Spatio-temporal correlation of Pleistocene palaeoflood deposits in Late Mousterian archaeological sites of Central Spain

Over the last three decades, more than a dozen sites with Pleistocene palaeoflood deposits in  river valleys of the central Iberian Peninsula (basins of the Duero and Tagus rivers and their tributaries) have been located, described, dated and interpreted. Some palaeoflood sediments were deposited next to, or within, Neanderthal occupation sites and, as in the present, those floods impacted on human activities, judging by the flood beds containing and covering Mousterian remains. Here, we characterize and spatio-temporally correlate these palaeoflood deposits within archaeological sites, including analysis of the topographic and geomorphological position of flood beds; detailed stratigraphic descriptions, sediment peels of the stratigraphic profiles; textural analysis, mineralogy and palynological sampling; and sediment dating (luminescence, radiocarbon).

These palaeoflood sites share some common characteristics, namely (i) geomorphological context of deposition in caves and rock shelters within fluvio-karst canyons cut on tabular and slope reliefs in Cretaceous carbonate rocks; (ii) location at high elevations (+11 to +25 m) in relation to the present position of the riverbed;  (iii) depositional sets with submetric to metric thickness, with alternation or succession of fluvial and colluvial beds; (iv) flood sequences made of sand, silt and clay, with sedimentary structures typical of slackwater depositional environments, such as stagnated flow and eddies; (v) chronology dated to the Late Pleistocene with a mode between 45,000 and 50,000 years ago; (vi) very high specific peak flows, between 6,02 m³·s^-1·km^-2 (Duratón River) and 12,32 m³·s^-1·km^-2 (Jarama River), based on estimations using the present topography; but these high specific peak flows significantly decreased when variations in valley geometry (based on geomorphological criteria) from Pleistocene to present are taking into account.

The high geographic extent of the Mousterian flood deposition around the Spanish Central System, covering multiple river basins (e.g. Eresma, Jarama, Duratón rivers), flood clustering in time, high magnitude and paleoflood sedimentology suggest common regional causes and effects of the triggering events. Two flood-producing mechanisms may explain such palaeoflood characteristics, (i) meteorological floods from intense precipitation episodes of mesoscale convective systems (isolated depressions at high levels), and (ii) breaches of proglacial lakes (outburst floods) dammed by moraine deposits or large landslides during the interglacial stage of Central Spain.

From their spatio-temporal correlation, it is not only possible to analyse the frequency and magnitude of catastrophic floods in a changing climate and to reconstruct the geomorphological configuration of these valleys during the Pleistocene-Holocene incision, but also to broaden the knowledge of their distribution and disruption on the Neanderthal populations.

This work has been developed in the framework of the archaeological research project: “Primeros pobladores de Segovia” (First settlers of Segovia) funded by the regional government (Junta de Castilla y León).