Abstract:
The New Mountain Sandstone forms the basal 300 m of the Beacon Supergroup (Devonian and older (?) to Triassic) in the McMurdo Sound region. Detailed field work has shown that a three part division of the formation into the Windy Gully sandstone Member, Terra Cotta Siltstone Member and New Mountain Sandstone proper, is consistent over a distance of about 100 km, from Mt Kempe in the south to Mt Jason in the north. It is therefore proposed to replace the present New Mountain Sandstone with the New Mountain Subgroup, consisting of the Windy Gully Sandstone (~40 m thick), resting nonconformably on basement rocks, the Terra Cotta Siltstone (~50 m thick), and the New Mountain Sandstone (~200 m thick), in a new restricted sense, at the top. The New Mountain Sandstone (restricted usage) includes two members, the Plane Table Member and the Boreas Subgreywacke Member.
The New Mountain Subgroup rests nonconformably on the Kukri Peneplain, which cuts Precambrian and Lower Paleozoic basement rocks. The Heimdall Erosion Surface separates the New Mountain Subgroup from overlying formations of the Taylor Group. Age control on the subgroup is poor but an age between Early Silurian and Early Devonian can be deduced from radiometric dates of basement rocks and fossil evidence from formations in the upper part of the Taylor Group.
Sedimentological evidence indicates that the Windy Gully Sandstone and New Mountain Sandstone were deposited by braided rivers, the Terra Cotta Siltstone and Plane Table member in lakes and the Boreas Subgreywacke Member by debris flows. Some aeolian deposition is recorded in the upper part of the New Mountain Sandstone.
A sequence of vertical facies changes in the New Mountain Sandstone begins with overbank deposits at the base of the formation that grade up into the middle part dominated by channel deposition which in turn grades into overbank deposits in the upper part. The stratigraphic relationship of these facies changes suggests a regional channel migration.
Analysis of cross-bedding directions shows that the mean flow direction in river channels throughout deposition of the New Mountain Subgroup was westerly to northwesterly. Tectonic stability of the area during deposition of the New Mountain Sandstone is indicated by stable, unimodal flow patterns at three localities and by the consistent lithologic character of channel sediments. Aeolian cross-bedding indicates a dominant southeasterly paleowind direction.
