Facies and palaeoecology of the late Viséan Actinopteria Black Shale Event in the Rhenish Mountains (Germany, Mississippian)

Abstract

The isochronous, mid-Brigantian (latest Visean) Actinopteria Black Shale is intercalated in between upper Visean calciturbidites within the northern part of the Rhenish Mountains (Germany). It shows characteristic mass-occurrences of the eponymous bivalve Ptychopteria (Actinopteria) lepida (Goldfuss). The case study of a classical, exceptionally well-preserved outcrop in the Ludenscheid Syncline documents the facies and palaeoecology of this event horizon. Hitherto, sediments had been regarded as deep-marine anoxic deposits, resulting from pelagic rain. Now, microfacies analysis of the mostly silicified mudstones revealed a heterogeneous sedimentary facies, showing ample evidence of current-driven micro-event deposits regarded as mud turbidites. Various but simple ichnofabrics indicate mostly dysoxic conditions of the soupy sediment. Discussion of palaeobiological constraints proves the earlier assumed pseudoplanktic mode of life for Ptychopteria (Actinopteria) lepida and contradicts epibenthic colonisation by that bivalve. Mass-occurrence might be a compaction artefact, but occurrence on few distinct bedding planes in an interval several centimetres thick is related to an extraordinary complete sedimentary record that protected the paper pectens from erosion. This variant of a taphonomic epibole is termed maximum completeness epibole. In the case of the Actinopteria Shale, in sequence stratigraphic terms a transgressive systems tract, this bioevent is postulated to represent the maximum flooding surface with minimum sedimentary input and reworking.