Nearshore transgressive black shale from the Middle Jurassic shallow-marine succession from southern Poland

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• 作者：Paulina Leonowicz
• 关键词：Black shale ; Oxygen ; deficient conditions ; Shallow ; marine mudstone ; Middle Jurassic transgression ; Ore ; bearing clay ; Silesian ; Kraków region
• 刊名：Facies
• 出版年：2016
• 出版时间：April 2016
• 年：2016
• 卷：62
• 期：2
• 全文大小：8,980 KB
• 刊物类别：Earth and Environmental Science
• 刊物主题：Earth sciences
  Sedimentology
  Biogeosciences
  Geochemistry
  Paleontology
  Ecology
  
• 出版者：Springer Berlin / Heidelberg
• ISSN：1612-4820

文摘

Five facies types are distinguished in the Middle Jurassic dark-grey mudstone of the Częstochowa Ore-Bearing Clay Formation on the basis of sedimentary structures, bioturbation intensity, and composition of trace fossil and benthic fauna associations. Three of them, laminated mudstone (M_l), laminated claystone (C_l), and alternating laminated and bioturbated mudstone (M_a), are varieties of black shale. They formed in relatively shallow water, several tens of meters deep, in an epicontinental sea, mainly during the early phase of Middle Jurassic transgression. Suboxic conditions developed beneath a temporary pycnocline in a narrow, proximal zone near flooded land, which delivered increased amounts of organic matter and nutrients, triggering plankton blooms. Oxygen-deficient conditions were recurrently interrupted by re-oxygenation events, linked with the activity of storm-generated bottom currents, which simultaneously redistributed significant amounts of sediment from the basin-margin shoreface zones. Oxygenation improvement varied in duration from single storms to periods lasting several tens of years. The association of laminated mudstone with the beginning of major transgression, relatively shallow water, frequent oxygenation of the sea floor, and the occurrence of better-oxygenated time-equivalent facies in more distal settings, indicates that these deposits represent nearshore transgressive black shale. The limited occurrence of black shale and its poor correlation with minor transgressive–regressive cycles show that its formation depended mainly on local conditions, which were only partly influenced by relative sea-level changes within the basin.