Geologic map of the Orchard 7.5' quadrangle, Morgan County, Colorado

Scientific Investigations Map 3331
By: , and 

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Abstract

The Orchard 7.5' quadrangle is located along the South Platte River corridor on the semi-arid plains of eastern Colorado, and contains surficial deposits that record alluvial, eolian, and hillslope processes that have operated through environmental changes from the Pleistocene to the present. The South Platte River, originating high in the Colorado Front Range, has played a major role in shaping the geology of the quadrangle, which is situated downstream of where the last of the major headwater tributaries (St. Vrain, Big Thompson, and Cache la Poudre) join the river. Recurrent glaciation (and deglaciation) of basin headwaters affected river discharge and sediment supply far downstream, influencing alluvium deposition and terrace formation in the Orchard quadrangle. Kiowa and Bijou Creeks, unglaciated tributaries originating east of the Front Range also have played a major role by periodically delivering large volumes of sediment to the river during flood events, which may have temporarily dammed the river. Eolian sand deposits of the Greeley (north of river) and Fort Morgan (south of river) dune fields cover much of the quadrangle and record past episodes of sand mobilization during times of drought. With the onset of irrigation during historic times, the South Platte River has changed from a broad, shallow, and sandy braided river with highly seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge. Along this reach, the river has incised into Upper Cretaceous Pierre Shale, which, although buried by alluvial deposits in Orchard quadrangle, is locally exposed downstream along the South Platte River bluff near the Bijou Creek confluence, in some of the larger draws, and along Wildcat Creek.

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Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic map of the Orchard 7.5' quadrangle, Morgan County, Colorado
Series title Scientific Investigations Map
Series number 3331
DOI 10.3133/sim3331
Year Published 2015
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geosciences and Environmental Change Science Center
Description Report: 1 p.; 1 Plate: 54.41 x 34.02 inches
Country United States
State Colorado
County Morgan County
Online Only (Y/N) Y
Additional Online Files (Y/N) Y