Latest Cretaceous cone-in-cone structures and soft-sediment deformation (Basque-Cantabrian Basin, north Spain): A record of deep-marine paleoseismicity?
Unusual cone-in-cone structures are preserved in fibrous celestite veins hosted by carbonatic rocks. The veins are early diagenetic features formed near the water-sediment interface in a deep-marine environment of latest Cretaceous (early Maastrichtian) age. New field and microscopic evidence on the veins and their host rocks indicates that they predate sediment compaction and were related to synsedimentary, soft-sediment deformation structures. The mechanism of cone-in-cone formation has been related to fluid-pressure drops following sediment overpressurization, but instantaneous loading during emplacement of submarine slides or other mechanisms might be viable alternatives. We argue that this cone-in-cone structure is an overprinting feature (penetrative conical fractures) that resulted from seismic wave propagation through a mechanically anisotropic medium. Submarine amplification of seismic surface waves (Rayleigh waves) induced shock wave formation in the contacts between veins and the bounding lithified calcarenite and unlithified marl sediment layers. Fracture front wave propagation (normal to the seafloor and the Rayleigh source) across the veins and along the celestite crystal fibers induced crack nucleation in heterogeneities and penetrative conical fracturation. The cone-in-cone structure might thus be regarded as a new paleoseismological indicator in deep-sea sediments.