Application of the Asymmetric Delta Model to Along-Strike Facies Variations in a Mixed Wave- and River-Influenced Delta Lobe, Upper Cretaceous Basal Belly River Formation, Central Alberta

Abstract

The early to mid-Campanian Basal Belly River Formation in the Ferrybank, Keystone, and eastern Pembina fields of central Alberta, reflects a mixed waveand river-influenced deltaic succession with strong storm overprinting. Prodelta deposits consist of mud-dominated heterolithic successions, characterized by a low abundance yet moderately diverse trace fossil assemblage attributable to a “stressed” expression of the Cruziana Ichnofacies. Proximal prodelta to distal delta front intervals comprise interbedded sandy siltstones and very fineto fine-grained sandstones exhibiting convolute bedding and sporadic bioturbation. Trace fossil assemblages range from very low to moderate abundance, and low to moderate diversity; the suites are attributable to a “stressed” expression of the Cruziana Ichnofacies. Distal delta front deposits coarsen upwards into fine– to medium-grained sandstones of the proximal delta front. High-energy conditions, coupled with strong storm influences, resulted in erosional amalgamation of tempestites, and led to sporadic distribution of ichnogenera. Proximal delta front intervals are weakly bioturbated, and trace fossil assemblages are characterized by low abundances and low to moderate diversity. Most forms within the sandstone facies represent the structures of deposit-feeding organisms. Suites reflect a “stressed” infaunal community and contain a mixture of elements characteristic of both the Skolithos and Cruziana ichnofacies. Analysis of more than fifty cored wells has revealed prodelta and delta front deposits that vary markedly along depositional strike. The along-strike variations fit well with the recently proposed asymmetric delta model. The model is based on observations of modern wave-influenced deltas such as the Danube. This study provides an ancient analogue. Continued research will seek to further delineate delta lobe asymmetry and concomitant along-strike facies variations, both attributable to longshore drift and deflection of river-induced stresses downdrift of distributary channel mouths. Organisms are exceedingly sensitive to fluvial influence and this “stress” is reflected in the relatively low diversity and low abundance assemblages that characterize many deltaic successions. Ethological preferences and burrow sizes further reflect the level of “stress” imparted on infaunal organisms within the subaqueous delta environment. Trace fossil suites of river influenced deltaic successions signify a departure from the archetypal ichnofacies characteristic to shoreface successions, and their mapped distributions may serve as a predictive tool for determining distributary channel proximity.