Pleistocene geodynamic evolution of the Arlanzón valley: implications for the formation of the endokarst system and the open air archaeological sites of the Sierra de Atapuerca (Burgos, España)

Abstract

The Arlanzón river is located in the NE Duero Depression (Castilian Plateau of northern Spain), where it drains the anticlinal ridge of the Sierra de Atapuerca, where a rich record of karstic and open air Palaeolithic archaeological sites are located, spanning the last 1.22 Ma. The geomorphological sequence of this valley is composed of fourteen fluvial terraces from T1 (+92-100 m), to level T14 (+2-3 m), ranging from the Early Pleistocene to the Holocene and mainly related to cold MIS. Available chronological data and terrace relative heights extracted from LiDAR data, have allowed the reconstruction of the downcutting phases of the valley, which show an acceleration of the incision rates throughout the Quaternary, probably related to tectonic uplift. In the Sierra de Atapuerca multilevel endokarst system, three sub-horizontal cave levels appear spatially and chronologically related to the Arlanzón fluvial terraces T2 (+82-91 m), T3 (+70-78 m) and T4 (+60-67 m)/T5 (+50-58 m). The association between fluvial terrace base levels and sub-horizontal phreatic caves suggests a relatively short period of time for the formation of these phreatic passages when compared to karstic vadose entrenchments related to the low incision rates of the Arlanzón river (< 0.038-0.045 m/ka). In the valley, the incision of 0.061 m/ka caused the progressive exhumation of a neogene limestone layer containing flint, which erosion and successive re-sedimentation phases by alluvial and colluvial processes on the valley slopes during the Middle and Upper Pleistocene caused many wide captation areas of raw material, which were exploited by the Neanderthals during MIS 3 and 4.