Fluvial sediments of Cretaceous Age along the southern border of the Cantabrian Mountains, Spain

This study deals with a sedimentological description and interpretation of a strip of continental deposits of Cretaceous age along the southern border of the Cantabrian Mountains. In this work they are designated as Voznuevo Formation (Evers, 1967). They belong to the type of deposits which have commonly been referred to as ‘sediments in Wealden facies’. As regards stratigraphy, palynological analyses (apart from pollen, the Voznuevo Formation proved completely barren) made it clear that the transition between the Lower Cretaceous and the Upper Cretaceous runs obliquely through the formation. Hence, the Voznuevo sediments are diachronic deposits, decreasing in age in a westerly direction and forming the continental counterparts of successively younger shallow marine deposits towards the west. In comparison with similar deposits in Wealden facies in other parts of Spain they represent one of the last vestiges of continental deposition in Upper Cretaceous times. Sedimentological investigations made it clear that the Voznuevo sediments are of purely fluvial origin, derived from a granitic source area, probably now partly hidden, which must have been situated on the arc of granitic rocks which runs from Galicia via northern Portugal to the Sierra de Guadarrama, N of Madrid. Especially paleocurrent directions indicate a transport from southwesterly directions. Typical final products of weathering of granites – kaolinite and quartz – form the main share of the sediments. The independence of the Voznuevo deposits of the Paleozoic rocks which they overlie is striking; only the very lower part of the formation testifies to a small contribution from the Cantabrian Mountains. In the extreme west of the area some supply could be ascertained from the Precambrian rocks present there. The transition into the overlying shallow marine deposits is very gradual; a very small amount of lagoonal deposits probably occurs in the upper part of the formation. Apart from these, no sediments transitional to a marine environment, such as beach deposits, could be observed. The fluvial sediments can be subdivided into braided and meandering river deposits, the former much coarser-grained than the latter. The braided river deposits show characteristic features such as channelling, cut-and-fill structures, absence of distinct grading etc. The meandering river deposits exhibit fining-upward grading cycles in which cross-bedding of the planar type with backflow phenomena in their bottomsets is common, and sub-environments such as point bar, natural levee, crevasse-splay and backswamp deposits. Towards the coastal side of the area of deposition (i. e. the eastern part of the area studied) meandering river deposits were found. They presumably represent sediments of a coastal plain in which meandering rivers, chiefly carrying sandy sediments, ran to the sea in the east. At the same time, much coarser-grained braided river deposits formed further westwards in a mountain foreland. In shifting towards the west in its entirety, this model has yielded the sediments as they can be observed today. The Voznuevo Formation clearly stands out against the deposits which are to be found at its bottom and its top. First of all, there is the complete absence of limestone, the predominance of quartz over quartzite and the abundance of kaolinite. Secondly, heavy minerals not resistant to chemical weathering, such as hornblende, epidote and augite are completely lacking. Erosion of fresher rocks in the source area during the latter part of the sedimentation is reflected by the increased appearance of metamorphic minerals such as andalusite and kyanite. The abundance of authigenic kaolinite favours the supposition that weathering took place after deposition. The high percentage of kaolinite, the ubiquity of organic matter, the flatness and roundness of the pebbles and the absence of any trace of laterization indicates weathering under temperate to warm, humid conditions with relatively strong leaching. The Voznuevo sediments form a clearly foreign element in the depositional history of the sediments along the southern border of the Cantabrian Mountains. Tertiary and Quaternary sediments, all having the Cantabrian Mountains as source area, have been partially supplied from the Voznuevo sediments. But hitherto enigmatical occurrences of, for instance, staurolite and kaolinite in Tertiary deposits can also be explained by considering them as erosion products of the Voznuevo Formation.