Coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification (Draken Formation, Upper Riphean, Svalbard).
Coastal lithofacies and biofacies associated with syndepositional dolomitization and silicification (Draken Formation, Upper Riphean, Svalbard).
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DOI：
10.1016/0301-9268(91)90071-h
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发表时间：
1991
影响因子：
3.8
通讯作者：
I. Fairchild;A. Knoll;K. Swett
中科院分区：
文献类型：
--
作者：
I. Fairchild;A. Knoll;K. Swett
斯匹次卑尔根（斯瓦尔巴群岛）东北部的德拉肯组（120-250米）是上元古代厚碳酸盐岩台地序列的一部分。它主要由信息白云岩砾岩组成，具有良好的结构保存。野外鉴定出6种主要岩相：石英砂岩、叠层石席、硅化内碎屑砾岩、干燥泥岩白云岩砾岩、鲕粒/泥质颗粒岩和窗缘白云岩。从硅化（和罕见的钙化）微化石中可识别出5个主要的层状生物相。生物相1代表低能潮下底栖生物（直立的丝状生物）和浮游生物（肢栖生物和花瓶状的微化石），而生物相2至5代表从基底潮间带到高潮间带/潮上的微生物席组合（包括丝状席建造者，以及相关的居住者和被冲入的浮游生物）。使用蒙塞尔图对锯切岩石样本的颜色值（一种衡量颜色阴影亮度的方法）进行了量化，并根据其他标准推断，在潮下到潮上沉积物的光谱中显示出明显的变化（主要群体的平均值从4.0到5.95不等）。在逐渐暴露的沉积物中，变亮与有机碳含量的降低有关，可能主要是氧化作用。早期水泥有六种类型。方解石微晶石（1型）是许多元古代地层中常见的潮下胶结物，而2型（潮下等径条纹）、3型（潮下硬地白云岩）和4型（潮间半月板白云岩）除了白云岩矿物学外，与显生宙的例子非常相似。类型5和类型6是复杂多变的白云岩生长，与膨胀和替换现象有关。它们具有窗缘岩相特征，并与现代潮上胶结物相比较。成岩结构和内碎屑截断结构表明，淋溶、白云化、硅化均是改变原有亚稳碳酸盐的重要同沉积过程。该数据集为潮汐周围环境的光谱提供了证据，包括浅滩、受保护的潮下、潮汐沙滩和受保护的碳酸盐泥滩。不同剖面显示出特定相的优势。海岸岩相连续体完全白云化，不同于邻近地层的近海至浅滩相。因此，白云化与沉积测深有关。虽然水动力学显然有作用，但也强调了白化降水在边际海洋环境中提高Mg/Ca的潜在重要性。
The Draken Formation (120–250 m) of northeast Spitsbergen (Svalbard) forms part of a thick Upper Proterozoic carbonate platform succession. It consists predominantly of intraformational dolomitic conglomerates, with excellent textural preservation. Six main lithofacies were recognized in the field: quartz sandstones, stromatolitic mats, conglomerates with silicified intraclasts, dolostone conglomerates with desiccated mudrocks, oolitic/pisolitic grainstones and fenestral dolostones.A series of five main gradational biofacies were recognized from silicified (and rare calcified) microfossils. Biofacies 1 represents low-energy subtidal benthos (erect filaments) and plankton (acritarchs and vase-shaped microfossils) whereas biofacies 2 to 5 are microbial mat assemblages (with filamentous mat-builders, and associated dwellers and washed-in plankton) ranging from basal intertidal to high intertidal/supratidal.Colour values (a measure of the lightness of the colour shade) of sawn rock samples were quantified using a Munsell chart, and exhibit a pronounced variation (means of major groups varying from 4.0 to 5.95) across the spectrum of subtidal to supratidal sediments as inferred from other criteria. The lightening in progressively more exposed sediments is related to lowering of organic carbon contents, probably mainly by oxidation.Six types of early cement have been recognized. Calcite microspar (type 1) is common as a subtidal cement in many Proterozoic formations, whereas types 2 (subtidal isopachous fringes), 3 (subtidal hardground dolomicrite) and 4 (intertidal meniscus dolomicrite) are very similar to Phanerozoic examples except for their dolomitic mineralogy. Types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena. They characterize the fenestral lithofacies and compare with modern supratidal cements. Consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching, dolomitization, silicification were all significant syndepositional processes altering the original metastable carbonates.The data set provides evidence for a spectrum of peritidal environments including ooid shoals, protected subtidal, tidal sandflats and protected carbonate mudflats. Different sections show a preponderance of particular facies. The coastal lithofacies continuum was completely dolomitized, unlike offshore to ooid shoal facies of adjacent formations. Dolomitization thus bears a relationship to depositional bathymetry. Although hydrodynamics clearly have a role, the potential importance of whiting precipitation in raising Mg/Ca in marginal marine environments is also stressed.