USING SOIL AND FOREST TYPE TO MAP BEDROCK AT NEMO, SOUTH DAKOTA

Bedrock mapping can be used to locate economic deposits, assess natural disaster (e.g., landslide) risk, and decipher the geologic history of a region. Methods such as direct sampling, aeromagnetic surveys, changes in topographic relief and soil cover can be used to map bedrock. This study examines a site at Nemo, SD to assess whether two contrasting types of bedrock produce different soil that result in different types of forest cover. Although bedrock varied, other soil forming factors, including climate, slope, elevation, and the time for soil development, were uniform. Within an approximately 1 km² area, quartzite and metagabbro bedrock, and ponderosa pine and quaking aspen forests were mapped. Samples of surface soil, soil profiles, and bedrock were collected. Elemental composition was measured by X-ray fluorescence. Soil organic content was measured by loss-on-ignition and soil nutrient content measured at a commercial soil testing lab. Both the quartzite and its overlying soil have less Mg and Fe, and greater Si content than the metagabbro and its overlying soil. The soil above the quartzite bedrock is thinner, sandier, has a lower cation exchange capacity, and lower phosphorus content than soil overlying the metagabbro bedrock. The forest above the quartzite is dominated by ponderosa pine (Pinus ponderosa). In contrast, the soil above the metagabbro bedrock is thicker, contains more clay, has a higher cation exchange capacity and more phosphorus than soil overlying the quartzite. The forest above the metagabbro is dominated by quaking aspen (Populus tremuloides) and contains an iris (Iris germanica). A soil with a higher cation exchange capacity holds more nutrients and water than a soil with a low cation exchange capacity. Phosphorus is an important nutrient needed for growth and development of plants. The increased cation exchange capacity and phosphorus content of the metagabbro-derived soil may account for the corresponding quaking aspen forest as quaking aspen require more water and nutrients to thrive than ponderosa pine. Ponderosa pine is more tolerant of nutrient deficient conditions such as the quartzite-derived soil. These preliminary data indicate that at this location soil properties and forest cover can aid in bedrock mapping.