| The lithologic characteristic of the opal-CT rich Basic City Shale member of the Tallahatta Formation in the vicinity of Meridian, Mississippi was determined through the analysis of 577 outcrop and core samples. Eighty percent of the analysed samples consist of opal-CT and clinoptilolite. Other constituents are smectite, quartz, glauconite and mica. Siliceous microfossils are also present but are rare. The density of those fossils increases eastward to Alabama, Georgia and South Carolina, but the abundances clinoptilolite decreases in that direction. Total silica is almost the same from Mississippi to South Carolina. The burial depth for the middle Eocene Tallahatta and equivalent formations in the southeastern belt is not more than 100 meters. This is not sufficient to develop the necessary burial temperature for the formation of opal-CT from diatoms. X-ray diffraction patterns show a possible progression from opal-CT to opal-C. The opal-CT, clinoptilolite and smectite mineral assemblage is common in rocks of volcanic origin. The possible volcanic sources are located in the Carribean and the volcanogenic material was distributed to the north and east due to atmospheric dispersal and movement of the ocean currents. So, it is believed that the primary source of the silica is volcanic and with the possible change of the microenvironment, the siliceous fossils have also contributed to the formation of opal-CT. |
