Stable isotopic investigation of clay minerals and pedogenesis in an interfluve paleosol from the Cenomanian Dunvegan Formation, N.E. British Columbia, Canada

Abstract

Mineralogical and stable hydrogen and oxygen isotopic compositions have been obtained for clay minerals comprising an interfluve paleosol (Clayhurst CH 2) from the early to middle Cenomanian Dunvegan Formation, British Columbia, Canada. This paleosol provides a special opportunity to determine the stable isotopic composition of early Late Cretaceous paleoprecipitation on the western coast of the Western Interior Seaway at a very northerly paleolatitude (65°). Detrital kaolinite and illite, and complex dioctahedral mixed-layer clay minerals of pedogenic origin are abundant throughout the paleosol in the <2-μm size-fraction, including ‘intergrade’ clays that range from swelling chlorite to hydroxy-interlayer vermiculite. Dioctahedral ‘intergrade’ clays consisting mostly of randomly interstratified illitic and hydroxy-interlayer vermiculitic layers comprise the <0.2-μm size-fraction. TEM observations suggest that the <2-μm size-fraction comprises a mixture of detrital and pedogenic clays, whereas the <0.2-μm size-fraction consists almost entirely of clays formed during pedogenesis. Using the stable isotopic compositions of the pedogenic clays and temperatures inferred from paleobotanical and micromorphological data, δD values of −93‰ to −81‰ and δ 18O values of −12.9‰ to −11.6‰ were generally obtained for the ancient soil water, and hence paleoprecipitation. For one thin interval near the top of the paleosol, higher δ-values are indicated for the ancient soil water. These results may indicate processes arising from a westward transgression of the Western Interior Seaway.