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KOTTER, Daniel L.
and DURBIN, James M., Dept. of Geology, Univ. of Southern Indiana, 8600
University Blvd, Evansville, IN 47712, dkotter27@yahoo.com
A pulseEKKO ground penetrating radar (GPR) system
with a 50 MHz antenna was used to collect data on subsurface profiles of Wesley
Chapel Gulf,
a karst feature on the Mitchell Plain
in south-central Indiana. GPR
data were collected along three profile lines on the alluvial floor of the
gulf. Lines 1 and 2 were 31 meters in length along the long axis of the gulf
and Line 3 ran perpendicular to the long axis. This research identifies the
thickness of the alluvial floor, depth to bedrock, and nature of
sedimentation and stratigraphy of the fluvial
deposits. In addition, hypotheses about the factors controlling deposition of
strata are postulated. GPR data were analyzed and then compared to previously
collected sediment cores (Jones et al., 1999: Durbin et al., 2003). The GPR
data support interpretations from core data that indicated limestone bedrock
or possible collapse rubble at depths of -10.8 m. Core analysis identified
two units, each with distinctive sediments. The upper unit (0 to -3.5 m) is
dominated by silt and the lower unit (-3.5 to 10.8 m) possessed a basal sand
that became finer-grained upward. GPR data compares well with the core stratigraphy. A transition zone in wave velocity change
occurred between 3.0 to 3.5 m depth correlative to the boundary between upper
and lower units. Distorted velocity changes at depths of 9.0 to 12.0 m in lines
1 and 2 and -7.0 to -12.0 m in line 3 are strong evidence for a probable
varying bedrock depth. Wide parabola-like features in several profiles at –7
m are interpreted as either a sediment contact (silt/clay to sand) and/or
subsurface reflective points. Close proximity to a large tree (roots),
pockets of high velocity sediment and/or buried limestone boulders is
hypothesized sources for the observed parabolic reflections.
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