Abstract

A simple theoretical model for generating proximal alluvial depositional sequences was developed from flume experiments and illustrated by seismic reflection geometries. During periods of low discharge, the transport system is inefficient, and the sediment is mainly deposited in proximal areas; at extreme conditions of hydrological deficit, maximum backfilling occurs. The rise of discharge begins to retransport sediment stored in proximal areas during low-efficiency periods. At maximum discharge, the maximum degree of incision is reached, concomitant with a progradation of coarse material toward the basin center. In this model, a transit cycle of the knickpoint occurs, caused only by transport efficiency changes and not by base-level change. A dominantly fining-upward sequence ending with a short coarsening-upward interval is thus deposited, bounded by truncation surfaces well developed at the basin margins. Flume experiments were designed to explore this model. Two runs were analyzed that differed in the rate of volume growth of the water-sediment system of the flume, with the first run at no volume growth and the second run at 0.38 L/min. These runs simulated different rates of generation of accommodation space. A single grain size (1 mm) was used to simplify the analysis and to avoid high-relief bed forms. During each cycle, four main behaviors occur. (1) The rise of discharge produces an immediate proximal erosion and progradation of the sedimentary wedge. (2) Within the interval of high discharge, a maximum erosion forms the truncation surface, and soon after that slow deposition at high regime occurs in proximal areas (minor onlap). (3) A fall of discharge produces a rapid onlap on the simulated basin margin, the sedimentary wedge moves upstream, and a surface of nondeposition or an interval of very low rate of accumulation is formed distally. (4) During the low-discharge interval, onlap proceeds together with the progradation over the nondeposition surface (a downlap surface) of the sediment wedge formed at the basin margin. These behaviors were observed in both runs, but the timing, degree of progradation and retrogradation, and the relief of stratal geometries differ slightly; however, the higher rate of volume growth (more accommodation space) led to less proximal erosion and to better downlap development. Application of the model is demonstrated with a natural example in a Neogene basin showing well-defined truncations in seismic lines at the unfaulted margin, and major proximal incisions in the active part of the basin. Downlap stratal geometry occurs as predicted by the model. Nondepositional surfaces or intervals deposited at low rates were located close to where the analog model predicted. A method for correlating these sequences is proposed. Using the global water cycle as a link, times of maximum alluvial incision (due to the cycle of peak discharge) can be correlated to the time of fastest eustatic rise and marine onlaps; however, when water is stored on the continents and not delivered to the oceans, hydrological deficits might produce alluvial onlaps.

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