Ichnology and Sedimentology of the Lower Permian Mackellar Formation at Turnabout Ridge and Buckley Island, Beardmore Glacier, Central Transantarctic Mountains (CTAM), Antarctica: A Shallow Deltaic Marine Environment

Jackson, Adam; Hasiotis, Steve; Flaig, Peter; Isbell, John

The Mackellar Formation (MF), deposited at 80-85° S paleolatitude, has been interpreted as an inland glaciolacustrine sea dominated by freshwater, and now as a shallow deltaic marine environment. Trace fossils with morphologies characteristic of marine organisms, and sedimentary packages indicative of submarine fan-channel complexes (lower MF) to deltaic deposition (upper MF) were found during the 2010-2011 Central Transantarctic Mountain field season at Turnabout Ridge (TR) and Buckley Island (BI), Beardmore Glacier area (BGA). In the BGA the MF is located between the earlier Lower Permian Pagoda Formation and later Lower Permian Fairchild Formation.

The MF in the BGA predominantly consists of two sedimentary packages- heterolithic lithofacies and sandstones with mudstone and siltstone interbeds. Each sedimentary package contains an associated ichnocoenoses. Within each sedimentary package, the ichnocoenoses are characteristic of epi- and endobenthic worms, and arthropods with a vast majority of the component traces being diminutive in size (i.e., diameter). Along with the diminutive size of changes, the tiering depth to which the burrows extend to is quite thin (< 2 cm). The apparent reduction in trace fossil morphology from "normal," and the shallow tiering depth are characteristic of benthic organisms in a stressed environment.

Over 20 ichnogenera occur in siltstone- and sandstone-dominated lithofacies, ripple, climbing ripple, planar, and trough cross stratification: Arenicolites, Bergaueria, Circulichnus, Cochlichnus, Conichnus, Cruziana, Diplichnites, Gordia, Haplotichnus, Kouphichnium, Lockeia, Palaeophycus, Phycodes, Planolites, Rusophycus, Sagitichnus, Scolicia, Skolithos, Taenidium, Teichichnus, and Undichna. The greatest diversity of traces occur in BI sandstones, often between mudstone and siltstone interbeds: Arenicolites, Bergaueria, Chondrites, Circulichnus, Conichnus, Cruziana, Diplichnites, Gordia, Haplotichnus, Kouphichnium, Lockeia, Palaeophycus, Phycodes, Planolites, Rusophycus, Sagitichnus, Taenidium, and Teichichnus. In the Mackellar at TR, traces from barforms include abundant surface and shallow subsurface traces, particularly Arenicolites, Cruziana, Gordia, Kouphichnium, Lockeia, and Rusophycus. Ichnological and sedimentological 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.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013