Paleogeographic and Tectonic Significance of Diachronism in Siluro-Devonian Age Flysch Sediments, Melbourne Trough, Southeastern Australia

Abstract

The features of these Siluro-Devonian age flysch sediments of the Melbourne Trough in southeastern Australia conform closely to those of the European Alps. The coarser of the alternating coarse-fine layers were deposited by turbidity currents and the fine layers by slower pelitic sedimentation. The turbidites intruded the deeper water shale environment with an average frequency of 1 turbidite per 2000 yrs from calculations of the thickness of the formation and the time deduced by graptolite zones. The flysch, shelf, and massive lithofacies are continuously diachronous from Lower Silurian to Lower Devonian, during which time the longitudinal axis of the zone of flysch deposition was displaced laterally across the trough a distance of 80 mi. A theoretical model of the sedimentation pattern indicates the flysch sedimentation to be the result of a balance sediment supply and accumulation in a holding area to reach a state of metastability, allowing a triggering mechanism to periodically displace the coarser sediment downslope into the quieter shale environment. The flysch sediments are considered to be an expression of the slow tectonic uplift of the western margin of the Melbourne Trough prior to the major Tabberabberan orogeny in Late Devonian time.