A Mass-Balance Framework for Down-System Facies Partitioning in Fluvial Successions
Theoretical and experimental work shows that sediment supply calibre and the rate of sediment extraction (subsidence) from the geomorphic surface to stratigraphy exerts a fundamental control on fluvial style, and thus on grain size characteristics and proportions of sedimentary facies at a given down-system location in a basin. Importantly, previous work suggests that this behaviour is independent of the length of the fluvial depositional system, and hence the system has a fractal character. Such fractal behavior provides an important link between modern processes and fluvial stratigraphy, and allows us to add a quantitative dimension to the established concept of accommodation-to-sediment-supply ratio (i.e. mass balance). We use outcropping fluvial successions of the Organ Rock Formation (Permian Paradox Basin, Utah, USA), Blackhawk Formation and Castlegate Sandstone (Upper Cretaceous Western Interior Basin, Utah, USA) to address this behavior. For each unit we have knowledge of: (1) the rate and spatial pattern of sediment extraction; (2) the rates of facies-proportion change down-system; and (3) the 2D down-system sediment budget, thus allowing units to be treated as closed systems. Importantly, our detailed grain size dataset within the Castlegate Sandstone shows that there is no down-system fining of any particular facies, but that facies are organised systematically, and therefore predictably, down-system to accommodate the overall down-system fining trend of the fluvial architecture. This result allows the development of a combined theoretical-empirical framework that uses the fractal behaviour of fluvial systems to make a quantitative assessment of the down-system fining architecture of the three studied fluvial successions. Therefore each unit permits us to validate theory and experiments with robust real-world examples using only two specified parameters: (1) spatial distribution of tectonic subsidence; and (2) input sediment supply characteristics. Down-system changes in stratigraphic architecture and vertical facies stacking patterns within the aforementioned units, and differences between them, are driven by changes in these two parameters. This mass-balance framework provides a powerful and practical tool for predicting regional-scale fluvial architecture from limited data, and therefore has direct applications to frontier exploration settings.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California