Application of Geochemistry to Stratigraphic Problems in Lower Cretaceous of Western Canada

Abstract

The detrital sediments that form the Lower Cretaceous series of the Interior Plains of western Canada were sampled at close vertical intervals in fourteen widely spaced sections in Alberta and western Saskatchewan. The soda and potash contents of these samples were determined and then corrected to a carbonate-free basis. logs are plotted for each section and show that throughout the area studied the series can be separated into three stratigraphic divisions, each of distinctive soda content. The lower division is poor in soda. The middle division is principally composed of sediments that are rich in sodic igneous detritus, and which interfinger with low-soda strata in the eastern part of the area. The upper soda division consists of sediments that have a low to intermediate soda content. Samples of different grain size, such as sand and shale, share a distinctively similar soda content if they come from the same division or sediment suite. Thus chemical contacts may be traced from section to section without being affected by the lateral facies changes from sand to shale that are so common in the Lower Cretaceous. The potash content of the series is controlled by grain size variations to a much greater extent than soda, and thus this oxide is much less useful for stratigraphic correlation. Correlations made on the basis of the chemical data are related to the different stratigraphic units into which the series has been divided in different parts of the area. Compositional variation between the sediment suites is ascribed principally to varying provenance, rather than to influences within the depositional basin. Plots showing regional chemical variation within the middle division indicate a western derivation for the highly sodic detritus and an eastern derivation for the soda-poor sediments, with the two suites mixing in eastern Alberta and western Saskatchewan. The upper division shales show a consistent decrease in potash and an increase in soda away from the presumed western shoreline of the time. Chemical methods of correlating detrital sediments appear to be useful, particularly when applied to shales or to mixed assemblages that include shales. The rapid development of more efficient methods of analyzing rocks makes the application of geochemical methods to stratigraphic studies increasingly attractive.