the Mont Blanc massif. Its southern and eastern flanks are steep with basalts cropping out, in parts with nearly vertical walls of some hundred meters. The west and north sides consist of terraces and plateaus covered with sediments at 140 m, 400 m, 2000 m, and 3500 m. The Horseshoe Seamount area is also remarkable as a kind of disturbed crossing of three major oceanic flow systems at different depths and directions with forced upwelling and partial mixing of the water masses. Most prominent is the Mediterranean Outflow Water (MOW) with its higher temperature and salinity between 900 to 1500 m depth. It enters the horseshoe unimpaired from the open eastern side but penetrates the seamount chain through its valleys on the west, thereafter diverging and crossing the entire Atlantic Ocean. Below the MOW is the North Atlantic Deep Water (NADW) between 2000 m to 3000 m depth flowing southward and finally there is the Antarctic Bottom Water (AABW) flowing northward below the two other systems.

Age determination of the rhyolites of the Lord Howe Rise in the Southwest Pacific Ocean

Drilling at site 207 (DSDP Leg 21), located on the broad summit of the Lord Howe Rise, bottomed in rhyolitic rocks. Sanidine concentrates from four samples of the rhyolite were dated by the 40Ar/39Ar total fusion method and conventional K-Ar method, and yielded concordant ages of 93.7 +/- 1.1 my, equivalent to the early part of the Upper Cretaceous. At this time the Lord Howe Rise, which has continental-type structure, is thought to have been emergent and adjacent to the eastern margin of the Australian-antarctic continent. Subsequent to 94 my ago and prior to deposition of Maastrichtian (70-65 myBP) marine sediments on top of the rhyolitic basement of the Lord Howe Rise, rifting occurred and the formation of the Tasman Basin began by sea-floor spreading with rotation of the Rise away from the margin of Australia. Subsidence of the Rise continued until Early Eocene (about 50 myBP), probably marking the end of sea-floor spreading in the Tasman Basin. These large scale movements relate to the breakup of this part of Gondwanaland in the Upper Cretaceous.