About This Item
Share This Item
A comparison of Holocene shelf sand ridges on the Georges Bank-Nova Scotia-Newfoundland-Labrador shelf system and the Texas shelf with Cretaceous shelf-bar sandstones from the Western Interior seaway provides new insights into the possible role shelf topography may have in controlling shelf-bar distribution, as well as providing a possible sand source on an otherwise muddy Cretaceous shelf. The Holocene ridges are elongate, asymmetric in cross section, 2 to 20 m (6.6 to 66 ft) in height, up to several kilometers in length, and are spaced from 90 m to 3 km (295 ft to 1.9 mi) apart in parallel sets. They occur up to a maximum of 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 s elves upon which the ridges sit are up to 400 km (250 mi) wide, and 100,000 km2 (39,000 mi2) in area, with water depths of 50 to 200 m (165 to 660 ft).
The commonly prolific hydrocarbon-bearing Cretaceous shelf-bar sandstones (e.g., Shannon, Sussex, Gallup, Hygiene, Viking, Cardium, etc) exhibit similar geometries and dimensions to these Holocene ridges and are thought to have been deposited tens to hundreds of kilometers from the paleoshoreline in comparable water depths. Material for these sandstones is commonly thought to have been transported long distances across a flat, muddy shelf, since the sandstones are usually associated with thick shale sections.
All of the Holocene ridges sit atop topographic highs on the shelf surface. They are generally part of a Holocene transgressive sand sheet derived by reworking of underlying substrate while contemporaneous mud is deposited in adjacent low areas. The above comparisons suggest the Cretaceous shelf-bar sandstones may have been deposited upon similar shelf highs. Such topographic highs could develop on the Cretaceous shelf surface by (1) recurrent folding or faulting, as has been proposed for some Wyoming sandstones, (2) fluvial erosion and sculpturing during lowered sea level, as on the Texas shelf, (3) deposition of thick accumulations of sediment (e.g., deltas), as on some Holocene Atlantic shelves, and (4) preservation of paleotopography from stratigraphically lower unconformity surfa es, as has been suggested for the Cretaceous of Alberta.
A sequence of events is proposed for the evolution shelf-bar sandstones which considers the role of shelf topographic highs coupled with fluctuations in water depth in providing the source, development and burial of shelf sand ridges. The initial stage is development of the topographic high on the shelf. With shallowing of water, sediment derived by erosion on the high forms a sediment sheet which later is molded into sand ridges by shelf hydrodynamic processes; during this stage mud is winnowed and
deposited in adjacent low areas. With deepening of water, sand ridges first stabilize then eventually become buried in shelf mud. Repeated cycles of development of highs and changing water depth can give rise to a series of locally sourced shelf-bar sandstones associated with shale. This sequence of events assumes erosion and deposition can occur on different parts of the shelf topographic high, which is of the same general size as northwestern Atlantic shelf highs upon which Holocene ridges sit.
End_of_Article - Last_Page 549------------