This paper reports the results of studies undertaken to test the theory that clay minerals undergo diagenetic alteration when ransferred from a non-marine to a marine environment.
The Bearpaw marine shale of southern Alberta was sampled at three localities and the clay-sized fractions of 165 samples were analyzed mineralogically. The samples are composed of montmorillonite, illite, and chlorite, almost always in that order of abundance. No significant relationships were found between the clay mineralogy of the shales and the distance to the ancient shoreline.
Samples were also collected across the formational contact between the marine Bearpaw formation and the underlying non-marine Oldman formation.. Chemical and mineralogical analyses of the clay-sized raction of 45 of these samples show that the Na2O content ofthesamples decreases by a factor of four from the non-marine to the marine strata. Only insignificant changes were found in K20, CaO, P205 and total iron as Fe203. The values for MgO, TiO2, Al203 and SiO2 are essentially constant, Illite increases slightly at the expense of montmorillonite going from the non-marine to the marine strata.
The results offer but slight evidence to support the theory that the sedimentary environment controls the diagenetic alteration of clay minerals. However, this may be partially due to the masking effect of heavy outfalls of volcanic ash which fell into this sedimentary basin during Oldman and Bearpaw time. It is significant that the ash altered to montmorillonite whether it fell into a marine or a non-arine environment.
The most remarkable feature of the shales is their mineralogical and chemical uniformity both laterally and vertically.