| ABSTRACT | Since the evaporites were discovered on the Mediterranean floor, an intense debate has arisen about the most appropriate scenario for the onset of their deposition, occurred at about 5.97 Ma.
The Sorbas basin (SE Spain) records a continuous and complete succession of events characterizing the Messinian salinity crisis (MSC). We focus on the astronomically tuned pre-evaporitic Upper Abad Mb. (UA) and the lowermost Primary lower gypsum deposits (PLG). The UA deposits are characterized by a well-defined precession-driven lithological cyclicity, commonly made up of quadripartite cycles (sapropel - marl - diatomite - marl). The PLG unit represents the evaporitic phase and is composed of shales - gypsum cycles. Samples were collected in the Perales section (cycles UA23 to UA34) and in the shales of the lowermost 4 PLG cycles. Calcareous nannofossils (CN) relative abundance is calculated in order to obtain information about the environmental condition leading to the gypsum deposition, usually interpreted as change (increase) in salinity. These data could also help clarifying the disappearance of calcifying plankton reported in many sections across the basin.
Preliminary results shows that the CN are present throughout the studied interval, except in PLG1 and PLG2 shales. A peak in abundance of Sphenolithus abies, followed or accompanied by a peak of Helicosphaera carteri, Umbilicosphaera rotula and Rhabdosphaera procera has been identified in an interval approximating the MSC onset. The identification in the western Mediterranean of this peculiar succession of CN paleobioevents, previously reported in Italy (Sicily, Emilia Romagna and Piedmont) and Greece (Cyprus) (Lozar et al., 2018 and references therein), strongly suggests that at approximately the time of the MSC onset, a strong increase in fresh water input and stratification of the water column affected the Mediterranean at basin-wide scale. This, at least in the Sorbas basin, culminates in the alternate deposition of shale – gypsum cycles. By contrast, in the shales of PLG3 and PLG4 cycles the CN assemblage reflects almost normal marine condition with high productivity in the water column, testified by the quasi monospecific assemblage made by Reticulofenestra spp and Coccolithus pelagicus.
Summarizing, we report the first continuous CN record at the time and before the MSC in the Sorbas basin. The finding of the CN paleobioevent in the West Mediterranean strongly suggests that the evaporitic stage was triggered by the same changes in all the Mediterranean basin, that points to an high fresh water input and stratification of the water column events.
Lozar, F., Violanti, D., Bernardi, E., Dela Pierre, F. & Natalicchio, M., 2002. Identifying the onset of the Messinian salinity crisis: a reassessment of the biochronostratigraphic tools (Piedmont Basin, NW Italy). Newsletters on Stratigraphy 51/1, 11-3 |