Date of this Version
CONTRIBUTIONS TO GEOLOGY, Vol. 22, No. 2, Fall, 1983, pp. 109-115 + cover.
Locally derived blocks and boulders (megaclasts) occur in conglomerate and in sand and gravel fills of channels in the Ogallala Group (Miocene) at localities in Banner, Kimball, and Morrill counties in western Nebraska. Some of the megaclasts are up to one or more orders of magnitude larger than the largest distantly derived grains in the sediments surrounding them. A number of these megaclasts probably moved downslope by mass wasting from outcrop sites on paleovalley sides and were later transported by running water. Other blocks and boulders were eroded by streams from bedrock along channel sides and then were transported by these bodies of water to their depositional sittls. The compositions and source formations of the blocks and boulders vary. Clasts of sandstone, caliche, and cemented volcanic ash were derived from erosion of older beds of the Ogallala Group. Interbedded sandstone and concretions were eroded from rocks of the Arikaree Group, and masses of siltstone were part of the Brule Formation.