Abstract:
Marine authigenic aragonite crystal fans and isopachous cements are mainly formed in anoxic-dysoxic seawater or pore waters with oversaturated CaCO3 derived from bacterial sulfate reduction(BSR).Their presence can be used as a potential indicator for redox conditions of the depositional environment.Carbonate units from the Mesoproterozoic Wumishan Formation(ca.1.5~1.45 Ga)in North China Platform show that aragonite fans and cements are common in deep subtidal deposits,but decline in shallow subtidal and disappear in intertidal deposits.Particularly the biolaminites deposited from deep-subtidal environments are composed of submillimeter-scale dark-light couplets of microbial mat and isopachous aragonite.Microscopically,the isopachous aragonites are composed of fibrous crystals or fan-shaped aggregates,similar to those recognized from the Archean strata of South Africa.Carbonates commonly form shallowing-upward cycles with shallow subtidal facies(Unit A) at the base,intertidal facies(Unit B) in the middle and supratidal facies(Unit C) at the top.The shallow subtidal facies(Unit A)contain abundant thrombolites,in which meso-clots consist typically of fabrous or fan-shaped aragonite aggregates surrounding organic-rich nuclei or pyrite grains,but rarely with carbonates micritic grains.Intertidal facies(Unit B)are dominated by laminated dolostone with interbeds of grainstones or packstones.Aragonite fans and cements are present but not abundant.Most of the aragonites in this unit grow on mat fragments or previously formed minerals,either as isopachous,fan-shaped or as botryoidal aggregates that fill cavities.Supratidal facies(Unit C)consists of micritic dolostone with pseudomorphs of halite and desiccated microbial mats but no aragonite crystal fans or cements. Geochemical analysis shows that V/Th,Mo/Th and U/Th increase from Unit C to Unit A,suggesting progressive anoxic condition with increasing water depth.Our study shows that during deposition of the Wumishan Formation(ca.1.5~1.45 Ga),the North China epeiric sea was likely anoxic below the fair-weather wave base,but the abundance of aragonite crystal fans and fibrous cements in deep-subtidal environments imply the availability of sulfate in shelf environments of the Mesoproterozoic epeiric sea.
Key words:
Mesoproterozoic,
palaeooceanographic conditions,
aragonite sea-floor precipitates,
redox sensitive elements,
North China Platform
摘要: 文石海底沉淀是地球早期大气高CO2浓度、海洋贫氧条件下,CaCO3过饱和而直接沉淀于海底所形成的自生碳酸盐,可反映古海洋贫氧状态。对华北中元古界雾迷山组(ca 1.50~1.45 Ga)碳酸盐岩的研究发现:潮下带下部黑色纹层石由针状文石假晶等厚层与微生物席层交互堆叠而成;潮下带中部凝块石白云岩(A单元)主要由针状文石假晶形成的环带或扇状集合体包裹有机质团块所组成,少见碳酸盐微粒。潮下带上部—潮间带下部纹层白云岩(B单元)以自然沉淀的碳酸盐微粒为主,含少量等厚层和扇状、葡萄状文石假晶集合体,以及以孔洞充填形式产出的针状文石假晶。而潮坪成因的泥晶白云岩(C单元)则完全由含有石盐假晶和微生物席裂痕的泥晶质微晶白云岩组成,不含针状文石沉淀。对构成雾迷山组典型副层序基本单元的氧化还原敏感元素比值分析显示:由C单元至A单元,V/Th、Mo/Th、U/Th值逐渐增大,在A单元可能达贫氧富集程度。研究表明,在雾迷山组沉积期,华北陆表海潮下带中部及其以下可能长期处于贫氧状态,而潮下带上部则处于氧化状态,氧化—还原界面接近于正常浪基面底。这个认识对进一步了解中元古代海洋条件、生物与环境相互作用过程及烃源岩勘查具有重要意义。
关键词:
中元古代,
古海洋条件,
文石海底沉淀,
氧化还原敏感元素,
华北地台
CLC Number:
Tang Dongjie, Shi Xiaoying, Pei Yunpeng, Jiang Ganqing, Zhao Guisheng. Redox status of the Mesoproterozoic epeiric sea in North China[J]. JOURNAL OF PALAEOGEOGRAPHY, 2011, 13(5): 563-580.
汤冬杰, 史晓颖, 裴云鹏, 蒋干清, 赵贵生. 华北中元古代陆表海氧化还原条件*[J]. 古地理学报, 2011, 13(5): 563-580.