GEOCHEMISTRY OF CHERT, SILICIFIED LIMESTONE, AND LIMESTONE OF THE OCALA LIMESTONE

Chert is a sedimentary silicate which can form on the bottom of marine environments from siliceous ooze made up of the skeletal remains of planktonic organisms such as diatoms and radiolarians; or, chert can form as a precipitate from solution. Nodular chert is commonly found in the Eocene upper Ocala Limestone, Florida. It occurs alongside soft, light-colored micritic finely pelletal limestone, and silicified limestone. Using chert, silicified limestone, and limestone were collected from the CEMEX Center Hill quarry, Center Hill, Florida, portable x-ray florescence (pXRF) was used to determine the geochemistry of the chert and the surrounding rock. Geochemistry can reveal where the chert was deposited and how its formation affected the geochemistry of the surrounding limestone. The limestone is high in Ca and low in Si. Silicified limestone is relatively high in both Ca and Si. Chert is high in Si, low in Ca, Al is below detection limit of the pXRF, and Fe₂O₃/SiO₂ ratios are below 1. The low Fe₂O₃/SiO₂ ratio suggests a shallow marine environment for the cherts. The high Fe and lack of Al suggests the cherts formed as precipitates in the limestone. This is in concordance with the depositional environment of the Ocala Limestone and the nodular structure of the chert deposits. There is a linear Si-Ca relationship between the chert, silicified limestone, and limestone which suggests a diogenitic replacement of the limestone with chert. This is visible in the rocks themselves as a banded gradient of dark cherts, lighter silicified limestone, and fossiliferous limestone.