ISOTOPIC AND CHEMICAL-COMPOSITIONS OF BENTONITES AS PALEOENVIRONMENTAL INDICATORS OF THE CRETACEOUS WESTERN INTERIOR SEAWAY

Oxygen and hydrogen isotopic compositions of volcanic ashes, extensive ly deposited in the Cenomanian-Turonian Greenhorn and Campanian Clagge tt seas of the Cretaceous Western Interior Basin, are commonly at vari ance with the isotopic compositions of the seas themselves as inferred from well-preserved fossils. The delta(18)O values of montmorillonite from bentonite beds in Manitoba, Montana, South Dakota, Wyoming, Colo rado, Kansas, Oklahoma, and New Mexico range from 13 to 28 and transla te into values for the Greenhorn and Claggett seas of -14 to 4 parts p er thousand at 10-25 degrees C. These values are significantly more va riable than the delta(18)O values of fossils, which translate into val ues of -8 to -2 parts per thousand for the same seas. The discrepancie s are attributed to post-formational alteration of the bentonites. The delta D values of Turonian and Campanian bentonites are unusually low at -120 to -110 parts per thousand relative to those to be expected f rom the respective late Greenhorn and Claggett seas, whereas some samp les of the Cenomanian ''X'' bentonite seem to be in oxygen- and hydrog en-isotopic equilibrium with the earlier Greenhorn sea. Mg and Ca cont ents of montmorillonite from the bentonites vary regularly with delta( 18)O values but not with delta D values, and SEM analyses show the pre sence of neoformed fibrous smectite in bentonites with the highest del ta(18)O and Mg and Ca contents. The chemical and isotopic data indicat e that the variations in mineralogy and alteration of the bentonites r esult from widely disparate depositional environments and alteration o f smectites by interaction with basinal brines.