Frasnian reef evolution and palaeogeography, SE Lennard Shelf, Canning Basin, Australia

Abstract

AbstractFrasnian reef complexes of the southeastern Lennard Shelf (northern Canning Basin) developed on tilt-block highs and their evolution was controlled by fault-related subsidence. Tectonic control on relative sea-level changes was, therefore, a major factor influencing Early–Middle Frasnian palaeogeography of the Lennard Shelf. However, palaeogeographic reconstruction is not consistent with simple landward (northward) backstepping and younging of reef complexes in response to basin extension and subsidence of fault blocks. Using a sequence-stratigraphic approach, in conjunction with sedimentological and biostratigraphic data, we propose that two neighbouring fault blocks (Lawford area on the eastern side of Bugle Gap and the Hull platform to the north) record a similar history and that the reef complexes on those blocks were initiated at similar times. Seven phases of Early–Middle Frasnian platform growth (Fr2–8) are identified. All are bounded by third-order flooding surfaces associated with backstepping of platform margins and three surfaces (defined by conodont Zone 4, Zone 6 and late Zone 6) correlate across the two fault blocks. Only one sequence boundary has been clearly identified and a second relative sea-level fall is proposed based on a major collapse event following progradation and associated coarse siliciclastic facies. We propose that the correlation of flooding events across the SE Lennard Shelf is related to episodes of basin-margin faulting centred on the large, long (c. 25 km) faults which border these blocks (shelf-parallel faults for the Lawford block and an oblique north-trending transfer zone for the Hull block). There is limited evidence for relative sea-level falls and those recognized most likely resulted from eustatic events. The correlation between the Lawford block and Hull platform suggests linkage between major NW-trending shelf-parallel and oblique transfer faults and an evolved rift system by the Early Frasnian.