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The islands of the Florida Keys are often viewed as an exploration model of stratigraphic traps which may form, during very high stands of sea level, several kilometers landward of shelf margins. A detailed study of one of these islands--Key Largo--has been undertaken utilizing cores from 10 boreholes drilled to depths of 7 to 21 m. The stratigraphic section cored is of late Pleistocene age and represents a linear accumulation of reefs enclosed by extensive, generally burrowed deposits of skeletal sand. Reef facies contain approximately 20 to 30% of the relatively large corals, predominantly Montastrea annularis, Diploria sp., Porites astreoides, and Porites porites. Skeletal-sand facies (packstones to grainstones) vary in detail but are characterized by pellets, Halimeda sp., mollusk, coralline algae, and foram debris, and are associated with both the reef facies and with finer grained, mollusk-bearing wackestones. Mudstones per se are scarce.
The stratigraphic succession is interrupted in the upper 10 m by two major discontinuities formed as the result of subaerial diagenesis during low stands of sea level. Evidence includes caliche crusts, angular substrate fragments, root casts, and pockets of reddish soil. The shallowest and commonly more subtle of these two breaks is generally developed within 2 to 3 m of the modern surface, whereas the deeper and more conspicuous discontinuity lies at a minimum depth of 7 to 8 m. This latter break represents a prolonged period of exposure and is overlain by a burrowed to locally cross-bedded zone of quartz sand containing varied amounts of coarse skeletal debris.
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