UPPER EOCENE MICROTEKTITES DISCOVERED IN ALABAMA: A FIRST-ORDER CORRELATION TO THE GPTS

The age and number of upper Eocene impact layers has been controversial despite several numerical age measurements of ~35.5 Ma on a microtektite layer. There appears now to be general agreement that there are 2 upper Eocene impact layers: (1) a younger microtektite dominated layer and (2) an older clinopyroxene-bearing microkrystite layer. Microtektites have previously been found in the North and South Atlantic, Barbados, and the Caribbean Sea. Microtektites and tektites are correlated to the North American Strewn Field based on geochemical evidence and are believed to originate from the Chesapeake Bay Impact Structure. Microkrystites have been found in the Atlantic, Pacific, and Indian Oceans, and in Europe. The microkrystites are stratigraphically older than the microtektites and appear to have originated from the Popigai crater in Siberia. One reason for the age controversy is the need for more high-resolution first-order correlation between the microtektite horizons and the geomagnetic polarity time scale (GPTS) in land-based sections. We have evaluated magnetobiostratigraphic correlations at Ocean Drilling Program Site 1090B in the South Atlantic and at St. Stephens Quarry (SSQ) in Alabama, providing first-order correlation between microtektites and the GPTS. The occurrence of microtektites and microkystites at Site 1090B can be directly tied to the GPTS using the published magnetostratigraphy. We have identified sixteen microtektites at SSQ (63.1-63.4 m) that range in size between 212 to 250 µm within the NP19/20 and P15 biozones. The microtektites are located in the North Twistwood Creek Formation and within the highstand systems tract of the Moodys Branch-NTC sequence. This is the first identification of stratigraphically in place (micro)tektites in North America. SSQ provides an excellent record of sequences and stable isotopes, providing direct evidence for the lack of long-term (>100 kyr) changes in sea level or climate in association with the impacts. At both SSQ and Site 1090, the microtektite layer occurs within the middle of Chronozone C16n.1n with a magnetochronologic age of 35.426 Ma. At Site 1090B the microkrystite layer underlies the microtektite layer and is also correlated with C16n.1n (magnetochronologic age of 35.430 Ma).