Nomenclature of lithotectonic features discussed in this report was chosen based on the following criteria (when applicable): 1) the names accurately describe the location of the feature, named after towns centered on or near the feature's axis; 2) the axis, center or extent of the more regional features is well defined by the nomenclature (e.g., Peninsular Arch and South Florida basin); 3) the names are consistent for genetically related Mesozoic features in the region (e.g., the Conecuh and Apalachicola Embayments; and 4) the nomenclature is well established in state and federal literature as well as in the oil and gas industry. Admittedly, revision of some of the nomenclature would give a more accurate lithotectonic description of these features. For example, the "embayments" might be better defined if called "grabens." However, due to the need for consistency and to avoid further confusion by adding to the list, no new names are introduced. The Florida basement nomenclature used in this report should be used in subsequent publications. All structural features shown on the Florida basement map (Figure 12) are discussed in the following text from northwest to southeast.
In the Florida panhandle, the Conecuh Embayment, the Chattahoochee Arch and the Apalachicola Embayment are thought to be a graben-horst-graben sequence formed during Mesozoic rifting (Miller, 1982). These grabens contain Triassic Newark Group equivalent sediments (Eagle Mills Formation) and Jurassic sedimentary rocks interlayered or cut by basalts and diabase. Contact metamorphic aureoles are associated with the intrusions (Milton, 1972). In the above context, the horst is represented by the Chattahoochee Arch, which is an uplifted block consisting of Triassic and Paleozoic sediments overlying an Upper Precambrian Lower Cambrian igneous terrane. Arden (1974) quantified the extent of Paleozoic sediments in the area with seismic data, identified the block as the western limit of the northwest trending Paleozoic Suwannee basin and suggested the presence of a thick Triassic section in the uplifted block. More recently, however, Arden (1987, written communication) notes that most of the sediments near the arch are Paleozoic (based on fossil evidence) rather than the uppermost part being predominantly Triassic, as previously reported (Arden, 1974). In contrast, lithologic descriptions and well-log interpretations suggest there is no Traissic in this area (G. Winston, personal communication, 1988).
The Suwannee basin sediments generally consist of Ordovician quartzitic sandstones and Silurian to Devonian black shales and siltstones, some of which are red and may be confused with Triassic red beds. Paleontological evidence constrains the age of these Paleozoic rocks and suggest an early Paleozoic connection of Florida to Africa and South America (Gondwana) (Cramer, 1973; Pojeta et al., 1976). More recent paleomagnetic (Opdyke et al., 1987) and geochronologic evidence (Opdyke et al., 1987; Dallmeyer, 1987) support this conclusion.
Felsic igneous (and metamorphic ?) rocks underlying the Suwannee sediments are directly below the top of the basement surface in the Florida panhandle. These crystalline rocks may belong to the same terrane as that of central Florida (Chowns and Williams, 1983). An unpublished K-Ar (feldspar) age determination of 709 + 25 Ma from a granodiorite in Gulf County (Earth Resource Consultants, Inc., 1981; W-12509, P-746), however, suggests that the panhandle igneous complex may be older than that of central Florida.
The northeast-trending Apalachicola Embayment lies east of the Chattahoochee Arch. Corehole data indicate that the Apalachicola Embayment contains secondary horst blocks (Barnett, 1975). The Triassic sediments located east of the Apalachicola Embayment (those centered on Taylor County; Barnett, 1975) may be erosional remnants of a separate basin or the preserved eastern extension of the Embayment separated by a secondary horst block. In contrast, if the red beds beneath Taylor County are Paleozoic rather than Mesozoic as suggested by Winston (1987, personal communication) then they represent part of the Suwannee basin.
Palynology of sedimentary rocks (written communication, D.L. Martin, Sohio, 1985) and radiometric data from basalts (Table 1) sampled from the Gainesville 707 offshore corehole (Figure 1, corehole location number 3) indicate that the Triassic graben(?) of the Apalachicola Embayment and/or Taylor County Mesozoic(?) sediments extend at least 40 miles into the Gulf of Mexico. Ironically, overlying the basalts at 12,440 feet in this corehole, rhyolitic cuttings yield a Rb-Sr whole rock age of 576 + 20 Ma (Sohio, 1986). These have been interpreted to represent coarse clastic deposits "whose source is an uppermost Precambrian to lowest Cambrian terrane of acid volcanics and fine grained granites" (written com34