Recognition and Interpretation of Cemented Subsurface Horizons in Sandy Paleosols of Cretaceous-Paleogene Age, Eastern Montana

Cemented paleosols of the Cretaceous Hell Creek Formation were examined in an attempt to discern origin of cementing agents. Six 3-m columnar sections were selected for detailed study. The soils were formed in interbedded sand and mud fluvial sediments and typically exhibit only weak signatures of pedogenesis. Furthermore, the sediments have been subject to low-temperature diagenesis that in some instances blurs the distinction between pedogenic and post-pedogenic features. In the sandstones, which are all interpreted as subsoil horizons, pedogenic features include oriented coatings of fine material and iron oxide coatings and hypo-coatings in intimate association with organic matter. In some horizons it appears that post-pedogenic phyllosilicates have replaced pedogenic coatings. Replaced grains and dense infillings of non-pedogenic voids by iron oxides were also identified. Calcite is evident in at least trace amounts in all sandy subsoils and is conspicuously abundant in medium textured horizons. However, the paleoenvironmental significance of calcite is difficult to assess because its abundance in any other context than as a framework constituent must be regarded at least in part as the result of repeated episodes of precipitation during diagenesis. © 1990