Lateral variability of ancient seismites related to differences in sedimentary facies (the synrift Escucha Formation, mid-Cretaceous, eastern Spain)

Lateral extent and continuity of deformation has been used as criterion for the recognition of a seismic origin of soft-sediment deformations. Particular problems are posed by ancient seismites developed in deposits that show lateral variability of sedimentary facies. In such a context, the lateral continuity of deformation will depend on the lateral variability of physical properties of sediments that ultimately results in the lateral variability of seismites in a single deformation event. A variety of decimetre-scale soft-sediment deformation structures occurs in the synrift Escucha Formation (late Aptian–middle Albian) in Teruel, Spain. These structures, which include pseudonodules, ball-and-pillow structures, slumps, water-escape structures, multi-phase deformed structures, folds, sagging-like structures and synsedimentary faults, were developed contemporaneous to the formation of dark grey silty mudstones and grey clayey siltstones in a shallow-marine environment. The soft-sediment deformation is constrained to a 1.2-m thick horizon exposed in four outcrops consisting of quiet water mudstones and heterolithic storm deposits. The deformed horizon can be sub-divided into two parts, each reflecting a deformation event that affected both cohesive and granular (non-cohesive) sediments. The deformed structures occur in the vicinity of the Gargallo normal growth-fault, which is several kilometres long. Activity along this fault controlled the facies distribution of the unit bearing soft-sediment deformations. High-energy facies were located in the fault tip while low-energy facies developed in both the down-tilted hanging-wall and footwall blocks. In addition, fault-induced shocks released enough kinetic energy to trigger soft-sediment deformation. The soft-sediment deformation in the Escucha Formation represents different responses of deformation-susceptible sediments to earthquakes, which reflect the lateral variability of sedimentary facies.