COMMENTS ON THE GEOLOGIC HISTORY OF THE OGALLALA FORMATION IN THE SOUTHERN PANHANDLE OF NEBRASKA

The southern Panhandle of Nebraska and the adjoining parts of southeastern Wyoming and northeastern Colorado are unusual places to examine the geologic history of the Ogallala Formation because they are much closer to the source areas of much of the sediment which makes up the unit than are other sites along the Ogallala outcrop belt. This study in Nebraska combined with results of earlier work there, in southeastern Wyoming, and ; northeastern Colorado, outlines the complex cut and fill history of the Ogallala close to Rocky Mountain source areas. Parts of the Ogallala consist of fills of sand and gravel, including very large, locally derived clasts, deposited in anastomosing channels carved into bedrock. These channels may be several kilometers wide, up to 50+ m deep, and may cut across other older Ogallala channels with similar geometries. Local tributary fills also occur in the Ogallala. These fills usually are composed of sediments eroded by streams from Miocene and Oligocene sandstone and siltstone formerly exposed along paleovalley sides near the sites of sediment deposition. Up to six volcanic ash deposits have been found in superposed sequences of the Ogallala in the study area. These deposits have blanket, lens, shoestring and irregular geometries. Ash colors are shades of yellow and gray. Shards vary from silt to coarse sand size. Ash deposits are up to 6-7 m thick. Caliche (calcrete and silcrete) occurs throughout the Ogallala but seems to be better developed toward the present top of the formation. The overall geologic history of the Ogallala in the southern Panhandle of Nebraska is one of multiple cycles of erosion and deposition. It is not the history of a single major episode of erosion followed by gradual valley filling that has been reported previously from elsewhere along the Ogallala outcrop belt