Bulletin Na. 64


 Both penecontemporaneous and diagenetic dolornitlization processes have apparently affecl. ed lower Floridan aquifer system carbonates in Brevard County- Dolostones associated with algal laminations and subaerial exposure surfaces are likely supratidal and at least partly peneconlemporaneous in origin, Partial, matrix selective dolomilizat[on is common in many of the lower Floridan aquifer system dolastones and may be an important factor in the developmeni of moldic porosity (Murray, 1960). Murray (1960) suggested that where only partial dolomitization occurs, porosity can be created by 1he dissolution of nan-replaced calcium carbonate remaining in the rock, possibly during periods of subaerial exposure. The origin of massive, regionalJy pervasive dolostone sequences within the lower Floridan is best explained by marine dolomitization, probably penecontemporaneous with deposition in a tidal flal environment.

 HYDROGEOLOGY

 General Hydrogeologic Summary of the
 Floridan Aquifer System

 Four major hydrogeologic units occur in peninsular Florida (SEGS 1986). These are the surti. cial aquifer system, the intermediate confining unit or intermediate aquifer system, the Floridan aquiter system and the sub-Floridan confining unit (Figure 10]. The Floridan aquifer system consists of the upper Floridan aquifer system, the middle confining unit and the lower Floridan aquifer system, The hydrogeology tor only the middle confining unit and lower Floridan aquifer system will be discussed in delail.

 The upper-most hydrologic unit in the study area is the surticial aquifer system which is comprised of a thin blankel of terrace and fluvial sands, shell beds and sandy limestones of PJiocene, Pleislocene and Holocene age. In Brevard Countly, the suificial aquifer system is a permeable unit contiguous with land surface which varies in thickness from 90 to 150 feet (Plate 3). The surficial aquifer system is an


unconlined aquiter under water table conditions and is an important source ot drinking water for more than half oi Brevard and Indian River counties (Scott et al., 1991) (Figure 12}.

 The intermediate confining unit in the study area is associated with the Hawttorn Group, The Hawthorn Group sediments separate the overly ir;g surficial aquifer system and the underlying Floridan aquifer system- These sediments consist of a low-permeability sequence ol siliclaslic sediments and carbonales which effeclively confine 1he Floridan aquifer system throughout most of Ihe study area (Plate 3). The intermediate aquifer system is not well developed in eastern ceniral Florida and is not an imporlanl source of drinking water. Locally, the intermediate confining unit may be breached due to sinkhole activity or erosion; thus, the upper Floridan aquifer system may be under confined, semiconfined or unconfined conditions.

 The Floridan aquifer system, as defined by Miller (1986), is a vertically continuous sequence of carbonate rocks of generally high permeability. These middle to upper Tertiary carbonates are hydraulically connected to varying degrees. Permeability is typically several orders of magnitude greater Ihan those rocks thal bound Ihe system above and below.

 In Brevard County, the Floridan aquifer system generally consists of Iwo major permeable zones (Plate 3) separated by a middle confining unit of lower permeability (Miller, 1986). The upper and lower Floridan aquifer systems and the middle confining unit are comprised of a sequence of Paleocene to Eocene carbonates. These carbonates may be hydraulically connected or separated based upon highly variable local geo logic conditions.

 In the lower Floridan aquifer system of southern Florida, there is a subzone of highly fractured and cavernous dolostone which exhibits high iransmissivities (Miller, 1986), The cay-