Pedogenic development at the Cretaceous-Tertiary boundary, Garfield County, Montana

Abstract The fabric, mineralogy, and geochemistry of a 0.5-1.5 cm-thick Cretaceous-Tertiary (K/T) boundary claystone parting from eastern Montana indicate that certain of its features are the result of contemporaneous pedogenesis. Although the two layers composing the parting show general similarities in moldic porosity and in major element compositions, the upper layer is dominantly composed of 2:1 clay minerals (smectite), while the lower layer is dominated by a 1:1 clay mineral (halloysite and/or poorly crystalline kaolinite). Moreover, the upper layer is richer in mobile and immobile trace elements than is the lower layer. The dominant fabric element of the 2:1 claystone is laminations with interbedded, horizontally aligned organic debris. This is interpreted as a primary depositional fabric with little diagenetic alteration. Extremely uncommon root traces penetrate the unit. In contrast, the 1:1 claystone is characterized by a complex arrangement of discrete mineral and organic components which are rounded or fragmented, and distributed throughout a microporous matrix. This fabric implies turbation. Pedogenic features, including root traces, void coatings, and void hypo-coatings of ferric oxyhydroxides and clays, are more abundant in the 1:1 layer than in the 2:1 layer. Subaerial weathering of the 1:1 claystone layer is therefore suggested. The Montana K/T boundary claystone parting is thus interpreted as preserving a diastem (i.e., micro-disconformity) between the 2:1 and 1:1 claystone layers. The temporal separation between these two layers, indicated by the contrast in the nature and arrangement of pedogenic features as well as by the textural, mineralogical, and geochemical distinctness of the two layers, militates against the two layers of the parting resulting from the same event. In this respect, our conclusions differ from those of previous workers.