MORPHOLOGIC FEATURES OF CONDUITS AND AQUIFER RESPONSE IN THE UNCONFINED FLORIDAN AQUIFER SYSTEM, WEST CENTRAL FLORIDA

Conduits within the unconfined Floridan Aquifer of west-central Florida include both horizontal and vertical components. In this paper, we investigate each and propose theories based upon cave survey data and a collection of over 300 Florida cave maps. First, we find that vertical portions of conduits visually correlate to fractures, and these fractures tend to be the dominant control of conduit directionality. Length-weighted rose diagrams of passage directions reveal a NW-SE and NE-SW pattern of conduit directions statistically similar to results found in remote sensing studies of photolinears. Secondly we note that horizontal elements of conduits occur at consistent horizons that are pervasive and laterally continuous. Their control is presently unknown but is potentially the result of some combination of lithology, fracture density, and water-table position. In this paper we demonstrate the variability of hydrographs from springs in karst areas. Traditional observations and interpretations of springflow hydrographs from karst areas are from telogenetic karst (e.g., midcontinent Paleozoic limestones), where the karst has developed after the limestone has been deeply buried and lithified. Limestones of telogenetic karst have very low porosities (less than 5%) and matrix permeabilities on the order of 10 – 10 m. Hydrographs of springs in telogenetic karst show a prompt response to precipitation events and a rapid return to base flow conditions. Because the matrix permeability is so small, only a very small percentage of the full volume of the aquifer appears to participate in the precipitation event. Springflow hydrographs from the eogenetic karst of Florida are in striking contrast to those of telogenetic karst. These springs respond very little to single storm events, but do vary according to seasonal or longer-period cycles. It appears that the huge interparticle pore volume (20-40%) and matrix permeability (10 – 10 m) of these Eocene and Oligocene limestones mutes, and even eliminates, the spiky responses that have come to be featured in conceptualizations of karst springs. Likewise, traditional classification of conduit morphology is from observations in telogenetic karst. In telogenetic karst aquifers, current classification is based upon the type of recharge to the aquifer (allogenic or hypogenic) and whether conduits are controlled by fractures or bedding planes. Recent observations in eogenetic karst (e.g., Bahamas) do not fit these classifications. We suggest that a more appropriate classification scheme of karst would incorporate diagenetic maturity of the rock, as well as recharge type and fracture density. In such a scheme, conduits of the unconfined Floridan Aquifer represent a mid-point in a range between Plio-Pleistocene karst on young carbonate islands and Paleozoic karst in the Appalachian lowlands. CONDUITS WITHIN THE FLORIDAN AQUIFER SYSTEM Tertiary carbonate units of the Florida platform have experienced a complex depositional and diagenetic history recorded by variations in lithology and by the development of macroporosity through dissolution. Dissolution The 12 Symposium on the Geology of the Bahamas and other Carbonate Regions 33 features are present throughout the platform. Paleokarst features are omnipresent at specific horizons in Florida and were formed during periods of prior exposure. Active karst processes are limited to regions of vigorous groundwater circulation within the Floridan Aquifer System. These conditions exist where the Eocene and Oligocene carbonates are semi-confined to unconfined; principally occurring in west-central Florida in a broad band bordering the Gulf Coast and stretching approximately from Tampa to Panama City (Figure 1).