A PEDOSTRATIGRAPHIC NONMARINE SEQUENCE MODEL

A basin-scale nonmarine sequence model has been established for the Cenomanian Dunvegan Formation that focuses on paleosols and their pedostratigraphic relationships. Paleosols across the sequence boundary vary depending upon their paleo-landscape position. The sequence boundary can be partitioned into three zones along an up-dip transect based upon both paleosol development and architecture. Weakly developed, hydromorphic paleosols within Zone 1 form as the coastal plain progrades in response to relative sea level fall (= falling stage systems tract time). Updip, within Zone 2, fluvial incision results in better drained interfluves that receive no new sediment, resulting in well-developed, welded paleosols. Slowly aggrading alluvial environments in the most proximal foredeep constitute Zone 3 where cumulative, compound and complex paleosol successions develop. Because accommodation, sediment supply and hydrological conditions vary both updip and along strike, these paleosol successions represent lateral facies equivalents. The sequence-bounding, soil-forming interval is a geosol composed of welded paleosols in Zone 2 that splay both up-dip and down-dip into less well-developed paleosol complexes. Above the sequence boundary, a high accommodation phase (= transgressive systems tract) is represented by widespread lacustrine and poorly-drained floodplain facies and weakly-developed hydromorphic paleosols. As accommodation rate decreases (= late highstand time), the succession becomes paleosol-dominated, comprising floodplain pedocomplexes that record a regional decrease in the accommodation/ sediment supply ratio. Updip variability along the sequence boundary, and within nonmarine sequences is controlled primarily by changes in the accommodation/sediment supply ratio, by hydrological variation associated with floodplain incision during valley formation, and by tectonic subsidence rate that varies in time and space.