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Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

Subsurface Characterization of Selected Water Bodies in the St Johns River Water Management District, Northeast Florida

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Identification of Karst Features from Seismic Patterns
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Lake Weir
Marion County, Florida

Introduction | Subsurface Characterization

Subsurface Characterization

Lake Weir
Lake Weir. Black lines show profile locations.
The subsurface disturbance features visible in seismic profiles acquired from Lake Weir generally fall into subsidence type categories (2 and 3) at two different depth levels (see Location of Possible Subsidence Map). The deeper structures could possibly correspond to subsidence within the Hawthorn Group (B-B' below), but detail is obscured by noise in the overlying record. Near-surface features show low-angle reflections that exhibit disturbed bedding (blue lines in profiles). Relationships between the upper and lower features are seldom apparent due to the generally poor acoustic return. Large areas in the central portion of the lake show a gassed out appearance. This could be due to a hard, sandy lake bottom or high organic content in the surficial sediments. The seismic records to the west show strong multiples, which also obscure the underlying record. This could also be due to a hard sandy lake bottom. Surface features include obvious dredge canals along the northern shore and a very large, long dredge-like structure across Little Lake Weir (A-A'). This feature is up to 300 m (984 ft) wide and 3 m (9.8 ft) deep (6 m, 19.7 ft water depth). It cannot be determined from the record whether this is a dredge canal or collapse structure. Possible near-surface faulting (red dashed lines) and what could be a real reflection dipping beneath the multiple could indicate subsidence in the area.

Seismic profile for A-A'
Profile A-A'

Seismic profile for B-B'
Profile B-B'

The top of the Ocala Limestone plunges to the east beneath Lake Weir, as indicated from interpretations of gamma-log profiles obtained from wells around the lake. The Ocala surface is at 4.6 m (15 ft) NGVD on the southwest side of the lake (well M-0070, see Index Map C and gamma log cross-section B-B') and decreases to about 30.5 m (-100 ft) NVGD on the east side (M-0079, M-0080, see Index Map C and gamma log cross-sections B-B' and E-E'). A well north of the lake (M-0078, see Index Map C and gamma log cross-section E-E') shows the contact to be at -5 feet NGVD. This irregular surface may indicate mature karst development beneath Lake Weir and the disturbed nature of the reflective horizons shown in seismic profile B-B' (blue and red lines) could be a result of more recent subsidence in the overburden. Well logs indicate that the Hawthorn Group crop out on the west side of the lake and dip to around zero feet NGVD to the east. This corresponds to approximately 16 m depth in seismic profile B-B' and would suggest the reflective horizons represented by the red and blue lines in the profile to correlate with the top of the Hawthorn Group.

Coastal & Marine Geology Program > Center for Coastal & Regional Marine Studies > Geologic Characterization of Lakes and Rivers of Northeast Florida > OFR 00-180

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Updated December 02, 2016 @ 11:25 AM (JSS)