Sedimentological and palynological evidence of regional climatic changes in the Campanian to Paleocene sediments of the Rocky Mountain Foothills, Canada

Abstract

Evidence of paleoclimatic variations in the upper Campanian to lower Paleocene post-Wapiabi sequence of strata is visible both laterally between the central and southern Foothills, and vertically in the stratigraphic record. These differences are expressed by the distribution of climatically sensitive sediments, i.e. coal and caliche, and associated palynological assemblages within floodplain facies, as well as by changes in the style of fluvial channels. The interpretation of a semi-arid environment for mature caliche paleosols is supported by the impoverished character of the associated palynological assemblages, both in terms of diversity and the number of specimens recovered, and by the conspicuous presence of Classopollis. The lateral extent of the semiarid floodplain facies and associated broad and mobile channel deposits is limited to the southern part of the basin. No signs of a caliche facies have been found in the post-Wapiabi strata of the central Foothills. Instead, some of the floodplain deposits associated with meandering streams in this part of the basin contain numerous, thick, coal-bearing intervals. The relative climatic differences between the more humid central part and the drier southern part of the basin prevailed throughout the entire post-Wapiabi interval. As this cannot be satisfactorily explained by the position of the sea shore or by orographic influences alone, there was probably another external factor. This could have been the pattern of atmospheric circulation, such as that responsible for the present-day climatic differences existing between the southern and central Foothills. Semiarid floodplain facies occur at two levels in the stratigraphic column of the southern Foothills (in the late Campanian, Belly River Formation and in the upper Maastrichtian to lower Paleocene, Willow Creek Formation). They correspond to regressive episodes of the epicontinental seaway. Intervening between these semiarid floodplain facies are the marine shales of the Bearpaw Formation and, both overlying and underlying them, thin coal beds. These represent recurrent periods of swamp growth and peat accumulation in back-barrier environments (lagoon-fill and supratidal marsh respectively). Because of the proximity of these swamps to the sea shore it is difficult to assess the influence of climate on the relative humidity of these settings on strictly sedimentological grounds. However, the terrestrial components of the palynological assemblages recovered from these marginal-marine settings and correlative lacustrine sediments (containing fresh-water stromatolites but no coal) in the central Foothills are in sharp contrast with the impoverished assemblages known from the caliche facies. The former are prolific, diverse, and contain a number of triprojectate pollen species, which indicate a relatively humid climate during the latest Campanian time.