Le site de Longgupo dans son environnement géologique et géomorphologique

On a regional scale, Longgupo is found within a karstic landscape of fenglin-ouvala developed in the Triassic limestones of the Yangtze platform. After several seasons, it is now possible to define and propose a scenario for the formation of the site. During one of the intense karstification phases during the Cretaceous or early Tertiary, karstic conduits were created deep in the substratum. In concert with these deep flows and by karstic collapse, a cave was created. With the start of Himalayan uplift during the Oligocene and Miocene, the beginning of fluvial cutting was reflected by the partial erosion of the upper sandstone formations and the creation of the Miaoyu Valley. This action opened the cave, which would later be occupied by the hominid groups that interest us here. Probably at the end of the Tertiary, the cave, open at the bottom of the valley slope and facing upstream, began to be filled with alluvial deposits from the valley. The basal deposits (C III 10 of the current stratigraphic sequence, levels 20-13 of the first sequence) begin with black clays; at the Plio-Pleistocene transition, deposits become clearly alluvial, increasingly marked by reworking of surface and karstic clays and by systematically coarser contributions. During the deposition period of archaeological complexes III/III’ (previously levels 12-6), karstic collapse appears to have still been active. During the Lower Pleistocene sensu lato, the coarse nature of the deposits becomes accentuated. The material of complex II (previously levels 5-2) is a deposit that is clearly heterometric and coarse, composed at the end mainly of gravels, pebbles and more or less rounded blocks in a clayey matrix. At the conclusion of this new stage, the cave was entirely filled and, purely and simply, disappeared from the landscape. As the valley deepened during the Middle and Upper Pleistocene, the Longgupo slope underwent profound changes, the last of the limestone blocks disappeared from the slope limit and the archaeological site was protected by the coarse breccias that carpeted the slope. The main cause for the sedimentation processes at the site of Longgupo is the hydrosystem of the Miaoyu he, although injections of karstic type sensu stricto cannot be excluded. All of the arguments effectively support alluvial deposition: pebbles, an alternation of coarse and clayey beds, systematic and repeated granulometric decrease. But the stratigraphic disposition was disturbed by deformations due to gravity and karstic origins; the alluvial sediments, rendered plastic by the high proportion of clayey beds, were deformed by gravity (deformation of beds, faults, slopes) following the direction of the slope and by karstic collapse along the two limestone walls north and south of the sites (in sub-parallel faults to the south and by circular aspiration to the north). To this is added a calcitic consolidation of the upper levels that made it difficult to excavate complex C II. The stratigraphic disposition of the archaeological levels at Longgupo is now well understood. It has thus been possible to collect samples for absolute dating, to be published soon.