Abstract

The margins of carbonate platforms have been broadly classified into those where sediment is deposited at the margin and those where sediment bypasses the margin. Although modern bypass margins have been described from north of Jamaica and the Bahamas, examples of ancient bypass margins are conspicuously absent. However, the recent discovery of parts of several bypass margins in the Jurassic carbonates of the Umbria-Marches Apennines in Italy allows a close look at the processes that create and shape these margins. The bypass margins in the Apennines were created on the escarpments of large normal faults, which broke up a large, continuous platform into a group of small, isolated platform remnants. The newly discovered outcrops allow us to recognize that the marginal escarpments of these platforms are characterized by a series of deep grooves that were cut into the escarpment by the subaqueous erosive action of bypassing sediment swept off the tops of the platforms and deposited in the basins. Erosion of the escarpments and sediment bypassing gradually diminished as the basins filled with sediments and the platforms submerged to greater depths, cutting off the rapid shallow-water sedimentation.

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