Foraminiferal biostratigraphic and paleoenvironmental results from the MDS1 borehole of the Guercif basin reveal the detailed history of its paleogeographic evolution during the late Neogene.

Four biostratigraphic events are recognised based on the study of planktonic foraminifera. In addition to the placement of the Tortonian-Messinian boundary at 798 m depth, they also allow correlation of the MSD1 borehole with other late Miocene sections in the Mediterranean area.

The diversity of benthic foraminifera is significant in the studied borehole. 111 species assigned to 55 genera, were identified.

Three successive assemblages of benthic foraminifera were recognised, each one conveying a different paleoenvironmental information. They thus recorded a number of fundamental changes in the environmental and water mass characteristics of the Guercif basin.

Following the oldest upper Tortonian transgressive marine deposits found at the bottom of the borehole, marls accumulated at lower epibathyal to upper mesobathyal depths (~400 to ~800 m). They recorded sea-floor dysoxia and associated stratification of the water column for the time interval between ~7.5 and ~7.3 Ma. During the latest Tortonian-early Messinian the basin experienced a rapid shallowing, first from the previously bathyal environments to deep neritic (~100 to ~200 m), and to shallow neritic afterwards (0 to ~30 m). The total absence of any foraminifera in the conglomerates situated at the top of the study borehole suggests that these sediments were deposited in subaerial environments. At this time the connection between the Atlantic Ocean and the Mediterranean Sea through the South Rifain Corridor was stopped. This local event contributed to the establishment of the particular conditions of the Messinian salinity crisis.

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