La sedimentación continental del Neógeno en el sector Centro-Septentrional de la depresión del Duero (Provincia de Palencia, España)

Abstract

Neogene deposits are well exposed around Falencia (Duero Basin, Spain, Fig. 1). From the study of ten sections, up to four informal lithostratigraphic units were recognized (Fig. 2, 3) including siliciclastic, carbonate and gypsiferous facies that were recorded and interpreted (Fig. 4).The BASAL UNIT (Astaracian) is made up of red to brownish mudstone with interlayered bodies of sandstone, limestone and marl. It was laid down by fluvial tile systems of high-sinuosity channel pattern and complex flood plain including backswamp and lacustrine realms (Fig. 5).The LOWER UNIT (Middle Astaracian) consists of mudstone and carbonate deposits with pedogenic layers both as rooted carbonate and black shale horizons (Fig.6). Two informal members can be separated: the lower one corresponds to palustral, fluviatile and lacustrine sediments, whereas the upper one was laid down in lakes with fluctuating water level and localized fluviolacustrine influx from the northern edge where flood plain facies are found (Fig. 9).The MIDDLE UNIT (Upper Astaracian to Upper Vallesian) consists of marly sediments with large crystal of gypsum and interlayered turbidite-hke clastic gypsum, limestone and dolostone in the eastern sector and interbedded green mudstone and carbonates in the western sector (Fig. 7). All them are the result of deposition in arid lacustrine environments. In the central part, a mixed terrigenous/gypsiferous assemblage is found: units of lenticular shape displaying coarsening upwards sequences are interpreted as lacustrine deltas (Fig. 7, 8). The source area of the remobilized gypsum should be placed towards the NE and that of siliciclastics, towards the N/NW.The badly-preserved sediments of the UPPER UNIT (Upper Vallesian-Turohan- Phocene?) consist of lacustrine limestone and mudstone, very much alike those of the lower unit: shallow lakes of fluctuating water level are assumed as well. The lack of gypsum and the common calcification of the underlying sediments of the Middle Unit, suggest that the salinity of the waters was not too high.The schematic palaeogeographical evolution has been expressed graphically for better understanding (Fig. 9).