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Black shale deposition and early diagenetic dolomite cementation during Oceanic Anoxic Event 1: The mid-Cretaceous Maracaibo Platform, northwestern South America

机译:海洋缺氧事件1期间黑色页岩沉积和早期成岩白云岩胶结作用:南美洲西北部白垩纪中期马拉开波平台

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摘要

Thin laterally continuous organic-rich dolomitic marlstones were deposited in the extended Late Aptian - Early Albian epicontinental sea of northwestern South America. These intervals are the proximal equivalents of thick hemipelagic black shale-ammonitic floatstone couplets, deposited in the distally stepped, differentially subsiding part of the Maracaibo Platform. The marlstones reflect the dynamic conditions resulting from orbital forcing mechanisms and can be genetically related to (1) minor sea-level changes, (2) proximal turnovers in marine productivity, and (3) sudden climate shifts affecting mid-Cretaceous shoaling upward, shallow marine, carbonate cyclicity. Therefore, the marlstones may well be linked to the multiple environmental perturbations collectively referred to as Oceanic Anoxic Event 1. The interstitial euhedral dolomite has a medium crystallinity, and exhibits unusual textural relations with framboidal pyrite and gypsum. The authigenic mineral assemblage also includes quartz, Ca-F apatite, and barite, which together with the chemical signals of dolomite, point to an unsteady climate regime. Bulk-rock biomarker parameters, rare earth element geochemistry, and iron speciation data point to widespread photic zone anoxia and transient shallow marine euxinia by the time of deposition, with climatic instability driving the delivery of oxidized detritus from the hinterlands. These conditions led to a schizohaline redox stratified environment favorable to dolomite precipitation. In such a depositional setting, the bio-utilization of Fe, Mn, and sulfur for organic matter respiration sustained elevated pore-water alkalinity and pH, and allowed for the pre-compactional growth of interstitial dolomite.
机译:薄的侧向连续的富含有机物的白云质泥岩沉积在南美洲西北部延伸的晚阿普天-早阿尔比陆上大陆海中。这些间隔是沉积在Maracaibo平台的远端阶梯状,有差异性沉降的部分中的厚的半地中海黑色页岩-氨水浮石stone联的近端等效物。泥灰岩反映了由轨道强迫机制引起的动态条件,并且可能与(1)轻微的海平面变化,(2)海洋生产力的近端周转量以及(3)突然的气候变化影响白垩纪中部浅滩向上,浅水浅滩有关海洋,碳酸盐循环。因此,泥灰岩很可能与被称为海洋缺氧事件1的多种环境扰动有关。间质性正面白云石具有中等结晶度,并表现出与黄铁矿黄铁矿和石膏的异常结构关系。自生矿物组合还包括石英,Ca-F磷灰石和重晶石,再加上白云石的化学信号,则表明气候条件不稳定。块岩生物标志物参数,稀土元素地球化学和铁形态数据表明,到沉积时,广泛的光合带缺氧和短暂的浅海游藻类毒素,气候的不稳定性驱动了腹地氧化碎屑的输送。这些条件导致了有利于白云石沉淀的裂藻碱氧化还原分层环境。在这样的沉积环境中,Fe,Mn和硫被用于生物质呼吸的生物利用维持了较高的孔隙水碱度和pH值,并允许间隙白云石的预压实生长。

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