The effect of flexural isostasy on delta architecture: implications for the Mungaroo Formation

Abstract

The fluvio-deltaic Triassic Mungaroo Formation, North West Shelf (NWS) of Australia, hosts vast resources of hydrocarbons. However, the mechanisms that generated its 4-6 km monotonous infill architecture (colloquially known as layer cake stratigraphy) remain elusive. The vertical fluctuation between fluvial and shallow marine deposits indicates that accommodation was created simultaneously with deposition. This seems to suggest that the stratigraphic style of the Mungaroo formation was significantly controlled by the isostatic compensation of the sediment load. To test this we use a basin and landscape dynamics model, BADLANDS that combines fluvio-deltaic processes (erosion and sedimentation) with flexural isostasy. To drive our simulations we use dimensions, gradient, water discharge and sediment flux from seismic and scaling relationships extracted from the Mungaroo Formation and different lithospheric elastic thickness (Te) to account for the effect of dissimilar lithospheric rigidities and flexural isostasy. Results show an increase in delta size and decrease in sediment thickness as the lithospheric elastic thickness increases. These models help explain how thick deltaic sequences can be generated in a lithosphere with low Te values. Future research will focus on comparing the synthetic stratigraphy extracted from the models with the stratigraphic record. This study provides a valuable quantitative approach for understanding how the isostatic compensation of the sediment load can control the architecture of fluvio-deltaic deposits, which has implications for reservoir modelling.