Lateral diapiric emplacement of Triassic evaporites at the southern margin of the Guadalquivir Basin, Spain

Abstract

Abstract The Guadalquivir Basin is the Neogene foreland basin of the central and western Betic thrust belt in southern Spain. At the boundary between the basin and the outcrops of thrust nappes of Mesozoic limestones of the Prebetic and Subbetic is a broad belt of outcrops of Triassic evaporitic sediments with scattered younger rocks: the so-called ‘Olistostrome’ unit. This is highly deformed, in places chaotic, and its mode of emplacement has been attributed by various authors to olistostromal debris flow, diapirism, or tectonic melange. Studies of outcrop data in conjunction with seismic and well data, integrated using restorable cross-sections lead us to propose the following sequence of emplacement mechanisms. (a) Loading above a Triassic evaporite formation, probably in the Intermediate Units depositional zone, by north vergent thrusting of thick nappes of Mesozoic sediments, causes northward expulsion of evaporitic sediments between a basal thrust and the base of the limestones. (b) Continued thrust loading drives the diapiric body forwards ahead of the thrust belt, into the floor of the deepening Miocene foreland basin. The body includes blocks of Triassic rocks in normal stratigraphic sequence, as well as blocks of younger rocks broken off the leading hanging-wall cutoffs of the nappes. (c) When the diapiric body reaches the sea-floor of the basin, its top becomes subject to modification by sedimentary processes such as dissolution of evaporites leaving a cap rock and debris flow, both submarine and subaerial but rarely, if ever, forming true olistostromes. (d) At the leading edge of the diapir, northward compression of Miocene basin sediments results in thin-skinned thrusting within these sediments, and formation of duplex structures with a north-dipping monoclinal deformation front. Results from analogue and numerical modelling match the main geological features observed in the study area, thus supporting the plausibility of the proposed lateral diapiric emplacement of the chaotic unit.