Storm-influenced deposition and cyclicity in a shallow-marine mudstone succession - example from the Middle Jurassic ore-bearing clays of the Polish Jura (southern Poland)

Abstract

Sedimentological analysis of bioturbated mudstones from the Middle Jurassic ore-bearing Częstochowa Clay Formation has revealed common relics of sedimentary structures, such as thin silt/sand laminae, bedding-plane accumulations of shell debris, small and medium silt-sand lenses, as well as silt-, sand- and shell-debris-rich levels, accompanied by erosion surfaces and minor scours. These features document periodical high-energy conditions of mud deposition, including storm stirring, event sedimentation and the activity of storm-generated bottom currents, which were responsible for seafloor erosion and sediment supply from shallower parts of the basin. The sea bottom was prevalently below the storm wave base, but it rose above it during exceptionally strong storms and in the Early Bathonian zigzag chron when the basin experienced considerable shallowing. The shallowest and highest energy conditions occurred in the late macrescens subchron, when the bottom was close to, or above the fair-weather wave base. The estimated depth of the sea did not exceed several tens of metres, but in some periods it could be even less than 20 m. Based on the vertical variation of the sand, silt and clay contents, seven transgressive-regressive cycles have been distinguished in the approximately 75-m thick succession spanning the Upper Bajocian - Upper Bathonian. Cycles TR1, TR2+3, TR5 and TR6+7 correlate with those distinguished in the coeval succession from central Poland, although the stratigraphic position of cycle boundaries is slightly shifted. These cycles record relative sea-level changes that affected the entire Polish Basin. Regressions R2 and R6 were smaller-scale-events, generated in response to a local tectonic activity and autocyclic shoreline progradation.