DIAMICT OF PROBABLE GLACIAL ORIGIN AT THE EOCENE-OLIGOCENE BOUNDARY ON SEYMOUR ISLAND, ANTARCTIC PENINSULA

There is a growing consensus that the onset of significant ice growth on Antarctica began at or near the Eocene-Oligocene boundary, but direct evidence for glaciation at this time has been limited and often controversial. Sediments at the top of the Eocene section on Seymour Island, Antarctic Peninsula, include pebbly mudstones, inferred dropstones, and a diamict. These sediments may record the first pulse of significant Cenozoic glaciation in the peninsular region.

Stratified, fossiliferous, cross-bedded, marine shelf sands of the La Meseta Fm are overlain by ~12 m of thin-bedded, dark brown-gray, compact silty mudstone containing frequent rounded dropstones several cm in diameter. This unit is truncated obliquely by a very dense, compact diamict ~1m or more thick. The diamict is entirely unsorted, unstratified, and cobbles and pebbles are mostly rounded and occasionally striated and/or teardrop shaped. Rare dinoflagellates within the diamict are all reworked Cretaceous and Paleogene species. The unit is abruptly overlain by more pebbly mudstone with dropstones, and the section is truncated by the erosional surface at the top of the island. Barnacles encrust the upper surfaces of cobbles at the top of the diamict, indicating that the unit was emplaced in or rapidly returned to a marine environment. The unit is exposed for at least 50 m along the outcrop face, but bad weather precluded further exploration of its extent.

Age constraints are difficult to place on the diamict, but collaborative work is ongoing. Calcareous taxa are absent in the muds. Strontium isotope ratios reported from bivalves in the underlying marine sands give ages ~35 Ma, but ⁸⁷Sr/⁸⁶Sr on barnacle calcite yielded mid-Miocene ages, suggesting alteration or freshwater influx. Dinoflagellates preserved in mud below the diamict suggest a late Eocene age, while those above indicate basal Oligocene, hence the unit appears to lie at or about the Eocene-Oligocene boundary.

If this diamict is in fact a subglacial till, it represents the first expansion of ice to sea level in the Cenozoic of the Peninsula. Significant ice in the northern peninsula at this time may indicate that the E-O pulse of glaciation was even more extensive than previously thought.