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.
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Conference of the Canadian Society of Petroleum Geologists) and in Dallas in 2004 (Annual Conference of the American Association of Petroleum Geologists). These sessions, entitled Facies Models Revisited, were intended to capture the state of the art with respect to facies modeling in several key depositional environments. This volume is focused on clastic depositional settings including continental (aeolian and fluvial), estuarine, shoreface, deltaic, shelf, and deep water. The approach that they encouraged with the authors to follow was a first-principles rather than a model-driven approach. Their philosophy was to provide the reader with the tools and rules to create their own models rather than providing them with “canned” models or “templates”. Following this approach, they believe that geoscientists will develop better and more predictive facies of depositional models. The editors believe this volume will find a niche with both academic as well as industry and government geoscientists.