The Lower Permian Mackellar Formation is well exposed in a 10,000 km 2 outcrop belt in the Nimrod, Beardmore, and Shackleton Glacier areas of the Transantarctic Mountains. This formation directly overlies glacial deposits and provides a unique glimpse of high paleolatitude conditions during the last icehouse to greenhouse transition. The unit records deposition in the Mackellar Lake or Inland Sea (MLIS), a fresh-water body at ~80° S paleolatitude that was broadly analogous to Glacial Lake Agassiz and was filled by fine-grained turbidites. Low total organic carbon (TOC) content and predominant vitrinite and inertinite are consistent with a low influx of organic matter from a sparsely vegetated, recently deglaciated terrain. A widespread but low-diversity ichnofauna and variable (although low overall) levels of bioturbation suggest oxic conditions and a bottom fauna restricted to areas of low sedimentation. Integration of sedimentologic, organic geochemical, and paleobiologic information with results of climate models and characteristics of modern lakes enhances reconstruction of parameters that controlled the functioning of the lake as an ecosystem. Regression equations relating mean annual temperature and mean depth of modern lakes to the number of ice-free days applied to the MLIS indicate ice cover from two to five months a year. Estimates of the depth of mixing and depth to the thermocline, based on maximum length, maximum width, and area, suggest a mixing depth of ~50 m and a thermocline of ~20 m. The MLIS probably was stratified during the summer and was dimictic, with overturns occurring after fall cooling and after ice melt; mixing was enhanced by turbidity currents. Productivity was low, as recorded by the low TOC, but organic matter fixed in the surface water of the lake may have been degraded and not recorded in the sediments. In spite of its high paleolatitude, the MLIS as reconstructed was dynamic and biologically active; the same probably was true of other Permian postglacial lakes.

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The Permian-Triassic Transantarctic basin, which occupied the Panthalassan margin of the East Antarctic craton, including the present Transantarctic and Ellsworth Mountains, evolved above a mid-Paleozoic passive continental margin basement through the following stages: (1) Carboniferous/Permian extension, (2) late Early Permian back-arc basin, (3) Late Permian and Triassic foreland basin, and (4) Jurassic extension and tholeiitic volcanism. A mid-Paleozoic (Devonian) wedge of coastal-to-shallow marine quartzose sandstone developed on the eroded roots of the Late Cambrian-Early Ordovician Ross orogen. A lacuna in East Antarctica during the Carboniferous was followed by the inception of Gondwanan deposition in a wide Carboniferous/Permian extensional basin. Volcanic detritus at the base of the late Early Permian post-glacial marine(?) shale and sandstone sequence in the Ellsworth Mountains is the first sign of a volcanic arc and subduction along the Panthalassan margin. A similar but much thinner non-volcaniclastic sequence accumulated in the Transantarctic Mountains. The introduction of abundant volcanic detritus to the cratonic side of the basin and a 180° paleocurrent reversal in the Late Permian in the Beardmore Glacier area are the earliest indicators of tectonism along the outer margin of the basin and the inception of a foreland basin that accumulated thick Late Permian and Triassic braided stream deposits of mixed volcanic and cratonic provenance. The Permian sequences in the Ellsworth and Pensacola Mountains were folded in the Triassic. The foreland basin was succeeded in the Early Jurassic by extension and initial silicic and then tholeiitic volcanism that led to the breakup of Gondwanaland.

Abstract Marked differences in the morphology of modem gastropod trails can be related to sediment consistency. In uncompacted sediment of tidal channels, tidal channel banks, and low tidal flats in Barnstable Harbor, Massachusetts, the gastropod Polinices duplicates produces deep V-shaped grooves as the animal moves several centimeters below the sediment surface. Bilobed, transversely ridged trails are formed in firmer substrate of the high tidal flat where P. duplicates moves nearer the surface. Results of laboratory experiments on P. duplicates moving through various sediment mixtures suggest that grain size and substrate consistency are two controlling factors of trail morphology. Trails, presumably formed by snails, occur in Pennsylvanian sandstones of tidal origin in Tennessee. Substrate characteristics of the rocks were determined qualitatively by examination of sedimentary features. Comparison of trail morphology with syn-depositional sediment mass properties suggests a close relationship between substrate consistency and trail morphology. In sandstones likely deposited on higher parts of a tidal flat, single trails change from (1) V-shaped furrows to (2) bilobed, transversely ridged trails to (3) longitudinal rows of tiny knobs. These morphologies apparently resulted from the snail moving at decreasing depth in a uniformly firm substrate. In sandstones representing looser sand of lower tidal flat origin the snails produced V-shaped and lobed trails that lack prominent transverse ridges. Recognition that different trace morphologies can be produced by the same organism is useful for paleoecological interpretation for at least three reasons. (1) The variety of trails produced may be a clue to syn-depositional sediment mass properties. (2) Variety (or degree of variation) of gastropod trail morphology may help identify the tidal flat environment of deposition. (3) Recognition of variation will result in a truer picture of trace producer diversity and development of more accurate compilations of ichnogeneric paleoenvironmental distributions.