Example of inner-shelf sand ridges from Upper Cretaceous Eagle Sandstone, central Montana uplift

Conference · Mon Aug 01 04:00:00 EDT 1983 · Am. Assoc. Pet. Geol., Bull.; (United States)

OSTI ID:6497324

Rice, D D; Gautier, D L; Shurr, G W

The Upper Cretaceous Eagle Sandstone of central Montana was deposited during a general eastward progradation of the western shoreline of the narrow, north-south-trending Western Interior epicontinental seaway. Cordilleran highlands to the west were episodically uplifted, and provided the main source for sediments deposited in the seaway. On the Central Montana uplift, the lower member of the Eagle consists predominantly of very fine-grained sandstone, which is exposed as a thick (average 100 ft, 30 m), continuous topographic rim. This sandstone gradationally overlies shales of the Telegraph Creek Formation. Within the rim, resistant beds and concretions of calcite-cemented sandstone define several smaller units, which can be traced laterally over distances of several miles. These units maintain fairly uniform thicknesses from north to south. However, in an east-west direction, the units thin, are imbricated, and become younger to the west. The lenses in the lower member are interpreted as landward-prograding (westward) sand ridges that were deposited on the inner shelf at distances of tens of miles from the shoreline. Laterally equivalent coastal sandstones of the Virgelle Sandstone Member prograded seaward (eastward) at this same time. The lenses are elongated in a north-south direction, generally parallel to the coast. However, the exact geometry of individual ridges is unknown. After bypassing the shoreface zone, the sand probably was transported parallel to the shoreline by geostrophic currents driven by wind-forcing. Storm waves reworked the upper, seaward-facing slopes of the ridges, whereas landward-facing parts of the ridges were more protected and subjected to bioturbation.

• Research Organization: Geological Survey, Denver, CO
• OSTI ID: 6497324
• Report Number(s): CONF-8309274-
• Conference Information: Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 67:8
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580100* -- Geology & Hydrology-- (-1989)
CRETACEOUS PERIOD
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC FORMATIONS
GEOMETRY
LITHOLOGY
MATHEMATICS
MESOZOIC ERA
MONTANA
NORTH AMERICA
ROCKS
SANDSTONES
SEDIMENTARY ROCKS
SEDIMENTATION
USA