Extensive soft-sediment deformation structures in the early Darriwilian (Middle Ordovician) shallow marine siliciclastic sediments formed on the Baltoscandian carbonate ramp, northwestern Estonia

Abstract

Abstract The nearshore Middle Ordovician (early Darriwilian) deposits of the Baltoscandian Basin, formed on the flank of tectonically stable Fennoscandian Shield, comprise numerous soft-sediment deformation structures. The areal extent of the deformed interval extends up to 9000 km 2 from NW Estonia to SE Sweden. The deformation structures occur within a meter-scale sandstone horizon that includes load casts, flame structures, ball-and-pillow morphologies, up to 2 m wide sedimentary dykes, autoclastic breccias, sand volcanoes, and other related soft-sediment deformation structures. Evidently, these deformation structures formed during a single seismic event that caused liquefaction and fluidization of the water-saturated unconsolidated shallow marine sediments within almost the entire sandstone unit (Pakri Formation). Probably a unique catastrophic earthquake of magnitude 7 or higher occurred in this region penecontemporaneously with the deposition of these sediments. The ultimate cause of seismic event of such magnitude in the tectonically stable Baltoscandian Basin is unclear. However, deposition and deformation of the Pakri Formation fall within the Middle Ordovician meteoritic bombardment period which resulted from the disruption of a large parent body in the asteroid belt approximately 470 Ma ago. This in turn suggests a potential impact-origin for the single catastrophic earthquake in the tectonically stable study region. Although impact crater of suitable age is currently not known from the region, then previous studies have documented some impact metamorphic shock lamellae (i.e. PDFs) and abundant extraterrestrial chromite grains from the deformed sediments.