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

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, mineralog ical and geochemical methods. The paleosols consist of alternating decimetr e-scale beds of red, grey-green and variegated mudstone. intermediate horiz ons (red and grey-green variegated mudstones) contain associations of pale yellow clay coatings, dark red clay coatings, Fe-depletion hypo-coatings an d Fe-oxide quasi-coatings. SEM-EDX-ray analyses indicate that the dark redd ish 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 t he paleosol profiles, the distribution of clay minerals indicates: (1) an i ncrease in discrete clay minerals downward, (2) a decrease in mixed layer i llite-smectite downward and (3) an increase in chlorite upward. X-ray diffr action indicates that the pale yellow and dark red clay void coatings are p olymineralic. Pale yellow clay coatings have a higher proportion of mixed l ayer illite-smectite and kaolinite, less chlorite and are less crystalline than dark reddish coatings. Despite a diagenetic overprint, including devel opment of diffusion gleyans, degradation of organic matter, burial reddenin g, and probably some smectite illitization, both compositional and morpholo gical gradients exist in the paleosol profiles and void coatings. These gra dients are most Likely related to depositional variations and to pedogenic development in the original soils under variable Eh-pH conditions in the pr esence of organic matter. Therefore, although the void coatings are altered (diagenetic features) their morphology, mineralogy and perhaps even some o f their microchemistry is attributable to the presence of pedogenic precurs ors initially formed in soils similar to modern, temperate Alfisols and glo ssic soils. (C) 1999 Elsevier Science B.V. All rights reserved.