Lower Permian Antarctic Marine Deltas of the Mackellar Formation: Turnabout Ridge, Beardmore Glacier Region, Central Transantarctic Mountains, Antarctica
Flaig, Peter P.; Hasiotis, Steve; Jackson, Adam; Isbell, John
Recent investigations into the facies and architecture of the Mackellar Formation (MF) in outcrops at Turnabout Ridge in the Beardmore Glacier Region of the Central Transantarctic Mountains indicate that sediments were likely deposited by deltas that prograded into a marine environment. The MF, deposited at 80-85° S paleolatitude, has previously been interpreted as deposits of a freshwater-dominated inland sea or glaciolacustrine deposits. Our collaborative study combining sedimentology with detailed ichnology instead reveals facies, architectures, and trace fossils indicative of marine submarine fan-channel complexes (lower MF) and prodelta and/or delta-front deposits (upper MF).
The MF overlies glacigenic deposits of the Lower Permian Pagoda Formation and is overlain by fluvial-deltaic environments of the Lower Permian Fairchild Formation. Facies of the MF at Turnabout Ridge include shale, wave-to current-ripple cross-laminated and planar-laminated siltstone to fine-grained sandstone, and trough cross-laminated fine-to medium-grained sandstone. Convolute bedding is pervasive. Siltstone and sandstone commonly exhibit one or more of the following traces: Arenicolites, Bergaueria, Circulichnus, Cochlichnus, Conichnus, Cruziana, Diplichnites, Gordia, Haplotichnus, Kouphichnium, Lockeia, Palaeophycus, Phycodes, Planolites, Rusophycus, Sagitichnus, Scolicia, Skolithos, Taenidium, Teichichnus, and Undichna. Trace fossil morphologies in the MF reflect marine epi- and endobenthic worms, arthropods, and echinoids. Most trace fossils are diminutive and exhibit a very shallow tiering depth which is indicative of a stressed environment.
Overall, the MF coarsens-upward, becoming sand dominated as it approaches the contact with the FF. Observed stratal geometries include meter-thick heterolithic interbeds of sandstone-siltstone and mudstone, coarsening-upward successions of wave-to-current-ripple cross-laminated siltstone and sandstone, and compensationally- stacked, erosionally- based lenticular sandstones capped by mudstone-siltstone interbeds. Facies, architectures, and stratal stacking combined with an environmentally- stressed trace fossil assemblage indicate that sediments at Turnabout Ridge were most likely related to progradation of marine deltas and not deposited in a freshwater-dominated inland sea or large lake.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013