A LARGE-SCALE TECTONICALLY-DRIVEN SLIDE IN THE PERMIAN MACKELLAR FORMATION, VICTORIA GROUP, ANTARCTICA

The Antarctic Gondwana sequence, comprising the Devonian Taylor Group and the Permian-Triassic Victoria Group, is well exposed throughout the Transantarctic Mountains. In the central Transantarctic Mountains, the Lower Permian section consists of the glacigenic Pagoda Formation overlain by the post-glacial lacustrine to marine Mackellar Formation, and then the deltaic to fluvial Fairchild Formation. Pagoda strata are interpreted as glaciomarine deposits associated with temperate glaciers and with overall paleoflow from the north. These beds are abruptly overlain by black shales, mudstones, siltstones and fine sandstones of the Mackellar Formation. Regional paleocurrent indicators again suggest southerly paleoflow. At Bunker Cwm Mackellar beds, with about 700 m of outcrop perpendicular to strike, display penecontemporaneous folds and thrusts which are confined by undeformed strata. Fold axes are near horizontal and oriented east-west; fold limbs and thrust sheets have similar strike trends. Some folds and associated cleavage are consistent with late-stage reverse slumping. The upper Mackellar strata are more sandy but do include fine-grained beds; the base of the overlying Fairchild Formation is placed at the base of a continuous vertical exposure of sandstone. The uppermost Mackellar beds were disturbed and broken but were planed off prior to Fairchild deposition. Fairchild and Mackellar beds are offset by a fault, but talus in the fault region covers the transition to thrust stacking in the toe region of the slide. Structural data suggest that a massive slide moved south, slowed to a stop and beds then were thrust at the toe forming a ramp of at least four slices, folded and faulted behind the toe, and thrust in the tail. The slide may have been more extensive than is apparent today. The slide is interpreted to be the result of slumping of cohesive strata on a depositional slope forming the margin of the basin. The slide could have been triggered by slope instability but the thickness of Mackellar strata below the slide is only a few tens of meters, and therefore the trigger is interpreted to have been tectonic, the result of earthquake activity or uplift steepening the depositional slope. This provides further support for inferred glacial and post-glacial basin geography and Early Permian Gondwanan tectonism.