Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

Configuration and relative vertical position of underlying basement fault blocks controlled distribution of Newcastle alluvial valleys in the east-central Powder River Basin. The Newcastle Formation rests unconformably on the Skull Creek Shale and is conformably overlain by the Mowry Shale. Valleys localized in structurally downdropped areas were cut into the Skull Creek Shale during a regional sea-level drop. When the sea level began to rise, stream gradients decreased and the alluvial valleys filled with sediments of the lower, middle and upper Newcastle. The lower member is restricted to narrow incised valleys of upper meander-belt streams. As the transgression progressed, lower meander-belt streams broke out of the narrow valleys and spread over a larger area depositing the middle member. The alluvial valley-fill stage ended when tidal influence became dominant. The upper member was deposited in estuaries within old alluvial valleys and in tidal flats and beaches marginal to the estuaries. No deposition occurred on the structurally high areas.

Measured surface sections (38) on the Clifton terrace in Weston County, Wyoming, and near Edgemont, South Dakota, along with well-log data (634 wells) were used to develop this tectonically influenced depositional model. Six regional (3,000 square miles) isopach maps demonstrate repeated, consistent thickening and thinning of all Lower Cretaceous intervals along major northeast-trending fault blocks and along minor north-south and east-west-trending fault blocks. In the Clifton and Edgemont areas, the Newcastle Formation is bounded by either present-day monoclines or fault zones. Sandstone dikes in the Mowry and Skull Creek Shales originating from the Newcastle indicate structural adjustments before lithification. These lines of evidence provide strong support in favor of tectonic control on sedimentation for the Newcastle Formation.