VARIATION IN ALLOCHEM TYPE AND AUTHIGENIC MINERAL ABUNDANCE IN METER-SCALE CYCLES: ORDOVICIAN BROMIDE FORMATION OKLAHOMA

The pattern and formation of meter-scale cycles has long been a source of debate in the geologic community. To test various cycle motifs, we conducted a point-counting analysis of the allochem type and mineral composition in two of these cycles from the mixed carbonate-siliciclastic Bromide Formation (Ordovician) in southern Oklahoma. Possible end-member models for meter scale cycles include R-cycles (parasequences), T-cycles, and symmetrical T-R cycles. We developed a set of hypotheses to test centered on the expected mineralogical changed associated with each end-member model. 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 allochem points per slide) on allochems indicates a strong dependence between grain or fossil type and sequence stratigraphic motif. For example, authigenic phosphate 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.