Geological setting of the lower Pleistocene fossil deposits of Chilhac (Haute-Loire, France)

Abstract

The Chilhac deposits (CH2 and CH3) have long been known to contain exceptionally well preserved late Pliocene (now lower Pleistocene) fossils, as well as alleged pebble culture artifacts (found in CH3) attesting to the presence of hominins. The geological complexity of the deposits in the small valley previously prevented ascertaining their stratigraphic position. Recent farm extension upstream the deposits provided a continuous profile and revealed previously unknown geological structures. Additional trenches dug at the margins of the Chilhac deposits confirm the superposition of a lava flow (ca. 1.8 Ma) on the CH2 sediments, after a major phase of erosion, and allowed correlation of these deposits with those of CH3. Another lava flow, topographically inverted, provides an upper limit of 2.5 Ma for the age of the deposits. These new observations also reveal that the valley underwent several cycles of erosion. The structural complexity of the CH3 deposits suggests a landslide event affecting surface deposits, volcanic tuffs, and the upper part of CH2 sediments. This landslide probably occurred shortly after the deposition of the CH2 sediments and the volcanic eruption. These observations are compatible with the age of 2.2 Ma inferred from the CH2 and CH3 fauna (MNQ17 biozone).