The Weimer Marker Bentonite-A Regionally Persistent Correlation Horizon within the Upper Cretaceous Lewis Shale, Eastern Greater Green River Basin, Wyoming and Colorado

摘要

Previous stratigraphic studies of the Upper Cretaceous Lewis Shale in the eastern Greater Green River Basin of southwestern Wyoming and northern Colorado have identified laterally continuous intervals that can be correlated over large areas. Correlations from many of these studies were compiled into a geological database that facilitated their comparison and evaluation. Bentonitic and organic-rich pelagic shale intervals within the lower Lewis Shale interval were found to be the most laterally persistent correlations. This study provides a structure map on a horizon, herein named the "Weimer Marker Bentonite," that has frequently been used as a datum in previous studies. An isopach map was generated for the interval between the Weimer Marker Bentonite and the deepest laterally persistent bentonite identified in the lower Lewis Shale. Isopach anomalies revealed on this map coincide with areas of other Upper Cretaceous interval isopach anomalies or present-day structure. Coincidence between isopach anomalies and structure suggests that syndepositional tectonics within the basin controlled sediment dispersal patterns. Core studies of the most widespread and thickest condensed interval within the lower Lewis Shale show that total organic carbon content ranges from 2 to 4 weight percent. Previous studies that utilized spot-sampling procedures did not provide representative source rock characterization due to high frequency vertical variations in total organic-carbon content. A continuous sampling methodology produced total organic-carbon content data that more accurately represents source rock characteristics.