Geologic Mapping in Missouri

The  Missouri Geological Survey  is continually conducting bedrock mapping across the state to better understand the geology of the state. These efforts further scientific understanding, economic development, aid in environmental protection, and hazard identification and mitigation. The nation is divided into 7.5' quadrangles by the  USGS . The Missouri Geological Survey conducts  bedrock mapping  at a 1:24,000 scale based upon these quadrangles. The boundaries of these quadrangles are defined by latitude and longitude and are approximately 58 square miles. Let's focus on one quadrangle, Bunceton, mapped in 2016.

The Bunceton 7.5' quadrangle is located in Cooper County in rural central Missouri. Missouri Route 5, Petite Saline Creek, and Stephens Branch are located in the study area. The Bunceton Quadrangle was selected for mapping in 2015-2016 as part of a larger effort of the Missouri Geologic Survey to provide high quality bedrock mapping along the I-70 corridor. Seven quadrangles were mapped in 2016 as a part of this effort. The mapping efforts of the Missouri Geological Survey are funded in part by the USGS  STATEMAP  grant.

Much of the information on the layers of rock in Missouri and their relationships has been determined over the course of the Missouri Geological Survey's 160 year existence. Layers, or groups of layers, are mapped in a given study area. Mapping helps the Missouri Geological Survey learn more about the details pertaining to these layers, including where they are found in the state, their range of thicknesses, and layer relationships.

These details are often valuable for studies on groundwater, economic mineral deposits, geologic hazards (sinkholes, landslides, etc.), and others.

The relationships between rock layers can often be complex. Some layers are reliably present over large areas, while others are spotty in their presence. The thickness of the layers, as well as the materials that make up the rock, may change drastically as well.

Bedrock mapping begins in the office. The Missouri Geological Survey has large amounts of data on file. Some of the more helpful data sets when conducting bedrock mapping are records relating to wells and  mines . These records tell us what commodities have been mined in an area as well as what layers of rock have been drilled through and their thicknesses. Academic publications, studies with specific goals (such as energy or groundwater) and  historic field notebooks  are also very helpful.

A great deal of information is included on well logs, including lithology (rock type), thickness of named units, elevation, and others. Minerals, fossils, and lithology may be included on thorough well logs.

 Well logs  and other historical information on file at the Missouri Geological Survey can prove to be very valuable, particularly if the bedrock is poorly exposed or there are difficulties gaining access to properties.

The information on file at the Missouri Geological Survey is supplemented and checked by data gathered in the field. Many outcrops are located along roads, waterways, and in residential or commercial developments. There are typically too few of these locations to meet the accuracy requirements for bedrock maps therefore access to private property is necessary to gather enough data. The data gathered includes: location, elevation, rock type, minerals present, fossils present, and the name of the layer of rock (if it is possible to determine in the field). Photos and samples may be taken for additional analysis if needed.

The laboratory analyses of rock samples collected by geologists in the field can be very powerful tools. Analysis may include identifying the large fossils present, dissolving the rock to recover microfossils, or cutting the rock into very thin (30 microns, or the thickness of a human hair) slices to be viewed under the microscope. These analyses help refine mapping, ensure accuracy, and strengthen the scientific understanding of Missouri Geology.

Field data collection may be supplemented by various other methods. For several years the Missouri Geological Survey has partnered with the Missouri Department of Transportation to drill in the map areas and extract core. Drilling projects typically occur in the right-of-way of roads, within medians of roads, or in state properties. Rock core can be helpful in determining which map units are present in an area and their thicknesses.  Geophysical studies  may be employed as well, often in partnership with universities.

Geologic data collected in the field and historic data on file at the Missouri Geological Survey are then plotted with computer software. Outcrops of bedrock are color-coded according to type. The contacts between layers are indicated by green triangles and well logs by large blue circles. This data helps geologists determine where the layers of rock are located, how they are oriented, and which ones are in contact with each other. This data also helps geologists locate, identify, and possibly predict the locations of folds, faults, mineralization, and karst features like sinkholes and springs.

The geologic data available in an area allows geologists to make confident interpretations on the underlying bedrock. The contacts between rock layers, referred to as Map Units on a geologic map, may be interpreted in areas with little data based on data gathered in other locations.

The various colors in this image represent the different Map Units on the Bunceton 7.5' quadrangle. In some cases a color represents a thick layer of rock that is largely the same throughout (Mbk, light blue) or may represent a series of thinner layers that are distinct from one another but too thin to map separately(Mk, pink). The rock layers mapped on the Bunceton 7.5' quadrangle are generally  limestones  and sandstones. These kinds of rock are originally deposited in horizontal layers with gradual changes in thickness and character.

This image nicely illustrates locations with enough data and locations with little data. In areas with little or poor data the contacts between map units are inferred based on the elevations at other locations or the topography of the earth's surface.

Various geologic structures were identified while mapping the Bunceton 7.5' quadrangle. This image includes two  faults  and an  anticline  (an arch-like fold). Faults are indicated by bold lines and circles. The faults identified on the quadrangle are normal faults, a fault in which one block of the earth's crust moves down relative to another. The black circles on the lines indicate with block has moved down.

The evidence of faulting can be striking or subtle. In the case of the northwest trending fault, layers of rock on one side of the valley were approximately 80 feet higher than on the other side of the valley.

The anticline is inferred by the pattern formed by the map units. The Jefferson City-Cotter is found along a portion of the Petite Saline Creek while younger units are located to the north and south. This indicates that the layers have been forced upward in a fold.

The mapping process is repeated across the entire quadrangle. The final product gives high-quality geologic data to decision makers, planners, and public and private entities. The mapping efforts in this area refined our understanding of the distribution and occurrence of the various geologic units as well as identified previously unknown geologic structures.

In addition to maps produced in a given study area geologists also produce geologic cross sections. Cross sections are graphical predictions of the geology in the subsurface. They are produced using surface as well as subsurface data. The Bunceton cross section (above) uses water well information to supplement the bedrock outcrops. This cross section illustrates how some of the map units have patchy distribution in their  distribution  or have irregular contacts with the units above or below. This cross section also highlights some of the structures on the quadrangle, the anticline and one of the faults.

Each geologic map is also accompanied by a  stratigraphic column . A stratigraphic column is a graphical representation of the layers of rock in an area, their relative thicknesses, materials, and relationships.

This stratigraphic column illustrates the irregular nature of the contacts between some of the units. These are the results of  unconformities , instances of erosion or a lack of deposition of new rock in the past. Unconformities result in drastic differences in thickness and distribution of some of the units. Which may impact geologic hazards, mineral occurrences, and sources of drinking water.

The final product combines the various elements discussed above as well as includes detailed descriptions of each of the map units, descriptions of the geologic structures and economic geology of the area, a map with the locations of the data points used in the final interpretation, and a correlation of map units that describes their relative ages. Other elements may be included as needed, such as more detailed maps of portions of the study area, photographs from the field, or details of other analyses used.

After a study area is mapped the data is available to the public as well as used for various in-house purposes including environmental assessments and energy or mineral studies. The data can be accessed online using the Missouri Geological Survey's  Geostrat  tool. We hope this informative guide to geologic mapping helps meet the goal of the Missouri Geological Survey and the Department of Natural Resources to better serve the people of Missouri. For more information please see our  website .