河南登封地区寒武系第三统馒头组二段中的核形石及其意义

Oncoids and Their Significance from the Second Member of the Mantou Formation(Cambrian Series 3),Dengfeng Area,Henan

河南登封地区寒武系第三统馒头组二段发育有三种类型的核形石:球状、椭球状核形石与大型柱状叠层石伴生,形成于高能的潮下带;长卵形核形石与小型柱状叠层石伴生,形成于低到中等能量的潮间带;不规则状核形石与近水平状、缓波状叠层石伴生,形成于低能的潮上带和潮间带。从核形石的成因可以看出,核形石等微生物成因构造与后生动物扰动构造存在耦合关系,水动力条件是核形石形态类型变化的决定因素,泥质(陆源物质)供应是影响核形石生长及消亡的直接因素。

The Second Member of Mantou Formation(Cambrian Series 3) exposed well in Dengfeng area of Henan and characterized by purple and isabelline thick-bedded mudstone interbedded with stromatolite limestones,oncloidal limestones and oolitic limestones.Abundant oncoids are well preserved in the lower part of the Member.The paper have done further researches on macroscopic morphological characters,microscopic cortex characters,microbial fossils and their calcification,and the formation and controlling factors of oncoids using the actualism of 'the present is the key to the past'.The study provides evidence for the reconstruction of Cambrian palaeoenvironments of western Henan and the understanding of co-evolution between microbe and metazoan.There are three types of oncoids from the Second Member of the Mantou Formation(Cambrian Series 3),Dengfeng Area,Henan: the spherical and axiolitic oncoids are found in association with big columnar stromatolites and formed in high-energy subtidal environment; the long olivary oncoids are related to small columnar stromatolites and preserved in low-to moderate-energy intertidal environment; the irregular oncoids and their companion nearly horizontal and gently wavy-shaped stromatolites are dominant in low-energy supratidal and intertidal environment.The oncoids are often preserved in carbonates together with stromatolites in the study area.The setting containing oncoids and stromatolites is composed of three developing stages form bottom to top.The first stage is dominated by long olivary oncoids and small columnar stromatolites.The microbes wrapped up the carbonate particles to grow around the long olivary bioclastic and gravel core and formed long olivary oncoids under low to medium-energy condition or small columnar stromatolites under low-energy condition.The second stage consists of spherical and axiolitic oncoids and big columnar stromatolites.The microbes wrapped up the carbonate particles by intertwined grow of filaments and trapping and bounding of sticky EPS and formed evenly alternating of light and dark laminae under high-energy condition which lead to the formation of regular spherical and axiolitic oncoids and big columnar stromatolites.The third stage is characterized by stoamtolite-like oncoids and wavy stromatolites.The spherical and axiolitic oncoids in second stage stopped to agitate as the descending of sea 1evel.The laminae of oncoids began to grow upwards in place at this time.The result is that the spherical and axiolitic oncoids can gradually become to stoamtolite-like oncoids.All oncoids totally become to wavy stoamtolites and eventually disappear in carbonate sediments as the continued descending of sea level.The genetic analysis on oncoids shows that there is a coupling relationship between microbiogenic structures and metazoan disturbed structures; water energy is the determining factor for the changes of oncoid types; the influx of terrigenous clays is the most direct factor for oncoid growth and their ultimate demise.