Sedimentary fabrics for the stromatolitic bioherm of the Cambrian Gushan Formation at the Xiaweidian section in the western suburb of Beijing
Cambrian Gushan Formation, belonging to the upper part of Series 3, is made up a general upward shallowing succession of sedimentary facies from shelf marls to grain-bank oolitic grainstones, which constitutes a third-order carbonate depositional sequence of the drowning-unconformity type. Within one bed of the grain-bank oolitic grainstones in the upper part of Gushan Formation, occur many dome-shaped carbonate structures with the distribution like a string of beads that can be described as the stromatolitic bioherm. Macroscopically, both the typical high topography and the abrupt-changing boundaries with host rock reflect the early lithification feature of the stromatolitic bioherm. Within column stromatolites making up these stromatolitic bioherms, in addition to bounding few bioclasts of trilobites, develop non-uniform some of benthic oolites that are mainly constituted by radiated-fibrous calcites ( or aragonites?). For these benthic oolites, many features such as the small grain size, diverse type, smooth and discontinue oolitic circle and an obvious micritic envelope in the exterior margin of the oolite, reflect the special microbial-relative microscopic fabrics that are different from the suspended oolites of the host rock. Since stromatolites are typical buildups of microbial mats, these stromatolitic bioherms with special macro- and microscopic fabrics can further be grouped into the microbial reef, and become the typical example for the further understanding of both the sedimentation pattern in the shallow skeleton-poor sea before the biological radiation of the Middle Ordovician and the microbial reef-building features in this special geological time.
Sedimentary fabricStromatolitic biohermSeries 3 of CambrianXiaweidian sectionBeijing
NETL
NSTL
万方数据
维普
