Deposition of the lower Feather Conglomerate, a Permian braided river deposit in southern Victoria Land, Antarctica, with notes on the regional paleogeography

Abstract

The Feather Conglomerate is a body of subarkosic coarse sandstone and conglomerate, 200 m thick, part of the Victoria Group (Beacon Supergroup), in southern Victoria Land. This paper discusses the lower part of the formation, deposited on an alluvial plain some time in the Middle to Late Permian, and considers some paleogeographic implications. Study of a well-exposed section at Mount Bastion has provided detailed data for facies and paleocurrent analysis, and on paleosols that are well-developed there. Vertical burrows ( Skolithos) are also common. The lower Feather Conglomerate at Mount Bastion is divided into five facies associations based on the braided stream facies defined by Miall. The most common (71% of the 125 m-thick section) is trough cross-bedded and horizontally bedded coarse sandstone, interpreted as flood-deposited channel sand. The other facies are mainly sand and mud deposited from waning flood waters. The South Platte River appears to provide a moderately close modern analogue for lower Feather rivers. Paleocurrent directions from trough cross-bedding indicate deposition in low-sinuosity channels, and bed thickness and scour depths suggest an average bankfull depth of 1 or 2 m. Schumm's equations yield the following values for average channel and flood-plain variables, assuming a bankfull depth of 1 m: mean annual flood discharge: 149 m 3 s −1; slope: 1:644; and flood-stage mean velocity: 1.9 m s −1. Drainage area and channel length upstream are estimated at 13,500 km 2 and about 300 km, respectively. The Mount Bastion section contains 13 paleosols, on average 1 m thick. The paleosols are characterised by up to four horizons, vein networks, and iron and clay pans. These features suggest development in a temperate subhumid or humid climate over thousands of years. Permian strata, 650 km to the north, were deposited from broad, shallow low-sinuosity rivers, like the lower Feather, on a west-sloping flood plain, probably part of the same drainage basin. In contrast, Permian strata only 400 km south of the Feather outcrop were deposited from meandering rivers flowing southward. From the lack of “room” for a headwaters region, we believe the two drainage basins have been juxtaposed since the Permian by left-lateral movement of at least several hundred kilometres, probably along the transform fault postulated to run down the Ross Sea and crossing the Transantarctic Mountains as the Byrd Fault. The lower Feather flood plain sloped inland from the Late Permian coastline, which ran approximately down the present eastern coast of Australia towards West Antarctica. However, lower Feather rivers probably reached the coast near Tasmania after an arcuate course of 2000 km. This implies deposition on an inland high plain (elevation about 100 m) with a subhumid and high seasonal climate, which is consistent with inferences from sand mineralogy and the paleosols.