Use of time-trend curves to interpret depositional environments in portions of the Chickamauga Group carbonates (Middle Ordovician, northeastern Alabama)

Abstract

Data from point counts of 376 thin sections from six stratigraphic sections in the Chickamauga Group carbonates (Middle Ordovician of northeastern Alabama) were smoothed by time-trend methods. The resulting curves were plotted against the stratigraphic sections to show depositional trends and to facilitate interpretation of depositional environments. The depositional environments in the lower third of the sequence studied were dominantly subtidal; in the middle portions they were interbedded subtidal, intertidal, and supratidal; in the lower upper third they were supratidal; and in the uppermost portions they were interbedded subtidal, intertidal, and supratidal. Time-trend methods proved to be useful in this study, for they are easy to use and eliminate much noise observed in the data, whereas preserving major trends. Correlation between outcrops was by means of two continuous bentonites; comparison of the time-trend curves show that they would have been useful for correlation if the bentonites had been absent, although small differences in the vertical positions of beds between localities (indicating lateral migration of environments) would not have been detected by time-trend methods.