Continental Shelf Topography: Key to Understanding Distribution of Shelf Sand-Ridge Deposits from Cretaceous Western Interior Seaway

Abstract

A comparison of Holocene shelf sand ridges on the Georges Bank-Nova Scotia-Newfoundland-Labrador shelf system and the Texas shelf with Cretaceous shelf sand-ridge deposits from the Western Interior seaway provides new insights into the possible role that Cretaceous shelf topography may have had in controlling shelf sand-ridge distribution, as well as providing a possible sand source on an otherwise muddy shelf. The Holocene ridges are elongate, asymmetric in cross section, as much as 35 m (115 ft) thick, up to tens of kilometers in length, and are spaced up to 15 km (9.3 mi) apart in parallel sets. They occur as much as 300 km (186 mi) from the present shoreline and, in different areas, appear to be actively influenced by tidal, storm, and/or oceanic currents. The ridges enerally are part of the Holocene transgressive sand sheet derived by reworking of underlying substrate while contemporaneous mud is deposited in adjacent low areas. All of the Holocene ridges are situated atop topographically high banks on the shelf surface. The shelves are as much as 400 km (250 mi) wide, with water depth ranging from 20 to 200 m (66-656 ft). Often prolific, hydrocarbon-bearing Cretaceous shelf sand-ridge deposits (e.g., Shannon, Sussex, Hygiene, Viking, Cardium, etc) exhibit similar geometries and dimensions to these Holocene ridges, are generally encased in shale, and are thought to have been deposited tens to hundreds of kilometers from the paleoshoreline in water depths comparable to those of the Holocene ridges. Sediments for these sandstones are commonly thought to have been transported long distances across a flat, muddy shelf. The above comparisons suggest that the Cretaceous shelf sand ridges may have been deposited upon topographic highs or banks similar to those upon which Holocene ridges are situated. Such banks could have been developed on the Cretaceous shelf surface by (1) recurrent folding or faulting, (2) deposition of thick accumulations of sediment (e.g., deltas), (3) preservation of paleotopography from unconformities by draping and/or compaction, and (4) fluvial erosion and sculpturing during lowered sea level. A sequence of events is proposed for the evolution of shelf sand-ridge deposits that considers the role of shelf banks, coupled with fluctuations in water depth, in providing the source, development, and burial of the ridges. The initial stage is development of the topographically high bank on the shelf by any of the above processes. With shallowing of water, sediment is formed on the bank by erosion and is then spread into a sand sheet. The sand is then molded into ridges by shelf hydrodynamic processes. With deepening of water by sea level rise and/or basin subsidence, the ridges stabilize, then eventually become buried in mud. Repeated cycles of development of banks and changing water depth can give rise to a series of locally sourced shelf sand ridges associated with shale. This s quence of events assumes that erosion and deposition can occur on different parts of a shelf bank that is the same general size as northwestern Atlantic shelf banks upon which Holocene ridges are situated.