Regolith-landform mapping, leucitite basalt and the landscape evolution of the Byrock region, NW NSW

The Byrock region is approximately 50 km NNE of Cobar in northwestern NSW. Valley-filling leucitite basalts (leucitites) occur northeast of Cobar, at El Capitan, and to the west of Byrock (Figure 1). The Byrock leucitites occupy an area of ca. 8 km2 . They occur as moderately weathered basaltic rises up to 35 m high in a low-relief area dissected by an ephemeral drainage system, including the north-trending Mulga Creek lying in the east of the mapping area. The Byrock leucitite is a part of the eastern Australian leucitite suite, which is a linear, age-progressive intraplate volcanic province that parallels the better known central volcanoes, which also form age progressive suites southward through eastern Australia. The leucitite province forms a line from Byrock in the north to Cosgrove, Victoria, in the south, which has been K-Ar dated from 16.8 Ma to 6 Ma (Sutherland 1983).
The Byrock leucitite forms three main outcrops: Bye Hill, a lava mound of ca. 5 km2 and 35 m relief; and two much smaller disconnected outcrops forming low rises to the N and NE. Other small remnants of leucitite float (up to about 100 m2 ) have been iscovered during mapping traverses undertaken during the course of this research. The main outcrop of leucitite, Bye Hill, is cut through by a quarry on its western flank, allowing for cross-sectional examination and interpretation. The quarry faces are slightly to moderately weathered and it is difficult to interpret internal flow structures and to differentiate flow layers. Small pods of pink-coloured porcelainite, presumed to be explosively-incorporated lacustrine sediments, occur in the lower parts of the quarry faces, allowing some differentiation of flow boundaries.