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Abstract

Standard carbonate facies models are widely used to interpret paleoenvironments, but they do not address how carbonate platforms are affected by relative changes in sea level. An understanding of how the subtidal carbonate "factory" responds to relative sea-level changes and the role played by other environmental factors towards influencing the formation of carbonate platforms allows one to differentiate platform types and it helps establish a basis for constructing depositional sequence and systems tract models. The combination of in-situ production of carbonate sediment, which is also subject to transport, and local variations in depositional processes result in the formation of a wide variety of stratal patterns, some of which are unique to carbonate systems.

Fundamental carbonate-depositional principles and geologic-based observations were used to construct depositional sequence and systems tract models for a variety of rimmed shelves and ramps. The models show how, for example, depositional sequences made up of (1) carbonate, (2) carbonate-siliciclastic, or (3) carbonate-evaporite-siliciclastic facies are produced by depositional systems responding to lowstand, transgressive, and highstand conditions. Lowstand: Carbonate sediment production is reduced on rimmed shelves because a relatively small area of shallow seafloor is in contact with the carbonate "factory." Reduced sedimentation and subaerial exposure foster the retreat of shelf edges and slopes by erosion and slope failure during lowstands. As a result, thick debris-flow deposits may form. Karst development is important in humid climates and can affect large areas of a subaerially exposed platform. If siliciclastic sediments are available, they are delivered to the shelf edge and slope by fluvial-deltaic systems or, in arid climates, by wadis and advancing ergs. Under arid conditions, lowstand evaporites may fill an isolated or completely silled basin. Transgression: Carbonate sedimentation initiates in restricted environments and later as more open conditions develop, open marine facies, including patch reefs, may locally develop atop flooded platforms and ramps. Retrogradational parasequences comprising shallow-water carbonates form and subsequently drown, and shelf edges tend to aggrade, backstep, and drown if the rate of sea-level rise is high. Highstand: Seaward-prograding carbonate or siliciclastic coastal sediments and landward-prograding carbonate rimmed shelf edges may partially fill inner to outer shelf seas. Under arid conditions, evaporites and red beds commonly fill wide and shallow salinas. These strata onlap subaerially exposed rimmed shelf edges and prograding grain-stone islands in ramps. Shelf edges and shorelines tend to prograde under the influence of high rates of carbonate sedimentation across the shelf and shelf edge. Slope and basinal environments receive excess shelf- and shelf-edge-derived sediment.

Factors listed above must be integrated with established facies models in order to arrive at comprehensive sequence and systems tract models. As should be the case with all models, however, they are not meant to serve as rigid templates within which all carbonate sequences must fit. Modification may be needed to accommodate each case. Once they are deemed applicable to a specific case, they function as working hypotheses to help geologists visualize how and why carbonate strata were laid down and fit together as they do. As a general predictor of facies, carbonate depositional sequence and systems tracts models may be used in conjunction with seismic records to identify depositional systems and to locate reservoir-, seal-, and source-prone facies.

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