Lower Wenlock black shales in the northern Holy Cross Mountains, Poland: sedimentary and geochemical controls on the Ireviken Event in a deep marine setting

Abstract

The stratigraphic variability and geochemistry of Llandovery/Wenlock (L/W) Series boundary sediments in Poland reveals that hemipelagic sedimentation under an anoxic/euxinic water column was interrupted by low-density bottom currents or detached diluted turbid layers that resulted in intermittent seafloor oxygenation. Total organic carbon values and inorganic proxies throughout the Wilków 1 borehole section suggest variable redox conditions. U/Mo ratios > 1 throughout much of the Aeronian and Telychian stages, together with an absence of pyrite framboids, suggest oxygenated conditions prevailed. However, elevated total organic carbon near the Aeronian/Telychian boundary, together with increased U/Th and V/(V + Ni) ratios and populations of small pyrite framboids are consistent with the development of dysoxic/anoxic conditions at that time. U/Th, V/Cr and V/(V + Ni) ratios, as well as Uauthig and Mo concentrations, suggest that during the Ireviken black shale deposition, bottom-water conditions deteriorated from oxic during Telychian time to mostly suboxic/anoxic immediately prior to the L/W boundary, before a brief reoxygenation at the end of the Ireviken black shale sedimentation in the Sheinwoodian Stage. Rapid fluctuations in U/Mo during the Ireviken Event are characteristic of fluctuating redox conditions that culminated in an anoxic/euxinic seafloor in Sheinwoodian time. Following Ireviken black shale deposition, conditions once again became oxygen deficient with the development of a euxinic zone in the water column. The Aeronian to Sheinwoodian deep-water redox history was unstable, and rapid fluctuations of the chemocline across the L/W Series boundary probably contributed to the Ireviken Event extinctions, which affected mainly pelagic and hemipelagic fauna.

Agglutinated benthic foraminifera in Ordovician and Silurian black mudrock facies of the Holy Cross Mountains (Poland) and their significance in recognition of oxygen content

“…The Upper Ordovician and Silurian mudrock succession in the northern HCM shows profound variations of the benthic oxygen level driven by eustatic fluctuations as well as delivery of finegrained clastic and carbonate material (Trela, 2007;Smolarek et al, 2016). Basing on the inorganic geochemistry and TOC analyses, the studied foraminifera-bearing black/dark shales and claystones are interpreted as dysoxic to anoxic facies related to the early Sandbian, early…”

“…It is noteworthy that some dark laminae consist of submillimeter mudstone clasts occurring as numerous light and rounded to lens-like spots in the dark sedimentary matrix. Mudstones laminae consist of fine silt-sized quartz and are apparently homogenous, however in microscale they show discrete bioturbational mottling, subtle normal grading and erosional bottoms (Smolarek et al, 2016). The subordinate feature of this facies are discontinuous laminae and clusters of coarse quartz silt accompanied by rare mica flakes.…”

“…Upper Ordovician and Silurian mudrocks in the northern HCM form up to 350 m thick succession deposited in the Caledonian foredeep basin developed along the SW margin of Baltica (Narkiewicz, 2002;Poprawa, 2006). They have been extensively studied, including stratigraphic, sedimentologic and geochemical approaches concentrated on reconstruction of the water column oxygen level (Trela, 2007;Zhang et al, 2011;Smolarek et al, 2016; and references therein).…”

“…They show millimeter to submillimeter lamination enhanced in some cases by a conspicuous alternation of dark and light laminae, however, some beds display a massive character. Black laminae are enriched in fine short fibres of organic matter which reveal wavy crinkly lamination, and are interpreted as remnants of tiny microbial mats and biofilms (Smolarek et al, 2016). It is noteworthy that some dark laminae consist of submillimeter mudstone clasts occurring as numerous light and rounded to lens-like spots in the dark sedimentary matrix.…”

“…Katian and early Sheinwoodian transgressions, respectively (Zhang et al, 2011;Smolarek et al, 2016). However, the sedimentary features indicate that their deposition was influenced by shortterm pulses of oxygen increase.…”