The Late Holocene parasequence of the Banc d'Arguin (Mauritania): Role of aeolian detrital inputs in the lithology of shallow water platform parasequences

Abstract

The Banc d'Arguin (BA - Mauritania) has the double originality of being a shallow water platform (SWP) located on the edge of a sandy desert. This arrangement is rare in the world: Shark Bay in Australia, the western edge of the Persian Gulf and the Namibian margin are the main examples. BA's detailed study both in the marine part of the platform and on the coastal plain has highlighted the peculiarities of the Late Holocene parasequence (LH). In the basin, it is made up of two units recognized in seismic reflection: U5 surmounted by U6 / U7 in lateral variation of facies. U7 also constitutes most of the coastal plain, the construction of which Dia (2013) described in 5 stages. On its arrival, the postglacial Nouakchottian sea invaded areas determined by the construction and progression of ergs during the glacial episode (Azefal to the north, Agneitir and Akchar to the south). Their orientation defined the axes of penetration of the sea in the inter-ergs (Ras el Mâ, sebkha Acheil). In addition, the estuaries of the paleo-wadis, functional during the wettest episodes, were occupied by the sea (Khatt des Ogols). The ingression thus exceeded 12 km in Ras el Mâ and 10 km in the sebkha Acheil. The subsequent development was characterized by the clogging of the bay heads. It mainly used wind sands pushed in the axis of thalwegs and inter-ergs. The stability of the sea level since its arrival at its current level limited accommodation. The accommodation / supply ratio of Cattaneo and Steel (2003) was well below 1. The LH parasequence is therefore generally thin but very extensive on the surface. It is made almost entirely of sandy deposits characteristic of the inter and supratidal coastal environments: beach ridges and sedimentary spits, coastal dunes, large sand-flats which evolved into sabkhas for the oldest, as progradation progressed. The occasional use of the high points of the shore by the Neolithic populations underlined the coastal morphology. The description of the arrangement of the sedimentary subunits formed during the five phases of the period shows that it was formed by progradation with little aggradation during a phase of normal high-level regression.The comparison of the conditions prevailing in the other SWPs identified in the world shows that it is not the transgressive or regressive tendency of the deposits which is their most striking character. The very different lithology of the deposits is more discriminating and underlines the role played by desert sand inputs in the expression or not of other modes of biogenic and/or chemical sediment production. In the case where there are significant wind and fluvial siliciclastic components (northwest coast of Namibia) or significant wind inputs (BA in Mauritania), the production of biogeochemical carbonate components is reduced (Namibia) or even prevented (Mauritania). On the contrary, when the detrital inputs from a nearby desert environment are low (Shark Bay in Western Australia) or inefficient to almost non-existent (west coast of the Persian Gulf), the biogeochemical processes involved in the development of carbonates become predominant and lead to the formation of carbonate platforms. This continuum of models offers the possibility of identifying the amplitude of the desert signature in the paleogeographic reconstructions of certain platforms.