|
 |
|
|
 |
Open File Report:
Stratigraphy of the Indian River Region |
|
|
|
|
|
|
| |
Results |
 |
|
|
|
|
|
|
 |
|
|
|
|
|
|
|
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.
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).
top of page
 |
next section
|
