Autogenic progradation of Bayhead deltas during sea-level risewIthin incised valleys : theory, experiment and field examples
Résumé
The evolution of sedimentary landscapes is primary driven by the interplay between the rate ofaccommodation creation A, controlled by sea-level and subsidence, and the rate of sedimentsupply S, controlled by erosion and sediment transport. In simple terms, the balance between Aand S can be used to predict periods of progradation (when sediment supply exceedsaccommodation) and periods of retrogradation (when accommodation exceeds sediment supply).However, a growing list of observations show that internal feedbacks within the sedimenttransport system can generate large-scale, autogenic stratigraphic patterns that are notanticipated by the A/S theory. These observations call for a reanalysis of several sequencestratigraphic precepts that assume a deterministic relationship between external forcings andstratigraphic products. Here, we focus on the filling of incised valleys during constant sea-levelrise, and by a constant sediment flux. We develop a simple conceptual model of valley filling andwe show that the classic sequence stratigraphic phenomenon of bayhead deltaic systems can begenerated by purely autogenic progradation during the late stage of valley flooding. This transient“auto-advance” event results from a strong decrease of in-valley accommodation as base-levelrises towards the valley apex. To test this model, we build a laboratory experiment thatsuccessfully reproduces the dynamics predicted by the model. Finally, we apply our model to twosimilar field examples, the Trinity and Brazos rivers incised valleys (Texas, USA). There systems arebroadly similar in dimension and sea-level history but were filled at different sediment rates. Wepropose that this led to auto-advance event in the Trinity River valley while no advance is observedin the Brazos system. We thus show by conceptual, experimental and natural examples that auto-advance can produce out-of-sequence regressive bayhead diastems during highstands similar to atransient change in allogenic forcing. Combined with other recent studies, our findings support theidea that meso-scale autogenic patterns are ubiquitous in the fluvio-deltaic record, and need to bemore extensively incorporated into reconstructions of Earth surface evolution and reservoirmodels