ANATOMY OF A METER-SCALE CYCLE: ORDOVICIAN BROMIDE FORMATION OF OKLAHOMA

The pattern and formation of meter-scale cycles has long been a source of debate in the geologic community. The object of this research project is to use a statistical analysis of the fossil and mineral composition in one of these cycles from the mixed carbonate-siliciclastic Bromide Formation (Ordovician), to determine the rate and direction of sea level change. Possible end-member models for meter scale cycles include R-cycles (parasequences), T-cycles, and symmetrical T-R cycles. Sampling at a fine scale has not been performed on this area previously, and the methodology could be used to facilitate inquiry in other sedimentary systems. Samples were taken at 12 cm intervals over a 3 meter section, three thin section replicates were created for each sample location for a total of 75 slides. Preliminary data from a point counting analysis (minimum of 600 points per slide) on allochems indicates a strong dependence between grain or fossil type and sequence stratigraphic motif. For example, authigenic apatite and hematite increase in abundance between the transgressive surface (TS) and flooding surface (FS), whereas siliciclastic fines increase in abundance during the regressive (“R”) portion of the meter-scale cycle. Allochem and fossil indicators both suggest that meter-scale cycles in the Bromide Formation are symmetrical and contain both transgressive (“T”) and regressive (“R”) components, and are not simple parasequences.