Magnetostratigraphic dating of an Upper Miocene shallow-marine and continental sedimentary succession in northeastern Morocco

Abstract

A high-resolution magnetostratigraphy has been developed for an Upper Miocene shallow-marine carbonate complex, equivalent basinal marls, diatomites, and lime mudstones, and an overlying succession of marine to continental carbonate and siliciclastic sedimentary rocks in the Melilla 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 Carbonate Complex (TCC) [1]. Lithologies from these units contain at least three 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 that 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 component and single-domain to pseudo-single-domain grain behavior. Rock-magnetic studies suggest biogenic magnetite may contribute to the magnetic remanence of the low-coercivity component. The Melilla Basin magnetostratigraphy, independently corroborated with new 40 Ar 39 Ar dating of volcanic ashes and foraminifer biostratigraphy, has been correlated to the recently developed geomagnetic polarity time scale of Shackleton et al. [2]. This correlation suggests deposition of the bioclastic platform began approximately at the Tortonian/Messinian boundary, which 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 overlying TCC and mixed carbonate and siliciclastic succession correlate to within chron 3r. The new magnetostratigraphy is not without minor ambiguities, possibly due to either remagnetization within the TCC or the presence of a short-duration normal subchron in the lower portion of chron 3r. Results of this study have important implications for correlating between Late Miocene shallow-marine carbonate sections and basinal sections within the Mediterranean Basin, for improving the understanding of the history of the Messinian Salinity Crisis, and for deciphering the roles of eustacy, tectonics, and regional processes in the development of shallow-marine carbonate sequences in the Mediterranean region.