Stratigraphic information from high-resolution seismic data obtained across the southwest Florida platform indicates that the modern shelf is a constructional platform with Pliocene(?)-Pleistocene and Holocene sediments resting on an eroded karstic Miocene platform. The Miocene surface dips away from the coastline with significant breaks in slope occurring at the center of the shelf and at the shelf edge. At the southwest corner of the platform, this surface crops out to form a terrace. This terrace lies along the west-facing continental slope of the Florida shelf and is progressively buried to the south by younger deposits—reefs and sediment—so that it has no surface expression in the Florida Straits. A paired reef complex rests on the thickest post-Miocene sediments that mark the edge of the modern shelf. The deepest reef forms a well-developed escarpment with its crest buried by approximately 15 m (50 ft) of sediment. The shallower reef is a low swale over most of its extent but develops into a large reef-spit complex (Howell Hook) in the central part of the study area. Within the Pliocene-Pleistocene and Holocene sediments, two stratigraphic units can be delineated: (1) a lower progradational unit of Pliocene-Pleistocene(?) age that can be traced under the shelf-edge reef and continuously onlaps the Miocene(?) surface, and (2) an upper unit of late Pleistocene-Holocene age which is composed of reef and pelagic sediment.
Two sedimentary “fans” have been identified on the northern slope and floor of the Florida Straits. The apices are set in at gaps in the carbonate ridge rimming the southern Florida shelf. At least nine complex sigmoidal-oblique sequences are identified within the fans. A reef buried by sediment of the last two sequences correlates with Howell Hook. Radiometric analyses of material dredged from this feature and from the outcrops of the second sequence yield ages of 7.7-11 ka, indicating a Holocene age for these sediments and suggesting a very high rate of shelf accretion.