The significance of mudstone fabric combined with palaeoecological evidence in determining sedimentary processes - an example from Middle Jurassic of southern Poland

Abstract

Analysis of the fabric of laminated black mudstones of the Ore-Bearing Częstochowa Clay Formation (Bathonian, southern Poland) is used for reconstruction of sedimentary processes and conditions. Small-scale sedimentary features indicate that ore-bearing clays were deposited below storm-wave base in a shallow epicontinental sea, but structures reflecting quiet settling from suspension are not common. Most of recognized lamina types record the activity of bottom currents generated by storms. The benthic faunal association indicates that suboxic conditions prevailed on the seafloor, restricting infaunal activity to cryptobioturbation. Recurrent short-lived reoxygenetion events, linked probably to storms, resulted in the development of dysoxic conditions, recorded by a more diverse trace fossil association (consisting of Chondrites, Trichichnus, pyritized burrows, Palaeophycus and Protovirgularia) and colonization of the seafloor by epifaunal bivalves (Bositra). Truly anoxic conditions might have been briefly established, leading to mass mortality of Bositra.