Surficial Geologic Mapping and Wetland Hydrology of a Portion of the Des Moines Lobe in Iowa
by D.J. Quade and E.A. Bettis III
In 1995, a mapping project of the geologic materials in the upper five meters at Camp Dodge was undertaken by the Geological Survey Bureau of the Iowa Department of Natural Resources (Figure 1). The project is supported by a U.S. Department of Defense LEGACY grant. The 1:12,000-scale mapping is being accomplished on screen by integrating geologic information obtained through research drilling, trenching, existing water well and engineering boring records with digital soils and elevation data using Geographic Information System (GIS) tools. The hydrology, water quality, flora and fauna of two wetlands, one existing and one undergoing restoration, are also being studied. Hydrogeologic investigations include detailed mapping of wetland deposits, as well as piezometric-surface maps constructed from a network of 30 monitoring wells located throughout the project area.
The project area encompasses a tributary of the Des Moines River that flows through a multi-channel, late-glacial outwash system and its adjacent uplands. The uplands exhibit stagnation topography, a linked-depression system, and other ice-contact landforms. Stratified diamictons of the Morgan Member (Dows Formation) are relatively thick and continuous across the uplands and sideslopes, and interfinger with glaciofluvial sediments of the Noah Creek Formation along the outwash channel margins. Wetlands and restored wetlands are associated with the outwash channels and linked-depression systems. The project was undertaken to provide facility managers with detailed geologic and hydrogeologic information that will improve land use and natural resource management decisions. The project is also providing a unique opportunity for intensive geologic study of Des Moines Lobe (DML) deposits and hydrogeology, a critical need for addressing environmental management decisions in north-central Iowa.
Figure 1. Location of Camp Dodge with respect to former ice margins on the Des Moines Lobe in Iowa.
Figure 2. Location map of the area surrounding Camp Dodge.
Camp Dodge is a 4,400-acre Iowa National Guard training facility located about nine miles north of the DML terminus in central Iowa. The facility encompasses portions of Beaver Creek Valley and a narrow divide separating the Des Moines and Beaver Creek valleys (Figure 2). Beaver Creek occupies the northwest-southeast trending pre-late Wisconsin Des Moines Valley that was buried by advance of the DML around 13.9ka. By 13.5ka the Des Moines Lobe glacier was north of this area at the position marked by the Altamont Moraine. At that time Beaver Creek Valley served as an outwash channel carrying water and sediment from the ice margin to the pre-Wisconsin Des Moines Valley south of the DML terminus.
Figure 3. Lithostratigraphy of the Des Moines Lobe region in Iowa.
Deposits that accumulated in direct contact with the Des Moines Lobe glacier compose the Dows Formation, which consists of two different kinds of glacial till. 1) Sediment deposited beneath or at the base of the glacier is a uniform, massive, dense loam textured, slowly permeable, subglacial till called the Alden Member (Figure 3). This deposit is overlain by 2) the Morgan Member, a less dense, variably textured till formed by mudflows, debris flows, and flowing water at the surface of the ice. The Morgan Member also contains continuous to discontinuous sand, gravel and silt deposits that accumulated in the former glacial karst system draining the stagnant glacier. These porous sand and gravel deposits may function as preferred pathways along which shallow groundwater can move faster than through the adjacent and subjacent finer grained tills (Figure 4).
The above east-west transect (Figure 5) is one of several well transects installed at the Camp Dodge training facility. The hydrogeologic data gathered from these well installations is providing useful information on groundwater gradients, seasonal water table fluctuations and hydraulic conductivity values of glacial tills within the Dows Formation. The information collected at Camp Dodge will be extremely useful for future hydrogeologic studies conducted in similar geologic settings on the Des Moines Lobe.
Figure 6. Portions of the Des Moines NW and Grimes 7.5 quadrangles showing the location of Camp Dodge. P numbers represent piezometers.
The early stages of map construction involve application of existing landscape models and subsurface data to identify landscape patterns. This analysis uses continuous geographic data such as topographic coverages, digital soil surveys and orthophotos, plus point data such as drill-hole and outcrop records. Figure 6 is an example combining a topographic coverage, Camp Dodge boundary coverage, and some well-location data to create a simple base map for the training area. Data sources available in digital form, such as soil surveys, enable the implementation of GIS analysis in the initial stages of mapping (Figures 7 and 8).
Figure 9. Preliminary landform/sediment assemblage map for the Camp Dodge area.
Landscape/material assemblages derived from interpretation of soil coverages are integrated with outcrop and subsurface drill-hole data to develop a general landform/sediment assemblage (LSA) map of the area (Figure 9).
The next stage of mapping will involve development of a 1:12,000-scale, stack-unit map depicting geologic materials of the top five meters. This map will be constructed using information from about 100 research core holes, engineering borings, outcrops, and backhoe trenches in selected locations. Data included with the stack units will include texture and bedding characteristics, Unified Soil Classification System designation, and hydraulic conductivity. The stack map will provide a coverage of basic geologic information that can be integrated with other coverages, such as piezometric surface, landfill sites, training areas, vegetation and habitats. GIS analysis will allow the National Guard to develop derivative products for planning development and use of the facility while protecting natural resources.