Recognition of Localized Fluvial Input into a Wave-Dominated, Prograding Shoreface Succession: Tern Formation, Bonaparte Basin, Timor Sea, Australia

Kerrie L. Bann¹, Olaf Kloss², Geoff Wood³, Simon C. Lang⁴, Jochen Kassan⁵, and Jim Benson^3
1 University of Alberta, Edmonton, AB
¹ SANTOS, Australia, Australia
¹ SANTOS, Adelaide, Australia
¹ Australian School of Petroleum, Adeliade, Australia
¹ Whistler Research, Brisbane, Australia

Ichnological nuances of facies characteristics in the Permian Tern Formation, Bonaparte Basin, Timor Sea, Australia, provide fascinating insights into the potential of recognizing localized fluvial input into successions that otherwise appear to reflect prograding, wave-dominated, strandplain shorefaces. The interval comprises seven component facies, interpreted as 1) lower offshore, 2) upper offshore, 3) offshore transition, 4) distal lower shoreface, 5) proximal lower shoreface, 6) middle shoreface, and 7) upper shoreface deposits. Facies designations are based on primary sedimentary structures, lithology, palynology, bioturbation intensity, and subtle but significant recurring changes in trace-fossil diversities and predominant assemblage constituents. Ichnofossil assemblages are extremely diverse (21 ichnogenera, comprising 23 ichnospecies) in the offshore to lower shoreface deposits, and indicate fully marine environments that provided conditions ideal for infaunal colonization, though punctuated by minimal but intermittent storm-induced disruptions.

Locally, intensely and uniformly bioturbated successions are punctuated by heterolithic intervals containing very sparsely to unbioturbated sandstone beds, mantled by comparatively thick, sporadically burrowed, mudstone drapes. Ichnological assemblages are impoverished in these intervals, suggesting that deposition was associated with environmental conditions that impeded infaunal colonization. Trace fossil suites also display size reductions of ichnogenera and a paucity of suspension-feeding structures. These environmental fluctuations are attributed to localized delta-induced stresses that may have included increased sedimentation rates, salinity changes (mainly reductions), heightened water turbidity, increased phytodetrital influx, and pronounced fluid mud deposition. The Tern Formation is interpreted as a prograding strandplain shoreface that accumulated within reach of the effects of fluvial discharge, along strike of a contemporaneous wave-influenced delta complex.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005