Magnetoestratigrafía de las unidades inferior e intermedia del Mioceno de la Cuenca de Madrid

Resum

The paper shows the first magnetostratigraphic data recorded from distal alluvial and lacustrine Miocene deposits of the Madrid Basin. Four sections that cover most of the stratigraphic record so called Lower and Intermediate Miocene sequential units of the Madrid Basin, have been analyzed. The composite magnetostratigraphic succession, reaching up to 185 m in thickness, spans from chron C5Br to chron C5n, which represents a time interval of about 6 Ma, in the middle and upper Miocene. The absolute age of the previously established genetic units (Lower, Intermediate I, Intermediate II and Intermediate III) in the area has been determined by dating their stratigraphic boundaries. The boundary between the Lower and the Intermediate I units is correlated with chron C5ADn, at an age of approximately 14,4 Ma. The boundary between the Intermediate I and Intermediate II units is placed within chron C5ABn, around 13,4 Ma. The limit between the Intermediate II and Intermediate III units can be situated in chron C5An, around 12,2 Ma. Associated with this limit, a stratigraphic hiatus spanning 0,4 Ma can be postulated, but it should be confirmed by additional evidence. Accumulation rates have been estimated to range from 4,7 to 2,3 cm/Ka, which indicates lower values than in other continental cenozoic basins from the Iberian Peninsula. Changes in accumulation rates have been detected in the boundaries between units, which could indicate that there is a causal relationship between the development of stratigraphic discontinuities and changes in accumulation rates throughout the sedimentary infill of the Madrid Basin during the Miocene.