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Open File Report:
Stratigraphy of the Indian River Region |
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Summary |
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Analysis of seismic data and natural gamma logs from wells
within the study area indicate the Hawthorn Group to be dipping
to the southeast in response to subsidence or dissolution in the
underlying carbonate rock. Fluid migration and rock movement and
dissolution along a deeper fault zone is a possible mechanism for
the subsidence. Bermes (1958) and Schiner and others (1988) used
well logs and water quality data to infer the presence of a fault
system within the area. The fault system would predate the
Miocene since the seismic profiles show no evidence of faulting
within the Hawthorn Group.
Comparison of trends in contours generated from the seismic
data with the cross section from the natural gamma logs show good
correlation. Correlating measured depths on the gamma logs with
depth-to-horizon on the seismic profiles indicate an average
sound velocity of 1,955 m/s through the Hawthorn Group. This
estimate is within the range of velocities suggested from other
studies of the Miocene sediments in Florida. Absolute
correlation between gamma log depth and two way travel time
could not be established since the gamma log cross section did
not intersect the seismic profiles.
Other features identified in the seismic profiles include
three collapse sinkholes within the Hawthorn Group sediments.
Two are located north of Johns Island in the Intracoastal
Waterway on profile SB_2
(Fig. 11, S1 & S2). The other is
located about one mile offshore east of the city of Vero Beach on
profile SB_1
(Fig. 12).
Acknowledgments
The authors would like to express their thanks to the
Governing Board of the St. Johns River Water Management District
(SJRWMD),and Douglas A. Munch of SJRWMD, for continuing support
of high-resolution seismic reflection studies within the
District. Thanks to William Osborn, P.G. and David Toth, P.G.
for assistance in planning and technical review. We would also
like to recognize Dana Wiese (USGS) for operating the seismic
equipment, Micah Weltmer (USGS) for graphics support, and Shane
Dossat (SJRWMD) for his support in the field.
References
Bermes, B.J. , 1958, Interim report on geology and ground-water
resources of Indian River County, Florida: Florida Geological
Survey Information Circular 18, 74 p.
Brooks, H.K., and Merrit, J.M., 1981, Guide to the physiographic
divisions of Florida: Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences, University of
Florida, Gainesville. 1 map 150x105 cm and text 16 p.
Kindinger, J.L., Davis, J.B., Flocks, J.G., 1994, High-Resolution
Single-Channel Seismic Reflection Surveys of Orange Lake and
other selected sites of North Central Florida: U. S. Geological
Survey Open File Report 94-616, 48 p.
Miller, J.A., 1986, Hydrogeologic framework of the Floridan
Aquifer system in Florida and parts of Georgia, Alabama, and
South Carolina: U. S. Geological Survey Professional Paper
1403-B, B91 p.
Sacks, L.A., Lee, T.M., and Tihansky, A.B., 1991, Hydrogeologic
setting and preliminary data analysis for the hydrologic budget
assessment of Lake Barco, an acidic seepage lake in Putnam
County, Florida: U. S. Geological Survy Water-Resources
Investigations Report 91-4180.
Schiner, G.R., Laughlin, C.P. , and Toth, D.J. , 1988,
Geohydrology of Indian River County, Florida: U. S. Geological
Survy Water-Resources Investigations Report 88-4073, 110 p.
Scott, T.M., 1988, The lithostratigraphy of the Hawthorn Group
(Miocene) of Florida: Florida Geological Survey Bulletin 59, 148
p.
Wiener, J.M. ,1982, Geologic modeling in the Lake Wauberg-Chacala
Pond vicinity utilizing seismic refraction techniques. University
of Florida M.S. thesis. 94 p.
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