MAGNETOSTRATIGRAPHIC DATING OF AN UPPER MIOCENE SHALLOW-MARINE AND CONTINENTAL SEDIMENTARY SUCCESSION IN NORTHEASTERN MOROCCO

A high-resolution magnetostratigraphy has been developed for an Upper Miocene shallow-marine carbonate complex, equivalent basinal marls, di atomites, and lime mudstones, and an overlying succession of marine to continental carbonate and siliciclastic sedimentary rocks in the Meli lla Basin, northeastern Morocco. Units of the carbonate complex, from bottom to top, contain a bryozoan/red algal ramp, bioclastic platform, fringing-reef complex and a unit referred to as the Terminal Carbonat e Complex (TCC) [1]. Lithologies from these units contain at least thr ee components of magnetic remanence: (1) a low unblocking temperature component that is likely carried by a viscous remanent magnetization, (2) a low-coercivity, intermediate unblocking temperature component th at is probably carried by magnetite or magnetite and maghemite, and (3 ) a high unblocking temperature component that is probably carried by hematite. Lithologies are typically dominated by the low-coercivity co mponent and single-domain to pseudo-single-domain grain behavior. Rock -magnetic studies suggest biogenic magnetite may contribute to the mag netic remanence of the low-coercivity component. The Melilla Basin mag netostratigraphy, independently corroborated with new Ar-40/Ar-39 dati ng of volcanic ashes and foraminifer biostratigraphy, has been correla ted to the recently developed geomagnetic polarity time scale of Shack leton et al. [2]. This correlation suggests deposition of the bioclast ic platform began approximately at the Tortonian/Messinian boundary, w hich is defined as the base of chron 3Ar [3], and continued into chron 3An.2n. The fringing-reefs span an interval that begins in chron 3An. 2n until just above the base of chron 3r (lower Gilbert). The overlyin g TCC and mixed carbonate and siliciclastic succession correlate to wi thin chron 3r. The new magnetostratigraphy is not without minor ambigu ities, possibly due to either remagnetization within the TCC or the pr esence of a short-duration normal subchron in the lower portion of chr on 3r. Results of this study have important implications for correlati ng between Late Miocene shallow-marine carbonate sections and basinal sections within the Mediterranean Basin, for improving the understandi ng of the history of the Messinian Salinity Crisis, and for decipherin g the roles of eustacy, tectonics, and regional processes in the devel opment of shallow-marine carbonate sequences in the Mediterranean regi on.