Discovery and geological significance of the Holocene seismites in the Jinsha site in Chengdu, Sichuan

The seismites are interpreted as a typical representative of catastrophic event deposits, and a general term for a group of genetically related rocks with the structures and sequences of the seismites. The seismites and seismic deposition are recognized for the first time in the Holocene strata in the Jinsha site, Chengdu, Sichuan. These strata are assigned to the alluvial deposits, and display gravel layers at the base, dark (carbonaceous) argillaceous layers (shale layers) in the middle part, and brown yellow soil layers in the upper part. There occurs a series of SE-trending normal faults on both sides of which well-defined faults at the top interface of the basal gravel layers constitute the horst structures. In the IT6511 and IT6610 horizons, there occur stepped faults composed of extensional normal faults with the slips of faults from 10 to 90 cm. In the IT6512, IT6613 and IT6614 horizons, there are many deformational indicators and fracture structures in the soft sediments such as sand liquefied veins, liquefied convolute deformational structures, load structures, microfaults, microfractures, and seismites. The above-mentioned soft sediment deformational structures and fracture structures are believed to be caused by the crustal movement-induced earthquakes. The discovery of the seismites in the Jinsha site in Chengdu is of great value for the approaches to the palaeoenvironments of the Sichuan Basin from the Holocene onwards.