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

Abstract

Frasnian 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 EarlyMiddle 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 sequencestratigraphic 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.

Oxic facies and the Late Devonian mass extinction, Canning Basin, Australia

“…Discrete infl uxes of siliciclastic sediment are commonly associated with these falls, e.g., Zone 6, base of Zone 12, base of Zone 13a, and latest Zone 13-earliest triangularis Zone and late crepida-rhomboidea Zones (George and Chow, 2002;George et al, 2009). Averbuch et al (2005) proposed that diminished water clarity from detrital infl ux could result in features such as a reduction in eye lens size in trilobites.…”

“…1) provides unweathered material for geochemical analysis, and is ideal for conodont biostratigraphy to identify the Frasnian-Famennian boundary and place it within the regional sealevel history (George et al, 2009(George et al, , 2013. Facies in the core (Virgin Hills Formation) are similar to the classic outcrop sites from which the Frasnian-Famennian boundary has been described, including the nearby well-dated Horse Spring section ( Fig.…”

“…2), and are most easily interpreted in this manner. Regional fl ooding led to backstepping platforms on the Lennard Shelf at that time (Becker et al, 1993;George et al, 2009) and created restricted conditions in interreef basins, resulting in enhanced preservation of organic matter as recorded in the HS core. In contrast, TOC2 and TOC3 precede EX2 and EX3, respectively (Fig.…”

Stratal architecture and platform evolution of an early Frasnian syn‐tectonic carbonate platform, Canning Basin, Australia

“…Sedimentological work has been undertaken on Givetian–Frasnian platform lithofacies (Kerans, ; Warren & Kempton, ; Copp, ) and cycle types (Read, ; Hocking & Playford, ; Shen et al ., ), as well as detailed examination of specific platforms, platform margins and patch reefs (Benn, ; Cooper et al ., ; Hall, ; Hurley, ; Holmes & Christie‐Blick, ; Hocking et al ., ; Brownlaw et al ., ; George & Chow, ; Ward, ; Wood & Oppenheimer, ; Shen et al ., ; George et al ., ; Adams & Hasler, ). Some of these studies have highlighted cycle types in addition to those originally described by Read (), as well as platforms or parts of platforms which have weakly developed cycles or apparently non‐cyclic intervals (Hurley, ; George & Chow, ).…”

“…Late Devonian index fossils (notably conodonts and goniatites) are most abundant in fore‐reef and basinal settings and relatively sparse in platforms. In the Canning Basin, well‐dated fore‐reef sections help define broad depositional ages for associated platforms (Nicoll, ; Becker et al ., , ; Nicoll & Playford, ; George et al ., , ; Klapper, ) although the timing of platform initiation is poorly constrained, commonly resulting in contentious age assignments, such as for the Hull platform (i.e. Givetian versus Frasnian).…”