Seismic-induced slump in Early Pleistocene deltaic deposits of the Baza Basin (SE Spain)

13 figures.
Early Pleistocene lacustrine deposits of the Baza Basin in SE Spain show a 2 m thick sheet with deformation structures intercalated within horizontal strata. Along a 180 m exposure we recognise an alternation of compressional (thrusts) and extensional (normal faults and tensional joints) structures. These structures are interpreted as a planar slump. Analysis of the different structures together with the sedimentary facies indicates that this slump moved towards the north in a subaqueous deltaic environment 2.5 m deep with a paleoslope of < 1°. An earthquake is presumed to have triggered the mass movement and was followed by other shaking of lesser intensity. These events occurred in rapid succession, during the active deposition of a single delta lobe, which then covered and preserved the structures. The general stratigraphic position of this subaqueous slump matches other nearby liquefaction structures (seismites), indicating a period of active paleoseismicity. This appears to coincide with renewal of tectonic activity in the adjacent Betic Cordillera, which changed the source area of detrital deposits, infilled the NE margin of the Basin, and modified the morphology of the paleo-Lake Baza. Paleomagnetic and biostratigraphic data constrain the age of the structures to the Early Pleistocene, between 1.2 and 1.1 Mya.
We thank Ayuntamiento de Orce for a grant to L. G. This research is supported by the research group n°217 from the Junta de Andalucía, the Acc. Esp. BTE2001-5230-E and the Earthwatch Institute. We thank J. Menzies and M. Moretti for review and useful comments on this manuscript.