First magnetostratigraphic results in the Aïn Beni Mathar-Guefaït Basin, Northern High Plateaus (Morocco): The Pliocene-Pleistocene Dhar Iroumyane composite section

Abstract

The Aïn Beni Mathar-Guefaït Basin, in the High Plateau Region (Morocco), is dissected by the Za River, the main eastern tributary of the Moulouya, which incises more than 150 m into Plio-Pleistocene sediments. The main goal of the present study is to provide an initial geochronologic framework for such basin infill based on a combination of magnetostratigraphy and electron spin resonance (ESR). The combined results have implications on the age of the paleontological record, the evolution of the Moulouya River, and the activity of the faults that delimit the basin. We have studied sedimentary rocks that are essentially flat-lying and of an alluvial and lacustrine/palustrine origin. An approximately 140 m-thick section has been sampled at an average of 2.5 m per site, allowing to build a local magnetic polarity stratigraphy that includes nine geomagnetic reversals. Although no fold test is available, the presence of dual polarities and rockmagnetic analysis give us confidence that magnetization directions are primary. We then anchored the obtained magnetozones to the Geomagnetic Polarity Timescale (GPTS) using the biostratigraphic data as well as local geological observations. Our proposed magnetostratigraphy-based chronology reveals a Plio-Pleistocene sedimentary infill spanning from Gauss to Olduvai Chrons. The lower, detrital formations mostly fall within the normal Gauss Chron, whereas the upper lacustrine and palustrine carbonates, which are almost widespread to the top of the sedimentary fill, are Olduvai in age. These results provide the first chronological constraints for the basin fill in one of the largest intermontane basins of the High Plateaus. The new magnetostratigraphy also reveals that the major environmental change that triggered a switch from alluvial to lacustrine-palustrine conditions in Northern Maghreb occurred near the Gauss-Matuyama reversal, ca. 2.6 Ma. In addition, it shows that the age of the paleontological site Guefaït-4 is approximately 1 myr older than the ESR ages obtained from the quartz grains. The possible sources for this underestimation are discussed. However, we also acknowledge and discuss other possible chronostratigraphic interpretations of the current data, although less likely.