Highstand Sandy Deepwater Fans: Incorporating Sediment Supply, Sea Level and Basin Architecture into The Prediction of Sand Bypass to The Basin Floor

Carvajal, Cristian R.¹, Ron Steel¹ (1) The University of Texas at Austin, Austin, TX

The accumulation of thick and sandy deepwater fans has typically been interpreted following the lowstand model. In this model, a low sea level causes river incision of the shelf edge and direct sand bypass to the basin floor. By focusing on sea level this model has been useful in explaining the development of some ancient deepwater successions. On the other hand, such emphasis has caused geoscientists to downplay the importance of sediment supply despite the driving role that supply can have in both shelf margin accretion and significant sand bypass to deepwater areas during sea level highstand.

The Lewis-Fox Hills shelf margin (Maastrichtian) in southern Wyoming represents an example in which the importance of sediment supply is clear. Progradation (>48 km/my) and aggradation (>267 m/my) rates in this margin are significantly higher than in other margins of similar dimensions around the world (e.g. in Spitsbergen, offshore Ireland, NW Australia, North Slope and South Africa). Such relatively high rates and also relatively high volumes of sandstone (<65 km3) on the basin floor unequivocally point to a high sediment supply. In addition, systematically rising shelf-margin growth suggests a high relative sea level regime. Nonetheless the high sediment supply caused delta progradation to the shelf edge and significant bypass of sand to the basin floor despite sea level highstand. Thus, a main lesson from the Lewis-Fox Hills shelf margin is that interpretation of deepwater deposits needs to be holistic and incorporate shelf margin architecture, accommodation and sediment supply, and not sea level alone.