Pedogenic and diagenetic influences on void coating formation in Lower Cretaceous paleosols of the Mill Creek Formation, southwestern Alberta, Canada

Abstract

Pedogenic and diagenetic influences on void coating formation were studied in two paleosols from the Lower Cretaceous (Albian) Mill Creek Formation in southwestern Alberta, Canada, using detailed micromorphological, mineralogical and geochemical methods. The paleosols consist of alternating decimetre-scale beds of red, grey-green and variegated mudstone. Intermediate horizons (red and grey-green variegated mudstones) contain associations of pale yellow clay coatings, dark red clay coatings, Fe-depletion hypo-coatings and Fe-oxide quasi-coatings. SEM–EDX-ray analyses indicate that the dark reddish coatings are dominated by Si, Al and Fe, with subordinate amounts of K, Na, Ca, Mg and Ti. Pale yellow clay coatings have a higher Si content, and lower Al, K, Na, Ca and Fe contents relative to reddish coatings. Within the paleosol profiles, the distribution of clay minerals indicates: (1) an increase in discrete clay minerals downward, (2) a decrease in mixed layer illite-smectite downward and (3) an increase in chlorite upward. X-ray diffraction indicates that the pale yellow and dark red clay void coatings are polymineralic. Pale yellow clay coatings have a higher proportion of mixed layer illite–smectite and kaolinite, less chlorite and are less crystalline than dark reddish coatings. Despite a diagenetic overprint, including development of diffusion gleyans, degradation of organic matter, burial reddening, and probably some smectite illitization, both compositional and morphological gradients exist in the paleosol profiles and void coatings. These gradients are most likely related to depositional variations and to pedogenic development in the original soils under variable Eh–pH conditions in the presence of organic matter. Therefore, although the void coatings are altered (diagenetic features) their morphology, mineralogy and perhaps even some of their microchemistry is attributable to the presence of pedogenic precursors initially formed in soils similar to modern, temperate Alfisols and glossic soils.