AAPG ACE 2018

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Accommodation – Not a Significant Control on Riverine Stratigraphy?

Abstract

According to sequence stratigraphic concepts, accommodation or base level is considered the key control on fluvial stratigraphy. Changes in accommodation are expected to occur due to subsidence or relative sea- or lake-level changes, whereas base-level control may also include creation or destruction of local depocenters. This widely used concept contradicts the fundamental principles on riverine deposition (e.g. Lane, 1955), that state that channels aggrade when sediment supply exceeds transport capacity of the discharge regime, and degrade when water discharge transport capacity exceeds the sediment supply, thus implying to updip rather than accommodation controls.

This contradiction is further illustrated by modern highly aggradational river systems that form fluvial fans by avulsions. Fluvial fans may be elevated 100 m above the surrounding floodplains. Subsidence in such systems is significant for long-term preservation rather than for accumulation. Furthermore, a river can continue avulsions within a zone (lobe) of a fluvial fan that is topographically higher than some other parts of the fan, as lateral river mobility by avulsions is controlled by channel super-elevation as linked to local depositional-erosional dynamics.

In ancient systems amalgamated sandy river channels are commonly interpreted as low-accommodation systems and floodplain-prone isolated channel systems as high-accommodation systems, and linked to either tectonic controls or higher frequency relative sea-level lowstands and highstands. Channel amalgamation however is controlled by avulsion rates and channel return frequency. Avulsion rates are in turn primarily driven by local channel bed aggradation rates and thus discharge transport capacity. Well-documented ancient examples illustrate that amalgamated sand-prone channels may occur in the thickest parts of preserved fluvial stratigraphy, whereas floodplain-prone isolated channel bodies occur in the thinnest portions, contradicting the accommodation control. Furthermore, amalgamated sand-prone channel systems can be laterally correlated into floodplain-rich isolated channels from axial to distal parts of fluvial fans.

We conclude that accommodation is not the key control on riverine stratigraphy and highlight the need to return to fundamental principles of river deposition.