ICHNOLOGY AND CALCRETE STRATIGRAPHY OF THE NEOGENE OGALLALA FORMATION, HIGH PLAINS OF WESTERN KANSAS

The Neogene Ogallala Formation in the high plains region is well known for its characteristic honeycomb-weathering pattern resulting from the juxtaposition of erosive siltstone and sandstone and resistant calcretes. Our previous research in the Ogallala in western Kansas showed that many prominent weathering patterns are controlled by carbonate-cemented trace fossils attributable to ants (Daimoniobarax). In an attempt to better understand the influence of meso- to macrofaunal activity in soils on calcrete formation, our recent research has focused on describing ichnology and calcrete stratigraphy at multiple outcrop exposures in Scott County, Kansas. Our results show that major calcrete beds are laterally continuous throughout the study area and that multiple developmental stages are represented. Calcretes are typically developed in massive, pedogenically modified, red silty sandstone and form a supportive matrix for dispersed sand grains. Many calcretes contain pockets of friable, grain-supported silty sand, leading us to the interpretation that carbonate precipitation was displacive and began in voids created by biotic activity. A diverse assemblage of ichnofossils is associated with calcretes and paleosols and includes Daimoniobarax, rhizoliths, smooth-surfaced ovoids interpreted as bee cells, large-diameter vertical to subhorizontal tubes interpreted as mammal burrows, large-diameter horizontal to subhorizontal tube networks interpreted as mammal burrows, vertically flattened subhorizontal tubes interpreted as scorpion burrows, and curved, finger-shaped tubes interpreted as fossorial vertebrate burrows. The vertical distribution of traces within individual paleosols suggests relatively long periods of landscape stability punctuated by periods of rapid aggradation. Investigations into time constraints on deposition are ongoing, and will benefit from vertical stable-isotope profiles associated with measured sections and a recently discovered volcanic ash bed containing identifiable vertebrate fossils.