ASSESSING RELATIVE PALEOSOL DEVELOPMENT IN THE CENOMANIAN DUNVEGAN FORMATION, ALBERTA AND BRITISH COLUMBIA, CANADA, USING A MICROMORPHOLOGICAL SOIL DEVELOPMENT INDEX

Paleosols are now widely recognized and have proven useful for paleoenvironmental interpretations, paleolandscape reconstructions, and as stratigraphic markers in the sedimentary record. A persistent problem is that existing paleosol-landscape models are qualitative and subjective; paleosol comparisons within and between basins are made on the basis of terms such as "weakly-developed", "well-developed", "immature", or "mature", despite the fact that these terms have no specified meaning, nor consistency in their useage. A more objective method of quantifying relative paleosol development would facilitate comparison of paleosols both within and between different basins. Soil morphology has been used successfully as an index of the degree of development of soils, and several field-based morphological indices have been proposed, but these indices have not been widely applied to pre-Quaternary paleosols in the rock record because of the lack of preservation, and/or degree of diagenetic alteration, of key features required for their calculation (e.g. color, texture, pH). A soil micromorphology index (SMI) has also been proposed that is based on micromorphological features (e.g. illuvial clay coatings, mottles, nodules, microstructure) that are typically preserved in paleosols.

Relative paleosol development is assessed using SMI values calculated from quantitative micromorphological data from a basin-scale paleosol study in the Cenomanian Dunvegan Formation. A detailed stratigraphic and paleogeographic framework has already been established that permits interfluve paleosols exposed in outcrop to be located in terms of distance from coeval shorelines and valley margins. Floodplain and interfluve paleosols, whose maximum development is similar to modern Alfisols, have been identified along a proximal-distal transect. High SMI values correspond to interfluve paleosols qualitatively assessed as well-developed or mature, whereas lower SMI values correspond to floodplain paleosols formed during phases of landscape aggradation. In the Dunvegan Formation, the best developed paleosols with the highest SMI values occur on well-drained interfluves far from paleoshorelines but close to valley margins where water tables were lowered, and drainage improved, during phases of valley dissection.