The margin of carbonate shelves and platforms is a critical zone. During construction these margins control both the geometry and the style of sedimentation on the adjacent platforms. After burial they remain a major interface between carbonate and detrital sediments, a preferred site for both hydrocarbon and metallic mineral entrapment. During later deformation the structural style commonly changes across this zone.

Fossil carbonate margins occur in two realms: (1) along the craton margins, bordering Paleozoic and earlier ocean basins that are now fragmented and deformed in major mountain belts or buried beneath Mesozoic and Cenozoic successions along modern continental margins, or (2) around the edges of Paleozoic platforms developed in major basins on large cratons.

Regardless of setting, two different sedimentary styles recur. Rimmed margins, those composed of an elevated rim of bioherms or carbonate sand shoals, exhibit a marked declivity between shelf and basin. This slope is either gentle, gradually changing from shallow into deeper water sediments (depositional slope), or is a steep incline, with little sediment accretion, over which sediments move on their way to the basin (bypass slope). Slope and toe-of-slope sediments in these settings exhibit a full spectrum of hemipelagic ribbon and parted limestones, hardgrounds, slides and slumps, and sediment gravity flows together with submarine erosion. Open margins or ramps, those in which there is no marked shelf break, gradually change from inner shelf to slope strata. The high-energy zone is l cated well inside the craton margin on the platform or at the strandline. Deeper water shelf limestones are particularly common in these situations while slope deposits are characterized by hemipelagic deposits but fewer examples of gravity flow deposits.

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