Volcanic influences in a storm- and tide-dominated shallow marine depositional system: The late Permian Broughton formation, Southern Sydney Basin, Kiama, NSW

Shallow marine sediments of the Broughton Formation are dominated by immature volcanic debris of intermediate to basic composition, generated in an adjacent subaerial environment by volcanism responsible for the nine shoshonite units intercalated within sediments of the Kiama region. Sediment was supplied to the offshore environment via periodic storm-generated, expanded high density turbidity currents. Initial deposition, represented by the Westley Park Sandstone Member, was below storm wave base, during which time the depositional surface was subjected to post-depositional tractional reworking by northerly directed, tidally influenced bottom currents. The resulting positive-relief sand bodies on the seafloor contain tractional sedimentary structures (the ‘tractional facies association’). Areas of the substrate between these sand bodies retained their turbidite bedding structure (the ‘rhythmically bedded facies association’) but were extensively bioturbated by a diverse deposit-feeding biomass. Upon emplacement of the lowest of the nine shoshonite units as a tri-composite, locally intrusive lava flow, the depositional surface was elevated, transgressing storm wave base. The body of the shoshonite flow also shielded the substrate from the northerly directed tractional currents, allowing the development and preservation of the hummocky cross-stratified sandstone facies in the Kiama Sandstone Member. Following burial of the shoshonite flow by continued deposition, this local shielding effect was overcome and tractional currents again reworked the entire depositional surface.