Bulletin No. 64


which is cross-bedded. Coarse to gravel-sized "lag" zones are common at the base of crossbedded intervals. The cross-bedded sequence probably represents shoreface sedimentation by currents flowing parallel to the shoreline (Scholle et al., 1983). A transition from offshore to shoreface sedimentation is reflected by decreasing amounts of mud in the sediments as wackestone grades upward into packstone and grainstone.

 The beach sequence has a geophysical character that is generally correlative throughout the study area. The carbonates here have an extremely low gamma-ray signal and abnormally high sonic-log porosities due to borehole washout in the moderately indurated limestones relative to the overlying and underlying well indurated dolostones.

 The equivalent section cored in the West Melbourne well (1,396-1,404 feet BLS) consists of interbedded grainstone, packstone, and wackestone similar to that in the Merritt Island well with the exception of sedimentary structures. No bedding features of any kind were evident as the section is apparently highly bioturbated. Bioturbation typically signifies lower-energy conditions and slower sedimentation rates (Friedman and Sanders, 1978). At approximately 1,400 feet BLS a one-foot-thick interval of burrowed, oolitic grainstone (oolite) is present suggesting high energy, shoaling conditions were nearby.

 Core from 985 to 995 feet BLS in the Merritt Island well consists of laminated, moldic, to vuggy dolostone. At 992 feet BLS in this interval, an approximately six-inch-thick section of irregular to wavy algal laminated and moderately indurated dolostone (dolomudstone) may be indicative of tidal flat deposition. Randazzo and Cook (1987) studied approximately 450 feet of upper Avon Park Formation core from west central Florida and concluded that except for a 10foot section, all of the cored interval was "characterized by tidal mudflat sedimentation."


Sedimentary structures indicative of tidal flat sedimentation included micritic crusts, rip-up clasts, contorted algal laminations, burrows, mottles and thin peat lenses.

 GEOPHYSICAL CHARACTER OF THE LOWER FLORIDAN AQUIFER SYSTEM

 The typical suite of borehole geophysical logs run as part of injection well-evaluation procedures includes gamma-ray, sonic, caliper, and induction resistivity. Lower Floridan aquifer system carbonates have characteristic responses to each geophysical tool which depend primarily on lithofacies type, mineralogy, porosity, water chemistry, and borehole conditions. The following discussion focuses on the general geophysical characteristics of representative lithofacies and certain key intervals within the lower Floridan aquifer system of Brevard County.

 Poorly- to moderately-indurated limestones can be in many cases distinguished from interbedded, highly-indurated dolostones by using borehole geophysical criteria. Boreholes commonly enlarge or "wash-out" across poorlyto moderately-indurated lithologies and remain in gauge across highly-indurated zones during drilling operations. The effect is most apparent on caliper logs where relative borehole size variations can be readily noted. Sonic logs can record erroneously long travel times (i.e., high porosity) across wash-outs due to increased sound wave travel distances making porosity determinations invalid (Gulf Research and Development Company, 1978). A slight effect on the gamma-ray log in terms of reduced radioactivity across these zones can also be recognized. Induction resistivities are typically low in moderately- to poorly-indurated limestones due to abundant saltwater-saturated pore space in the rock.

 A unique mineralogical assemblage within the uppermost Oldsmar Formation imparts a distinctive geophysical property to the interval resulting in a highly correlative marker horizon (Plates 1,