A paleopedological approach to the interpretation of terrestrial sedimentary rocks: The mid-Tertiary fossil soils of Badlands National Park, South Dakota

Abstract

Sedimentology is concerned with processes, environments, and products of sedimentation, whereas paleopedology is concerned with soil formation on these materials between times of sedimentation. Both approaches can contribute much to an understanding of ancient terrestrial environments. Fossil soils can be recognized from irregular vertical tubular structures (fossil root traces and burrows), massive bioturbated layers with gradational contacts (soil horizons), complex cracking and veining (peds and cutans), and distinctive petrographic textures (sepic plasmic fabrics). In Badlands National Park, South Dakota, numerous other features, such as patterns of fossil-bone accumulation, horizons of calcareous nodules and layers, and local variations in mineralogy, and trace and major-chemical elements were also evidence of fossil soils. In the Pinnacles area of Badlands National Park, there are at least 87 successive fossil soils in 143 m of stratigraphic section. Ancient soil formation is a kind of early diagenesis, which can be obscured by additional changes after burial of the soil (late diagenesis). In Badlands National Park, those few late diagenetic features identified did not seriously alter traces of ancient soil formation. Fossil soils can be mapped and named in the field using the soil-mapping procedures of the U.S. Department of Agriculture. At least ten such nongenetic units (fossil soil series) can be recognized among fossil soils of the Pinnacles area of Badlands National Park. Each represents a particular ancient environment. In most cases, these fossil soil types could be identified within modern soil classifications, such as those of the U.S. Department of Agriculture and the Australian C.S.I.R.O. In addition, fossil soils can be evidence of past soil-forming factors, such as climate, organisms, topographic relief, parent material, and time of formation. Climatic interpretations from fossil soils may be useful confirmation of other data, although some immature or waterlogged soils reveal nothing of climate. The kind of former vegetation (such as woodland or grassland) can be interpreted from fossil soils, even where direct fossil evidence of its floristic composition is lacking. Fossil soils may be a guide to the degree, nature, and location of preservational biases of animal and plant fossils. Fossil soils can also be used to assess the degree of adaptation of associated fossil mammals, because they are evidence of vegetation independent of other fossils. From features of fossil soils related to waterlogging or drainage, it may be possible to assess paleotopographic changes, as well as some aspects of their tectonic control. Little-altered parent material is usually preserved under fossil soils. In some cases, as in the region of the South Dakota Badlands during the Oligocene, parent material appears to have been complexly interrelated with vegetation, climate, and fluvial regimen. Times of formation of fossil soils can be estimated by comparison with studies of modern soil formation. From such estimates, which for the sequence in the Pinnacles area of Badlands National Park appear to be crude but not unrealistic, the completeness and rates of sediment accumulation of different parts of a sequence can be compared, providing insights into changing factors in the long-term behavior of fluvial systems.