Depositional character and preservation potential of coarse-grained sediments deposited by flood events in hyper-arid braided channels in the Rift Valley, Arava, Israel

Abstract

Four coarse-grained braided channels undergoing rapid deposition in the hyper-arid Arava Rift Valley have been studied to determine the dynamics of strata deposition by monitored flood events and to ascertain the sedimentologic character of these event deposits. The systems span granule-sand to sandy-gravel bed material having longitudinal bars with simultaneously increasing grains size, slope and bed relief. Scour and fill were monitored along cross-sections using scour chains and repeat surveys, while event strata (=fill) were described with reference to sediment calibre, type of support, grading, thickness and location. Unlike many previous investigations of gravel sedimentology, the source of information for this study involves not only stratigraphic description but also observation of the concomitant dynamics of the channel bed as expressed by the extent of event deposition. The texture of the deposited sediment generally coarsens in sympathy with the surface bed texture of the studied channels. The thickness of event-deposited sediments increases as the magnitude of flow events increases. The stratigraphic record is characterized by: (1) sub-parallel stratification, (2) no grading, (3) clast-supported gravel infilled with sand-granule matrices and (4) rapid sedimentation rates. This character is hypothesized to mirror the elevated bedload fluxes in this hyper-arid fluvial setting. Lacking densely-vegetated banks, the proximal braidplain of this arid environment often contains no extensive fine-textured floodplain successions. The depth of fill as well as the extent of strata obliteration increase with flood magnitude, explaining the increased preservation potential of event strata with increase in flood magnitude. It also explains why most event strata, including large ones, are only partially preserved.