LAUSR

Abstract Botryoidal dolostones are abundant in the Second Member of the Upper Ediacaran Dengying Formation in the Western Yangtze Platform. Although they have been extensively discussed as important host rocks for giant gas reservoirs, there is no consensus on the microbial impact on mineralization processes in the botryoidal laminations. Typical botryoidal laminations were studied under optical microscopes and scanning electron microscopes (SEMs) to investigate their origin. There are three macroscopic morphological types of botryoidal lamination: stromatoid lamination, cavity-lining lamination, and spherical-encrusting lamination. These three types of lamination are composed of fibrous and bladed crystals growing perpendicular to the botryoidal laminae, which have hitherto been implicitly considered to be from abiogenic chemical precipitation. There are many lines of new evidence that demonstrate microbial activities in the interbedded dark and bright laminae constituting the laminations. In the dark laminae, euhedral dolomite rhombs are observed sticking to linear and tubular filaments, or wrapped in sheets of extracellular polymeric substances (EPS) with dolomite crystal sizes of generally