DOI: 10.5066/f77942r3
title: Land and marine seismic profiles used to assess the seismic stratigraphy and structure of the intermediate confining unit and Floridan aquifer system, Broward County, Florida

description: In southeastern Florida, the need exists to more accurately map previously defined stratiform permeability units of the Floridan aquifer system and map areas of possible vertical cross-formational fluid flow. Primarily, this is because at some deep-well injection sites in southeastern Florida upward cross-formational movement of effluent injected into the saline lower Floridan aquifer has been detected above the base of the U.S. Environmental Protection Agency underground source of drinking water within the brackish upper Floridan aquifer. New high-resolution, 2D, water-based, seismic-reflection data acquired from canals in eastern Broward County have been used to characterize the stratigraphy and geologic structures (karst collapse structures and faults) that could be components of cross-formational fluid-flow pathways of high hydraulic conductivity that enable vertical movement of effluent. In most cases, the karst collapse structures manifest as vertically stacked sagging seismic reflections; where displayed as a plan view, the sag has a semicircular shape. These seismic-sag structures are commonly visible on seismic-reflection profiles spanning early Eocene to Miocene age rocks that are equivalent to much of the Floridan aquifer system and an overlying lower part of the intermediate confining unit. In some cases, the seismic-sag structures extend upward into rocks and sediments of Pliocene age. In the eastern Broward County study area, an approach that integrated interpretation of seismic-reflection data (including multi-attribute analyses) with borehole geophysical and geologic data from wells has enabled the characterization and delineation of seismic sequences and equivalent depositional sequences, seismic-sag structures, and one tectonic fault over an area of about 725 square miles. Twenty seismic-sag structures, which are thousands of feet in vertical scale, have been identified on seismic profiles. Both multi-attribute analysis and visual detection of offset of seismic reflections within the collapse structures indicate faults are associated with many of the karst collapse structures. Multi-attribute analysis also suggests that the collapse structures have a high probability of vertical cross-formational fluid flow along the vertical extent of the structures. Comparison of the seismic-sag features to similar structures in other carbonate basins of the world provide supporting evidence that they are related to karst collapse. Hypogenic karst processes by ascending artesian flow of subsurface fluids are most likely to have produced the karst collapse of strata. Epigenetic karst related to major regional unconformities within the Florida Platform has also generated collapse structures that are of a much smaller scale than the vertical seismic-sag structures. Stratigraphic analysis of the seismic-reflection data identified numerous regional-scale seismic units, which extend from the uppermost rocks of the Lower Cretaceous to the base of Pleistocene rocks and sediments. Many of the seismic-stratigraphic units are seismic sequences sensu stricto and are informing the construction of a new Late Mesozoic to Cenozoic seismic-sequence framework in southeastern Florida. The approximate upper boundary of four principal permeable units of the Floridan aquifer system (Upper Floridan aquifer, Avon Park permeable zone, uppermost major permeable zone of the Lower Floridan aquifer, and Boulder Zone) have seismic-reflection signatures that can be mapped, and therefore the seismic stratigraphy provides a new means for the accurate mapping of these hydrogeologic units. The permeability of these hydrogeologic units is related to megaporosity generated by stratiform epigenetic karst in all cases, but interparticle porosity also contributes to the permeability, which is especially noteworthy for the Upper Floridan aquifer. The input of the new seismic stratigraphy allows, for the first time in southeastern Florida, 3D geomodeling of hydrogeologic unitsnever before accomplished using well data alone in Broward County. Notably, the 3D geomodeling of the units could be used to update groundwater simulations designed to address the sustainability of the water resources of the Floridan aquifer system.