渝东北陡山沱组碳酸锰微生物岩沉积环境初探

沉积型碳酸锰矿床多被认为沉积于水体较深、局限且较还原的沉积环境,特别是黑色页岩型碳酸锰矿床。在扬子北缘城口地区,埃迪卡拉系陡山沱组顶部黑色页岩中也发育碳酸锰沉积,黑色页岩及锰矿石Ce元素异常被认为是指示沉积环境的关键证据,但缺乏系统的沉积学证据,其沉积环境还存在争议。本次研究通过野外剖面、钻井岩心观察和室内薄片观察,详细描述了城口地区陡山沱组碳酸锰岩石的沉积结构、构造特征,对其展开了详细的沉积学研究,确定了其沉积环境并建立了沉积模式。研究表明,城口锰矿是典型的微生物岩沉积,菱锰矿以层状构造为主,内部发育叠层石、核形石、树形石以及凝块石。其中,核形石菱锰矿常黏结生长,菱锰矿微观特征记录了凝块状、球粒状、丝状、管状、纤维状以及放射状等微生物岩微结构。关键钻孔岩心沉积记录显示,研究区陡山沱组菱锰矿与胶磷矿交互沉积,菱锰矿中发育各种微生物岩旋回沉积,少见正粒序层理,内部偶有记录代表水动力变强的内碎屑层,而胶磷矿以强水动力内碎屑沉积为特征,常发育于菱锰矿上部,显示含锰岩系—含磷岩系旋回沉积特征,揭示陡山沱组末期城口地区沉积水体逐渐变浅的特征。结合区域陡山沱组地层沉积特征,认为扬子北缘城口地区陡山...

Preliminary Study of Manganese Carbonate Microbialite Sedimentary Environment of the Doushantuo Formation in Northeast Chongqing

Sedimentary manganese carbonates， especially the black shale type， are generally considered to have been deposited in a deepwater， restricted and reductive sedimentary environment. Deposits of manganese carbonate are well developed in the black shale near the top of the Ediacaran Doushantuo Formation in the Chengkou area of the northern Yangtze Craton. The black shale host rock and cerium anomaly of manganese carbonates is thought to be vital evidence of the sedimentary environment of manganese carbonates， but there is a lack of systematic sedimentological evidence， with the result that the sedimentary environment of the manganese carbonates in the Chengkou area remains controversial. In this study， the sedimentary textures and structures of the manganese carbonates in this area are described in detail， based on field and core observations and thin section microscopy. This work included a detailed sedimentological study of the Chengkou manganese deposit， and a model of the sedimentary environment was constructed. It was found that the Chengkou manganese carbonate deposit is mainly layered， and contains well-developed stromatolites， oncolites， thrombolites and dendrolites. The frequent bonding growth of the oncolites indicates that the Chengkou manganese deposits are typical microbialites with several kinds of microstructure: clotted， spherical， filamentous， tubular， fibrous and radial. The sedimentary records from drill cores show alternate deposits of rhodochrosite and collophanite. In the rhodochrosite， all types of microbialite are deposited cyclically. Normal grain bedding and intraclast layers indicative of stronger hydrodynamic force are rarely recorded. The collophanites are characterized by intraclast depositions in a strong hydrodynamic environment and often overlie the rhodochrosites， evidence of the cyclic sedimentary nature of the series of manganese-bearing rock to phosphorus-bearing rock and indicates gradually shallowing sedimentary water at the end of the Doushantuo Formation in the Chengkou area. When considered together with the sedimentary characteristics of the Doushantuo Formation in adjacent areas， Member I of the formation in the Chengkou area evidently developed a barrier-free coastal sedimentary system without a dolomite cap. Member II was a carbonate ramp sedimentary system with mixed siliciclastic sediments. The manganese carbonate microbialites mainly developed in the barrier and bonding reef sedimentary environment in the lower part of the inner carbonate ramp. It is notable that the development of barriers and bonding reefs at the end of the Doushantuo Formation was an important dynamic factor in the evolution of the sedimentary basin in the study area， promoting the evolving regional sedimentary system from carbonate ramp to carbonate platform.