Large-Scale Slumps and Associated Resedimented Deposits in Miocene Lake Basins from SE Spain.

Slump, breccias, and turbidite deposits interbedded with diatomaceous marlstone and limestone occur in the upper part of the Late Miocene lacustrine succession of the Cenajo and Las Minas Basins. The basins display half-graben geometries where the northern margins consist of a network of large and deep, listric normal faults. The largest single deformational unit, reaching up 40 m in thickness, crops out in the Cenajo Basin and consists of soft-sediment deformed slump deposits displaying a variety of fold styles and geometries. This represents possibly the largest slump body hitherto described in fossil lake systems. The amplitude of the folds varies from meters in major folds to centimeters in the intrafolial minor ones. Brittle deformation of the slump strata is also common and cross cuts the ductile fabrics. Measured major fold axial traces indicate preferential N-S to NW-SE slide movement. The predominant strain in the deformed deposits is simple shear. Outcrops in the Cenajo Basin allow observation of extensional structures at the head slide, large sheath, and recumbent folds in the intermediate zone and tight compression structures at the toe, where folds and thrusts overrun the slightly deformed down slope strata. In contrast, the slump deposits in the Las Minas Basin are seen to pass down-dip into breccia and turbidite beds, which is indicative of flow transformation and internal disaggregation of the folded beds with distance. A number of laterally linked turbidite facies are recognized in the uppermost resedimented unit of the Las Minas Basin. The occurrence of these deposits points to marked instability in the lake basins. This period was preceded and followed by pelagic sedimentation of varve-like laminated, diatom-rich deposits. Most probably, basin instability was related to earthquake shocks, perhaps coeval to the extrusion of volcanic lamproite rocks in the region during the late Tortonian.