CONTROLS ON DEEP-WATER SAND DELIVERY BEYOND THE SHELF EDGE: ACCOMMODATION, SEDIMENT SUPPLY AND DELTAIC PROCESS REGIME (Invited Presentation)

Stratigraphic models typically predict accumulation of deep-water sands where coeval shelf-edge deltas are developed in reduced accommodation and/or high sediment supply settings. On seismic data, these relationships are commonly investigated on a small number of clinothems, with a limited control on their lateral variability.

Advanced full-volume seismic interpretation methods now offer the opportunity to identify high-order (i.e., 4^th to 5^th) clinothems and to evaluate the controls on shelf-to-basin sediment transfer mechanisms and deep-water sand accumulation at these high-frequency scales.

This study focuses on the Lower Barrow Group (LBG), a shelf margin that prograded in the Northern Carnarvon Basin (North West Shelf, Australia) during the Early Cretaceous. Thanks to high-resolution 3D seismic data, 30 clinothems (average time span of ~47,000 years) are used to establish quantitative and statistical relationships between the shelf-margin architecture, paleoshoreline processes and deep-water system types (i.e., quantitative 3D seismic stratigraphy).

The results confirm that low values of rate of accommodation / rate of sediment supply (δA/δS) on the shelf are associated with sediment bypass, whereas high δA/δS conditions are linked to increasing sediment storage on the shelf. However, coastal process regimes at the shelf edge play a more important role in the deep-water sand delivery behavior.

Fluvial-dominated coastlines are typically associated with steep slope gradients and more mature, longer run-out turbidite systems. In contrast, wave-dominated shorelines are linked to gentle slope gradients, with limited turbidite system development (except rare sheet sands and mass-transport deposits), where longshore drift currents contributed to shelf-margin accretion through the formation of extensive strandplains. In this context, reduced volumes of sand were transported offshore and mud belts were accumulated locally.

This study highlights that variations from fluvial- to wave-dominated systems can result in significant lateral changes in shelf-margin architecture (i.e., slope gradient) and impact the coeval development of deep-water systems (i.e., architectural maturity). Quantitative 3D seismic stratigraphy represents a novel approach in assessing at high resolution the controls on deep-water sand delivery, and potentially predicting the type and location of reservoirs in deep water based on the shelf-margin architecture and depositional process regime.