Fluvial response in a sequence stratigraphic framework: Example from the Montserrat fan delta, Spain

Exceptional exposure of the Montserrat fan-delta system (Eocene) in northeastern Spain provides an excellent framework to evaluate the alluvial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles (∼ 10 ⁴ yr) and eight fourth-order cycles (∼ 10 ⁵ yr). Fifth-order cycles are characterized by long-distance shoreline migrations and, occasionally, by incised basal scour surfaces but not by changes in fluvial style, lithofacies, or channel stacking pattern. Fourth-order cycles are composed of stacked fifth-order cycles and have non-erosional basal boundaries. Vertical sedimentation rates and channel-stacking patterns change significantly within fourth-order cycles. The lower parts of these cycles, referred to as the transgressive facies tract, show overall shoreline transgression, and associated alluvial deposits contain abundant overbank materials with isolated (ribbon) channel bodies. During this time the supply of terrigenous material to the shoreline was reduced, as indicated by sediment starvation offshore. The overlying middle part of fourth-order cycles, the lower regressive facies tract, differs only in that shoreline is overall regressive, and there is increased clastic supply to the offshore. In the upper part of these cycles, the upper regressive facies tract, channel-stacking geometries become denser and more sheet-like, shoreline regression is more pronounced, and vertical aggradation rates are inferred to be reduced. Changes in the alluvial system during fourth-order cycles are most pronounced adjacent to shoreline and die away upstream over just a few kilometers - indicating that the base-level signal decays away over the distance of a few backwater lengths (channel flow depth/slope). Higher-frequency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) changes have more impact on the channel stacking architecture of the alluvial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland basins because of their back-tilted geometry. We propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea-level rise. Progradation occurs as erosion rates in the mountain belt increase, and rates of subsidence and relative sea-level rise diminish. Changes in alluvial architecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream. Hence, changes in channel-stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal part of the basin and are not necessarily driven by eustatic sea-level changes.