PRELIMINARY REPORT OF TRACE FOSSILS AND SEDIMENTOLOGY INDICATING A SHALLOW MARINE DELTAIC ENVIRONMENT FOR THE LOWER PERMIAN MACKELLAR FORMATION AT TURNABOUT RIDGE AND BUCKLEY ISLAND, BEARDMORE GLACIER, CENTRAL TRANSANTARCTIC MOUNTAINS (CTAM), ANTARCTICA

The Mackellar Formation (MF), deposited at 80–85° S paleolatitude, has been interpreted as an inland sea dominated by freshwater, and more recently as a glaciolacustrine environment. The rare occurrence of the trace fossils Cochlichnus, Planolites, and Treptichnus were used, in part, to support these paleoenvironmental interpretations. New research conducted during the 2010–2011 CTAM field season at Turnabout Ridge (TR) and Buckley Island (BI), Beardmore Glacier area, has found trace fossils with morphologies characteristic of marine organisms, and sedimentary packages indicative of submarine fan-channel complexes (lower MF) to deltaic deposition (upper MF).

The MF at TR and BI overlies the Lower Permian Pagoda Formation and is overlain by the Lower Permian Fairchild Formation. The MF is composed of coarsening and thickening upward sedimentary packages of heterolithic lithofacies that become sand-dominated up section. Heterolithic packages typically consist of interbedded mudstone and siltstone capped by planar to ripple laminated fine-grained sandstone; some units are convolute bedded. These lithofacies are overlain by compensatorily stacked and lenticular-shaped sandstones with mudstone and siltstone interbeds interpreted as submarine fan-channel complexes, prodelta and delta front deposits.

At least 20 ichnogenera are found in the siltstone- and sandstone-dominated lithofacies: Arenicolites, Bergaueria, Circulichnus, Cochlichnus, Conichnus, Cruziana, Diplichnites, Gordia, Haplotichnus, Kouphichnium, Lockeia, Palaeophycus, Phycodes, Planolites, Rusophycus, Sagitichnus, Scolicia, Skolithos, Taenidium, Teichichnus, and Undichna. Many of these trace fossil morphologies are characteristic of epi- and endobenthic worms, arthropods, and echinoids. The vast majority of these trace fossils are diminutive in size (i.e., diameter) compared to typical examples of these ichnogenera. Tiering depth is very shallow (< 2 cm). Diminutive trace fossil sizes and tiering depths are characteristic of benthic organisms in a stressed environment.

Sedimentological and ichnological characteristics of the MF indicate a basinal to marine deltaic environment with short-lived communities composed of small-bodied organisms influenced by freshwater input and high sedimentation rates.