EVIDENCE FOR HYPOGENIC KARST USING SEISMIC-REFLECTION DATA FROM THE SUBSURFACE OF THE SOUTHEAST FLORIDA PLATFORM

The integration of geologic and geophysical borehole data from 44 wells and approximately 80 miles of water-based, high-frequency, seismic-reflection data acquired in canals in Broward County along the southeast Florida Platform, detected 18 buried seismic-sag structures that are related to hypogenic karst. Also, 3D seismic-reflection data from the Miami Terrace further inform a 3D conceptualization of the seismic structures and provide insight into the karst origin of these structures. The seismic-sag structures are commonly characterized by vertically stacked, concave-upward arrangements of parallel seismic-reflection patterns where, in some cases, the seismic reflections are interpreted as vertically cylindrical volumes of chaotic reflection patterns or seismic chimneys. Typically the seismic-sag structures are expressed over heights of up to several thousand feet within lower Eocene to Pliocene strata, and in one case extend upward over a vertical distance of about 7,500 ft from lower Cretaceous into Pliocene strata. These towering karst structures are the dominant structural element in the subsurface ‘landscape’ of the study area. Both multi-attribute analysis and visual detection of seismic-reflection offset within the collapse structures indicate faults and fractures are associated with many of the karst collapse structures. Multi-attribute analysis also indicates that the seismic-sag structures have a high probability of vertical cross-formational fluid flow along the vertical extent of the structures. Over much of the area, thick late Miocene to Pliocene siliciclastics, which have prograded across a regional middle Miocene unconformity capping the thick stack of karsted platform carbonates, both partly fill and bow downward into depressions along the unconformity created by the underlying tall, vertical, carbonate karst collapse structures. This indicates collapse during both pre- and post-burial of the unconformity and progressive karstification from at least the middle Miocene until after deposition of the late Miocene to Pliocene siliciclastics. Post-burial collapse rules out epigenic karstification and indicates hypogenic karstification. Offshore seismic-reflection data provide further evidence for upward subsurface fluid flow and hypogenic karstification.