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Two carbonate units from Southwest Florida, the Caloosahatchee and Fort Thompson Formations, were studied using X-ray instruments to understand more about their origins. The Caloosahatchee Fm. is late Pliocene to early Pleistocene in age while the Fort Thompson Fm. is late Pleistocene in age. Powdered X-ray diffraction (PXRD) was used to obtain the mineral composition of these formations. PXRD revealed the Caloosahatchee Fm. consists predominantly of calcite with lesser quartz and aragonite. PXRD of the carbonate rocks from the Fort Thompson Fm. are also predominantly calcite with lesser quartz and aragonite. Fossils were picked by hand from poorly cemented Caloosahatchee Fm. samples and analyzed on the PXRD. These fossils were predominantly aragonite with minor calcite. Carbonates from the Caloosahatchee and Fort Thompson Fms. were also analyzed for their major and trace element geochemistry using a portable X-Ray fluorescence (pXRF). The Caloosahatchee Fm. has a Mg/Ca ratio which ranges from 0.07 to 0.10, Sr which ranges from 368 to 1650 ppm, and Al which ranges from 0.08 to 0.35 wt.%. No As was detected in the Caloosahatchee Fm. The Fort Thompson Fm. was divided into lower and upper units for pXRF analysis. The lower Fort Thompson Fm. has a Mg/Ca ratio which ranges from 0.08 to 0.10, Sr which ranges from 197 to 1097 ppm, and Al which ranges from 0.05 to 0.38 wt.%. The upper Fort Thompson Fm. has a Mg/Ca ratio which ranges from 0.07 to 0.10, Sr which ranges from 92 to 1015 ppm, and Al which ranges from 0.21 to 1.33 wt.%. The lower Fort Thompson Fm. has no As detected in it, but the upper Fort Thompson had 5 ppm As. The low (< 0.8) Mg/Ca ratio and calcite being the predominant mineral in both formations indicate they are limestone. The presence of quartz, and the Al values suggest both formations have terrestrial contributions and are not pure marine limestone. The higher Al of the upper Fort Thompsons suggest it might have the largest terrestrial contribution. The Sr values for the Caloosahatchee Fm. are higher than the Fort Thompson Fms. This could be due to a higher marine influence in the Caloosahatchee Fm. The PXRD suggests the aragonite is originating from the shells within the formations. The lack of significant As in all rocks suggests leaching of this metal into the environment is not a concern.
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This content will become publicly available on April 7, 2026
Plio-Pleistocene stratigraphy, paleoenvironments, and sea-level history at Florida Shell Quarry, Charlotte County, Florida, U.S.A.
ABSTRACT Plio-Pleistocene sediments from the southwestern Florida Peninsula contain an extraordinary density and diversity of marine mollusk and vertebrate fossils which, collectively, document major faunal shifts on the Florida Platform through a period of profound environmental change. Systematic study of these fossil assemblages and the environments in which they lived has been limited, however, by: i) a lack of outcrop sections spanning the full Plio-Pleistocene stratigraphy of the region and ii) major uncertainties in correlation between previous study sites due to extreme lateral variability in coastal paleoenvironments. Here, we describe a new stratigraphic section from Florida Shell Quarry in Charlotte County, Florida, which contains fossil-rich deposits of each major Plio-Pleistocene unit in the area (the Tamiami, Caloosahatchee, Bermont, and Fort Thompson formations). Bulk sediment samples collected from 22 horizons were used to broadly characterize stratigraphic variations in lithology and faunal content. Predation intensity was estimated from drill-hole frequency among populations of the bivalve Chione spp. While all studied formations were mainly deposited under marine conditions, both lithologic and faunal facies shifts within the Caloosahatchee and Bermont units indicate periods of pronounced freshwater influence. Faunal diversity is relatively high in the Tamiami, Caloosahatchee, and Bermont units but declines in the Fort Thompson. Similarly, predation intensity is high in the Caloosahatchee and Bermont units but lower in the Fort Thompson at the sampled sites. In addition to characterizing changes in the local paleoenvironment, we propose a sequence stratigraphic model for the section based on inferred local sea-level fluctuations. We leverage this sequence stratigraphic framework to correlate the Florida Shell section with other studied sections in the Charlotte Harbor area. The development of this new site provides a workable basis for more detailed studies of the long-term paleoecological and paleoenvironmental evolution of southwestern Florida.
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- Award ID(s):
- 2237429
- PAR ID:
- 10582522
- Publisher / Repository:
- Journal of Sedimentary Research
- Date Published:
- Journal Name:
- Journal of Sedimentary Research
- Volume:
- 95
- Issue:
- 2
- ISSN:
- 1527-1404
- Page Range / eLocation ID:
- 367 to 382
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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