ORIGIN OF CLAY MINERALS AND THEIR SIGNIFICANCE IN PEDOGENETIC ENVIRONMENTS OF THE LATE CRETACEOUS PRINCE CREEK FORMATION, NORTH SLOPE, ALASKA

Mineralogical and chemical analyses of the clay-size fraction of paleosols from the Late Cretaceous Prince Creek Formation, North Slope, Alaska, constrain the pedogenic environment of Late Cretaceous soils formed in an arctic greenhouse where dinosaurs thrived. We studied clays from Late Cretaceous (Maastrichtian) paleosols and bentonites to determine their origin (diagenetic, pedogenetic or detrital) and reconstruct the pedogenic conditions of clay formation. Climate has important repercussions on the genesis of clay minerals, and variations in humidity can be recorded by clay mineral associations. Pedogenic clays are important because they record weathering conditions in situ during paleosol formation. In this study, the main detrital minerals observed in the clay fraction are smectite, discrete illite, kaolinite, chlorite, and quartz. Analysis of illite polytypes and chemical composition of the clay fraction suggests that the clay mineral assemblage is derived from detrital inputs associated with pre-existing sediments and low grade metamorphic rocks of the Colville Basin, mixed with reworked volcanic ashfall-derived bentonites which are compositionally similar to discrete bentonite layers within the Prince Creek Formation. We also observed a small fraction of mixed layer illite/smectite, which we infer to be the product of illitization of smectite during pedogenesis. Diagenetic transformation of smectite to illite is unlikely in the Prince Creek Formation since published vitrinite-reflectance values indicate a maximum burial temperature of ~ 48° C, less than the temperature required to transform smectite into illite during diagenesis. Illitization of smectite is known to occur in pedogenic environments with seasonal precipitation; this is consistent with the acidic groundwater regime, redoximorphic features and limited evidence for clay illuviation observed within these paleosol profiles. In the Prince Creek Formation, widespread smectite development was favored by the volcanic parent material and its accumulation in poor drained floodplain deposits. The clay mineral associations and the predominance of smectite suggest warmer climatic conditions with alternating wet and arid seasons during the Cretaceous.