Architecture and stratigraphy of alluvial deposits, Morrison Fomation (Upper Jurassic), Utah

This article presents a unique cross section of a 13.5-km (8.3-mi)long by 150-m (492-ft)-thick stratigraphic interval containing braided stream and associated flood-plain deposits. The cross section is oriented approximately parallel to depositional strike. This cross section is a resource for geoscientists and engineers interested in the measurements of stratigraphic architectural elements, such as dimensions and continuity of facies tracts and facies associations, stratigraphic and geographic changes in sandstone/mudstone proportions (net to gross), and frequency and cause of vertical fluid communication between superposed reservoirs. In addition to presenting this rich data resource, observed and documented stratigraphic relationships add to our conceptual understanding of certain attributes of the stratigraphic process-response system. For example, we show that, like meandering rivers, aggradational braided river systems also build levees and alluvial ridges, providing the supra-elevation above the adjacent floodplain to create extensive crevasse splay and channel complexes. We show that superposed channel sandstone reservoirs may be brought into physical contact not only by the erosion of an upper sand into a lower sand but also by the vertical aggradation of the lower sandstone, producing a pyramid on which a younger sandstone is deposited. Four stratigraphic cycles of increasing and decreasing accommodation/sediment supply (A/S) regimes are identified. These cycles are recognized from systematic vertical changes in stratigraphic and sedimentologic attributes. At the largest scale, there is a progressive downhill shift in facies tracts recording a basinward stepping of the four stratigraphic cycles.