Abstract

Over much of the northern Rocky Mountain region generally drab, lignitic, fluvial, upper Paleocene strata are overlain by variegated, fluvial, lower Eocene rocks. In the Bighorn Basin of northern Wyoming the bright yellow, purple, and red colors of lower Eocene rocks result from the translocation and oxidation of iron compounds during soil formation on ancient floodplains. Regional replacement of drab by variegated rocks at approximately the Paleocene/Eocene boundary probably reflects widespread climatic drying that in turn modified soil forming processes.

However, superimposed on this climate-generated pattern are local variations in the time of onset of the pedogenic processes that produced variegated beds. Correlating strata in different parts of the Bighorn Basin through a framework of vertebrate and floral biostratigraphy, we can demonstrate substantial differences in local environments within the basin. Such local environmental differences are responsible for the markedly diachronous (~10⁶ yrs) onset of red banding in strata that are geographically close (20 km). We discuss possible structural and/or topographic explanations for these local variations in paleogeography.