CLAY MINERALOGY OF SOME HIGH LATITUDE PALEOSOLS FROM THE PRINCE CREEK FORMATION (MAASTRICHTIAN), NORTH SLOPE, ALASKA

Paleosols and dinosaur bones are known from the Late Cretaceous Prince Creek Formation (PCF), North Slope, Alaska. We are interested in the palaeosol clay content of this formation, since clays may record variations in paleoclimatic conditions and the Cretaceous is a time of global warmth and mass extinction. The PCF is interpreted as alluvial sediments deposited on a coastal plain. X-ray diffraction (XRD) analysis of floodplain sediments along a north-south transect of the PCF determined that the main detrital minerals are kaolinite, plagioclase (albite) and quartz. Smectite is the major clay mineral in the more distal areas (northern section), while the south (more proximal) shows more variability in clay mineralogy, which includes chlorite, illite, and smectite. Illite/smectite (I/S) is the predominant component of the mixed-layer clay fraction. Vertical changes in clay mineralogy within one compound to cumulative alluvial paleosol profile (NKT) in the southern part of this transect shows that illite might not be diagenetically altered. Vitrinite reflectance data from previous studies suggests low temperatures (<60°C) and shallow burial of sediments. The trend in the NKT profile has a positive correlation with the whole-rock geochemistry showing at least one discontinuity marked by the Ti/Zr ratio. The paleosol is a composite paleosol with Bg, Bt and Bw horizons. In the lower pedogenic phase (LPP) we found that smectite, I/S, and illite have an irregular distribution, while in the upper pedogenetic phase (UPP), we found a regular ratio between smectite, I/S and illite. These results suggest that: 1) the vertical variability in clay minerals of the LPP may be primarily due to differences in parent material sources resulting from periodic additions of alluvial sediment and 2) in the UPP, illitization is one of the main pedogenetic processes and may be related to fluctuation of the water table. We, therefore, conclude that clays of the LPP are probably largely of detrital origin, whereas those from the UPP are most likely transformed by pedogenesis. From the UPP results we conclude that illitization is the product of high seasonality in pedogenic environments at high latitudes, since this process occurs when smectite can be transformed into I/S and illite at surface temperatures by repeated wetting and drying cycles.