陕南寒武纪早期仙女洞组生物礁灰岩微相序列

MICROFACIES SEQUENCES OF THE EARLY CAMBRIAN(SERIES 2)XIANNDONG FORMATION REEFS IN SOUTHERN SHAANXI PROVINCE,NW CHINA

扬子地台北缘在寒武纪筇竹寺期—沧浪铺期出现海退,该时期的仙女洞组属浅海碳酸盐岩与碎屑岩的混合沉积单元,分布于陕西南部及四川北部。陕南南郑福成和碑坝剖面组成完整仙女洞组序列,其中包含8期生物礁相单元。单层礁体为米级厚度,形态特征显示丘状隆起或平缓延伸。水体的清澈度、深度以及水动力强度变化对各期礁相单元生长的控制力度因时而异,陆源碎屑数次侵入导致海水浑浊度增高,点断了礁体生长。礁灰岩微相类型包括钙质微生物粘结岩、古杯-钙质微生物绑结岩、古杯漂砾-砾屑灰岩、粒泥状-泥粒状灰岩以及泥状灰岩。早期礁灰岩中钙质微生物粘结岩含量丰富,之后古杯绑结岩在礁灰岩构建中具有丰度增长的趋势,且上部礁灰岩层位中不规则古杯占主要地位。仙女洞组沉积晚期海水变浅和水动力条件增强,常见漂砾灰岩和砾屑灰岩。

There developed a succession of Cambrian（series 2）reefs in the Xiannüdong Formation at the Fucheng and Beiba sections,Nanzheng,southern Shaanxi,NW China which provides significant material for analyzing microfacies of early Cambrian reefs.The archaeocyathan reefs of the Xiannüdong Formation are the earliest record of metazoan reefs in South China.According to the trilobite biostratigraphic information,the Xiannüdong Formation was formed during the Cambrian Chiungchussian to Tsanglangpuan interval（Cambrian Series 2）.Regressional tract of the Cambrian Chiungchussi and Tsanglangpu stages trigged the mixed depositional sequences from the north margin of the Yangtze Platform.Carbonates and terrigenous clastics of the Xiannüdong Formation occurred within a shallow marine setting.Eight reef units were recognized throughout the Fucheng and Beiba sections.Reefs with meters in thickness are domical or flatly stretched morphologically.Microfacies analysis indicates that the Xiannüdong reefs can be subdivided into calcimicrobial bindstones,archaeocyathan-calcimicrobial boundstones,archaeocyathan float-rudstones,wacke-packstones and mudstones.Archaeocyathan-microbial boundstones and microbial bindstones constitute the reef core,float-rudstones and wacke-packstones being the main components of the reef flank,while calcimicrobial bindstones and mudstones form the basement of reefs.Some reef units are covered by mudstones,which reflects a deeper water（subtidal）environment on the shelf,suggesting that the growth of reefs was interrupted due to the changes of sea level.Reef units show obvious cycles from bottom to top in development,reflecting the regressional tract of the Cambrian Chiungchussi and Tsanglangpu stages in the north margin of the Yangtze Platform.Sedimentary analysis reveals that the terrigenous clastics deposition had occasionally interrupted the growth of reefs due to the decrease of water clarities.Abundant calcimicrobialites appeared in reef boundstones in the lower reef units,but archaeocyathan framestones dominated by the irregular archaeocyaths are quite common in the upper reef units.Shallow and turbulent conditions were favorable for the rudstones and floatstones close to the top of the Xiannüdong Formation.The calcimicrobial bindstones,formed by calcimicrobes,such as Epiphyton,Renalcis and Girvanella,are the main component of the Xiannüdong Formation reefs.Calcimicrobe bound together with archaeocyaths,and formed the archaeocyathan-calcimicrobial bindstones.Trilobites,crinoids and phosphatic small shelly fossils were also discovered in the reef units.