Abstract:

In the region from Jixian County of Tianjin City to Xinglong County of Hebei Province in eastern Yanshan, the sedimentary structures which present the spindle-shaped crack on the surface are widely developed in the fine clastic strata of the of the Changcheng Group of the Mesoproterozoic, and they have been explained as the metazoan trace fossils or the seismic-induced liquefied veins etc. The structures always co-exists with the palimpsest ripples, the wrinkle structures,and so on. The preliminary studies indicate that the spindle-shaped cracks might be the products of degasing and dewatering of the sediments during early diagenesis, which are caused by the seal of overlying microbial mat, thus, they can be grouped into the category of the microbially induced primary sedimentary structures （the matground structures）. Spindle-shaped cracks and the other matground structures provide an important evidence to reconstruct the sedimentary environment of the Precambrian. These structures also illuminate the metabolism diversity and enormous biomass of microbes in the Precambrian on the earth surface, and thus form important marks in the sedimentary records.

Key words: spindle-shaped crack, microbially induced primary sedimentary structure, original analysis, clastic rock；Changcheng Group, eastern Yanshan