LITHOFACIES AND TRILOBITE BIOFACIES ON A LATE ORDOVICIAN CARBONATE RAMP: BROMIDE FORMATION, SOUTHERN OKLAHOMA

The Late Ordovician Bromide Formation of southern Oklahoma records a spectrum of lithofacies along a carbonate ramp associated with the southern Oklahoma aulacogen. Above a basal sandstone unit, the Mountain Lake Member is composed of deep subtidal parasequences that grade upward from lime mudstone and shale to bioclastic wackestone and packstone. Low diversity trilobite assemblages dominated by isotelines, Failleana and Remopleurides occur as thin (cm-thick) bioclastic packstone to rudstone horizons, and are joined by bryozoa and brachiopods in the shallower, upper portions of parasequences. With shallowing towards the top of the member, some parasequences are capped by rippled, echinoderm-rich packstone and grainstone. Deepening at the base of the overlying Pooleville Member is recorded by the appearance of poorly fossiliferous lime mudstone and a shift to blanketing as the dominant depositional process. Monospecific horizons of articulated exoskeletons of the isoteline trilobite Vogdesia are interpreted as reproductive aggregations. Shallowing in the upper Pooleville is indicated by the occurrence of thin, storm-winnowed, bioclastic rudstone that yields a diverse trilobite fauna dominated by "Encrinuroides", Lonchodomas, and Calyptaulax. Brachiopods, bryozoa and echinoderm debris are also conspicuous components of the rudstone. Along the margins of the aulacogen, the Pooleville is capped by peritidal carbonate, although subtidal deposition continued in the basin center. Abrupt deepening at the base of the overlying Viola Springs Formation led to regional extirpation of the Pooleville trilobite fauna. However, most genera reappear as shallow subtidal conditions became established in the Viola. As with other recent studies, our work demonstrates that trilobites remained significant, diverse components of shallow subtidal paleocommunities, even during the height of the Ordovician Radiation.