Reciprocal Lowstand Clastic and Highstand Carbonate Sedimentation, Subsurface Devonian Reef Complex, Canning Basin, Western Australia

Abstract

Integrated sequence analysis of seismic and well data of the Givetian to Tournaisian sedimentary succession on the outer margin of the Lennard Shelf and adjacent Fitzroy Trough has recognized 18 third-order stratigraphic sequences in a major transgressive-regressive facies cycle. A Givetian phase of crustal extension initiated the transgressive-regressive cycle. The cycle terminated in the Tournaisian, following a phase of slow thermal subsidence. The transgressive half-cycle comprises at least four third-order sequences; an initial Frasnian-Givetian sequence followed by three backstepping Frasnian sequences. The regressive half-cycle comprises 14 basinward-advancing third-order sequences. A period of tectonic uplift and erosion immediately prior to the Frasnian-Famennian boundary resulted in a major basinward shift in coastal onlap in the initial stages of the regressive half-cycle. Systems tracts and facies are partitioned according to their position on the transgressive-regressive cycle; in the transgressive half-cycle, lowstand deposits are subdued and transgressive and highstand deposits accentuated. In the regressive half-cycle, lowstand deposits are accentuated and provide a foundation for the overlying transgressive and highstand deposits. The extent of basinward highstand progradation was limited by the break point of their underlying lowstand deposits. Sequence and systems tract geometries, their stacking patterns, and component facies define two distinct styles of sedimentation: (1) a Givetian to early Famennian, reef-rimmed platform complex; and (2) a late Famennian to Tournaisian mixed carbonate and siliciclastic ramp complex. The reef complex demonstrates marked reciprocal sedimentation. During lowstands, terrigenous sediments by-passed the exposed platform to be deposited in End_Page 157------------------------ the basin as basin floor fans, slope fans, and prograding complexes. During transgressions and highstands, carbonate sediments were deposited on the platform, allodapic carbonate particles were shed into proximal marginal-slope settings, and clastics were trapped on the inner platform. Lowstand carbonate production occurred locally in areas starved of terrigenous clastic influx. The recognition of relative sea-level cycles in these strata has led to a subsurface model of reef development, which provides new insights into the third-order cyclicity within the larger Pillara and Nullara cycles recognized from previous outcrop studies. The outcrop model of backstepping and advancing reef complexes emphasizes transgressive and highstand depositional systems, and fails to recognize phases o lowstand deposition.