Geochemistry, ichnology, and sedimentology of omission levels in Middle Triassic (Muschelkalk) platform carbonates of the Germanic Basin (southern Poland)

Abstract

Sedimentology and paleoecology of omission or discontinuity surfaces in carbonate strata is well documented, but the geochemistry of such surfaces is poorly recognized. This work describes morphology, fabrics, geochemistry, and biota succession in omission levels from Middle Triassic (Muschelkalk) platformal carbonates in Upper Silesia, southern Poland. Three types of omission levels are distinguished: firmgrounds, hardgrounds, and intraformational conglomerates. (1) Firmgrounds are characterized by burrows of Balanoglossites triadicus . Some burrows are enclosed by a dark and/or (minor) bright diagenetic halo. The prevailing dark color, lower δ 13 C and δ 18 O ratios, and increased concentration of redox-sensitive elements (U, As, Mo, Cu, Zn, Pb, Ni) in the halos compared to the host micrite indicate a significant enrichment in organic matter, which most likely resulted from impregnation of the burrow walls with organic mucus from the burrowing organism, a process observed in modern burrows. The burrow walls were in contact with stagnant saline water (recorded by elevated 87 Sr/ 86 Sr), but were locally bleached by oxygenated fresh seawater pumped by the burrow producer. (2) Hardgrounds show encrustations of the bivalve Placunopsis ostracina and borings Trypanites weisei . In many hardgrounds, the uppermost 5-mm-thick layer has a different color and higher δ 18 O values than the underlying micrite. This heavier δ 18 O signal reflects a higher content of 18 O-enriched early-marine microcrystalline cement precipitated from colder bottom seawater. (3) Intraformational conglomerates consist of flat pebbles derived from reworked firmgrounds and hardgrounds. Many pebbles contain the same encrustations and borings as observed in the hardgrounds, which occur on both sides of the clasts and mimic their shape, indicating that they were developed after pebble formation. All studied types of omission level are laterally discontinuous at some scale and are thus unsuitable for regional correlation. • Firmground burrows are surrounded by dark and/or minor bright early diagenetic haloes. • Dark haloes are enriched in redox-sensitive elements and 12 C and 87 Sr stable isotopes. • These proxies suggest organic matter in burrow walls and stagnant water in burrows. • Hardground top surfaces show discoloration, bedding-parallel halo, and elevated δ 18 O. • This elevated δ 18 O implies early cementation by cold bottom seawater.