A Hierarchical Classification Scheme for the Sedimentary Architecture of Fluvial Crevasse-Splay Deposits

Abstract

Crevasse-splay deposits represent an important component of the stratigraphic record of fluvial overbank systems. Preserved sand-prone splay bodies constitute volumetrically significant parts of otherwise predominantly fine-grained floodplain successions. This work presents a hierarchical scheme with which to classify and interpret the origins and paleoenvironmental significance of splay deposits based on quantitative facies and architectural-element analysis of 15 ancient successions from the Mesaverde Group (Upper Cretaceous, Utah) and the Morrison Formation (Upper Jurassic, Utah and Colorado), supported by analysis of 20 modern fluvial systems. Hierarchical levels at various scale are recognized and represent the following components: (i) lithofacies arising from accumulation via fundamental flow processes; (ii) individual event beds (∼1 m thick) comprising one or more lithofacies proximal to major parent channels but thinning and fining laterally, representing the deposits of individual flood events (100 to 101 yr timescale); (iii) splay lobes comprising genetically related cosets of strata up to 3m thick that record the initiation, growth, and abandonment of individual parts of splay bodies (101 to 102 yr timescale); (iv) splay elements comprising genetically related lobes that have a common breakout point (102 to 103 yr timescale); (v) ‘splay successions’ comprising a group of splay elements that are not genetically related but which accumulated in a single flood basin (103 to 104 yr timescales). The extent, thickness and stacking of bodies that comprise each tier of the hierarchy is directly related to the following: (i) the size of the parent river and its flood basin, (ii) local gradient changes in the flood basin arising from processes such as channel super-elevation and levee development, (iii) rate of accommodation generation, which influences whether splay elements are laterally offset due to compensational stacking. 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 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.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016