Natural Resources, School of


Cenozoic Geology of the North Platte River Valley, Morrill and Garden counties, Nebraska

James B. Swinehart, University of Nebraska - Lincoln



Mapping of the Cenozoic deposits (Oligocene through Recent) on the north side of the North Platte River Valley near Broadwater and Lisco Nebraska was combined with analysis of drill-hole records and mineralogical and textural data. The stratigraphic framework thus established permits a reevaluation of earlier concepts regarding these deposits. The Nhitney Member of the Brule Formation (Oligocene), the oldest deposit exposed in the area, is an eolian, loess-like siltstone consisting of a lower clayey facies and an upper sandy facies. Vitric, crystal, and lithic volcaniclastic material--probably derived from distant western ash flow eruptions--makes up a minimum of 75 percent of both facies. The Arikaree Group (Late Oligocene-Early Miocene) overlies the 'V'Jhitney Hember and is divided into two units. The lower is the Gering Formation, which primarily is a fluvial volcaniclastic silty sand containing local accumulations of pumice pebbles derived from North Park, Colorado. Overlying it is a sequence of massive, volcaniclastic fine-grained sands--the Harrison-Monroe Creek formations, undifferentiated. This upper unit, like the Whitney, is primarily an eolian, loess-like deposit but contains a greater proportion of epiclastic material.

A hiatus representing about 10 million years duration separates the Arikaree Group from the overlying Pussy Springs channel complex (informal name) of the Ash Hollow Formation (Late Hiocene). The arkosic, poorly sorted silty and pebbly sands of the Pussy Springs unit resulted from the rapid filling of deep and narrow "arroyos" cut through the Arikaree Group and almost 200 feet into the Whitney Hember. The remainder of the Ash Hollow Formation consists of sand and gravel, silty sand, mudstone, caliche, and siliceous root cast horizons representing fluvial, lacustrine, and pedogenic deposits. Acidic volcanic clasts suggest a Colorado Front Range source. A hiatus representing several million years separates the Ash Hollow Formation from the Broadwater Formation (Pliocene-Early Pleistocene), an extensive sand and gravel containing several lenticular fine-grained units (including diatomites) that contain Blancan fossils. The Broadwater was deposited in a large east-trending paleovalley. Anorthosite and mafic igneous clasts indicate that the Laramie Mountains were a primary source of sediments. A series of isolated deposits of very coarse gravel and sand, Some filling deep narrow channels and containing clast types similar to the Broadwater Formation, are interpreted to repr~sent the basal fill of the Broadwater paleovalley.