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Autogenic Response of Shoreline Migration to Sea Level Rise in the Incised Valley Depositional System

Abstract

Valley fills generally contain a complex alluvial architecture from laterally amalgamated sandy channel belts in lowstand systems tracts (Martinsen 1994, Zaitlin 1994, Posamentier 2001) to avulsion process and backstepping deltaic deposit (Harris 1994) in highstand systems tracts. The stratigraphy records in the valley fills are commonly difficult to decipher and resolve due to the lack of quantified understanding of the “noise” – autogenic signatures – and their interaction with allogenic forcing in the sedimentary records. Many studies (Muto and Steel 2002, Muto and Steel 2004, Kim, Paola et al. 2006, Kim and Muto 2007) have been investigated the autogenic response of the fluvial system to sea level variation by both numerical and experimental works. In this study, we investigate the autocyclic sedimentary signals during steady relative sea level rise in the incised-valley depositional stratigraphy. To address our question, several experiments of incised valley infilling deposition during a single base-level rise were performed in the Saint Antony Falls Laboratory (SAFL, University of Minnesota). The experimental setup consists in a non-erodible V-shape valley of 2.05 m × 0.8 m × 0.10 m, with an original valley slope of 0.05, and placed inside a 5 m × 5 m tank. The sediment load (4 × 10-6 m3/s) and water discharge (7 × 10-6 m3/s - 2 × 10-5 m3/s) is carried in by a computer-controlled sediment feeder and water pump respectively. Water discharge, the rate of base level rise and the model slope were changed systematically in the series of experiments. Overhead images are collected every 30 s. In addition, the topography is digitally acquired every 10 min by a computer-controlled laser scanner. The geometry of the channel sandbodies is acquired by cross-section photos after each runs. In our experiments, the prismatic geometry of a theoretical V-shape widening seaward incised valley imposes a reduction of accommodation creation once sea-level rises above the slope. The diminution of accommodation space volume triggers a progradation of the alluvial feeders system over the valley fills. Our results imply that, the extensive fluvial progradation deposits (Claret Conglomerate unit) in the Esplugafreda section (Spain) might be a signature of autogenic process in the incised-valley stratigraphy. The coarse-grained progradation of the Claret Conglomerate is actually a consequence of reduced accommodation imposed by the incised valley geometry.