LANDSCAPE EVOLUTION DETERMINED FROM SURFICIAL AND 3D GEOLOGIC MAPPING IN THE VICINITY OF FORT MORGAN, EASTERN COLORADO

A new Quaternary geologic map and drill hole data in the vicinity of Fort Morgan, eastern Colorado, are particularly amenable to 3D modeling because lithologic characteristics of the geologic units generally are unique. Geologic units must be well defined to understand and interpret the depositional history modeled from subsurface data. A 1:100,000-scale geologic map was constructed using NRCS digital soil maps and field confirmations. Water-well drill hole data from 3550 sites were compiled and simplified in terms of lithology. A 3D model based on these lithologic data was constructed. Before evaluating the subsurface, the DEM-trimmed model surface was visually compared to the geologic map for veracity.

The South Platte River flows from west to east across the map area and forms units dominated by alluvium derived from the source area in the Colorado Rocky Mountains. Lesser tributaries also contribute alluvium to the system. Bijou Creek, sourced from the drainage divide of the South Platte and Arkansas Rivers, flows north to its confluence with the South Platte west of Fort Morgan. Although the Bijou Creek fan is correlative in age to units associated with the South Platte River, alluvium in the fan is much finer grained. Mostly old eolian sand thinly mantles bedrock topography north of the South Platte. South of the South Platte, old sand is exposed primarily as interdunes because the unit is buried by younger eolian sand. Large bodies of loess occur near the southern margins of the mapped area. Bedrock is less than 20 to 30 m below the surface except under Bijou Creek fan, where it is more than 200 m below.

Cross sections and contour, fence, and 3D diagrams of the subsurface were used to reconstruct the erosional and depositional history of the mapped area. The contoured top of the bedrock closely resembles the surface topography except under the Bijou Creek fan, where the river scoured the bedrock to form a broad alluvial floodplain that is now filled with Bijou Creek fan sediment. Cross sections display paleoterrace levels eroded into bedrock and capped with gravelly deposits that have been subsequently buried by thin deposits of eolian sand and loess. In places, sand occurs in abrupt vertical contact with loess, suggesting that the two may have been deposited at approximately the same time, perhaps due to a common climate forcing.