--> ABSTRACT: Tidal Point-Bar Deposit in Cretaceous Eagle Sandstone at Billings, Montana, by William B. Hansen and David R. Hearn; #91033 (2010)

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Tidal Point-Bar Deposit in Cretaceous Eagle Sandstone at Billings, Montana

William B. Hansen, David R. Hearn

The Cretaceous Eagle Sandstone (Campanian) exposed in the rimrocks at Billings, Montana, contains at least three members. The lowermost member of the Eagle has been described as a barrier island deposit that formed 50 mi offshore, in the Western Interior Cretaceous seaway.

Sedimentary structures and fossils within the upper member suggest nearshore tidal processes were at work during latest Eagle deposition at Billings. The occurrence of Teredolites-burrowed wood along a scour surface; steep, east-dipping, large-scale foresets (up to 50 ft); herringbone cross-stratification; flaser bedding; and abundant organic matter (logs) are recognized in one outcrop of the upper member. This outcrop represents the lateral migration of a point-bar in a tidal channel.

Considering the Eagle in light of a tidal depositional environment could affect future gas exploration plays in Montana.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988