Bulletin No. 64


 STRUCTURAL GEOLOGY

 Regional Structural Framework

 In Florida, Mesozoic and Cenozoic sediments overlie an eroded basement rock complex ranging from Precambrian to Jurassic (Barnett, 1975). The Peninsular Arch (Figure 4), the dominant structural feature of Florida, is a northwest-southeast trending positive basement element cored by a large block of Precambrian rock covered by Paleozoic strata (Barnett, 1975). The Peninsular Arch has been a positive feature affecting sedimentation from the Jurassic into the early Cenozoic (Miller, 1986).

 Structural Framework of Brevard County

 Brevard County lies on the eastern flank of the Peninsular Arch. Depth to basement ranges from approximately -7500 feet in the northwest to -11,000 feet National Geodetic Vertical Datum (NGVD) in the southeast portion of the county (Barnett, 1975). Barnett's (1975) subZuni subcrop map shows that basement rock in Brevard County consists of Middle Cambrian Osceola Granite with possible Jurassic volcanics in the extreme southern portion of the county. An apparently significant subsurface basement fault trends northwest-southeast from near the Florida-Georgia border down through the central portion of Brevard County according to Barnett's (1975) basement structure map (Figure 5). The interpreted normal fault is downthrown to the east.

 The most prominent structural feature influencing the lower Floridan aquifer system in Brevard County is the Brevard Platform, described originally by Riggs (1979). Scott (1988) characterized the Brevard Platform as a low relief ridge or platform that plunges gently to the south-southeast and southeast (Figure 6). Both the Ocala Limestone and Hawthorn Group sediments erosionally thin across the Brevard Platform and have erosional upper surfaces (Brown et al., 1962; Scott, 1988). The observed


degree of thinning increases to the north into Seminole and Volusia Counties where both the Ocala Limestone and Hawthorn Group are missing after erosionally wedging out along the flanks of the Sanford High (Vernon, 1951). Riggs (1979) considered the Brevard Platform a southern extension of the Sanford High.

 West of the Brevard Platform is the Osceola Low, described by Vernon (1951) as a faultbounded low with a significant thickness of Miocene sediments. Vernon's postulated fault that forms the eastern boundary of the Osceola Low trends north-northwest roughly following the Brevard Osceola County line and is upthrown to the east. Subsurface structure maps constructed on top of the Ocala Limestone for this area indicate anomalous apparent dip directions with possible dip reversals in the vicinity of Vernon's proposed fault. Scott (personal communication, 1991) interpreted the feature as "a possible flexure or perhaps a zone of displacement with 'up' on the east and 'down' on the west."

 Strata of the lower Floridan aquifer system in Brevard County dip generally to the southeast away from the Brevard Platform axis at an average angle of 0.1 degree (Figure 7). Apparent dip angles are greater in the Melbourne Port Malabar vicinity ranging from 0.2 to 0.5 degrees locally. Several apparent dip reversals occur along a southeasterly trend from the West Melbourne site to the Port Malabar site.

 Several lines of evidence indicate the possibility of normal faulting in Brevard County. The concentration, amount and quality of data in the Melbourne vicinity is much greater than that available in other areas making fault identification more confident. However, faulting is probably not restricted to this area. After detailed correlation of injection well geophysical logs (gamma-ray and sonic), a sequence of correlative marker horizons can be recognized and the thickness of specific stratigraphic intervals relative to the marker horizons can be determjned