PALYNOFACIES AND THERMAL MATURATION OF EARLY PALEOZOIC ROCKS IN SOUTHERN MISSOURI: PRELIMINARY COMPARISON BETWEEN THE REELFOOT RIFT AND ADJACENT PLATFORM

The Reelfoot Rift in the U.S. Midcontinent was actively subsiding from Late Precambrian time through the Early Ordovician, and had a major effect on the depositional and diagenetic patterns in the Cambro-Ordovician rocks in southern Missouri. Very little paleontological work that can be applied to bathymetric reconstruction has been done in this area. More than 50 samples of mostly carbonate rocks in seven bore holes in the Reelfoot Rift and the Viburnum Trend (New Lead Belt) were studied for their organic content, in order to qualitatively and quantitatively assess paleoenvironmental conditions and thermal evolution. The samples contain well-preserved organic-walled microplankton and dispersed organic debris, in spite of widespread dolomitization. The organic categories identified include acritarchs (acanthomorphs and sphaeromorphs), algal clusters, filaments and sheets, amorphous organic matter, and unstructured black fragments. Multivariate statistical analyses of the components have yielded several palynofacies assemblages that indicate changes in paleoenvironmental parameters that are consistent with a rapidly subsiding basin. Thermal alteration indices of acritarchs studied using published color standards indicate that the organic components within the Reelfoot Rift display a significantly higher level of thermal maturation in comparison with those from the shallower platform carbonates. The rift organic components range from dark brown to black, whereas the platform components are light brown to medium brown. This study will be used to establish a paleobathymetric reconstruction for the Reelfoot Rift and adjacent platform areas.