Abstract:

Upper Cretaceous strata of marine origin, which are commonly called the Frontier and Niobrara Formations in southeastern Wyoming and the main body of the Benton Shale and the Niobrara Formation in north-central Colorado, enclose two widespread disconformities. The position of these disconformities and the duration of the two corresponding hiatuses in this sequence are interpreted from lithologic logs of outcrops and invertebrate fossils collected in the region. The lower unconformity separates a dominantly shale unit of Belle Fourche and Greenhorn age from an overlying dominantly sandstone unit of Carlile age. The hiatus indicates progressively less erosion in a southeasterly direction. From central Natrona County, Wyoming, where the magnitude of the lower hiatus is greatest, to eastern Larimer County, Colorado, where the magnitude is least, the age of the rocks directly below the unconformity decreases and the age of the rocks directly above the unconformity increases. The upper unconformity is generally at the contact of the Frontier or Benton and the overlying calcareous shale and limestone of the Niobrara (between beds of Carlile and Niobrara ages, respectively). The corresponding hiatus indicates more erosion in the southeastern part of the region. In contrast to the lower hiatus, the upper hiatus may be absent in central Natrona County and northwestern Carbon County, Wyoming, and seems to be greatest in eastern Larimer County, Colorado. In Natrona and Carbon Counties, the beds underlying the Niobrara are much younger and the basal strata of the Niobrara are older than rocks below and above the unconformity in Larimer County. The variation in the age of the rocks overlying both unconformities seems to indicate slow transgressive marine overlap in the two basins.

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