Early Devonian synorogenic alluvial-fan deposits of the Maccullochs Range, western New South Wales

Abstract

The Early Devonian, Maccullochs Range beds (new) of the Winduck Interval largely comprise non-marine fine-grained sheet-flood-deposited sandstones which lie in the southeast sector of the Darling Basin Conjugate Fault System. Deposition of the >2.5 km-thick sequence occurred on the Wilcannia, Towers and Coolabah Bore alluvial fans, that were sourced largely from lightly indurated sandstone caps overlying a large basement high lying north of the Darling River Lineament, and also from west of Maccullochs Range (Coolabah Bore Fan). Four lithofacies are recognised. Lithofacies 1, massive sandstone, is proximal and was deposited from hyperconcentrated sheet floods. The more distal lithofacies 2 is partly massive, partly laminated and partly affected by soft sediment slumping during its deposition. It contains 1.3 – 3.5 m-thick sheet-flood successions that rarely show cross-bedding. Lithofacies 3 and 4 are minor: lithofacies 3, stream-flood deposited, comprises coarse-grained, pebbly sandstone and lithofacies 4, transient playa lake deposits that are locally intercalated with lithofacies 2. In lithofacies 2, thick massive fine-grained sandstone is commonly overlain by laminated sandstone that was deformed when soft. Incised channel deposits in lithofacies 2 deposits are rare and palaeosols were not discovered. Permanency of the positions of two of the alluvial fans, and by inference their feeder streams, remained unchanged for ∼9 million years. The fans overlie probable floodplain deposits observed in a quarry in the easternmost part of the study area. Marine fossils are very minor in the range—the brachiopod Howellella jaqueti at one locality indicates an Early Devonian age for one of the brief marine incursions into what was normally an alluvial-fan environment. Very brief marine incursions elsewhere in the group are deduced from the presence of very rare fossil gastropods.

Key words:

• alluvial fans
• Darling Basin
• Darling River Lineament
• Devonian
• Lake Poopelloe Sub-basin
• Maccullochs Range beds
• sheet-flood deposition

Acknowledgements

The Howards of Goonalga Station are thanked for hospitality. G. Packham generously provided a copy of his unpublished geological map of Maccullochs Range, J. Vaughan and G. Young assisted in the field, and S. Braddy and A. Cook identified a fossil eurypterid and three gastropods, respectively. An earlier draft of the paper was reviewed by K. Marsaglia, K. Mills and P. Southgate, and this manuscript was reviewed by P. Rickwood and M. Katz: all are thanked for their critiques. Also thanked are journal reviewers K. Mills and G. Packham.