A Neogene mixed siliciclastic and carbonate foundation for the Quaternary carbonate shelf, Florida Keys

New core holes and marine seismic profiles show the distribution and paleoenvironments of a siliciclastic foundation sandwiched between the much-studied Quaternary carbonates of the Middle and Upper Florida Keys and Oligocene to Miocene carbonates of the Arcadia Formation, a distance of some 160 km. Throughout this area the Quaternary carbonates are generally no more than 30 m thick and are underlain by up to 145 m of Plio-Pleistocene mixed limestones and sands, and marine siliciclastics (Peace River Formation). Quartz sands, days, and minor reworked phosphatic grains of the Peace River are the distal deposits of an almost peninsula-wide blanket that interrupted prolonged Tertiary carbonate deposition in southernmost Florida. In the Lower Keys the Peace River siliciclastics change facies to very fine- to fine-grained carbonates composed of bioclastic, planktonic-foraminiferal grainstones and packstones. The position of the present southern Florida shelf margin, with its discontinuous coral reefs, is determined by the seaward paleoslope of siliciclastics and/or carbonates of the Plio-Pleistocene section defined in the Florida Keys. Lithostratigraphy of five continuous cores on the Keys and southeastern mainland, each about 300 m long, combined with numerous well logs and sample descriptions from the Keys and mainland, establish the southern limit of Peace River siliciclastics to at least the Middle Keys. The well data show intervals of quartz of coarse sand to granule size localized as a north-south pathway of maximum paleocurrents that extend from the central part of the peninsula approximately 350 km to the Middle Keys. This pathway of maximum paleocurrents, about 100 km wide, is interpreted as the palimpsest record of unusually strong, southward-moving shoreline and channeled deposition or a giant prograding spit. The ultimate source of these siliciclastics is from the Appalachians, approximately 1000 km to the north. We propose that the Plio-Pleistocene marine siliciclastics and carbonates (including the Peace River Formation) recovered in wells on the Keys extend seaward beneath the present southern Florida outer shelf and are present in seismic profiles acquired across the shelf-margin slope. A series of profiles forming an offshore traverse from the Lower to Upper Keys show seaward-dipping clinothems that locally dip obliquely to the arcuate strike of the Florida Keys. These clinothems downlap on a prominent reflector that can be traced to an exposed submarine surface of upper Tertiary carbonates, the Pourtales and Miami Terraces. Some reflectors show configurations similar to contourites, and others, to channels. We contend that southward entry of the siliciclastics into southernmost Florida was deflected eastward by west-to-east channeled flow of an ancestral Florida Current.