Alluvial fan and flood‐basin sedimentation in the Triassic Red Cliff Coal Measures, New South Wales

Abstract

The Late Triassic Red Cliff Coal Measures on the north coast of New South Wales contain coarse poorly sorted conglomerate interbedded with diamictite, mudstone, fossiliferous fine sandstone and siltstone, bleached rootlet beds and coal. The conglomerates are composed of locally derived basement clasts and form sheet‐like and lenticular bodies, some of which are inversely graded. These locally derived conglomerates are replaced higher in the section by large lenses of sandy conglomerate that are cross‐bedded and contain mineralogically more mature pebbles than the conglomerates low in the section. The Red Cliff Coal Measures were deposited at the intersection of an alluvial fan with fluvial flood‐plain sediments. Coals accumulated as raised bogs on the fan surface. Initially, the fan was short, steep and dominated by debris flows. Slumping of fan sediments produced diamictites. Scarp retreat and lowering of slopes in the catchment area and on the fan resulted in replacement of debris flow conglomerates by ...