Tectonic control on third-order sequences in a siliciclastic ramp-style basin: an example from the Roper Superbasin (Mesoproterozoic), northern Australia

The Roper Group is a cyclic, predominantly marine, siliciclastic succession of Calymmian (Early Mesoproterozoic) age. It has a distribution of at least 145 000 km(2) and a maximum known thickness of similar to 5000 m. In the Roper River district the middle part of the Roper Group (similar to 1300 m thick) is characterised by the cyclical alternation of mudstone and sandstone units, and can be divided into six third-order depositional sequences. A typical sequence is broadly progradational in aspect, and comprises a lower, mudstone-rich. storm-dominated shelf succession (up to 330 m thick), and a sequence-capping unit dominated by tidal-platform cross-bedded sandstone (up to 80 m thick): both are interpreted as highstand systems tracts. Transgressive strata are poorly represented but where present are characterised by paralic to fluvial redbed assemblages that include ooidal ironstone. Roper Group sequences lack a distinct condensed section and sequence boundaries are mostly conformable. Erosional contacts separate mud-rich shelf facies from sequence-capping sandstones. We infer that these erosion surfaces were generated by episodic flexural tectonism, which also generated the accommodation and sediment supply for Roper sequences.