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Depositional History of the Newcastle Formation (Lower Cretaceous), Williston Basin, North Dakota and Montana

Ulishney, Adam J. 1, Richard D. LeFever2 (1) University of North Dakota, Grand Forks, ND (2) University of North Dakota, Grand Forks

 

The Newcastle (Muddy) Formation (Lower Cretaceous) is a widespread sandy unit between the Skull Creek and Mowry Shales. To interpret the stratigraphy and the depositional history for this study, we examined a total of about 9,300 geophysical wireline logs in eastern Montana, western North Dakota, and southern Saskatchewan, and constructed a series of isopach maps and detailed cross-sections.

 

Prior to Newcastle deposition, the Skull Creek seaway covered the study area and much of the Western Interior. Subsequently, the sea retreated to the north and west. The Skull Creek Shale was exposed to erosion, and a major drainage system developed.

 

The maximum lowstand shoreline position trended approximately north-northeast along a line from northern Rosebud County to eastern Daniels County, Montana, and northward into Saskatchewan; the shoreline position is farther east than has previously been reported in this area. Most Newcastle deposition took place during the lowstand, and produced valley-fill deposits (up to 160 ft thick) along the North Dakota-Montana border, and shallow marine/shoreline deposits (usually less than 40 ft thick) in eastern Montana. Although the lowstand drainage emptied into the seaway in eastern Montana, there is no clear evidence of deltaic deposition.

 

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana