The geometry, genesis, and stratigraphic framework of the Colgate Sandstone Member of the Fox Hills Formation, Northeastern Montana

Abstract

The Upper Cretaceous Colgate Sandstone Member of the Fox Hills Formation near the Fort Peck Reservoir, eastern Montana is of particular scientific interest. These rocks record the last major regression of the Western Cretaceous Seaway. The detailed interpretation of these stratigraphic units yields greater insight into the causes and dynamics of this important geologic event. The purposes of this research were to: (1) map the extent of the Colgate Sandstone Member in the study area, (2) describe and interpret the facies comprising the Colgate Sandstone Member, (3) determine the facies architecture, and sequence stratigraphic framework of the Colgate Sandstone Member, and (4) to interpret the depositional system active to deposit the Colgate Sandstone Member and those factors that influenced its distribution. Twenty-four stratigraphic sections were measured near Devil's Creek area, approximately 65 km (40 miles) NNW of Jordan, MT. Additionally, a series of isopach and structure maps were generated using data from geophysical wireline well logs from nearby boreholes to determine tectonic influences on Colgate Sandstone deposition. The Colgate Sandstone Member of the Fox Hills Formation is part of an incised valley system that formed in response to a relative sea level drop of the Western Interior Seaway during the Maastrichtian stage in northeastern Montana and western North Dakota. The valley fulfills the criterion set forth by Zaitlin (1994, 1995) to characterize a depositional system as an incised valley-fill. The valley-fill strata onlaps the unconformity forming the valley floor and sides and demonstrates an abrupt basinward shift of facies relative to those of the eroded substrate (Van Wagoner et al., 1988). Further, these lowstand and transgressive system tract valley fill strata are deposited during a 3rd order relative sea level cycle that correlate to Haq et. al.'s (1987) UZA-4.5 depositional sequence. This sequence lasted approximately 1 Ma, and is considered the last sequence of the Zuni transgression. There is also sub-surface and field evidence that regional subsidence (the Blood Creek Syncline) and local subsidence along the Plum Creek lineament were significant in determining the location of the valley system and potentially increasing accommodation beyond that created by fluvial incision alone.