Deformed stromatolites in marbles of the Mesoproterozoic Wumishan Formation as evidence for synsedimentary seismic activity

The Mesoproterozoic Wumishan Formation consists mainly of dolomites that represent diagenetically transformed limestones deposited in a shallow peritidal environment. Although the environment was tectonically stable, the study area in the Changping District (north of the Beijing area) was affected by synsedimentary earthquakes. Later, the dolomites in the study area were intruded by a granite. This resulted in contact metamorphism of the dolomites, changing them in a halo around the intrusion into marble. This marble, which is exposed near Taoyukou Village, shows some levels with well-developed stromatolites. The stromatolites contain structures that point at deformation when the stromatolites had not yet been lithified, and thus represent soft-sediment deformation structures. The stromatolites apparently built up during a time-span when repeated deformational processes affected the still unlithified sediment, from the sedimentary surface down to maximally a few decimeters. The deformed stromatolites thus represent seismites, which feature is exceptionally rarely recognizable in marble.