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首页> 外文期刊>Facies >Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: Implications for basin analysis
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Blackened bioclasts and bituminous impregnations in the Koněprusy Limestone (Lower Devonian), the Barrandian area, Czech Republic: Implications for basin analysis

机译：捷克共和国Barrandian地区Koněprusy石灰岩（下泥盆统）中发黑的碎屑和沥青浸渍：盆地分析的意义

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摘要

Carbonate reef talus facies of the Koněprusy Limestone (Pragian, Lower Devonian, Barrandian) locally exhibit widespread impregnation by organic matter resulting in a partial to complete blackening of the limestones. Two contrasting types of impregnation are recognized: blackening of individual carbonate fossils and bioclastic layers within the limestone originated very early during diagenesis. The blackening is due to finely dispersed organic matter and possibly some iron sulphides and clay minerals that selectively adhered to the outer layers of corals, bryozoans, and crinoid fragments, leaving other fossils unaltered. These darkened fossils are similar to black pebbles-i. e., reworked, dark to black limestone clasts and bioclasts that are known to occur exclusively in shallow-water zones of both ancient and modern carbonates. The alteration of fossil fragments may have taken place in very shallow-water environments, possibly those of saline and reducing back-reef lagoons or supratidal-intertidal zones, with organic matter being derived from decayed algae and microbes, or early vascular terrestrial plant material. Following the coloration, the blackened fossils were removed from their original position by waves or storms and transported into relatively deeper-water reef slope settings to form graded, "salt-and-pepper"-colored bioclastic beds. The presence of blackened fossils in the carbonate succession may point to episodic emergence and indicates a vanished vegetated siliciclastic hinterland that may once have existed to the west or south from the present-day erosive edge of the Barrandian Devonian strata. Subvertical veins cutting the Koněprusy Limestone and filled with black solid bitumen and blackened calcite resulted from a subsequent but substantially later diagenetic event, which is a testament of aqueous and petroleum fluid migration through the succession during deeper burial. Microthermometric characteristics of the aqueous inclusions embedded in vein calcite indicate that the veins were precipitated by brines of low to moderate salinity (0. 5-9. 5 wt% NaCl equiv.) with temperatures in the range of 87-116°C. The bitumen in the veins is epi-impsonite (R _r = 0. 70-1. 90%), which is interpreted as degraded petroleum residuum that experienced thermal alteration at around 120°C. The AFT modeling combined with fluid inclusion microthermometry and wider geological considerations indicate that the veins originated during the Variscan orogeny, most probably upon deep burial of the Lower Paleozoic strata in Carboniferous time.
机译：Koněprusy石灰岩的碳酸盐礁距骨相（普拉纪，下泥盆纪，巴兰阶）在当地表现出广泛的有机质浸渍作用，导致石灰石部分变黑。两种不同的浸渍方式被认识到：单个碳酸盐化石和石灰岩中生物碎屑层的变黑是在成岩作用的早期就开始的。变黑的原因是有机物分散很细，可能还有一些硫化铁和黏土矿物，它们选择性地附着在珊瑚，苔藓动物和海百合碎片的外层，而其他化石却未变。这些变黑的化石类似于黑色的鹅卵石。例如，经过改造的深黑色石灰岩碎屑和生物碎屑，仅在古代和现代碳酸盐的浅水区发生。化石碎片的改变可能发生在非常浅的水域环境中，可能是盐水和还原礁泻湖或上潮间带区域的有机物，这些有机物来自腐烂的藻类和微生物，或早期的陆生陆生植物材料。着色后，变黑的化石被海浪或暴风雨从其原始位置移走，并运送到相对较深的水礁坡度环境中，形成渐变的“盐和胡椒粉”色生物碎屑床。碳酸盐岩层中变黑的化石的存在可能指示了偶发的出现，并指示了消失的硅质碎屑腹地，该腹地可能曾经存在于现今Barrandian泥盆纪地层的侵蚀边缘的西部或南部。随后的成岩作用（但后来基本上是后来的成岩作用）造成了切割Koněprusy石灰石并充满黑色固体沥青和黑色方解石的垂直下静脉，这证明了在深埋期间水和石油流体通过演替的迁移。嵌入方解石中的含水包裹体的微热学特征表明，静脉是由盐度低至中等（0. 5-9。5 wt％NaCl当量）的盐水沉淀的，温度范围为87-116°C。静脉中的沥青是表皮次生石（R _r = 0. 70-1。90％），这被解释为降解的石油残渣，在120°C左右经历了热变化。 AFT模型结合流体包裹体热计量法和更广泛的地质考虑因素表明，这些静脉起源于Variscan造山运动，最可能是在石炭纪时代下古生界地层的深埋。

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