Shallow lacustrine carbonate microfacies document orbitally paced lake-level history in the Miocene Teruel Basin (North-East Spain)

摘要

Results are presented of a detailed carbonate petrographic study of an Upper Miocene lacustrine mixed carbonate-siliciclastic succession in the Teruel Basin (Spain) with the aim of constraining lake-level variability at different stratigraphic scales. Regular alternations of red to green mudstone and lacustrine limestone, termed the 'basic cycle', reflect lake-level variations at the metre-scale. In an earlier study, the basic cycle was shown to be controlled by the climatic precession cycle. Petrographic analysis made it possible to distinguish two main carbonate microfacies groups characteristic of very shallow transient and shallow permanent lake environments, respectively. In addition to the basic cyclicity, the microfacies analysis reveals lake-level variations on a larger scale. As a consequence, the astronomical forcing hypothesis of the cyclicity in the Cascante section is explored further. A climate modelling study of orbital extremes indicates that high lake levels could relate to enhanced net winter precipitation and runoff during precession minima, consistent with Mediterranean geological data. Using this phase relationship, an astronomical tuning of the cycles is established starting from astronomical ages of magnetic reversal boundaries. Subsequently, successive basic cycles are correlated to precession minima. The tuning reveals an identical number of basic cycles in the Cascante section as precession-related sapropel cycles in the deep marine succession at Monte dei Corvi (Italy), corroborating the precessional control of the basic cycles at Cascante. Lake-level highstands in the large-scale cycle identified by the microfacies analysis relate to maxima in both the ca 100 and 405 kyr eccentricity cycles, again consistent with Mediterranean geological data. Subtraction of the identified astronomically related (lake-level) variations from the palaeoenvironmental record at Cascante indicates a shift to deeper and more permanent lacustrine environments in the upper half of the section. The cause of this shift remains unclear, but it may be linked to tectonics, non-astronomical climate, long-period astronomical cycles or autogenic processes.