Tertiary and Quaternary sedimentation in a part of the Duero Basin Palencia, (Spain)

Sediments in the foreland of a mountain chain are sometimes suited to reconstitute the conditions in these mountains at the time of deposition of the sediments. The present study gives the results of a sedimentological investigation of the Tertiary and Quaternary deposits in a part of the Duero basin, situated south of the Cordillera Cantabrica, which supplied the sediments. The aim was to determine both the conditions in the source area, and the environments in the area of deposition. The investigated area is situated in the province of Palencia, between the rivers Pisuerga and Carrión. The area being a part of the so-called Meseta, has a simple relief. Two levels at different heights can be distinguished. The higher level, páramo, is strikingly flat, the lower, campiña, is more undulating. The Tertiary basin sediments are of various types, and can have six different facies. Along the mountain foot the sediments are chiefly conglomerates with some sandstones, united into the Cuevas facies. The pebbles consist of limestones derived from the Cretaceous limestones, which in the E occupy extensive areas but in the W are only exposed in a narrow strip along the southern border of the mountain chain. At that time they must have formed the southern flank of the Cantabrian Mountains. Pebble roundness and flatness indicate for the greater part a deposition as river fans in a warm and rather dry climate. These conglomerates have been brought into an overturned position in the W of the investigated area, and were strongly tilted in the E. This tectonic deformation is thought by several authors to have occurred during the Savian orogenic phase. The younger beds, having the Cuevas facies, are nearly horizontal, and were deposited during and after this folding as appears from the presence of sandstone pebbles derived from the sandstone layers within the folded conglomerates. South of the limestone conglomerate belt a wide zone with red beds occurs. These sediments, consisting of an alternation of quartzite conglomerates and sandy layers, represent the Vega de Riacos facies. The change of deposit from a limestone conglomerate into a quartzite conglomerate may be due to changes in the supply area, the Mesozoic limestones having been eroded and having become covered with soils, and the Palaeozoic quartzites and conglomerates becoming largely exposed. A typical feature is the absence in all basin sediments of pebbles derived from the Carboniferous limestone, the so-called Brezo-limestone, which at present froms a great part of the southern flank of the Cantabrian Mountains. The sediments presenting the Vega de Riacos facies were deposited in a warm and humid monsoon climate, as appears from (1) the red colour, (2) the shape of the quartz sand grains, and (3) the clay mineral associations. The remainder of the investigated area is characterized in the N by yellow sandy and clayey deposits, covered by similar, but yellow and red, sandy and clayey deposits, and in the S by yellow clayey deposits overlain by white and grey gypseous marls, alternating with limestones. The underling yellow sandy and clayey sediments, typical for the Carrión de los Condes facies, are dated as Vindobonian on account of the fossils found near Palencia and Saldaña. The upper yellow and red, sandy and clayey layers found in the N, having the Relea facies, have a Pontian age, based on fossils found near Saldaña and Relea. In the E of the investigated area this Relea facies shows a local divergent aspect, called Zorita facies, characterized by an alternation of red, sandy deposits and white, marly deposits. The gypseous marls and the limestones in the S, which show the Páramos facies, overlying the yellow clayey sediments in Carrion de los Condes facies, have a cover of a very hard, bluish limestone, the Páramos-limestone, which provided some freshwater molluscs indicating also a Pontian age. The sediments in the four last-named facies represent chiefly deposits of rivers and shallow temporary lakes (lagunas). A rather regular sedimentation went on from Vindobonian into Pontian times, meanwhile influenced by changes of climate in the basin. After the warm and humid climate in which the red beds were deposited, the climate became more arid, with an increased evaporation. First the yellow sediments in Carrión de los Condes facies were deposited, in the N being still sandy, in the S becoming more clayey. They are clearly deposits of rivers which did not supply very coarse material, but some deposition in temporary lakes must also have occurred. At the end of the Vindobonian evaporation became stronger, as can be concluded from the lime crusts found in the