Trace Fossil and Lithofacies Associations, Lower Triassic Fremouw Formation, Beardmore Glacier, Central Transantarctic Mountains, Antarctica

Hasiotis, Stephen T.; Flaig, Peter P.

Ichnological investigations of the Lower Triassic Fremouw Formation at Wahl Glacier, Wyckoff Glacier, Lamping Peak, Coalsack Bluff, and Graphite Peak in the Beardmore Glacier Area, Central Transantarctic Mountains, Antarctica, exhibit trace fossil-lithofacies associations that have been preserved in other Pangean fluvial strata deposited at low and mid-latitude settings. These associations are used to build ichnocoenoses that can be used to reconstruct environments of deposition (EOD) in outcrop and core. Fluvial and overbank deposits of the Lower Triassic Fremouw Formation were deposited at ~65-70 degrees S paleolatitude; fluvial deposits are represented amalgamated, medium- to coarse-grained, channel-form sandstone bodies, whereas overbank deposits are composed of poorly sorted alluvium composed of mostly sand and silt with some mud that are pedogenically modified. Channel-form sandstones are composed mostly of planar tabular crossbeds that in some cases contain dense accumulations of the vertical burrow Maconopsis or the horizontal burrow cf. Aulichnites, but not both types of traces. The former represents long-term, subaerially exposed side attached and midchannel bars, while the latter represents shallow water table conditions in subaerially exposed abandoned channels. Ripple-laminated sandstones contain the association of Arenicolites, Fuersichnus, Haplotichnus, and Planolites, or primitive archosaur, derived archosaur, and therapsid surface tracks with or without shallow, thin, and high-density rhizoliths. The former represents shallow-water, low energy environments with deposit- and suspension-feeding insect nymphs and locomotion trails of insect larvae, whereas the latter represents subaerially exposed channels traversed by small reptiles and therapisds. Thin-bedded, planar laminated fine-grained, interbedded sandstones and siltstones in lenticular scours contain Cochlichnus, Diplichnites, and Kouphichnium. This trace fossil association represents abandoned channels where the sediment-water-air interface recorded small annelid trails and horseshoe crab and millipede and/or centipede trackways. Poorly sorted heterolithic intervals contain rare to common rhizoliths and one of four size classes of subhorizontal elliptical burrows (diameters = ~2, 5-7, 10-15, and 50-60 cm) that contain a median furrow. This association represents vegetated proximal floodplain environments with therapsid burrows; higher burrow densities reflect longer-term stability.

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