Palaeogeography, Palaeoclimatology, Palaeoecology, 143(1998), 87-110

PAUL J. MCCARTHY¹ , I. PETER MARTINI¹ and DALE A. LECKIE²

1Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1
²Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303-33rd St., N.W., Calgary, Alberta, Canada T2W 2A7

Field observations are often not sufficient for process-based interpretations of palaeosols, particularly where they form parts of thick aggradational pedocomplexes within alluvial successions. Under such conditions micromorphology provides genetic, temporal and spatial information on soil-forming processes that is critical to an understanding of past environmental conditions. Thick alluvial successions of the Albian Mill Creek Formation contain abundant evidence of pedogenesis, but few well-developed palaeosol profiles, and therefore, provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation. The micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations, structure and fabric. Papules, evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions. While individual features can provide some palaeoenvironmental information, the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events. A common, recurring sequence of palaeoenvironmental events, subject to local variations, can be recognized throughout the Mill Creek Formation. The presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained. Dark reddish clay coatings indicate clay illuviation under freely drained conditions, while pale-yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions. The presence of iron-depletion coatings, iron nodules and quasiferrans indicates that these units were at least periodically saturated, and the occurrence of multiple, overlapping phases within single thin sections demonstrates that redox conditions fluctuated, strongly suggesting development in the vadose zone. Recent soils containing similar assemblages of features develop under warm temperate seasonal climates. Alternating phases of well-drained and saturated conditions on the Mill Creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change. This type of process-based, micromorphological analysis should have broad application in other complex, pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments.

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