Stratigraphic architecture and hierarchy of fluvial overbank splay deposits

Splay deposits represent an important sand-prone component of the otherwise fine-grained stratigraphic record of fluvial overbank systems. This work presents a hierarchical approach to the classification and palaeoenvironmental interpretation of ancient preserved splay deposits supported by the analysis of the stratigraphic architecture of 11 exhumed examples from the Jurassic Morrison Formation and the Cretaceous Mesaverde Group (western USA) and analysis of the morphology of splays from nine modern fluvial systems. A hierarchical arrangement of splay deposits is proposed, categorized into lithofacies, beds, elements and complexes. Recognition criteria for each tier of the hierarchy include identification of bounding surfaces, thinning and fining trends of splay elements and complexes, and palaeocurrent variability. Progradational and compensational stacking trends control the stratal architecture of splay deposits, and these are influenced by the following factors: (1) the rate of local accommodation generation, which influences the erosive power of floodwaters and whether splay elements are laterally offset owing to compensational stacking; (2) the nature of the topographic confinement of the floodplain; (3) the preservation potential linked to migration direction of channel. Splay bodies can contribute volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby enhancing reservoir connectivity.