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Seismic Stratigraphy of the Central Indian River Region

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Stratigraphy of the Indian River Region
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Stratigraphy of the Indian River Region - Results
Jim Flocks
  Approximately 82 line-km of seismic profiles were collected from the central Indian River region and adjacent offshore areas (Fig. 1) in an attempt to identify the fault postulated by Bermes (1958) and Schiner and others (1988). Bermes' (1958) identification of the fault was based on well logs. Schiner and others (1988) included water quality differences to delineate the fault. The fault is reported to strike parallel with the lagoon in a NNW direction and turns NE towards the Atlantic Ocean north of Johns Island. Water samples taken from Floridan Aquifer wells located east of the fault had chloride concentrations between 1,400 to 2,900 parts per million (ppm). The Floridan Aquifer wells that were sampled to the north and west of the fault had chloride concentrations of less than 700 ppm. The chloride gradient suggests that the fault may provide a pathway for upward migration of saline water. The location of the fault as suggested by well logs (Bermes, 1958) indicate a -60 to -90 m offset (-200 to -300 ft) of the top of the Ocala Limestone. The Hawthorn Group thickens from ~73 m (240 ft) north of Johns Island to ~153 m (500 ft) south of the proposed fault. The cross section of natural gamma logs (Fig. 4) shows the dramatic change in elevation of the top of the Ocala Limestone and the relatively constant elevation of the top of the Hawthorn Group.

  Figure 4
Figure 4
Where possible, survey tracklines were run parallel and perpendicular to the proposed fault trace. The tracklines acquired within the lagoon were constrained by navigable waters. Data quality varied from good to poor depending on the geologic and environmental conditions. Examination of gamma logs indicate a major lithologic change from sediments with high percentage clay to carbonates (Fig. 4, orange line). This change, interpreted to be near the top of the Ocala Limestone, could provide sufficient amplitude and velocity contrast to produce reflectors. In the seismic profiles, a series of strong reflections are laterally continuous at approximately 100-120 ms (Fig. 9, 10, 11; shown as orange line).

Plotting the depths to the Ocala Limestone from the gamma logs versus TWTT from the orange horizon on the seismic data throughout the study area yields a best fit curve that can be used to calculate an average velocity of 1,955 m/s for the Hawthorn Group sediments (Fig. 3). The peaks in the gamma logs are interpreted to lie just above the top of the Ocala Limestone. Though the orange reflector cannot be positively identified as the top of the Ocala Limestone, it is sufficiently close and can be used to identify morphology and/or structural trends in the Group. Due to the massive nature and the irregular surface of the Ocala, the high-frequency acoustic signal used in the HRSS generally provide poorly resolved reflectors in the carbonates below the Hawthorn Group (Kindinger and others, 1994), but resolution was better than expected in these data. In some areas a horizon was identified that may correlate with the Avon Park formation, as interpreted from the gamma logs. This is represented by the blue horizon on the profiles. The blue horizon can be seen in the northern section of the profiled area and is lost where it dips steeply to the south and east near Johns Island (Fig. 8, 9, 10, 11, 12). Above the orange horizon another reflector can be traced throughout the study area (purple line). This horizon is laterally continuous and remains level to gently dipping to the south and east (Fig. 6). The horizon can be correlated with the gamma logs to represent a laterally continuous unit within the Hawthorn Group. The shallowest reflector (red) traced throughout the study area represents a surface that truncates deeper, low-angle bedding and levels off subsidence or channel fill (Fig. 11). This stratigraphic and seismic character is indicative of a flooding surface. An average velocity of 1,955 m/s places the reflector at approximately 120 meters depth adjacent to wells IR00498, IR00699 and IR00024. This correlates very well with the interpretations of the gamma logs from those wells (Fig. 4), which indicate the top of the Hawthorn Group at that depth.

The Schematic Cross-section A-A' (Fig. 13) and corresponding seismic profiles of Figures 9 and 11 show how the sediments between the orange and the red reflectors thicken dramatically from the north-northwest to the south-southeast. The red reflector dips somewhat, but not nearly as much as the orange and blue reflectors. The thickening of the units below the red reflector suggest deposition into a basin or subsidence or faulting occurring during deposition (Fig. 14). There are smaller subsidence and solution/collapse features found beneath the red horizon throughout the study area (Fig. 10, 11 - S1 & S2, 12 - S3). The deeper area of the thickened sequence is much too large to simply be subsidence into a single collapse sinkhole. This trend is of a magnitude that could effect water quality such as identified by Schiner and others (1988).

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