Abstract: Stratigraphy and Sedimentary Petrography of Subsurface Tertiary Rocks in Western Nebraska

James B. Swinehart

Study of recent test holes drilled in western Nebraska, particularly Box Butte County, suggests that several revisions of the Tertiary stratigraphy in the central panhandle of Nebraska are in order.

A rock unit exposed in the Niobrara Valley and formerly mapped as Monroe Creek Formation, Arikaree Group, may represent a previously unrecognized unit in Nebraska. This unit is more closely related to the White River Group than to the Arikaree Group and may be equivalent to the Sharps Formation of South Dakota.

Rocks previously assigned to the Marsland Formation appear to consist of two major units with contrasting mineralogic characteristics. The older unit consists of predominantly volcaniclastic rock types, with a marked eolian imprint, typical of the White River and Arikaree Groups. The younger unit signals the appearance of predominantly fluvial deposition, derived mostly from plutonic source rocks. This unit is typical of the Ogallala Group.

On the basis of fission-track dating and vertebrate faunas, this change in mineralogy and depositional style probably took place around 18 m.y. B.P.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado