Late highstand patterns of shifting and stepping coastal barriers and washover-fans (late Messinian, Sorbas Basin, SE Spain)

Abstract

The late Messinian Sorbas Member, up to 75 m thick, consists in its type area of a parasequence set of three prograding coastal barriers (sequences I–III), associated with lagoon and washover sediments. Around the town of Sorbas these strata can be studied exceptionally well due to absence of burrowing by raised Messinian salinities and exposure along a network of up to 30 m deep canyons. Fifteen vertical sections were logged and carefully correlated. This permits to reconstruct and discuss patterns of relative sea-level movements between decimetres, up to 15 m within a parasequence. Excellent examples of non-tidal transgressive facies are characterized by lagoon and washover sediments instead of the usual combination of washover and tidal deposits (channel and flood-tidal delta). Implications for the sandstone connectivity are given. The lower two sequences are deposited in a relatively large, tectonically enhanced wedge-shaped accommodation space. They show both fining-up, deepening sequences, followed by prograding coarsening-up shoaling sequences and can be compared to the classical parasequences of the Western Interior Basin (USA). Progradation of sequence II was interrupted by a major slide event (most likely triggered by an earthquake), which caused more than 400 m seaward slumping of a stretch of 1 km of coastal sands. The architecture of sequence III is more complex due to limited accommodation space characteristic for the late highstand, so that this setting was very sensitive to sea-level fluctuations. This resulted in an intricate pattern of juxtaposed and superposed lagoonal muds, washover fans and swash zones. So-called ‘stranded’ coastal barriers occur, which were left behind after seaward jump of the coastline over more than 1.5 km during forced regression. The pattern of reconstructed sea-level positions is well comparable to the sequential pattern shown by the correlated equivalents along the northern basin margin, which belong to the so-called Terminal Carbonate Complex. The problem of ranking the complex sequence III as one or more parasequences and its consequences for cyclostratigraphy are shortly discussed. Two models of washover formation are given, respectively during more rapid and more slow sea-level rise. The influence of synsedimentary folding on the location of barriers is discussed and also the source area of extra- and intraclasts supplied to barriers and washovers. After deposition of the Sorbas Member the sea withdrew from the Sorbas Basin, probably as a result of the major downdrop in the Mediterranean at the maximum isolation during the Salinity Crisis. It is suggested that the semi-enclosed setting of the basin resulted only in limited, localized erosion, in contrast to the deeper adjacent Vera Basin, which was more open to the Mediterranean.