Soft-sediment deformation structures from Lower Ordovician strata of the eastern Pyrenees

Résumé fourni par la source

Soft-sediment deformation structures from basement rocks of the Pyrenees are reported for the first time. They occur in the Lower Ordovician Font Frède Member (Serdinya Formation), a heterolithic siliciclastic unit truncated by the Late Ordovician “Sardic” unconformity. These strata were deposited on a siliciclastic platform that fringed northwestern Gondwana. In proximal areas of this platform soft-sediment deformation structures are represented by disharmonic folds and pillow structures restricted to distinct sandy, partly burrowed interbeds of the La Molina area, southern Canigó massif. These liquefaction-induced structures indicate the loss of sediment strength in water-saturated, unconsolidated deposits, most likely triggered by earthquake-induced seismic shaking of shallowly buried marine substrates. We interpret these deformation structures record the earliest tectonic manifestations of the forthcoming ”Sardic” thermal doming, which culminated in the uplift in the Canigó massif during Mid Ordovician times in this margin of Gondwana. This uplift and doming likely generated tectonic instability and seismic activity, acting as trigger mechanisms for the onset of the described soft-sediment deformation structurespartly burrowed substrates of the La Molina area, southern Canigó massif, represent roughly packed sediments losing strength due, probably, to seismic shaking. The south-verging synsedimentary folds point to a south dipping palaeoslope, opposite to the proximal-distal palaeogeographic trends (S to N) recorded for this formation in the eastern Pyrenees. This change in the palaeoslope dip may be related to the forthcoming of a thermal doming located northwards, responsible for the onset of the "Sardic" unconformity in this area of the Gondwana margin. Uplifting doming may be responsible for tectonic instability and seismic activity, acting as trigger mechanism for the onset of the described soft-sediment deformation structures.

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