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Bulletin de la Societe Geologique de France; July 2005; v. 176; no. 4; p. 373-380; DOI: 10.2113/176.4.373
© 2005 Societe Geologique de France
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Sr isotopic tracing in a lagoonal system : example of surficial carbonate sediments in the Nador lagoon (northeastern Morocco)

Rachida Mahjoubi1, Saïd Kamel1, Yves Noack2, Annie Michard2, Daniel Nahon2 and Christian Carruesco3

1 UFR Géologie de Surface et de l’Environnement, Faculté des Sciences, Université Moulay Smail, Meknès, Maroc. r.mahjoubi{at}fsmek.ac.ma, kamel{at}fsmek.ac.ma
2 Université Aix-Marseille III, CEREGE-CNRS, Aix-en-Provence, France.
3 Université Bordeaux1, Dépt. de Géologie et Océanographie-EPOC, Bordeaux, France.

The Nador lagoon is the largest one in Morocco and along the Mediterranean. It is located on the northeastern coast near the Rif chain (North Morocco), and it is surrounded by volcanic and sedimentary rocks of various lithofacies. The watershed has an area of about 2200 km2 and is drained by a dense river network. The lagoon dimension is of 115 km2 (25 x 7.5 km) with a depth not exceeding 8 m. The island barrier is 25 km in length and 350 m in average width.

The carbonate fraction is the dominant component of the present-day sedimentation in the Nador lagoon. It represents 13 to 48 % of total sediments. Its distribution within the lagoon reflects the nature of marine and continental watershed.

The carbonate fraction is composed, in decreasing order of importance, of calcite, magnesium calcite, dolomite and aragonite. The Sr isotopic signatures of lagoonal carbonates sediments show that they are dominated by marine biogenic sediments deposited, and by chemical precipitation. In addition, other carbonates of external origin and less radiogenic Sr isotopic are also present, coming from aerosols and ancient carbonates and transported by streams and rivers. From northwest to southeast, the Sr concentration of these lagoon carbonates increases, showing a slow renewal of waters in the southeastern zone because of its remoteness from communication with sea.

Key Words: Morocco • Nador • Lagoon • Watershed • Sediments • Carbonates minerals • Isotope tracing • 87Sr/86Sr






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