Quantitative prediction of stratigraphic architecture in fluvial overbank successions

Most outcrop-based studies of fluvial successions predominantly focus on sand-prone channel complexes; less attention has been directed towards fluvial overbank successions. Crevasse-splay deposits represent an important component of the stratigraphic record of fluvial overbank systems and yield information about the size, form and behaviour of formative fluvial systems. Quantitative facies and architectural-element analysis was undertaken on outcrop successions from the Morrison Formation (Upper Jurassic) and the Castlegate and Nelsen formations, Mesaverde Group (Upper Cretaceous), this was then supported by analysis of 10 modern fluvial systems to better constrain the planform variations in overbank areas. Lithofacies arrangements are used to establish the following: (i) recognition criteria for splay elements; (ii) criteria for the differentiation between distal parts of splay bodies and flood plain fines; and (iii) empirical relationships with which to establish the extent (ca. 280-500 m long by 180-1000 m wide) and planform shape of splay bodies in the Morrison Formation (teardrop) and Castlegate and Neslen formations(semi-elliptical). A nested, hierarchical stacking of the deposits of fluvial overbank successions are recognized and records accumulation of the following components: (i) lithofacies; (ii) individual event beds comprising an association of lithofacies; (iii) splay elements comprising genetically related beds that stack vertically and laterally and represent the deposits of individual flood events; (iv) splay complexes comprising one or more genetically related elements that have a common breakout point and represent the deposits of multiple flood events. Splay accumulations occur as parts of larger successions in which floodplain-dominated intervals accumulate and become preserved in response to longer-term autogenic controls, such as rate of lateral migration and avulsion frequency of parent channels, and allogenic controls, such as changes in subsidence, climate, base-level and sediment supply. Sandy splays contribute ‘hidden’ volume to fluvial reservoirs and may form significant connectors that link otherwise isolated primary channel bodies, thereby contributing to reservoir connectivity.