No abstract

The Paleogene glacial history of Antarctica has been inferred largely from indirect evidence of glaciation gathered from the oceans beyond that remote, ice-shrouded, and inhospitable continent. This evidence includes the "proxy" stable isotopic record from the world's oceans; the occurrence of ice-rafted debris (IRD) in the Southern Ocean; inflections in sea-level curves; the presence of hiatuses in the deep-sea record; and changes in clay mineral assemblages, in the diversities of microfossil assemblages, and in the steepness of latitudinal biotic gradients. Ocean Drilling Program Leg 120 has added an important dimension to this growing body of evidence through the discovery of lowest Oligocene IRD at Site 748 on the Southern Kerguelen Plateau at a record distance north of the Antarctic continent (58°S latitude) and within a pelagic biosiliceous-carbonate ooze sequence that has yielded a complementary oxygen isotope record of the cryospheric event. We deduce that an ice sheet reached sea level during the earliest Oligocene (35.8-36.0 Ma) and that the effect was immediate and profound. In addition to the IRD, this event was manifested at Site 748 by a dramatic cooling of the surface waters surrounding the continent as indicated by a sharp increase in the percentage of cold-water calcareous nannoplankton, an increase in planktonic foraminiferal δ 18 θ values, and an increase in the percentage of biosiliceous material in the sediment. The temperature of the bottom waters over the plateau also decreased, and the volume of ice on the continent increased.We have reviewed the accumulated evidence from this and other investigations of the past two decades on and around Antarctica, and have concluded that ice sheets were present on the continent during the Oligocene, including the earliest Oligocene. Not necessarily permanent features, they probably went through many advances and contractions and may have disappeared completely at times. The rapid ice advance during the earliest Oligocene reached sea level at several widely scattered localities around the continent, but only for a brief period of time at some of these localities. Differences of opinion among investigators concern primarily the size and persistence of the Oligocene ice sheet(s).Tantalizing evidence of undated till deposits subjacent to well-dated Oligocene glaciomarine sequences along the Antarctic margin and possible occurrences of IRD beyond suggest the presence of glaciers during the late Eocene and possibly even during the early-middle Eocene. Included among the studies in this Scientific Results volume are new age calibrations from Eltanin piston cores taken at relatively low latitudes (38° to 58°S) in the southeast Pacific from which Eocene IRD has been reported. There is little agreement, however, among investigators as to whether an ice sheet was present at any time during the Eocene, particularly during early-middle Eocene times.

Appreciable lower Oligocene clastic detritus interpreted to be ice-rafted debris (IRD) was recovered at Ocean Drilling Program (ODP) Site 748 on the Central Kerguelen Plateau in the southern Indian Ocean. Site 748 is located in the western part of the Raggatt Basin, east of Banzare Bank at 58°26.45'S, 78°58.89'E (water depth = 1250 m). The physiologic and tectonic setting of the site and the coarse size of the material rule out transport of the elastics by turbidity currents, nepheloid layers, or wind.The IRD occurs between 115.45 and 115.77 mbsf within a stratum of siliceous nannofossil ooze in an Oligocene sequence otherwise composed exclusively of nannofossil ooze with foraminifers and siliceous debris. Glauconite and fish skeletal debris (ichthyolith fragments) occur in association with the IRD. According to planktonic foraminifer, diatom, and nannofossil biostratigraphy and magnetostratigraphy, the IRD interval is earliest Oligocene in age (35.8-36.0 Ma). The sedimentation rate throughout this interval was rather low (approximately 6.3 m/m.y.). The IRD consists of predominately fine to coarse sand composed of quartz, altered feldspars, and mica. A large portion of the quartz and feldspar grains is highly angular, and fresh conchoidal fractures on the quartz grains are characteristic of glacially derived material. Scanning electron microscope and energy-dispersive X-ray studies plus light microscope observations document the presence of a heavy-mineral suite characteristic of metamorphic or plutonic source rocks rather than that derived from the devitrification of a volcanic ash. Benthic foraminifer δ 18 θ values across this interval show a marked enrichment.This direct physical evidence of lower Oligocene IRD this far north of the Antarctic continent (the lowest latitudinal occurrence known) and the association of the IRD with the globally recognized shift in δ 18 θ argue strongly for the presence of an earliest Oligocene ice sheet on the Antarctic continent. This corroborates other recent drilling evidence of coeval glacial activity along the Antarctic margin (ODP Legs 113 and 119) and in the Ross Sea (CIROS-1), as well as previous indirect evidence based on stable isotope studies from the world's oceans.The co-occurrence of the IRD, biosiliceous-rich nannofossil ooze, fish skeletal debris, and glauconite probably denotes changes in patterns and/or intensity of oceanic circulation associated with a profound climatic cooling. These changes apparently enhanced upwelling and surface-water productivity and the development of a trophic structure that supported abundant vertebrate marine life, perhaps an important step in the evolution of vertebrate marine faunas.

