Florida Geological Survey


stone, mudstone, and dolostone and a lower section of predominantly well-indurated, crystalline dolostone. Benthic foraminifera and echinoderm fragments are the principle allochems composing the packstones and wackestones of the upper interbedded sequence. Clay is a common, but minor accessory mineral in the glauconitic wackestones of the upper Oldsmar Formation. Limestones are moderately cemented with a grain-fringing rim of microspar cement; however a pore-occluding, fresh-water, phreatic-spar cement was observed in a well-indurated packstone in the Merritt Island core at 1,720 to 1,723 feet BLS and the West Melbourne core at 1,967 feet BLS. Limestone porosity is generally high (20 to 30 percent) and permeability, based on visual estimates, is moderate to high.

 Dolostones of the upper Oldsmar Formation are microcrystalline to finely crystalline and fossils are typically not preserved. Laminations, burrows, mottles, and spar-filled root traces are common in dolostones of the upper interbedded sequence. Porosity is generally five percent or less and permeability is low. Matrix-selective dolomitization is apparent in some Oldsmar Formation dolostones where unaltered calcitic allochems appear to "float" in a finer grained dolostone matrix. Partial replacement imparts a speckled nature to some dolostones in this interval. Cores and geophysical logs indicate the dolostone beds range from approximately five to ten feet thick.

 The lower Oldsmar Formation is characterized by grayish-brown, microcrystalline, dense, and generally non-fossiliferous dolostone. The top of this sequence is a distinctive marker horizon ("C" marker bed) on gamma-ray and sonic logs and is correlative throughout the study area (Plates 1 and 2). The gamma-ray log shows increased radioactivity below this horizon relative to the overlying section. The sonic log shows a marked decrease in interval transit time at this point, which is indicative of the low porosities (less than five percent) and permeabilities prevalent throughout much of this interval.


 Middle Eocene Avon Park Formation

 Miller (1986) defined the Avon Park Formation as "the sequence of predominantly brown limestones and dolomites (dolostones) of various textures that lies between the gray, largely micritic limestones and gray dolomites (dolostones) of the Oldsmar Formation and the white foraminiferal coquina or fossiliferous micrite of the Ocala Limestone." In Brevard County, the Avon Park Formation is characterized by white limestones ranging from grainstone to mudstone interbedded with grayish-brown to grayishorange dolostones commonly containing organics. Cherty dolostones are typical of the lowermost Avon Park Formation.

 In Brevard County, the top of the Avon Park Formation is marked by a slight radioactive peak on the gamma-ray log which characteristically coincides with the first occurrence of Dictyoconus .. foraminifers (Plates 1 and 2). The top of the Avon Park Formation varies from
-232 feet NGVD in the Merritt Island well to -443 feet NGVD in the Harris Corporation well. Formation thickness averages approximately 1,500 feet across the study area.

 The uppermost Avon Park Formation consists of very light orange to white, moderately indurated wackestones and packstones containing abundant Dictyoconus a. foraminifers. Below this interval, the Avon Park Formation is characterized by interbedded dolostones and limestones. The principal allochems are whole foraminiferal tests and echinoderm skeletal fragments. Organic flecks are common throughout much of the formation. Ooids were present in a middle Avon Park "oolite" in the West Melbourne well. The range of allochemical and textural alteration in dolostones varies from complete preservation to total destruction depending on the particular diagenetic mechanism. A gammaray marker bed, designated the "B" marker bed (Plates 1 and 2), occurs approximately midway through the Avon Park Formation and serves as an excellent reference datum for correlation