Abstract

A fundamental architectural element of deltas is the mouth bar. Although process-based facies models have been developed to reconstruct the influence of different external controls on mouth bar geomorphology, depositional architecture, and grain-size distribution, few studies have documented the internal architecture of ancient mouth bars and mouth bar complexes, in order to analyze extrinsic and intrinsic controls on these parameters. Two exceptionally well-exposed ancient examples show that the increasing influence of inertial forces in friction-dominated mouth bars results in increasing deposition from gravity flows (hyperpycnites and turbidites), with increasing bypass of the mouth bar foreset and deposition in a detached frontal lobe on the basin floor ahead of the mouth bar. The increasing influence of inertial forces also results in increased bed length, and the better development of clinothem bottomset beds. Within these friction-dominated mouth bars, following initiation and aggradational-progradational growth, choking results in lateral accretion on the mouth bar flanks, but discharge may not be maintained symmetrically on both flanks. Additionally, “choking” of the feeding distributary can result in its upstream avulsion and abandonment of the mouth bar. This process generates laterally accreted fining-up successions that downlap onto the floor of the receiving basin, contrasting with standard coarsening-up facies successions predicted for mouth bars. Within mouth bar complexes, superposition of individual mouth bars causes gradual shallowing of the water column, reducing gradients in, and increasing confinement of successive mouth bars. Hence, early mouth bars are more strongly influenced by inertia, flows have long runout distances, and they are more likely to develop a succession of detached prodelta turbidite lobes. Later mouth bars are more strongly dominated by friction, and flows have short runout distances, since they are less able to achieve autosuspension. Earlier mouth bars display more “normal” aggradation-progradation, lateral accretion, and retrogradation in an unconfined setting, whereas later mouth bars are more strongly confined and progradational. The two case studies presented here illustrate that upward changes in mouth bar architecture and facies distributions within a mouth bar complex are a predictable product of shallowing and increasing confinement during delta progradation.

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