Paulina Leonowicz

Ichnofabrics of shallow-marine mudstone, the result of changing environmental conditions: an example from the Middle Jurassic ore-bearing clay from southern Poland

Analysis of the trace fossil association and ichnofabrics from a Middle Jurassic shallow-marine mudstone, referred to as the Ore-Bearing Częstochowa Clay Formation, is used to reconstruct the seafloor conditions and identify the factors responsible for their changes. The trace fossil association contains Chondrites, Trichichnus, Palaeophycus, Planolites, Protovirgularia, ?Rosselia, Schaubcylindrichnus, Taenidium, cf. Tasselia, Thalassinoides, and five types of pyritized burrow. The association represents the distal expression of Cruziana ichnofacies, temporarily changing to a more proximal development. The ichnofacies suggests that the depositional site was located in the lower offshore to offshore-transition zone, periodically passing to the distal lower shoreface. Eight subtypes of the ichnofacies are recognized, reflecting variations in seafloor oxygenation, sedimentation rate, and benthic food content. The sequence of ichnofabrics 1a–1b–2a–2b reflects increasing oxygenation; ichnofabrics 4a, 4b, and 5 record high sedimentation rates, whereas ichnofabric 3 reflects sediment starvation. Vertical changes of ichnofabrics reflect major fluctuations in basin depth and distance from the shoreline, which partly correlate with previously recognized transgressive–regressive cycles. However, they do not record minor variations of the shoreline position.

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

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 (Ml), laminated claystone (Cl), and alternating laminated and bioturbated mudstone (Ma), 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.