MASSIVE SANDSTONES, DUNE DEGRADATION AND THE RECORD OF CLIMATE CHANGE IN DEVONIAN EOLIAN SANDSTONE OF ANTARCTICA

Massive sandstone bodies are located throughout the middle to upper part of the eolian deposits of the Devonian New Mountain Sandstone, Antarctica. There are two types of massive beds: 1) meter-scale, channel-based units within large cross-bed sets, and 2) lenticular- to tabular-shaped beds within interdune deposits. The presence of steep-sided to undercut margins, indistinct low-angle stratification, and rare angular blocks of wind-ripple strata, suggest that channel-based massive beds were deposited in sediment-laden, hyperconcentrated flows. Interdune sequences dominated by massive-tabular beds are 3 to 12 m thick. Individual tabular beds are typically 20-30 cm thick with thin mudstone caps that commonly contain either desiccation cracks or flame structures. Tabular beds extend laterally for 100's of meters before interfingering with large-scale cross beds. Tabular beds are interpreted as gravity-flow deposits that were generated on the slopes of large dunes by slumps and hyperconcentrated flows (channel massive beds), and emplaced in shallow, ephemeral interdune ponds.

Potential scenarios for the generation of massive sandstone bodies within eolian settings include 1) marine inundation, 2) fluvial interaction, or 3) intense rainfall events. The nature and architecture of the massive sandstones are consistent with deposition during periods of extreme rainfall. Analogies to modern eolian systems suggest that the massive sandstone bodies of the New Mountain Sandstone were deposited as alluvial fans on the flanks of large dunes during prolonged periods of dune degradation caused by wet climate. Furthermore, the stratigraphic arrangement of massive sandstone units in the New Mountain sequence indicate that at least four alternating wet- and dry-climatic episodes occurred during the deposition of about 50 m of sandstone, with an overall upward trend towards more humid climate.