Holocene molluscan succession and paleoenvironmental reconstruction of the upper Moulouya basin: New data from the Sidi Bouramdan site (Morocco)

Abstract

This work focuses on litho-stratigraphic and malacological studies of the fluvial deposits of the Middle Moulouya, which are used to reconstruct the paleoenvironments and paleoclimates of Morocco during the Holocene. The objectives were to deduce environmental and climatic changes from the study of fluvial deposits known for their richness in paleontological remains and their spatial and temporal variations. The chronostratigraphic framework of the fluvial record is based on the radiocarbon dating of a dozen charcoal assemblages. Sidi Bouramdan is located in the Midelt region in the Ksabi basin (the Middle Moulouya) of Morocco. The Holocene plain morphodynamics are characterised by a cut-and-fill terrace system associated with a progressive deepening of the riverbed. At the level of Sidi Bouramdan, a geomorphological and lithological study identified five Holocene nested terraces of over 10-m sedimentary thickness, dated to between 10248 and 10649 cal BP and 782–921 cal BP. These show a strong chronostratigraphic discontinuity with formations of the middle Holocene period being locally absent, probably because they were destroyed by the evolution of the alluvial plain. Forty-four mollusc species were identified in the malacological study of this fluvial Holocene sequence, with changes in the malacological communities reflecting environmental variations in the fluvial system. The Upper Holocene terrace T0 was characterized by marshland-preferring species, thereby revealing a wet environment. In the other three lower Late Holocene terraces, aquatic species tended to disappear, and terrestrial species predominated, indicating the appearance and development of a dry environment in phase with paleoenvironmental data from various regional marine and continental sedimentary archives.