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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. Excellent exposures of these imbricated lenses occur along a rim that extends 12 mi (19 km) southwest from the town of Winnett.
Although the sandstone of the lower member is very fine-grained throughout the rim, systematic changes occur within a single lens. These changes include: (1) thinning and grading into shale and siltstone to the southwest: (2) bioturbation decreasing upward and to the northeast; (3) oblique Asterosoma burrows predominating in the lower part of each lens and to the southwest, with horizontal Ophiomorpha burrows being more common in the middle part of each lens and to the northeast; (4) parallel bedding and hummocky cross-stratification successively occurring in the upper part of each lens and to the northeast; and (5) relatively straight-crested symmetrical ripples generally capping each lens. The sedimentary structures within each lens indicate increasing energy and shoaling upward, bu do not indicate subaerial exposure.
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.
Ridges that occur on the Central Montana uplift are comparable in many aspects to sand ridges on the modern Atlantic inner shelf. However, the modern sand ridges differ from those of the Eagle in two ways: (1) they occur at angles oblique to the shoreline, and (2) they resulted from "shoreface detachment" during the Holocene transgression.
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