An important current reversal (influx) in the Rifian Corridor (Morocco) at the Tortonian-Messinian boundary: The end of Tethys Ocean


  • Richard H. Benson,

  • Kruna Rakic-El Bied,

  • Gioacchino Bonaduce


Atlantic psychrospheric and temperate mesopelagic faunas found in the lower Messinian marls in Morocco indicate that a strong, eastward flowing, bottom current was present in the Rifian Corridor before the Salinity Crisis. This influx began just before diatomite deposition in the Paleo-Mediterranean, continued during a decrease in species diversity in coral reef formation, and diminished with the initial stages of “brine” concentration in the deep-water phase of the Crisis. The influx is most readily studied in a condensed section of marl in the Bou Regreg valley near Rabat. The beginning of this “siphon event” coincides with the Tortonian/Messinian boundary (6.4 Ma, subchron 6N1). It is identified by (1) a change in the planktonic foraminifera from dominance of warm, tropical, epipelagic Globorotalia menardii with Globigerinoides to the temperate, mesopelagic Gl. miotumida plexus with conomiozea; (2) the sudden appearance of an upper psychrospheric ostracode fauna with Agrenocythere pliocenica and Oblitacythereis ruggierii, (3) a change in nannoflora; and (4) beginning of the 6.3 Ma Global Carbon Shift. The initial strong influx stage of the siphon lasted at least 0.7 m.y., decreasing after the middle of Chron 5, ca. 5.7, to be lost ca. 5.3 Ma. Conditions for the siphon formed when the continental climate created a deficit in the water budget of the Paleo- Mediterranean Sea. The reversal took place when tectonic movement in the foredeeps of the Betic- Rif Orogene changed the thresholds of the twin straits, the Rifian Corridor and the Iberian Portal. Inflow increased rapidly in the southern Corridor to draw in waters from beneath the rising Atlantic pycnocline, while Paleo-Mediterranean Overflow Water (PMOW) continued out of the northern Iberian Portal. The invasion of “nappes” or olistoliths, first into the portal and then into the corridor, led to the end of the outflow of the PMOW terminating the need for the siphon, and then to the isolation of the Paleo-Mediterranean.