TRACE FOSSILS AND PALEOENVIRONMENTS OF THE DEVONIAN NEW MOUNTAIN SANDSTONE, ANTARCTICA: MARINE OR NON-MARINE?

The lower Taylor Group in South Victoria Land consists of quartzose sandstones and intervening siltstones. Long-standing debate on the marine or non-marine nature of the unit has focused on the presence of varied and abundant trace fossils.

Sedimentological analyses of the New Mountain Sandstone at Table Mountain (225 m thick), revealed 3 major stratigraphic subunits, based on lithofacies and to a lesser extent trace fossils. Deposition occurred primarily in an eolian environment, with subordinate fluvial deposition in the lower and upper parts of the formation. The lower subunit was deposited in a low-relief eolian environment, which contained small barchan dunes and was episodically flooded by braided rivers. The middle subunit contains transverse eolian dune (crossbeds up to 8 m thick) and interdune (massive sandstones, ripple marks, and thin mudstones with desiccation cracks) deposits. Massive beds are sediment gravity-flow deposits derived from large dunes and emplaced in intermittent interdune ponds. Although the upper subunit is heavily bioturbated, structures suggest mixed eolian and fluvial deposition.

Three ichnoassemblages were recognized at Table Mountain: Heimdallia (lower), Diplichnites (middle), Skolithos (upper). Trace fossils are not limited to associated facies subunits or ichnoassemblages. As examples, Diplichnites is found within sediments of all ichnoassemblages, whereas the middle and upper subunits both contain Skolithos. Heimdallia is found mostly in sandy interdune deposits, and vanishes with the appearance of the thin mudstone beds of the middle unit. Trace fossils are relatively rare in the middle unit, but the transition between the middle and upper assemblages is complicated. It includes Cylindricum, Beaconites, additional broad trackways and many examples of a new kind of trace fossil, an excavation ‘pit and pile’ structure.

Trace fossil evidence strongly supports a non-marine environment: 1) both the lower and middle subunits contain trace fossils considered unique to non-marine environments; and 2) although some trace fossils in the upper subunit (Skolithos) by themselves do not support a particular environment, their association with non-marine trace fossils in the lower and middle subunits support a non-marine environment for the entire New Mountain Sandstone.