Origin of the tertiary red beds in the Northern part of the Duero Basin (Spain), II. Composition and genesis

In this second paper the red beds outcropping in the northern part of the Duero basin have been treated regarding their mineral and pebble composition, chemical parameters, and surface textures of quartz sand grains, taking as basis the results reported in the first paper. These deposits originate from soils in the source area, and have been rapidly supplied into the basin by braiding rivers. Heavy mineral associations and pebble composition prove the source area to be lying north and west of the area of deposition. Ferric iron oxides, clay mineral associations, and hydrogen ion concentrations point to a red soil formation in the source area which had not yet attained the laterite stage, but which had already suffered alkaline leaching. The presence of frosted and pitted quartz sand grains and the occurrence of marls are due to the high carbonate content of the waters in the area of deposition, which is caused by dissolution of limestones in the source area. The general conclusions from the analyses are: (1) that the red beds are “primary detrital” in the sense of Krynine; (2) that the climate in the mountain area during the red soil formation is presumed to have been a tropical savannah climate, that is, warm and fairly humid, at least seasonally; (3) that the climate was drier in the basin, which favoured the preservation of the red beds. Furthermore, from the presence of blue tourmaline grains within a limited zone, an ancient course of a river in the basin at that particular time could be reconstructed, which gives another indication for a south-easterly drainage direction.