湖北三峡地区黄陵隆起北缘灯影组微生物碳酸盐岩类型及古环境演化

Calcimicrolith types and paleoenvironment evolution of the Sinian Dengying Formation in northern margin of Huangling Uplift,Three Gorges of Hubei Province

黄陵隆起北缘震旦系灯影组不仅记录了震旦纪晚期环境与生命的协同演化,而且大量发育微生物碳酸盐岩,成为研究新元古代—古生代转折期古环境的重要对象。文中通过对黄陵隆起北缘震旦系灯影组开展详细的野外勘查及室内岩相分析解释,识别出云质微生物岩、云质微礁岩、云质微屑岩等的9个岩石微类,以云质微生物岩为主。根据微生物碳酸盐岩类型及其组合关系分析,认为发育多种向上变浅的潮下带—潮间带—潮上带沉积序列以及若干颗粒滩、微生物丘构成的米级沉积旋回,并以此为基础详细刻画出4次海侵—海退次级旋回,建立了黄陵隆起北缘震旦纪晚期以潮坪相、丘滩体、颗粒滩为典型沉积特征的浅水碳酸盐岩台地模式。该发育模式与现代微生物碳酸盐岩沉积模式相似,明显特征为凝块石形成于正常浪基面以下的潮下带低能环境,较深于云质层纹石和云质叠层石。综合微生物碳酸盐岩沉积特征、沉积组合特征及沉积相演化规律,认为该地区古环境演化主要受次级海平面波动影响。推测微生物碳酸盐岩的发育程度可能为新元古代氧化事件之后氧含量增高以及微生物大量发育的海洋岩石形成的响应,同时也可能与石板滩生物群的出现和以微生物为食的后生动物演化导致微生物含量减少密切相关。分析黄陵隆起北缘震旦系灯影组微生物碳酸盐岩发育类型,不仅可以约束震旦纪晚期全球海平面变化、古海洋环境演化,而且对于研究早期生命演化具有重要意义,同时可为下一步油气储集层的探索提供有力的依据。

The Sinian Dengying Formation at the northern margin of the Huangling Uplift not only records the co-evolution of the environment and life during the late Sinian,but also contains a large number of calcimicroliths,which serve as important indicators for studying the paleoenvironment of the Neoproterozoic-Paleozoic transition.In this paper,based on the detailed field investigation and indoor lithofacies analysis and interpretation of the Sinian Dengying Formation along the northern margin of Huangling Uplift,nine types of sedimentary microfacies are identified,including dolomitic microbialite,dolomitic microlithite and dolomitic microallolite,with dolomitic microbialite being the most dominant type.Based on the analysis of calcimicrolith lithology types and their combinatory relationships,it is proposed that a variety of subtidal-intertidal-supertidal stratigraphic sequences have developed,characterized by upward stacking and meter-scale stratigraphic cycles composed of multiple grain beaches and microbial mounds.On this basis,four transgression-regression secondary cycles are described in detail.A shallow water carbonate platform model,exhibiting typical lithofacies characteristics of tidal flat facies,mound beach bodies,and grain beaches during the late Sinian period at the northern margin of the Huangling Uplift,has been established.This carbonate sedimentary model is similar to that of modern microbial carbonate,with a notable characteristic being that the thrombolites are formed in a low-energy subtidal environment,situated below the normal wave base and deeper than dolomitic laminated stone and dolomitic stromatolites.Based on the lithofacies characteristics,lithofacies assemblage,and the evolution of sedimentary facies of calcimicrolith,it is inferred that the palaeoenvironmental evolution in this area is primarily influenced by secondary fluctuations in sea level.The development of calcimicrolith in the Sinian Dengying Formation,located at the northern margin of the Huangling uplift,may respond to the increased oxygen content and the formation of marine rocks abundant in microorganisms following the Neoproterozoic oxidation event.Additionally,it may be closely associated with the emergence of the Shibantan biota and the decrease of microbial content due to the evolution of metazoans.Analyzing the types of calcimicrolith development in the Sinian Dengying Formation at the northern margin of Huangling uplift not only constrains global sea-level changes and the evolution of paleo-ocean environment during the late Sinian,but also holds significant implications for studying the early life evolution.The Dengying Formation is characterized by a high abundance of calcimicrolith,which may provide a substantial basis for future exploration of oil and gas reservoirs.