The South Platte and Arkansas Rivers have shaped much of the Colorado Piedmont landscape during late Tertiary and Quaternary time. North and east of Denver, the South Platte and its tributaries excavated valleys in the soft sedimentary rocks of the Denver Basin and, during brief pauses in excavation, allowed thin deposits of alluvium and eolian sediment to accumulate. Thin terrace, alluvial-fan, and eolian deposits are the only records of aggradation in this otherwise degradational landscape. Despite their fragmentary nature, these deposits offer a means of studying the history of landscape development. To better understand this history, we have reconstructed stratigraphic facies from borehole logs, mapped sediment-dispersal patterns from pebble counts, and identified fluvial processes from sedimentary features. This report is an outgrowth of recent studies of gravel deposits of the South Platte River and its tributaries for the U.S. Geological Survey (USGS) Front Range Infrastructure Resources Project. The stratigraphy of late Pleistocene and Holocene alluvium in the South Platte and the Cache la Poudre River valleys was delineated previously in valley cross sections. Additional sections showing the stratigraphy of valley fill in the South Platte River and some of its tributaries, principally Clear Creek and Bear Creek, were prepared for this report. The lithology of gravel, first mapped by Colton and Fitch (1974) and later by the Infrastructure Resources Project, is supplemented here by additional data and used to identify the provenance of gravel. Sedimentary features, described from gravel pits in late Pleistocene and Holocene alluvium of the South Platte and Cache la Poudre River valleys, are redescribed and interpreted, and new information on sedimentary features in older Pleistocene alluvium is reported here.
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| Posted September 2005 |
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