Textural Characteristics of Cenozoic Preglacial and Glacial Sediments at Site 270, Ross Sea, Antarctica

Abstract

The Deep Sea Drilling Projects Site 270 was located to pass through the oldest part of a planed-off gently dipping sedimentary sequence in the south central Ross Sea, and successfully penetrated calc-silicate gneiss basement at 412 meters subbottom. The upper 20 meters (Unit 1) is largely sand-silt-clay of Gauss age, with pebbles scattered throughout. It is capped with a 20-cm-thick veneer of diatom silty clay of Bruhnes age. Unit 2 comprises 364 meters of silty claystone with sparsely scattered pebbles, 16-25 m.y. in age on the basis of magnetic reversal stratigraphy. Units 3 and 4 are moderately sorted sandstones totalling 2 meters in thickness, and are in sharp contact with the glacial sediments above and a fossil soil developed on 26 meters of sedimentary breccia beneath. The breccia rests on basement gneiss. The older carbonaceous sandstone (Unit 4) is fine-grained and moderately sorted and is believed to be an estuarine deposit. The younger glauconitic sandstone is similar in texture and is a shallow marine sediment. The contact with the overlying glacial sequence is sharp, but a K-Ar age of 26 m.y. on the glauconite indicates that there is no significant time break. The texture of the sediments of Units 1 and 2 is very similar to that of continental tills from Ohio, showing that the glacial marine sediments at Site 270 have been eroded and transported by ice with little or no modification to their size distribution. The size distribution in some samples, however, shows a slight bimodality, with broad modes in the fine sand-coarse silt and in the clay grades. Samples of this type are associated with faintly stratified parts of the sequence, and it is believed that both bimodality and the stratification are the result of size sorting due to low intensity traction or bottom currents. Therefore, the several alternations of unstratified and faintly stratified lithological units in Unit 2 are interpreted as changes in current regime, though the cause of the changes is not known. The consistently poor sorting shows that the sea floor was never above wave base (60-70 m) throughout the deposition of Units 1 and 2. Long-term changes in pebble abundance and parallel variations in percent of coarse sand seem independent of bedding features, and are attributed to erosional processes in the source region. Data from Sites 270, 271, and 272 show that sedimentation was more or less continuous and relatively rapid (40 m/m.y.) from 25 to about 5 m.y. ago, when erosion truncated the sequence. Since then there has been only one interval, the Gauss magnetic epoch, with a similar sedimentation rate. The high sedimentation rate is believed to indicate a time of dominantly wet-base glaciation in Antarctica, with alpine glaciers calving directly into the Ross Embayment, whereas periods of erosion and very slow sedimentation, as in early Pliocene and late Quaternary times, are regarded as the result of dominantly dry-base glaciation on the continent with an extensive ice shelf in the Ross Embayment.

Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

“…Low seismic velocities indicate that the till layer and the underlying sequence are poorly lithified. The sedimentary sequence beneath the ice stream may represent a southeastward extension of the Ross Sea basin, where a c. 20-m-thick layer of Plio-Pleistocene tills (Ross Sea Tills) overlies a several-hundred-meterthick Late Tertiary glacimarine sequence Anderson et al 1984;Barrett 1975a;Kellogg et al 1979;Rooney et al 1991). Regional gravimetric and magnetic studies also suggest that the Ross Sea basin extends beneath part of the WAIS (Jankowski and Drewry 1981;Savage and Ciesielski 1983).…”

“…Deep drilling in the Ross Sea (Fig. 1) shows Miocene glacimarine pebbly muds and diatomaceous muds lying unconformably beneath widespread Plio-Pleistocene diamictons, generally believed to have been deposited by grounded ice Anderson et al 1984;Barrett 1975a;Kellogg et al 1979;Savage and Ciesielski 1983). We hypothesize that a similar stratigraphy exists beneath Ice Stream B at the UpB camp.…”

Late Wisconsin Reconstruction for the Ross Sea Region, Antarctica

“…Indeed analysis of RIGGS d ata (Bentley and others, in press) points to such geological influences on the gravity field of the R oss Ice Shelf and indicates that the gravity resul ts cann ot be unambig u o usly interpreted in terms of recent large-scale ice loading. Substantial vertical movements during the mid to late Cenozoic are already documented (Barrett, 1975 ).…”

Late Mesozoic-Cenozoic geology of the Ross Sector, Antarctica (extended abstract)

“…is documented at present. At Site 270, in the central Ross Sea, approximately 29 m of breccia (?Oligocene), greensand and carbonaceous sandstone is preserved above a Palaeozoic crystalline basement (Ford and Barrett, 1975;Barrett, 1975a). This succession was deposited during a late Oligocene marine transgression.…”

“…Currents within the Transantarctic Strait moved debris-laden icebergs from the Ross Sea periphery to open-sea melt zones in the central and northern Ross Sea. On the basis of grain-size analyses Barrett (1975a) and Webb (1979b) concluded that marine sediments derive from glacial debris eroded from continental ice and were deposited with little modification during ice-rafting and subsequent marine deposition. Clast compositions and 87Sr/86Sr ratios of feldspars point to a Mesozoic source somewhere between the Transantarctic Mountains and Marie Byrd Land.…”