The base of the Messinian stage is defined at the base of the reddish layer of sedimentary cycle no. 15 at Oued Akrech, Morocco, NW Africa. This point coincides closely with the first regular occurrence of the planktonic foraminifer Globorotalia miotumida and the first occurrence of the calcareous nannofossil Amaurolithus delicatus, and falls within the interval of reversed polarity that corresponds to C3Br.1r. The base of the reddish layer and, thus, the Messinian GSSP has been assigned an astronomical age of 7.251Ma, later revised to 7.246.
The Oued Akrech is located 7km SSE of Rabat in a road-cut along a steep bluff next to the Oued Akrech ("oued" = "wadi" = valley), at a latitude: 33°56'13" north, and a longitude: 6°48'45" west. The exposure is along the narrow road that connects the P22 from Rabat (direction Rommani) and the S203, about 3km from the junction with the P22. Oued Akrech is a tributary of the Bou Regreg River that forms a deep valley in southward direction as the river turns to the east (topographic map NI-29-XII-3C, coordinates 370.2-370 to 371).
The Oued Akrech section consists of deep marine marls known as Blue marls after their distinct fresh color. The weathered color of these marls at Oued Akrech, however, is a yellow-beige with reddish color bands (color cycles). The Blue Marls were deposited in the Gharb Basin which represents the westward extension - and opening to the Atlantic - of the Rifian Corridor. The corridor acted as an extensional foredeep during the late Miocene to early Pliocene.
The basic sedimentary cycles are precession-controlled but precession/obliquity interference patterns are present as well. The characteristic cycle pattern allows the section to be astronomically tuned by matching the color cycles to correlative patterns in the astronomical target curve (Hilgen et al., 2000a; 65° N lat. summer insolation), thereby providing highly accurate ages for the Tortonian/Messinian boundary. The base of the reddish layer and, thus, the Messinian GSSP has been assigned an astronomical age of 7.251Ma, later revised to 7.246.
The turnover of dominantly dextrally coiled assemblages of Globorotalia menardii by dominantly sinistrally coiled assemblages of the Globorotalia miotumida group can be used to recognize the Tortonian/Messinian boundary in the Mediterranean and adjacent North Atlantic.
The first occurrence of Amaurolithus primus and Amaurolithus delicatus predate the boundary, while the first occurrence of Amaurolithus amplificus s.s. postdates the boundary. The first common occurrence of Reticulofenestra rotaria also predates the boundary.
The recorded two normal polarity intervals and intervening reversed interval correspond - from bottom to top - to C3Br.1n, C3Br.1r and C3Bn.
Planktonic foraminifer first regular occurrence of Globorotalia miotumida, and calcareous nannofossil FAD Amaurolithus delicatus
Astronomical cycles in sediments
Hilgen, F. J., Iaccarino, S., Krijgsman, W., Villa, G., Langereis, C. G., and Zachariasse, W. J., 2000. The Global Boundary Stratotype Section and Point (GSSP) of the Messinian Stage (uppermost Miocene). Episodes, 23/3, 172 - 178.
Hilgen, F. J., Bissoli, L., Iaccarino, S., Krijgsman, W., Negri, A., and Villa, G., 2000. Integrated stratigraphy and astrochronology of the Messinian GSSP at Oued Akrech (Atlantic Morocco). Earth Planet. Sci. Lett., 182, 237-251.