A Comparative Analysis of the Ichnology of Wave- and River-dominated Allomembers of the Upper Cretaceous Dunvegan Formation

ABSTRACT The Upper Cretaceous (Cenomanian) Dunvegan Formation is a siliciclastic unit, which lies in the subsurface of western Alberta and crops out in the west-central portion of the province. The Dunvegan is deltaic in origin. Previous workers have subdivided the Dunvegan into 7 allocyclic members (A through G) in the subsurface, based on detailed sedimentological and stratigraphic analyses. Each allomember corresponds to a deltaic lobe and exhibits varying degrees of wave and river dominance. Allomembers D and E were chosen to test ichnological variations in deltaic systems because D is strongly wave-dominated and E is strongly river-dominated. Tidally influenced end members were not included in the scope of this study. The study area comprises Townships 59 to 67, and Range 20 W5M to 8 W6M. Fifteen cores, totaling 269 m in length, were analysed sedimentologically and ichnologically in order to compare the ichnological characteristics of river- and wave-dominated deltas. Allomember E is characterized by an overall lower intensity of bioturbation, a lower diversity of ichnogenera within proximal facies, and a general suppression of the Skolithos ichnofacies. In contrast, Allomember D shows a greater intensity of burrowing, a generally higher ichnogeneric diversity, and a diverse Skolithos ichnofacies. The contrasting ichnology provides insights into the ethology exhibited by infauna in the different depositional settings. More importantly, the differences observed are independent of bathymetry and emphasize the influences that environmental parameters such as variability in salinity, temperature, sedimentation rates, water turbidity, and substrate consistency play in an organism's selection of an ethological survival strategy. As such, ichnological data can be successfully integrated with physical sedimentological data in order to form predictive models that may be used to better comprehend and ultimately differentiate between these two depositional systems. RESUME Le Formation Dunvegan du Cretace superieur est une unite siliciclastique qui git dans la subsurface de l'ouest de l'Alberta et affleure dans la partie centre-ouest de la province. Le Dunvegan est d'origine deltaique. Precedemment, les chercheurs ont subdivise le Dunvegan en 7 members allocycliques (A a G) en subsurface, sur la base d'analyses sedimentologiques et stratigraphiques detaillees. Chaque allomembre correspond a un lobe deltaique et montre a des degres divers des facies a dominance de vagues ou de riviere. Les allomembres D et E ont ete choisis pour tester les variations ichnologiques dans les systemes deltaiques parce que D est a forte dominance de facies a vagues et que E est a forte dominance de facies de riviere. Les facies marginaux a influence de marees n'ont pas ete inclus dans le cadre de cette etude. La region d'etude comprend les Tsp. 59 a 67 et les Rge. 20W5 a 8W6. Quinze carottes de forage, totalisant 269 m de longueur, ont ete analysees des points de vue sedimentologique et ichnologique, dans le but de comparer les caracteres ichnologiques de deltas a dominance de riviere et de vagues. L'allomembre E est caracterise par une bioturbation globalement de plus basse intensite, une plus basse diversite ichnogenerique a l'interieur des facies proximaux et une suppression generale du facies a Skolithos. Par contraste, l'allomembre D montre une plus grande intensite de bioforage, une End_Page 51------------------------- diversite en ichnogenres generalement plus grande et un ichnofacies a Skolithos plus varie. Cette ichnologie contrastee donne un apercu de l'ethologie de la faune presente dans les differents environnements de depot. De maniere plus importante, les differences observees sont iondependantes de la bathymetrie et soulignent l'influence que joue les parametres tels que la variabilite de la salinite, de la temperature, du taux de sedimentation, de la turbidite de l'eau et la consistance du substrat dans la selection d'une strategie ethologique de survie par un organisme donne. A cet egard, les donnees ichnologiques peuvent etre integrees avec succes aux donnees sedimentologiques en vue d'elaborer des modeles predictifs qui peuvent etre utilises pour mieux comprendre et ultimement differencier ces deux systemes de depots. Traduit par Lynn Gagnon.