The Rakhat al Madrh sedimentary archive in the periphery of the Bat oasis, Oman: paleoenvironmental evolution and human occupation

The Bat oasis is located in the valley of the Wadi al Hijr, in the western piedmont of the Hajar Mountains. Occupied since the Neolithic period, the Bat archaeological complex is well-known for its Bronze Age funerary and domestic structures related to the first oasis of Oman and has been classified as a UNESCO World Heritage Site. While the central oasis of Bat is beginning to be well studied, there is scarce information on its network with peripheral sites as well as local-scale environmental change across mid- to late Holocene. The sediment-filled depression of Rakhat al Madrh, c. 7.5 km NE of the central oasis presents five remote Bronze Age dwellings associated with courtyards, represents a key site to fill this gap.

This circular depression filled with khabra-like deposits covers 1,600 m². It is surrounded by an outcrop of sandstone and quartzite to the east and calciturbidite to the west. The eastern part is topped by remnants of Pleistocene fluvial conglomerates. To the north-east, the depression is connected to the Wadi al Hijr by an alluvial fan. This depression has functioned as a sedimentary trap since at least the Upper Pleistocene (OSL dating: 46,400 ± 5600 BP).

Nine soundings were opened down to a depth of up to 310 cm and sampled for sedimentological (grain-size distribution), geochemical (X-ray fluorescence and photometric analyses), mineralogical (X-ray diffraction) and micromorphological analyses. The chronostratigraphy is based on radiocarbon and optically stimulated luminescence (OSL) data. The upper part of the soundings concerns the Bronze Age period, enabling us to connect the evolution of the depression with the Early Bronze Age occupation of this specific micro-environment.

We follow a working hypothesis of two main types of sedimentary input, one mainly coming from the Wadi al Hijr through the alluvial fan (ex situ) characterized by pyroxenes, olivines, serpentines and poorer particle sorting, the other one from the immediate local sub-catchment of the depression (in situ) characterized by higher carbonate and quartz content as well as finer and better sorted particles. Whether the regional wadi and local sub-catchment inputs reflect more humid or arid phases, respectively, needs to be tested by further research.

On a regional scale, the occupation of Rakhat al Madrh by Early Bronze Age societies is remarkable due to its persistence up to the mid-Umm An Nar period, in spite of aridification. Adapting to the restrictive conditions of the arid environment meant exploiting the distinctive local features of the environment. Rakhat al Madrh could belong to a network of grazing area around the main oasis, which, in addition to a few groundwater sites, provided the water resources, thus justifying the designation of Rakhat al Madrh as an oasian peripheral site.

Keywords : geoarchaeology, geomorphology, sedimentology, Holocene, arid environment, Oman