Análisis sedimentológico y caracterización paleoclimàtica de la sucesión cíclica de Orerà, Mioceno continental de la Cuenca de Calatayud

Resumen

The representation of continental and continuous cyclically bedded sequences for long time intervals in the sedimentary record of the Iberian Peninsula is very scarce. The middle Miocene sedimentary fill o f the Calatayud Basin (NE Spain) consists of proximal to distal alluvial fanfloodplain and shallow lacustrine deposits. A 160 m-thick lacustrine succession showing an exceptional cyclical bedding (Orera Composite Section, OCS) is exposed close to the Orera Village, in the northern margin of the basin. The correlation of the magnetostratigraphy of the OCS with the Geomagnetic Polarity Time Scale (GPTS), resulted in an age o f 10.7-12.8 Ma for the entire section, which is supported by biostratigraphical data. The cyclic shallow lacustrine succession comprises 91 superposed small-scale cycles, each consisting mainly of an alternation of greygreenish mudstone and white dolomite beds. Based on the number of cycles, the average periodicity o f the basic small-scale cycles is approximately 23,000 yr. This periodicity suggests that deposition of the sedimentary cycles was controlled by astronomically induced climate changes causing lake-level fluctuations. The deposition of these cycles took place in a low gradient, shallow lake basin developed in an "inter-fan" zone (Miedes and Orera alluvial fans). The small-scale cycles are interpreted as a result of the evolution from mudflat deposits (red and green-grey clays), accumulated in lowstand periods of the lake, to shallow lake deposits (dolomites) during relatively high lake level stages.