Extrinsic and intrinsic controls on mouth bar and mouth bar complex architecture: examples from upper Carboniferous of the central Appalachian Basin, USA

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... [ view full 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 analyse intrinsic and extrinsic controls on these parameters. Two exceptionally well exposed ancient mouth bar complexes, from the Upper Carboniferous Breathitt Group of the central Appalachian Basin, USA, show that, following initiation and aggradational-progradational growth of the bar, “choking” of the feeding distributary channel results in its avulsion preferentially to one flank of the mouth bar, promoting lateral expansion of the mouth bar top. Gradual abandonment of the distributary channel produces a laterally accreted fining-up succession which downlaps on to floor of the receiving basin (sea/bay/lake). This motif contrasts with the standard coarsening-up facies succession predicted for mouth bars. Within mouth bar complexes, superposition of individual mouth bars causes gradual shallowing of the water column, leading to reducing gradients in, and increasing confinement of successive mouth bars. Hence, early mouth bars within the complex are more inertia dominated; flows have long run-out distances and are more likely to develop a succession of prodelta turbidite lobes. Later mouth bars are more friction dominated and flows have short-run out 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 due to depositional topography and therefore progradational. The two case studies 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.