Evidence for Late Messinian seismites, Nijar Basin, south-east Spain



The Feos Formation of the Nijar Basin comprises sediments deposited during the final stage of the Messinian salinity crisis when the Mediterranean was almost totally isolated. Levels of soft-sediment deformation structures occur in both conglomeratic alluvial sediments deposited close to faults and the hyposaline Lago Mare facies, a laminated and thin-bedded succession of whitish chalky marls and intercalated sands alternating with non-marine coastal plain deposits. Deformation structures in the coarse clastics include funnel-shaped depressions filled with conglomerate, liquefaction dykes terminating downwards in gravel pockets, soft-sediment mixing bodies, chaotic intervals and flame structures. Evidence for soft-sediment deformation in the fine-grained Lago Mare facies comprises syndepositional faulting and fault-grading, sandstone dykes, mixed layers, slumping and sliding of sandstone beds, convolute bedding, and pillar and flame structures. The soft-sediment deformed intervals resemble those ascribed elsewhere to seismic shaking. Moreover, the study area provides the appropriate conditions for the preservation of deformation structures induced by seismicity; such as location in a tectonically active area, variable sediment input to produce heterolithic deposits and an absence of bioturbation. The vertical distribution of soft-sediment deformation implies frequent seismic shocks, underlining the importance of seismicity in the Betic region during the Late Messinian when the Nijar Basin became separated from the Sorbas Basin to the north. The presence of liquefied gravel injections in the marginal facies indicates strong earthquakes (M ≥ 7). The identification of at least four separate fissured levels within a single Lago Mare interval suggests a recurrence interval for large magnitude earthquakes of the order of millennia, assuming that the cyclicity of the alternating Lago Mare and continental intervals was precession-controlled. This suggestion is consistent with the present-day seismic activity in SE Spain.