PALEOGEOGRAPHY AND GEOMORPHOLOGY OF A DEVONIAN ISLAND-REEF SYSTEM, CANNING BASIN, WESTERN AUSTRALIA

The Devonian island-reef complex of the Canning Basin in Western Australia provides a unique opportunity for geomorphologists to explore an intact biophysical system preserved in three dimensions. Burial of the currently exposed Devonian reefs by 2,500 m of carbonates after the Frasnian-Famminian boundary and subsequent geological quiescence allowed structures such as skerrys, islands, and clastic deposits to be preserved in detail. The island-reef complex consists of (1) the highly folded quartzite-phyllite Oscar Range, which formed a chain of islands in Devonian times, and (2) the stromatoporoid reef structure, which includes a classic shoreward sequence as well as an inner and outer lagoon, which are separated by the Oscar Range. The 2005 discovery of a fan-delta deposit composed of siliciclastics apparently sourced from the Oscar Range spurred a return trip to the region to map the deposit and surrounding bio-facies. Stratigraphic sequences show episodic siliciclastic sedimentation events with provenance in the Oscar Range that decrease in magnitude with time. A transition to reef growth is diagnostic of a rise in sea level near the close of Frasnian time. The reconstructed paleogeographic relationship between the fan-delta deposit and the paleoislands of the Oscar Range suggests that the stream channels passing immediately alongside the fossil fan-delta are inherited from those which originally delivered sediments from the Oscar Range in Frasnian times. Quantitative assessment reveals that differential erosion rates between quartzite and limestone, derived from studies of landscape denudation in arid Australia, would not result in enough erosion within the Oscar Range to override structural controls on stream erosion. Analysis shows that stream flow is controlled almost entirely by folds in Proterozoic quartzite and phyllite, therefore demonstrating that modern stream channels are likely to be very similar in location to those active during the Devonian.