Oncoids And Their Biogenic And Depositional Features From Cambrian Carbonates Of Henan Province

Abundant oncoids occur in the carbonates of the Second and Third Member of Mantou Formation and the Zhangxia Formation(Cambrian Series 3)in Henan Province.The macroscopic morphology,microscopic fabrics,microbial fossils and their calcification,depositional features,controlling factors and distribution model of oncoids are studied in the dissertation using the actualism of “the present is the key to the past”.The study provides evidences for the formation of oncoid and the understanding of co-evolution between microbe and metazoan in Cambrian.The classification index is established in the dissertation based on the morphology,core and cortex of oncoid.Eight kinds of oncoids can be found in study area,they are: type I-spherical and ellipsoidal oncoids with regular cortex,type II-spherical and ellipsoidal oncoids with irregular cortex,type III-spherical and ellipsoidal oncoids with gravel nuclei,type IV-long olivary oncoids with irregular cortex,type V-cap-like oncoids with regular cortex,type VI-flower-like oncoids with regular cortex,type VII-long olivary oncoids with bioclastic nuclei and type VIII-lath-shape oncoids with irregular cortex.Abundant Girvanella fossils occur in the Cambrian oncoids of Henan province.Girvanella filaments appear as flexuous,unsegmented,and unbranched tubular objects which are mainly densely winding,being intertwined with each other.Each individual tube consists of dark grey tube wall(sheath)and bright tube core(cell row)under polarizing microscope.Viewed under electronic microscopy,the tube wall of Girvanella consists of radial-arrayed granular or short columnar calcites which have a diameter of 1–2 ?m and are considered to be biominerals induced by photosynthesis of Girvanella.The tube core is made up of cylindrical calcites with the diameter of 10–20 ?m and the variation length of 50-200 ?m,which are the products of recrystallization during diagenesis.According to the microstructure of Girvanella in combination to the features of modern tubular cyanobacteria,Girvanella is made up of short rod cells instead of coccoid cells.Its microstructure includes strands of short rod cells attached end-to-end,cell walls and extracellular sheaths(EPS).The formation and distribution of oncoids in study area are much different among the Mantou Formation Member II,III and the Zhangxia Formation.The strata with oncoids consist of oolitic limestones with intercalation of oncolites during the depositional period of the Mantou Formation Member II.The oncolites mostly occur as lenses in study area and can be transformed with stromatolites frequently on regional scale.They often have the thin,unstable thickness and the short extended length and die out quickly.They are preserved in coastal oolitic inter-shoal or back-shoal facies.The strata with oncoids are composed of oolitic limestones and microbial mounds with oncoids during the deposition period of the Mantou Formation Member III.The oncolites mostly occur as thick-bedded and display mosaic distribution with stromatolites and trombolites on regional scale.They often have the thick,stable thickness and the long extended length.They are formed in oolitic inter-shoal of restricted platform facies.The strata with oncoids are dominated by alternative development of oolitic limestones and oncolites during the deposition period of the Zhangxia Formation.The oncolites mostly occur as multilayered and have the thick,stable thickness.They are formed in oolitic inter-shoal of platform facies.The genetic analysis on oncoids shows that barren of metazoan and special palaeoenvironmental situation in Cambrian Epoch 3 are the determining factors for the formation of oncoids in study area;the diversified shapes of oncoids are formed by different water energy;the high-energy condition of sustained agitation,influx of terrigenous clays or the destruction of depositional substrates by metazoan are the direct controlling factor for the ultimate demise of oncoid.