Integrated Ichnology and Sedimentology of Mixed River-Wave Influenced Delta Complexes, Upper Cretaceous Basal Belly River Formation, Central Alberta, Canada
The Upper Cretaceous (early-mid Campanian) basal Belly River Formation of central Alberta comprises eight progradational, mixed river- and wave-influenced delta lobes (cycles A-H). Successive cycles intertongue with the underlying marine shales of the Lea Park Formation, and young to the east-northeast. A detailed subsurface analysis of cycles E-G was initiated within the Ferrier, Willesden Green, Gilby and Wilson Creek fields of central Alberta, to complement a previous study in the Ferrybank and eastern Pembina/Keystone fields. Expansion of the study area and study interval will serve to enhance our understanding of facies characteristics and depositional architectures within ancient, mixed river- and wave-influenced deltaic systems. Such refinements in the facies model will shed light on the degree of predictability in reservoir heterogeneities within such settings.
Fifty subsurface cores were selected, with the aim of optimizing spatial coverage and cycles intersected. Cored intervals were evaluated using ichnology and sedimentology in order to establish a high-resolution facies framework. Initial results reveal 12 discrete facies, which can be distilled into 3 distinct facies associations: (1) mixed river- wave- and storm-influenced deltas; (2) river-dominated, storm-influenced deltas; and (3) fluvio-estuarine distributary channels. Differentiation between FA1 and FA2 is based on the prevalence of river-generated features, such as soft-sediment deformation, syneresis cracks, current-generated structures, and drapes of fluid mud origin. FA3 is erosionally based, coarser-grained, and dominated by high-angle cross-stratification and current ripples. Although both FA1 and FA2 exhibit stressed ichnological suites consistent with a deltaic signal, FA2 is characterized by lower diversities and sporadically distributed traces dominated by facies-crossing elements. FA1 suites are more diverse, whereas FA3 units are mainly devoid of bioturbation.
Preliminary evaluations indicate that sandier deposits of FA1 are concentrated in updrift positions, whereas more heterolithic deposits of FA2 prevail in positions downdrift of the distributary channels. These distributions compare favorably with those predicted by the asymmetric delta model. Continued research will focus on along-strike variations in facies distributions for these Belly River lobes, in order to construct a paleogeographically accurate depositional model.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California