Massive Sandstone Facies in The Hawkesbury Sandstone, A Triassic Fluvial Deposit Near Sydney, Australia

Abstract

ABSTRACT Massive sandstone is a common facies in the Hawkesbury Sandstone, a Triassic formation attributed to deposition by a large braided river. The massive sandstone occurs as sheets and in elongate depressions trending perpendicular to the paleoslope, as indicated by the unidirectional mean orientation of abundant cross-strata. Some of the depressions filled with massive sandstone also contain angular mudstone intraclasts, up to several meters in length. Massive sandstone is also associated with deformed cross-strata, formed by progressive loss of lamination during mass movement down foreset slopes. Foreset failure is attributed to liquefaction due to falling water level, or to collapse of adjacent mud banks. The latter mechanism resulted in rapid loading of bedform foresets, and introduce large mudclasts into the massive sand. Sand and mudclasts accumulated in transverse scours at the foot of the foreset slopes, but also travelled along the scours, or spilled out to form massive sheet sands. The abundance of massive sandstone in the Hawkesbury Sandstone is attributed to deposition in a large river, in which flood stage bedforms were probably up to 15 m high. Because of the size of the bedforms, large volumes of liquefied sand were generated when foreset slopes failed.