Isotopic and chemical compositions of bentonites as paleoenvironmental indicators of the Cretaceous Western Interior Seaway

Abstract

Oxygen and hydrogen isotopic compositions of volcanic ashes, extensively deposited in the Cenomanian-Turonian Greenhorn and Campanian Claggett seas of the Cretaceous Western Interior Basin, are commonly at variance with the isotopic compositions of the seas themselves as inferred from well-preserved fossils. The δ18O values of montmorillonite from bentonite beds in Manitoba, Montana, South Dakota, Wyoming, Colorado, Kansas, Oklahoma, and New Mexico range from 13 to 28 and translate into values for the Greenhorn and Claggett seas of −14 to 4‰ at 10–25°C. These values are significantly more variable than the δ18O values of fossils, which translate into values of −8 to −2‰ for the same seas. The discrepancies are attributed to post-formational alteration of the bentonites. The δD values of Turonian and Campanian bentonites are unusually low at −120 to −110‰ relative to those to be expected from the respective late Greenhorn and Claggett seas, whereas some samples of the Cenomanian “X” bentonite seem to be in oxygen- and hydrogen-isotopic equilibrium with the earlier Greenhorn sea. Mg and Ca contents of montmorillonite from the bentonites vary regularly with δ18O values but not with δD values, and SEM analyses show the presence of neoformed fibrous smectite in bentonites with the highest δ18O and Mg and Ca contents. The chemical and isotopic data indicate that the variations in mineralogy and alteration of the bentonites result from widely disparate depositional environments and alteration of smectites by interaction with basinal brines.