Data release for the geologic map of the Fort Morgan 7.5' quadrangle, Morgan County, Colorado

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.

Tags: alluvium, argillic-b-horizon, bijou-creek, bijou-no-2-reservoir, braided, broadway-alluvium, bull-lake-glaciation, colluvium, colorado, cretaceous, discharge, dune, empire-reservoir, eolian, floodplain, fort-morgan, front-range, geologic-map, greeley, holocene, jackson-reservoir, kersey, kiowa-creek, laramie-formation, lignite, louviers-alluvium, luminescence, masters, meandering, milliron-draw, morgan-county, nussbaum-alluvium, orchard, osl, peoria-loess, pierre-shale, pinedale-glaciation, piney-creek-alluvium, pleistocene, pliocene, quaternary, radiocarbon, rocky-flats-alluvium, sand-sheet, sheetflood, slocum-alluvium, soil, south-platte-river, terrace, u-series, united-states, usa, usgs-5a02006fe4b0531197b72d31, verdos-alluvium, weld-county, weldona, wildcat-creek
Organization: Department of the Interior