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

Scientific Investigations Map 3408

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https://doi.org/10.3133/sim3408

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Abstract

The Fort Morgan 7.5′ quadrangle is located on the semiarid plains of northeastern Colorado, along the South Platte River corridor where the river has incised into Upper Cretaceous Pierre Shale. The Pierre Shale is largely covered by surficial deposits that formed from alluvial, eolian, and hillslope processes operating in concert with environmental changes from the late Pliocene to the present. The South Platte River, originating high in the Colorado Rocky Mountains, has played a major role in shaping surficial geology in the map area, which is several tens of kilometers downstream from where headwater tributaries join the river. Recurrent glaciation (and deglaciation) of basin headwaters has affected river discharge and sediment supply far downstream, influencing deposition of alluvium and river incision in the Fort Morgan quadrangle. Distribution and characteristics of the alluvial deposits indicate that during the Pleistocene the course of the river within the map area shifted progressively southward as it incised, and by late middle Pleistocene the river was south of its present position, cutting and filling a deep paleochannel near the south edge of the quadrangle. The river shifted back to the north during the late Pleistocene. Kiowa and Bijou Creeks are unglaciated tributaries originating in the Colorado Piedmont east of the Front Range that also have played a major role in shaping surficial geology of the map area. Periodically during the late Pleistocene, major flood events on these tributaries deposited large volumes of sediment at and near their confluences, forming a broad, low-gradient fan composed of sidestream alluvium that could have occasionally dammed the river for short periods of time. Wildcat Creek, also originating on the Colorado Piedmont, and the small drainage of Cris Lee Draw dissect the map area north of the river. Eolian sand deposits of the Sterling (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 prolonged drought. With the onset of irrigation and damming during historical times, the South Platte River has changed from a broad, shallow, and sandy braided river with highly variable seasonal discharge to a much narrower, deeper river with braided-meandering transition morphology and more uniform discharge.

Suggested Citation

Berry, M.E., Taylor, E.M., Slate, J.L., Paces, J.B., Hanson, P.R., and Brandt, T.R., 2018, Geologic map of the Fort Morgan 7.5' quadrangle, Morgan County, Colorado: U.S. Geological Survey Scientific Investigations Map 3408, 2 sheets, scale 1:24,000, https://doi.org/10.3133/sim3408.

ISSN: 2329-132X (online)

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Additional publication details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Geologic map of the Fort Morgan 7.5' quadrangle, Morgan County, Colorado
Series title:
Scientific Investigations Map
Series number:
3408
DOI:
10.3133/sim3408
Year Published:
2018
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Geosciences and Environmental Change Science Center
Description:
2 Sheets: 48.14 x 35.72 inches and 31.76 x 23.78 inches; Georeferenced sheet; 2 Data releases; Read Me
Country:
United States
State:
Colorado
County:
Morgan County
Other Geospatial:
Fort Morgan 7.5' quadrangle
Online Only (Y/N):
Y
Additional Online Files (Y/N):
Y