--> --> A Modern-Ancient Comparison of Inclined Heterolithic Stratification Across the Tidal-Fluvial Transition in Rivers: Modern Fraser River and Lower Cretaceous McMurray Fm, Canada

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A Modern-Ancient Comparison of Inclined Heterolithic Stratification Across the Tidal-Fluvial Transition in Rivers: Modern Fraser River and Lower Cretaceous McMurray Fm, Canada

Abstract

Inclined heterolithic stratification (IHS) is an important depositional element of tidal-fluvial deposits, and consists of interbedded sand/sandstone and mud/mudstone that preserves depositional dip. IHS reflects lateral and downstream accretion of channel bars positioned within the tidal-fluvial transition. The sedimentological and ichnological character of the IHS reflects the complex spatial and temporal fluctuations in river flow and tides, as well as the mixing of marine and fresh water. In this study we compare IHS deposits from the modern Fraser River, Canada to the Lower Cretaceous McMurray Fm from the Athabasca Oil Sands region of Alberta, Canada, and we propose linkages between facies characteristics and depositional processes. Sedimentological, ichnological, palynological, carbon-isotope geochemistry, and quantitative bed-thickness analyses from the Fraser River are distilled down to construct a facies model for IHS deposited in tidally influenced river channels. In the McMurray Formation, approximately 1400 m of subsurface core from a single valley complex are compared with results derived the lower Fraser River. Fraser River and McMurray results indicate that 3 broad zones recur through the tidal-fluvial transition: a freshwater tidal zone, a transitional freshwater to brackish-water tidal zone, and a brackish-water tidal zone. Key trends in IHS successions that indicate increasing brackish-water and tidal conditions are: (1) increasing bioturbation intensity, (2) increasing burrow size and diversity, (3) increasing proportion of mud and thickness of mud beds towards the mixing zone between fresh- and saltwater, and (4) increasing depositional cyclicity between sand and mud beds. In addition, carbon-isotope signatures and palynomorph populations reflect greater marine influence, though these signatures differ in their details between systems. These trends provide a means to determine the paleogeographic position of channel bars and define the range of bedset-scale heterogeneities that can occur in IHS-prone reservoirs. This modern-ancient comparison is iterative and builds upon the full-system analysis of the Fraser River. The intent is to develop predictive tools for high-resolution reservoir characterization of hydrocarbon-bearing sedimentary strata, by linking sedimentological and ichnological characteristics of IHS-dominated deposits to depositional process, and hence constrain the depositional position of paleo-channel deposits.