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ABSTRACT

The shelf-slope break is the zone which controls the evolution of fossil carbonate platforms and shelves because it is the locus of most rapid carbonate fixation, both organic and inorganic. The fossil record of carbonate platforms and shelf margins is biased. The best known and most studied shelf-slope breaks are of middle and late Paleozoic age, occuring in intracratonic basins. Those of early Paleozoic and Mesozoic through Cenozoic age, which occur mostly along continental margins, are poorly known. Shelf-slope breaks of Precambrian age are poorly known. Five recurring types of carbonate shelf-slope break are found in the fossil record: (1) stationary, (2) offlap, (3) onlap, (4) drowned and (5) exposed. The reefs and carbonate sand shoals at the break are the line source of most sediment deposited on the foreslope. In the case of drowned or exposed margins this sediment production is arrested, resulting in starved slope and basin margin sedimentation. Most examples in the rock record are a combination of these types. The nature of the break through time depends upon the types of organisms present and their paleoenvironment. When large skeletal metazoans were alive, barrier reefs formed at the margin and reef mounds grew on-shelf or downslope, but when only diminutive skeletal organisms occurred, the break is generally formed by sand shoals. Lithologies at the break are particularly prone to diagenetic alteration. Intensive early diagenesis tends to preserve texture but decrease porosity, whereas intensive late diagenesis generally destroys texture but creates good reservoir rock.

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