Facies Models Revisited: Clastic Shelves
A consistent, commonly accepted facies model for clastic continental shelf deposits has proven elusive. Sand ridges of various types are ubiquitous on Quaternary shelves, but recognition of ancient examples has been intermittent, as models emerge, evolve, and undergo relative degrees of acceptance and rejection. Continental shelves lie in the zone between highstand and lowstand shorelines, and are greatly affected by sea-level changes. Shelf deposits per se are thus limited to and are most volumetrically significant during periods of high or rising base level, the transgressive and highstand systems tracts. The continental shelf is roughly defined from the base of the shoreface to the shelf margin or upper continental slope. These defining depths, and consequent shelf widths, are highly variable, depending largely on tectonic setting. Primary shelf sediments may be derived from erosional scour of preexisting deposits, biological and/or chemical precipitates, or supplied from adjacent shorelines by direct fluvial, deltaic, or estuarine input, or by tidal flux or storms. Finegrained shelf sediments, constituting potential source and seal facies, are less controversial than the origin and distribution of potential reservoir facies. Coarse-grained shelf deposits are believed to be primarily relict, reworked from shoreline and nearshore deposits laid down during previous regressive episodes, although notable exceptions exist. These deposits can be quite complex, taking on geometries reflecting reshaping by wave, tidal, storm, and oceanic currents, with internal stratigraphics reflecting multi-stage depositional and erosional histories.