upper layers in the area of the Carrión de los Condes facies, and more to the S, in the area of the Páramos facies, from the deposition of gypsum bearing marls, when the drainage was more or less restricted. This climate persisted during the first part of the Pontian as can be concluded from lime crusts occurring in the lower beds in Relea facies, the depositional environment, that is rivers and lagunas, remaining the same. Later in the Pontian the humidity of the climate increased, as appears from the increasing number of red layers in this Relea facies. In the S this increasing humidity caused the precipitation of gypsum to cease, and at this time the Páramos-limestones were formed. The Zorita facies, which laterally replaces the Relea facies, is chiefly determined by a supply from a different source area, namely the Mesozoic calcareous rocks exposed a few kilometres N of the deposits in Zorita facies. The heavy mineral associations (chapter VIII, part 1) are monotonous, practically consisting of resistant minerals. They seem to indicate a supply from NW to SE. Also the pebble supply followed this direction, as may be concluded from their size distribution within the red beds having the Vega de Riacos facies. This NW—SE direction was the main drainage direction in Pre-Rhodanian times. The clay minerals in the sediments presenting the various facies allow to draw some conclusions on the climates at the times of deposition. For instance, the rather righ percentage of kaolinite in the sediments in the Cuevas and Vega de Riacos facies, may indicate a warm and rather humid climate during and after deposition. But later alterations also influenced the clay mineral associations, causing a preponderance of illite (see chapter VIII, part 3). After the deposition of the Páramos-limestones the Duero basin became a non-depositional area. During the Rhodanian orogenic phase the bordering mountains were uplifted, and the basin was tilted towards the W. This caused a switch of the drainage pattern which before was directed towards the Mediterranean, and now became directed towards the Atlantic Ocean. During the whole of the Pliocene strong bevelling occurred, through which the páramolevel in the basin and pediments at the foot of the mountain chains were formed. Next, a warm and dry climate characterized by sheetfloods must have prevailed all over the Meseta, causing the deposition of the angular quartzitic raña pebbles, so well exposed in the investigated area on the raña of Guardo. These rañas are presumed to be of Villafranchian age. Soon the influence of Quaternary changes of climate became evident. Certainly the younger river terraces, found at five various levels, are due to these Pleistocene climatic changes. Pebble analyses could confirm the opinion of various authors who admit only two real glaciations in the Spanish mountains, namely the last and the penultimate. Indeed, the two lower terraces contain pebbles which may have been formed in a periglacial climate, whereas the deposits of the three upper terraces only contain evidences of a humid, temperate climate. The sedimentological data on which the conclusions on the depositional environments, as given above, are based can be found in the following chapters: (a) grain size distribution (chapter V), (b) pebble analyses (chapter VI), (c) morphometrical sand analysis (chapter VII), (d) mineralogy of the sands (chapter VIII, parts 1 and 2), (c) clay minerals (chapter VIII, part 3). The development of the drainage pattern (see chapter IX) was reconstituted with the help of a number of captures, which can be observed in the field. In this way a gradual adjustment of the drainage to the present direction can be demonstrated. In the investigated area this adjustment occurred rather late during the Quaternary. At that time also the campiña-level was formed. Finally, in the last chapter (X), an attempt is made to establish the palaeoclimates, and the relief in the source area, though there remain many uncertainties. The Cordillera Cantabrica, being a mountain area, must always have had a more humid climate than the basin. Even during the Upper-Vindobonian and Lower-Pontian, while the basin was arid, the climate in the mountains must have been more humid. This appears from the clastic sediments supplied into the basin (Relea facies). Though the drainage was restricted, it will not have been totally interrupted, because only calcite and gypsum were deposited in the basin centre, and no halite. There will have been a certain relief in the source area during the whole time. The sediments give no indications for a fully developed peneplain. During the whole of Vindobonian and Pontian times clastic sediments have been supplied by the Cantabrian Mountains.