USE OF MICROMORPHOLOGY FOR PALEOENVIRONMENTAL INTERPRETATION OF COMPLEX ALLUVIAL PALEOSOLS - AN EXAMPLE FROM THE MILL CREEK FORMATION (ALBIAN), SOUTHWESTERN ALBERTA, CANADA

Field observations are often not sufficient for process-based interpre tations of palaeosols, particularly where they form parts of thick agg radational pedocomplexes within alluvial successions. Under such condi tions micromorphology provides genetic, temporal and spatial informati on on soil-forming processes that is critical to an understanding of p ast environmental conditions. Thick alluvial successions of the Albian Mill Creek Formation contain abundant evidence of pedogenesis, but fe w well-developed palaeosol profiles, and therefore, provide an ideal c ase study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation. The micromorphological f eatures of greatest interpretive value are types of clay coatings and ferruginous segregations, structure and fabric. Papules, evidence of b ioactivity and ferruginous concretions provide information on geomorph ic surface stability and assist in reconstructing temporal changes in drainage conditions. While individual features can provide some palaeo environmental information, the relationships of features to one anothe r and assemblages of features provides additional information when ana lysed hierarchically to establish a sequence of sedimentologic and ped ogenic events. A common, recurring sequence of palaeoenvironmental eve nts, subject to local variations, can be recognized throughout the Mil l Creek Formation. The presence of illuvial clay requires that water p ercolated through the soil and that the soil periodically dried out so that the translocated clay was retained. Dark reddish clay coatings i ndicate clay illuviation under freely drained conditions, while pale-y ellow and silty clay coatings suggest that phases of free drainage alt ernated with phases of poorly drained or saturated soil conditions. Th e presence of iron-depletion coatings, iron nodules and quasiferrans i ndicates that these units were at least periodically saturated, and th e occurrence of multiple, overlapping phases within single thin sectio ns demonstrates that redox conditions fluctuated, strongly suggesting development in the vadose zone. Recent soils containing similar assemb lages of features develop under warm temperate seasonal climates. Alte rnating phases of well-drained and saturated conditions on the Mill Cr eek floodplains are attributed to changing sediment supply and local p alaeogeomorphology rather than to any major regional climate change. T his type of process-based, micromorphological analysis should have bro ad application in other complex, pedogenically modified alluvial succe ssions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments. (C) 1998 Elsevier Science B.V. All rig hts reserved.