Formation of Parasequence-like Successions at Different Depositional Hierarchical Scales

Boyan Vakarelov, Bruce Ainsworth

The interpretation of parasequences is often complicated by ambiguities in the definition of key surfaces, uncertainties in interpretation of facies successions, varying data quality, and different rules of thumb (heuristics) used for identification. The number of interpreted parasequences in an area can, for example, be directly influenced by factors such as the number of data points, the spatial coverage, and the depositional processes governing sedimentation. While abrupt deepening events associated with flooding surfaces may occasionally be identified objectively through vertical facies relationships, they are often inferred, which leads to both allogeneic and autogenic successions being treated as parasequences. This paper explores the various mechanisms that produce shallowing-upward successions that can resemble parasequences across different depositional hierarchy levels, including: (1) allogenic changes in the balance between rates of sediment supply and accommodation, (2) autogenic trunk channel avulsions driving significant sediment redistribution at the shoreline, and (3) local-scale autogenic processes, such as levee and barrier breaches near river mouths. Modern examples of each type are provided, and their implications for interpreting ancient parasequences are discussed. It is argued that misinterpretation of the hierarchy level of such parasequence-like successions can significantly impact the selection of depositional analogues and the reconstruction of depositional history of an interval. Although each mechanism discussed may result in architectural units resembling parasequences, we recommend that the term "parasequence" be reserved for only regressive-transgressive units formed on a regionally significant scale, while alternative terminology should be used for intra-depositional system deposits formed on a more local scale.