Reich, Lynn T.1, S. George Pemberton1
(1) University of Alberta, Edmonton, AB
ABSTRACT: Transgressive Surfaces of Erosion in the Viking Formation of Southeast Central Alberta
During Albian time, Cretaceous shorelines in southeast central Alberta experienced major transgressions and regressions overprinted by minor sea level fluctuations. As a result, there are transgressive surfaces of erosion (TSEs) in the shallow to marginal marine deposits of the Viking Formation that signify both major and minor flooding events. The TSEs are commonly overlain by a pebble lag, conglomerate or coarse sandstone unit. Some wells contain several discontinuous conglomerate or grit units signifying multiple transgressions. However, a major transgression across the Western Canada Sedimentary Basin deposited a relatively continuous lag demarcating the top of the Viking Formation, which is then overlain by the Colorado shales.
Between the cores, the transgressive lag at the top of the Viking varies in grain size, thickness, underlying contact type, and matrix to clast ratio. In some cases there is also a relatively thick sideritized zone beneath the lag, which is significant in differentiating a sequence boundary. Other lags or grits within the Viking overly a Glossifungites surface that is commonly distinguished by Skolithos burrows. Curiously, some Glossifungites suites are emplaced within a sideritized unit. In instances where the Glossifungites ichnofacies is not present, there is only a sharp, undulatory surface below the transgressive lag.
By documenting the characteristics of transgressive deposits and surfaces, the substantiation of a basinwide transgression and implications for major eustatic sea level changes or tectonic subsidence can be resolved. Accordingly, mapping the TSEs can be fundamental to understanding the depositional history and geometry of the Viking Formation in the subsurface.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.