Stratigraphy, Paleoenvironments and Thermal Maturity of Cambrian Strata, Reelfoot Rift, Southeastern Missouri: A Case Study of Integrated Palynology, Sedimentology and Geophysics

Gordon D. Wood, Rick C. Tobin, J. Todd Stephenson

Regional stratigraphic, paleoenvironmental and thermal maturity interpretations of Cambrian strata in the Reelfoot Rift area were enhanced through the integration of palynologic, sedimentologic and seismic data. Outcrops in the St. Francois Mountains of Missouri were compared with mineral cores and exploration wells in southeastern Missouri and northeastern Arkansas. This comparison indicates that paleoenvironments range from inner ramp (outcrops) to middle ramp (mineral cores) to distally steepened ramp margin to basin (deep exploration wells). All productive palynological samples from this area were dominated by clusters, filaments, and "sheets" tentatively considered algal in origin. Acritarchs recovered can be assigned to Elektoriskos, Granomarginata, Leiosphaerid a, Lophosphaeridium, Micrhystridium, Timofeevia, Vulcanisphaera and several new forms. This assemblage is the first record of Granomarginata squamacea Volkova 1968, Timofeevia phosphoritica Vanguestaine 1978, and Vulcanisphaera turbata Martin in Martin and Dean 1981 from the Upper Cambrian warm-water provincial realm. Synchronous sequence boundaries were established using palynological "extinctions". These horizons were used to correlate age-equivalent strata from inner ramp to basin. In a regional context there is a slight increase in the abundance and diversity of organic-walled microphytoplankton from nearshore to offshore environments. Anomalies in palynological distribution trends can be explained using geological and seismic evidence.

Regional patterns in thermal maturity were also determined through the integration of palynology and fluid inclusion thermometry. A visual determination of thermal maturity and its relationship to dip location and burial history was assessed using various organic-walled microfossils. For example, the color of Timofeevia phosphoritica recovered from outcrop samples is colorless-to-light yellow, indicating low maturity (pre-oil window). In shallow mineral cores, this species is yellow-to-light brown (oil window), and in samples from deep penetrations it is dark brown-to-black (gas window). Other organic-walled microfossils (e.g., algal clusters, filaments, etc.) also exhibit this same color trend. Fluid inclusion homogenization temperatures were also utilized to help constrain hermal maturity interpretations. Fluid inclusion data were derived from dolomite and calcite host minerals taken from exploration wells. These data were tied to petrographic observations to elucidate regional tectonic history and facilitate isopaching of maturation indices.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995