Soft-sediment deformation structures in Frasnian and Famennian tempestites and related sediments (southern Cantabrian Mountains, Spain).

In this study the types and forms of soft-sediment deformed in storm beds and related sediments of the Upper Devonian of the southern Cantabrian Mountains are investigated, as well as their possible genetic processes. The storm bed successions consist of very thin to medium bedded tempestites, indicating an almost equilibrium in which a more or less constant bathymetry was maintained over a long period, with a consistent unidirectionally downwelling storm flow transport in response to coastal setup, perpendicular to the E-W running palaeoshoreline. The soft-sediment deformed storm beds are not spread equally across the Upper Devonian succession, but occur in specific stratigraphic horizons. Based on their forms, shapes, and stratigraphic relation with overlying and underlying strata, they can be grouped into five types, each associated to deformation processes which act in specific parts of the depositional profile of the Late Devonian coastal zone: (1) the chaotic contorted and folded mixture of very fine-grained sand, sandy mud, and mud (Complex mixture), (2) the folded very fine-grained sandstone beds (Folded sandstone), (3) the faulted and folded silty shales, with folded sandstone beds, and lithoclasts (Muddy debris), (4) the folded sandstone beds with brittle-ductile deformed shear zones and imbricated toes (Sand flow rolls), and (5) the ball-like bodies of sandstone with internal lamination and upcurled edges (Pseudonodules). The depositional mechanisms of tempestite sand-mud layers contain several soft-sediment deformation driving forces and agents. The geological data of the deformed bedforms does not point to a single deformational cause. The various deformation processes involved act simultaneously or as successive events. The overall characteristics of the soft sediment deformation types are (1) a storm bedded succession of the transition zone, (2) or a storm bedded-related succession of the offshore zone, (3) a steep depositional slope and, (4) a tectonical active area, probably with earthquakes. [ABSTRACT FROM AUTHOR]