The terminal Miocene event

The Messinian salinity crisis in the Mediterranean Basin, which resulted in a paleontological and stratigraphical hiatus between the Lyellian Miocene and Pliocene faunal sequences, has a relatively brief history (∼1.5 m.y.) coincident with upper Zone N17 planktonic microfauna and within calcareous nannoplankton Zones NN11 and NN12. In the Andalusian Stage stratotype, located in the Atlantic-connected Guadalquivir Basin of southwestern Spain, paleobathymetric analysis of benthonic foraminifera indicates a marked reduction in water level, from ca. 70–100 m to ca. 30 m, in uppermost N17 levels, and a return to former water depths coincidentally with the beginning of the Pliocene (Zone N18 microfauna) which is calibrated tot = 5 Ma. An apparently simultaneous regression is evident in upper Zone N17 levels of the Neogene basins of southeastern Spain studied by C.Montenat, and alkaline volcanics at Barqueros (Murcia) within the N17 sequence have K-Ar ages indicating that the regression began less than 6.5 Ma. The Spanish regression is clearly linked with the Messinian salinity crisis, and it can also be biochronologically correlated to a period of marked eustatic lowering and refrigeration recorded in Kapitean strata of New Zealand and thereby to a great, perhaps maximum temporary expansion of grounded ice on the Antarctic continental shelf. It is clear nevertheless that no eustatic sea-level change of even this magnitude could have isolated the Mediterranean Basin unless it had already been nearly isolated by tectonism. Therefore two stages in Messinian history are envisioned following Ruggieri: (1) An initial deep-water phase marked by refluxive concentration controlled by a tectonically elevated sill, during which evaporites and associated sediments accumulated in marginal sea level areas and in great saline bodies in the depths of the basin; this event lasted about 1 m.y. and spanned the interval between 6.5 and 5.5 Ma. (2) A terminal phase of total isolation caused by eustatic lowering of the world sea level, during which erosion and desiccation features were developed which fit the “deep-basin, shallow-water” model of Hsu¨, Cita and Ryan. This second phase spanned the interval approximately between 5.5–5.0 Ma.