Soft-sediment deformation structures induced by seismic activity in the Upper Cretaceous Tena Formation - Oriente basin of Ecuador

Soft-sediment deformation structures (SSDS) form during or shortly after sediment deposition and can result from various triggering mechanisms, including glacial activity, bioturbation, storm waves, tsunamis, gravitational detachment, and seismic events. The Upper Cretaceous Tena Formation preserves a variety of SSDS, categorized by their rheological behavior. These include SSDS such as pseudo-nodules, loop bedding, ball-and-pillow, deformed lamination, load cast, and flame-like structures, as well as brittle deformation structures like micro-faults. These structures are observed at the interfaces of mudstone, claystone, and sandstone sedimentary deposits. The morphological characteristics of the SSDS, combined with the tectonic setting of the Tena Formation deposition, analogies with seismically induced structures documented in other regions, published experimental models, and the stratigraphic confinement of deformation, suggest that the primary deformation mechanisms involved liquefaction and/or fluidization processes, triggered by seismic activity. These SSDS are interpreted as seismites, providing evidence of intraplate seismic activity in the South American Plate during the Upper Cretaceous in the Ecuadorian Amazon. This seismicity is likely associated with tectonic subsidence in the Andean Amazon Basin, potentially linked to the initial uplift of the Andes. The identified deformations, attributed to seismic events with an estimated magnitude of Mw > 5, offer the first insights into understanding the interaction between seismic activity and sedimentary processes, contributing to the understanding of the complex tectonic evolution of the Andean Amazon Basin during the Upper Cretaceous.