Northeast Florida Lakes & Rivers Home Open File Report: Seismic Reflection Surveys   Abstract   Introduction   Karst Formation   Methods   Regional Geology   Orange Lake:   Seismic Investigations   Lake Geology   Karst & Sinkhole Features   Southeast Area   North & Central Area   Southwest Area   Kingsley Lake   Lowry & Magnolia Lakes   Drayton Island   Conclusions   References Contact: Jim Flocks   The general geologic framework at Orange Lake is similar to that discussed earlier in the regional geology section. The surficial sediments include sands and clays of Plio-Pleistocene age. These are underlain by clays, sandy clays and carbonates of the Miocene age Hawthorn Group. The Ocala Group carbonates are the oldest units exposed in this area and can be seen in roadcuts and the numerous quarries. The contact between units exhibits highly irregular surfaces typical of karst. Surficial sands and clays range from a few feet thick (~1 m), over most of the area, to about 60 feet (~20 m) thick in the Fairfield Hills. Sands from the Fairfield Hills are eroded and deposited in the Orange Lake bottom as well.   Below - Figure 12: Gamma log cross section A-A' Orange Lake. See Figure 6 for locations of cross sections A-A' and B-B'. Above - Figure 13: Gamma log cross section B-B' Orange Lake. The erosional remnants of the Miocene Hawthorn sediments underlie Orange Lake. The Hawthorn functions as an aquiclude, preventing the lake from totally draining into the Floridan aquifer. Natural gamma logs from boreholes adjacent to Orange Lake (M-0351, M-0347, M-0154, M-0157, A-0456 and M-0361) (Figs. 12, 13) indicate that the clays range in thickness from about 11 meters at the south and southwest areas of the lake to over 25 meters thick on the east side at A-0456. The thick section at A-0456 may be the result of the well being located on a buried sinkhole. Additional well logs from this area are needed to confirm this interpretation. Figures 12 and 13 identify the depth from the January 1994 lake stage to the top of the Ocala Group as determined from natural gamma logs of adjacent wells. For undisturbed sections near borehole M-0361, for example, the top of the Ocala Group should be seen at approximately 23 milliseconds on the seismic records depending on which velocity is chosen for the calculations. Borehole profiles in the vicinity of the seismic profiles are needed to confirm the conversion of the two way travel time to depth of a reflector. There is no borehole data available for the north or northeast sections of the lake at this time. As shown below, the top of the Ocala Group carbonates range from about 20 feet NGVD to greater than -20 feet NGVD. The overall trend reflects a dip to the northeast with highs and lows superimposed because of the karst. Dissolution of the carbonate is very active and can be seen at the Heagy-Burry Park. For years, dirt fill has been periodically brought to an actively subsiding sinkhole on the east side of the park's boat ramp. This sinkhole is probably representative of the many cover subsidence sinks in Orange Lake.   top of page  |  next section Coastal and Marine Program > Center for Coastal Geology > Northeast Florida Lakes & Rivers Home > Seismic Reflection Surveys > Orange Lake Geology U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology http://coastal.er.usgs.gov/stjohns/ofr616/orange1.html Address questions and comments to Trent Faust - Webmaster Updated February 29, 2000 @ 03:57 PM (THF)