Florida Geological Survey


(Plate 1). Marker-bed constrained stratigraphic intervals can then be compared on a well-to-well basis. Any significant variations in thickness within a particular stratigraphic interval can then be evaluated in terms of a possible fault, unconformity, or other geological mechanism.

 Anomalously shortened stratigraphic sections are evident in the D. B. Lee and West Melbourne boreholes (Plates 1 and 2). Approximately 70 feet of strata are missing in both wells at two different stratigraphic levels. The omitted section occurs at a depth of approximately -2,086 feet NGVD in the D. B. Lee and
-1,368 feet NGVD in the West Melbourne well. The structure top of the Oldsmar Formation (glauconite marker bed) based on geophysical logs shows that the West Melbourne well is 51 feet higher compared to the D. B. Lee well (Figure 7) which is consistent with the appropriate footwall/hanging wall geometric relationship of a possible normal fault cutting both wellbores where the omitted sections occur (Plate 2). The West Melbourne and Grant Street wells are both located on the footwall or "upthrown" block of the fault and are on strike with respect to the Oldsmar Formation top. Marker beds above the fault cut in the West Melbourne well are structurally lower than the equivalent intervals in the Grant Street well indicating their position on the "downthrown" block of the fault (Plate 2). The fault apparently "dies out" upward above the Ocala Limestone in the Hawthorn Group somewhere between the West Melbourne and Grant Street wells (Plate 2). The similarity in the amount of shortened stratigraphic section or "throw" occurring in the D. B. Lee and the West Melbourne wells and the structural relationships between the West Melbourne and Grant Street wells suggest that the probable fault strikes north-south and is downthrown to the west.

 Difficulties encountered during drilling operations, and unusual pump test results in the D. B. Lee injection well, could be a reflection of anomalous structural conditions in this vicinity. Extremely poor recovery on several attempts to


core could be an indication of highly fractured rock that may be associated with faulting in this wellbore. Pump tests (see Figure 33, Hydrogeology Section) conducted on the D. B. Lee injection well showed an almost immediate response in all three surrounding monitor wells (Knapp, 1989) indicating an unexpected high degree of vertical communication within the Floridan aquifer system at this location. This could be the result of a highly fractured injection and confining sequence, direct communication along a fault plane, or injection well mechanical problems. Sonic log cycle skipping and an erratic caliper log observed from approximately 1,150 feet below land surface (BLS) to 2,185 feet BLS could be explained by the presence of fractured rock (Plates 1, 2 and 3). Fracturing is also apparent on borehole videos beginning at a depth of 1,100 feet BLS down to 2,176 feet BLS.

 Alternatively, the shortened stratigraphic sections in the D. B. Lee and West Melbourne wells could be interpreted in terms of two unconformities rather than a single fault. However, as the missing sections occur at significantly different stratigraphic levels in the two wells, such an interpretation would have to involve two separate unconformities representing two unique episodes of uplift. This interpretation appears less likely, over such a small stratigraphic interval, than one involving faulting given that the amount of missing section is approximately the same in both wells and the unique circumstances of the D. B. Lee.

 Shortened stratigraphic sections in Brevard County wells could be an artifact of karst collapse structures. Conceptually this explanation would entail a sinkhole-like collapse and sediment in-fill with subsequent differential compaction and subsidence across the karst feature (Figure 8). A shortened section could occur between the hypothesized subsided sediment and the karst depression floor. However, the sediment package overlying the karst feature should be thicker overall relative to non-karst well locations. Detailed correlation of marker beds indi-