From turbidites to evaporites: the Messinian Salinity crisis record of the Tabernas Basin (SE Spain)

The Messinian Salinity crisis (MSC) was a major ecological crisis triggered by a combination of climatic and tectonic drivers that led to the progressive restriction of the Mediterranean, and culminated with the formation of large evaporite deposits. The Tabernas Basin (SE Spain) presents an exceptional Miocene sedimentary record, key for understanding the evolution of a turbiditic basin during the MSC. A composite stratigraphic section (>200m long) from the early Messinian to the early Pliocene shows different phases of the MSC in the Yesón Alto area (~20 km from Almeria City). Our study allows recognizing pre-evaporitic, syn-evaporitic, and post-evaporitic units. Two stratigraphic sections, one in the Yeson Alto and the other in Rambla de Lanujar were measured, sampled and documented for their petrological, geochemical and sedimentary facies characterization. Paleontological data, obtained through the analysis of foraminifera and marine macrofossils, enabled bio-stratigraphic dating. Magnetostratigraphic sampling along the pre- and post-evaporitic units was executed, although only the post-evaporitic deposits allowed the isolation of the primary Characteristic Remanent Magnetization. The preliminary results indicate that sedimentation rates during the pre-evaporitic phase were approximately four times greater than those of the correlated Abad member in the neighbouring Sorbas Basin The examined pre-evaporitic unit (90 m-thick) predominantly comprises fine-grained deposits intercalated with levels of sandstones and limestones. Abundant benthic and planktonic foraminifera, together with other marine fauna, facilitated the identification of 13 bioevents in the succession. The presence of Turborotalita quinqueloba and Orbulina taxa allows correlation with the last Messinian biozone (d) of Mediterranean biostratigraphy. Decimeter-thick beds of fossiliferous packstones/wackestones and barren mixed-siliciclastic carbonates occur toward the uppermost part of the unit, at the transition with the evaporites, indicating the initiation of evaporitic conditions preceding the deposition of the first gypsum bed. Soft-sediment deformation in these transitional beds suggests the occurrence of an important seismic event in Tabernas basins during initial stage of the MSC. The evaporitic unit in this area comprises only three cycles of massive selenitic gypsum beds intercalated with mudstones, in contrast with the Sorbas Basin, where up to 15 cycles have been described. δS analysis of these selenites reveals values expected for Miocene marine evaporites. A level with abundant marine fossils in the second inter-evaporitic level indicates at least an episode of dilution from >150gr/l to <40gr/l during the evaporitic deposition phase, where normal marine conditions prevailed. Towards the basin margins (Rambla de Lanujar section), the pre-evaporitic unit is characterized by the alternation of siliciclastic mudstones and gravity flow deposits, including boulder-grained breccias. The evaporitic unit is represented here by two beds of secondary nodular gypsum and centimetriclenite crystals forming Selenite supercones up to 2.5 m in diameter. Conformably overlying the last gypsum bed, a cyclic sequence of interbedded conglomeratic sandstones, matrix-supported conglomerates, and siliciclastic mudstones occurs. The paleomagnetic analysis reveals a reverse polarity for this unit, suggesting its correlation with the chron C3r. The presence of a fossiliferous assemblage, indicative of marine conditions with high salinity, also supports the conclusion that the post-evaporitic phase began during the Messinian times.