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Quantitative Analysis of Vertical and Lateral Trends in Fluvial Megafans


Fluvial megafans have been extensively studied what concerns the shape, progradation across alluvial plains, building extensive sedimentary bodies, channel amalgamation degree and channel size and type. However, the vertical and lateral facies, grain size and bed thickness trends are not well understood, except for the large-scale across-fan changes. Fluvial megafans build by channel avulsions, due to high deposition and channel bed aggradation rates. Individual upward-coarsening and –thickening avulsion packages, consisting of floodplain fines, overlain by successively thicker and more amalgamated splay deposits, followed by thick channel sandstones, are the basic building blocks of these sedimentary bodies. Modern megafans indicate that avulsions tend to cluster in certain areas than swipe the whole fan surface systematically. These major avulsion clusters form lobes, and the lobe avulsions build the megafans. As fans are considerably narrower in their proximal apex, and this is also where the channels are largest, amalgamated channel successions with thick-bedded coarse channel fills form due to high channel return frequency. The channel size and amalgamation degree decrease outward in the fans. Thus, as fans prograde, overall upward-coarsening and –thickening successions form, or as fans retrograde the successions fine and thin upward. Modern megafans tend to contain 3–4 lobes, ca 50 km wide. Similar to the individual avulsion packages, the lobes show upward-coarsening and thickening trends, but on a larger scale. Thus, there are at least 3 different scales of grain size and bed thickness trends. This study aims to quantify these vertical and lateral grain size and bed thickness trends in the megafans of the Williams Fork Fm in the Piceance Basin and the Green River Fm in the Uinta Basin. Statistical correlation analysis and Fourier transform algorithm are used to determine the covariance and cyclicity. Understanding and quantifying these vertical trends and their lateral scales is a powerful tool for hydrocarbon reservoir prediction and allows for better understanding of the megafan dynamics, such as avulsions, lobe switching, progradation and backstepping. We also explore whether there are intermediate scale packages, between the individual avulsion and the lobe scales, with the aim to link vertical grain size and bed thickness trends to specific lateral scales. This study links the specific grain size and bed thickness trends to facies changes.