--> Morphodynamic Stratigraphy of River-Dominated Deltaic Bars

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Morphodynamic Stratigraphy of River-Dominated Deltaic Bars

Abstract

The ability to predict the stratigraphic architecture of deltaic deposits, which form significant hydrocarbon reservoirs, depends on understanding how deltaic morphodynamics influence stratigraphy. However, the connection between delta stratigraphy and morphology remains incomplete. Here, we seek to define the coupled morphodynamics and stratigraphy for two different modern deltaic bars. We focus on bars because they are the fundamental building blocks of deltaic morphology and stratigraphy. Results are presented from a comparative study of bars with different growth processes and morphologies found on the braided Goose River delta, Canada, and the elongate Beanblossom Creek delta, USA. Geomorphically, these bars cause bifurcations on the delta topset and form at the river mouth or within distributary channels. From serial aerial imagery we know that the Goose River delta is sculpted by highly mobile bars morphologically similar to bars found in braided river channels, whereas the Beanblossom Creek delta is created by channels that elongate via levee growth and bifurcate around stable river-mouth bars that are morphologically similar to those deposited within expanding turbulent jets. To characterize the internal stratigraphy of each bar, ground-penetrating radar was collected and tied to sediment cores. The Goose River delta bar stratigraphy is dominated by concave-up bounding surfaces, a homogenous distribution of stratal bodies with similar geometries, and a homogenous spatial grain-size distribution of medium sand. The Beanblossom Creek delta bar stratigraphy, on the other hand, is dominated by flat to slightly dipping bounding surfaces, a heterogeneous distribution of stratal bodies with a range of geometries, and a heterogeneous spatial grain-size distribution of mostly mud and fine sand with some medium sand. Our results indicate that different growth processes and morphologies result in different stratigraphies, and that these sedimentary models for different deltaic bar types may be used to enhance predictions of the stratigraphic architecture of deltaic deposits that formed under a variety of environmental conditions.