Girvanella in Oncoids from the Mantou Formation (Cambrian Series 3), Mianchi,Western Henan
Girvanella filaments in the microbialite are widely distributed in Cambrian,which provide more visual evidences for the participation of microorganism in the formation of the microbialite.Abundant oncoids,a kind of microbialite,occur in the carbonates from the Second Member of the Mantou Formation (Cambrian Series 3),Mianchi area,western Henan.The Girvanella filaments which are preserved in the nuclei,cortices and surrounding rocks exhibit large density and high integrity. The microscopic analysis on the oncoids shows that the Girvanella filaments are preserved in both of the nucleus and cortex of oncoids.The filaments appear as a flexuous,unsegmented and unbranched tubular colony.Each individual is about 20μm in diameter and 400μm in maximum extended length.They are mainly densely winding,being intertwined with each other with irregular distribution.The nucleus of oncoids is dominated by filaments surrounding lime clumps and debris,occasionally the bioclasts.The filaments in the cortex of oncoids mainly occur in dark laminae and exhibit four kinds of growth patterns inclu-ding winding and intertwined,trans-laminar,convex and bedding-arrangement.Girvanella filaments share the similar growth pattern no matter where they occur in oncoids,showing that they have similar growth condition.The mass occurrence of Girvanella filaments in oncoids has much to do with their growth condition of low-energy coastal oolitic inter-shoal or back-shoal facies.This kind of low-energy condition faclitates the growth and preservation of Girvanella filaments.Further studies on the distribution characteristics and micro-fabrics of theGirvanella filaments which were well preserved in the oncoids in the study area are expected to offer significant evidences for the genesis analysis of other microbialite in Cambrian and for the inter-action between microorganism and environment.
Mianchi areaCambrian Series 3oncoidsGirvanellaMantou Formationstratigraphy
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