Five hundred meters of a unique Upper Cretaceous Cr-rich glauconitic sequence (Unit III) that overlies a 3-m-thick alkali-basalt flow with underlying epiclastic volcanogenic sediments was drilled at ODP Leg 120 Site 748. The Cr-rich glauconitic sequence is lithostratigraphically and biostratigraphically divided into three subunits (IIIA, IIIB, IIIC) that can also be recognized by the Cr concentration of the bulk sediment, which is low (<200 ppm) in Subunits IIIC and IIIA and high (400-800 ppm) in Subunit IIIB. The Cr enrichment is caused by Cr-spinel, which is the only significant heavy mineral component beside Fe-Ti ores. Other Cr-bearing components are glauconite pellets and possibly some other clay minerals.The glauconitic sequence of Subunit IIIB was formed by reworking of glauconite and volcanogenic components that were transported restricted distances and redeposited downslope by mass-transportation processes. The site of formation was a nearshore, shallow inner shelf environment, and final deposition may have been on the outer part of a narrow shelf, at the slope toward the restricted, probably synsedimentary, faulted Raggatt Basin.The volcanic edifices uncovered on land were tholeiitic basalts (T-MORB), alkali-basaltic (OIB) and (?)silicic volcanic complexes, and ultramafic rocks. The latter were the ultimate source for the Cr-spinel contribution. Terrestrial aqueous solutions carried Fe, K, Cr, Si, and probably Al into the marine environment, where, depending on the redox conditions of microenvironments in the sediment, green (Fe-and K-rich) or brown (Al-rich) glauconite pellets formed.The Upper Cretaceous glauconitic sequence at Site 748 on the Southern Kerguelen Plateau constitutes the transition in space and time from terrestrial to marine, from magmatically active subaerial to magmatically passive submarine conditions, and from a tranquil platform to active rifting conditions.

During the drilling of Hole 603B on Deep Sea Drilling Project Leg 93, an unexpected series of sand-, silt-, and claystone turbidites was encountered from Cores 603B-45 through -76 (1224-1512 m sub-bottom depth). Complete and truncated Bouma sequences were observed, some indicating deposition by debris flows. Sand emplacement culminated with the deposition of a 30-m-thick, unconsolidated sand unit (Cores 603B-48 through -45).The purpose of this preliminary study is to determine the nature of the heavy mineral suites of this sediment in order to make tentative correlations with onshore equivalents. The heavy mineralogy of Lower Cretaceous North American mid-Atlantic coastal plain sediment has been extensively studied. This sediment is classified as the Potomac Group, which has a varied heavy mineral suite in its lower part (Patuxent Formation), and a limited suite in its upper part (Patapsco Formation). The results of this study reveal a similar trend in the heavy mineral suites of sediment in Hole 6038. Hauterivian through lower Barremian sediment has a heavy mineral suite that is dominated by zircon, apatite, and garnet, with minor amounts of staurolite and kyanite. Beginning in the mid-Barremian, a new source of sediment becomes dominant, one which supplies an epidote-rich heavy mineral suite. The results of the textural analyses show that average grain size of the light mineral fraction increases upsection, whereas sorting decreases. The epidote-rich source may have delivered sediment with a slightly coarser mean grain size. This sediment may represent a more direct continental input at times of maximum turbidite activity (mid-Barremian) and during deposition of the upper, unconsolidated sand unit.