Geochronology and geoarchaeology of Pleistocene fluvial deposits in the Prados-Guaten Depression (Madrid Basin, Central Spain)

摘要

This study deals with the geochronological, geoarchaeological, sedimentological and geomorphological analyses of sandy Pleistocene fluvial deposits exposed throughout the Prados-Guaten Depression, occupying the wide drainage divide of the Guadarrama and Jarama rivers draining towards the Tagus River (SW Madrid, Central Spain). Pleistocene fluvial deposits within the depression record two different fluvial systems draining the zone during the Pleistocene. Older fluvial deposits (T1), thick stacked arkosic sand sequences (～ 15-17 m) with fossil fauna attributable to the Lower Pleistocene (c. 1.0-0.9 Ma; Mammuthus meridionalis NESTI), exposed at elevations of +15-40 m above the present Guaten stream thalweg. This terrace was deposited by an ancient fluvial system draining the Depression north to south, flowing directly to the Tagus (Paleo-Manzanares). Overlapping the older ones, the younger deposits studied are inset fluvial terraces at +13-16 m (T2) and +8 m (T3). T2 Upper-Middle Pleistocene (500-200 ka) fauna, Equusferus cf. Mosbachensis, is related with the abandonment of the valley, the onset of a stage of relevant intra-valley dissection, and subsequent refilling linked to the early organization of the present drainage leading to the development of T3. Paleomagnetic data from T2 and T3 terraces display an overall normal "Brunhes" polarity, although anomalous reverse polarities are recorded in the younger terrace (T3). TL and OSL dates range between c. ＞126-75 ka for T2 (MIS 5) and c. 20-17 ka for T3 (MIS 2), indicating a mainly Upper Pleistocene age for the development of these two more recent terraces. The dates obtained indicate that the establishment of the present divergent fluvial arrangement within the Prados-Guaten Depression started during the Middle-Upper Pleistocene transition, after it was abandoned by the Lower-Middle Pleistocene Paleo-Manzanares fluvial system (T1). T2 contains a variety of Lower to Middle Palaeolithic flint flaking products, indicating chronocultural transition during the Upper Pleistocene in the area, correlated with a drastic change in the fluvial landscape from the end of MIS 5 (＜75 ka).