Abstract
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.