Environment

Geomorphology

Since the pioneering work of S. Daveau (1959) and work by Beaudet et al (1981), the Bandiagara Plateau has not been the subject of more recent geomorphological or geological studies. Considerered too hastily as a simple gravelly plateau covered with a thin layer of sediment from the disintegration of pre-Cambrian sandstone, the area offers the researcher the possibility of demonstrating the impact of recent large palaeoclimatic fluctuations. In fact, whether on the plateau, upstream from the Yamé Valley or at the cliff that limits the plateau to the east, the sedimentary formations can clarify and date the broad phases of geomorphological and stratigraphic evolution during the Pleistocene and Holocene. Initial results have been recently published (Rasse et al, 2004 et 2006, Lespez et al, 2008) or have been the subject of dissertation research (Le Drezen, 2008); other research is currently in progress and/or in preparation for publication. Nevertheless, further work is necessary to fully uncover the richness of the sedimentary record preserved.

Chronostratigraphy

As one of the major interests of the Ounjougou site complex is its access to sedimentary units from the Middle Paleolithic to modern times, the development of a general chronostratigraphic sequence had a central role in the research themes. This diachronic approach required the establishment of strict collaboration between several disciplines (geomorphology, sedimentology, archaeology and geoarchaeology, paleobiology, OSL and radiocarbon dating).

After several missions to analyze the geology of the site, several questions remain. Sedimentological analyses of the 15-16 m thick Quaternary unit are in progress: texture, color of the deposits, grain size distribution and sedimentary structure are all being analyzed and results are as yet only in some measure useable. However, the principal characteristics of the formations and the analysis of sedimentary contacts make it possible to reconstruct the broad outlines of the stratigraphy.

All of the geomorphological evolution of the plateau having been several times dependent on the same surface conditions, the repetition of forms and figures of sedimentary overlap is constant. The stratigraphy of the Ounjougou site complex can only be interpreted with a necessary distinction between surface processes, which affect the flat surfaces and the superficial reworking of soils, and the processes of embanking the fluvial arteries that influenced the principal forms.

At the surface, the material is almost exclusively silty, whether on the principal glacis or on derived forms. These are forms associated with successive reworking of eolian silts that were deposited on the plateau, probably particularly during dry phases. Conversely, in the channels, coarse sequences, composed mainly of sands and gravels, characterize deposition. Heavy precipitation fueled the powerful flow that caused a real size sorting of grains: fine particles were transported downstream and coarse material was slowly deposited in the base of the channel. This distinction is simple to recognize in the Holocene levels, and while it helps to understand the stratigraphy, it remains hypothetical for the Pleistocene formations.

Landscape and vegetation

The Holocene sequence at Ounjougou is particularly rich because of its exceptional preservation of macro-and micro- botanical remains. It provides us thus with a great opportunity to study the ecosystem transformations during this period in West Africa.

A combined approach was developped with the aim to study vegetal palaeoenvironments and landscapes dynamics: palaeoanthracology, carpology, palinology, phytoliths analysis, sedimentology and micromorphology. The primary objective is the reconstruction of the history of societies and climatic variations influences on Holocene landscapes and vegetation in Dogon Country.