Allostratigraphy of the Upper Cretaceous Lea Park - Belly River transition in central Alberta, Canada

The Upper Cretaceous (Campanian) Lea Park - Belly River transition in the study area of central Alberta is composed of a downlapping wedge of eight regressive-transgressive shoreline allomembers (A-H) which young to the east/northeast The six youngest allomembers (C-H) are fully developed within the study area. These allomembers are interpreted to reflect high frequency fluctuations in base level. There were superimposed on the base level rise phase of a longer term cycle which created the accommodation space necessary for the Belly River wedge of sediment to migrate into the basin. The bounding discontinuities which define each allomember are marine regressive surfaces of erosion (RSEs) and nonmarine subaerial unconformities (SUs). These RSEs are interpreted to have been created by forced regressions caused by high frequency base level falls which resulted in the shoreline shifting rapidly in a basinward direction before deposition of the shoreline allomember. Nonmarine subaerial unconformities (SUs) underlie fluvial channels and associated nonmarine deposits landward of the shoreface allomembers. These SUs are created by the same drop in base level as the marine RSEs, and as such the two surfaces are genetically related to each other. Marine flooding surfaces (FSs) are generally nonerosive in nature, cannot be correlated as successfully as the RSEs and have no easily identifiable genetic equivalent in the nonmarine section. This makes the FSs less suitable for defining the allostratigraphy. Deposition in the marine realm during the base level rise phase of each allomember is relatively minor except in Allomember G. which contains extensive base level rise deposits. The older allomembers (C-E) form a strongly progradational stacking pattern, and are interpreted to have been deposited during a period of minimum rate of long term rise in base level. The youngest allomembers (F-H) form a progradational/aggradational stacking pattern reflecting a period of higher rate of long term base level rise The rate of long term rise is interpreted to have reached a maximum during deposition of Allomember G, allowing for extensive transgressive sedimentation during the base level rise phase of the high frequency base level cycle.