Tide-Influenced Regressive Shelf-Margin Deltas Deposited 300 Km Seawards of Lowstand River Mouths: Importance of Tidal Transport on Giant Continental Platforms (Bonaparte Basin, NW Australia)

The Bonaparte Basin (NW Shelf of Australia, Timor Sea) forms a shallow-water continental shelf extending for more than 630 km from the coastline to the shelf-edge. Neogene to recent deformation had a great influence on the platform geometry and during sea-level falling-stage and lowstand periods of the Quaternary, shaped a >10.000 km2 intra-shelf basin that forms an epeiric (epicontinental) sea in the middle of the shelf. Therefore lowstand fluvial deltas are restricted along the epeiric sea shoreline in the middle shelf, about 350 km from the present-day (highstand) shoreline, but more than 300 km upstream of the lowstand shelf-edge. As a consequence fluvial incised valleys do not reach the shelf-margin area during falling-stages and lowstand sea-level periods.

Despite this, relatively thick regressive shelf-margin deltas have been deposited during the Late Pleistocene. Sediments are composed of a mixture of clastic muds and carbonate particles of local origin. Sediments are drained from the epeiric sea by a 10-km wide valley which is thought to be dominated by tidal processes. Falling-stage and lowstand periods are associated with the development of a distributary tidal channel network observed along the delta tops, delta fronts and prodeltas. Individual ~ 100 ka regressive sequences (resulting from one phase of sea-level drop and lowstand) can be locally characterized by the accumulation of up to 100-120 ms TWT thick (about 100 m) deltaic and pro-deltaic sediments associated with channel complexes. The regressive tidal channels of the Bonaparte Basin are typically 50 m - 1000 m wide and 5 - 20 ms TWT deep (i.e. up to 20 m), with a width and depth decreasing basinward. The overall size and architecture of the channels resemble those described in ancient tide-dominated/influenced regressive deltas such as in the Sego Sandstone (Willis and Gabel, 2003). Basinward, the area is marked by a steep slope and poor turbidite system development.

These results highlight the potential importance of tides during falling-stage and lowstand sea-level periods and their impact on source-to-sink sediment transfer and shelf-margin sedimentation processes along very wide continental shelf settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California