Only one major interruption has occurred in the long history of shallow-water carbonate deposition that has prevailed in southern Florida since the Jurassic. This break resulted from a substantial incursion of the finer siliciclastic sands interbedded or mixed with surprisingly coarse quartz sands during the late Cenozoic. Along the southeastern margin, this influx was succeeded by the development of reefs during the Quaternary.

The siliciclastics occur in the subsurface beneath a section of Pliocene to Holocene shallow-water carbonates. Recent study of well samples shows that these sediments are thickest (120-200 m) along a north-south trend that extends from the central part of southern Florida to the upper Florida Keys. These sediments are largely composed of quartzose grains ranging in size from very fine sand to granule (0.06-4 mm), with minor proportions of calcareous clays, phosphatic grains, and marine faunal fragments. The medium sand to granule-sized sediments are composed of well-rounded quartzose grains and occur either interbedded or mixed with finer fractions.

The sudden influx of siliciclastics in southern Florida beginning in the Miocene is quite unexpected considering the remoteness of the Appalachians, the postulated source. This southward transport may have been accomplished by rivers and/or longshore currents. The siliciclastic section extends southward slightly beyond the curving arc of Quaternary reef deposits. The coincidence of the southeastern edge of siliciclastics with the arc of Quaternary reefs suggests that reef development may have been localized on the siliciclastic margin.

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