CRAIG, T.L., and FRANK G. ETHRIDGE, Department of Earth Resources, Colorado State University, Fort Collins, CO; and MICHAEL D. HARVEY, Resource Consultants and Engineers, Inc., Fort Collins, CO

ABSTRACT: Configuration and Internal Architecture of the Cenozoic Platte River Valley, Northeastern Colorado

Much attention has been given to paleovalleys, principally because of the importance of valley-fills as oil reservoirs. Few studies, however, have been published on modern valleys and their fill deposits. Using mainly subsurface data, a 55 mile length of the South Platte River alluvial valley in northeastern Colorado has been investigated to (1) characterize the valley fill in terms of distribution of coarse and fine-grained sediment and geometry of sand bodies, (2) determine the configuration of the bedrock valley floor, and (3) determine how tributaries effect grain-size distribution

within the fill and the valley floor profile.

The bedrock valley of the South Platte River is between four and nine miles wide, and it has a meander wavelength of approximately 30 miles. The longitudinal profile of the valley floor is irregular, with steep reaches associated mainly with tributary junctions and changes in bedrock lithology. Cross-sectional profiles are asymmetrical, and the area of deepest incision is not confined to a particular location within the tributary junctions. The dimensions of tributary valleys at their junction with the South Platte Valley are much larger than would be expected from the size of the modern streams. Most likely, the position of the confluences have shifted over time; and, both the tributaries and the South Platte carried larger discharges under different climatic regimes within the Pleis ocene and Holocene. Distinct packages of tributary sediment overlying Platte Valley fill are easily recognizable.

The valley fill reaches 290 feet in thickness and consists of unconsolidated sediment ranging from cobble to clay in size; the largest proportion, however, is sand and gravel. Overall the fill fines up, but in places displays two or more fining up sequences. Some of the sequences have cobble at the base, but the majority of the sediment overlying the bedrock valley floor is gravel size.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.