The Messinian post-evaporitic deposits of the Gafares area (Almerı́a-Nı́jar basin, SE Spain). A new view of the “Lago-Mare” facies

Abstract

The exceptional exposures of the evaporitic and post-evaporitic Messinian deposits in the Gafares area (NE Almerı́a-Nı́jar basin, SE Spain) make it possible to elucidate those processes affecting this region during the critical period of the Messinian Salinity Crisis. The contact between the pre-evaporitic and the evaporitic units is an erosional unconformity. The transition from the evaporitic to the post-evaporitic units is gradual, since the gypsum beds of the evaporitic unit thin upward and the siliciclastic interbeds concomitantly thicken, thus passing gradually to the overlaying post-evaporitic unit. The presence of Globorotalia miotumida group in the last pelitic intercalation of the evaporitic unit as well as throughout the post-evaporitic one indicates a Messinian age. The post-evaporitic unit consists of a variety of sediments formed in continental and marine settings. Continental facies are characterised by fluvial channel-filled conglomerates and alluvial-plain sands and silts. Marine sediments are represented by silty marls with foraminifers. A river-dominated delta developed in the transitional zone at the NW edge of the Gafares area. Localised fan-delta deposits coming from the ESE also occur in the southeastern part of the study area. Microbial carbonates locally colonised the fan-delta conglomerates. During the deposition of the lower half of the post-evaporitic unit, sedimentation took place in a sheltered embayment, with restricted water circulation and relatively isolated from the Mediterranean Sea by an E–W-trending sill located at the south of the study area. In this palaeogeographic context, freshwater influx from the delta exerted a certain influence on the distribution of microorganisms. Thus, brackish settings developed close to the river outflows that were occupied by caspibrackish ostracod assemblages. The presence of gastropods of the family Cerithiidae as well as fishes attributable to the genus Aphanius, organisms that preferentially inhabit sheltered lagoons, is consistent with these settings. Beyond the freshwater influx, foraminifers dominated the microfossil assemblages. In the upper half of the post-evaporitic unit, microfossil assemblages dominated by planktonic forms are consistently found, thus revealing a better connection of the semi-enclosed Gafares area with the marine domain. This is related to the deepening of the sill due to a progressive rise of the sea level. The continental and brackish deposits (Zorreras-like facies) have been equated with the “Lago-Mare” facies, which were supposed to spread throughout the whole Mediterranean at the end of the Messinian. Nevertheless, in the Gafares area, these deposits change laterally to and interbed with marine deposits. This stratigraphic architecture and facies relationship has two important implications. On the one hand, marine waters at the latest Messinian occupied at least the western Mediterranean, predating the long-claimed early Pliocene deluge. On the other hand, the continental-brackish “Lago-Mare” facies were spatially restricted. As a consequence, neither the presence of the caspibrackish ostracods nor the continental-brackish facies can be plausibly taken to support the hypothesis that widespread “Lago-Mare” facies occupied the whole Mediterranean synchronously at the end of the Messinian. Therefore, this type of facies is expected to be found (albeit with varying degrees of development) in marginal basins at different times but not throughout the Mediterranean.