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 , S chaubcylindrichnus , 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.