Ordovician-Devonian black slates from the Rhenohercynian and Truchas basins – deposition and diagenesis

Mud-rich successions can be challenging to interpret accurately, since sedimentary structures such as cross-bedding and ripple marks are often scarce or absent. This makes it difficult to determine the exact environmental and depositional parameters. In order to gain a better understanding of the conditions of such environments, this study aims to compare two of the most important roofing slate deposits in Europe: The Devonian-age “Mosel Slates” and the Ordovician-Silurian-age Truchas Domain.

Until 2019, Rathscheck Schiefer mined the “Mosel Slates” from the Katzenberg mine in the SE Eifel, while companies such as Samaca actively exploit roofing slates near Valdeorras, making Spain one of the largest exporters of roofing slate. These slate mines provide a unique opportunity to gather new insights into the geology of the areas of Mayen (Rhenohercynian Basin) and Valdeorras (Truchas Basin). Both of these areas contain sediments which were deposited in marine shelf settings, although the precise mechanisms of deposition and the precise depositional environment are poorly understood.

This study presents the results of high-resolution facies analyses, both above and below ground, geochemical analyses, including XRD, XRF and CNS measurements, and micro-CT measurements of framboid populations. In both settings, deposition occurred in a low-energy regime on a passive continental margin. Sediments were derived and reworked from the Laurussian and Avalonian continents, respectively. Background sedimentation and density flows were the main mode of deposition. The water column was oxygenated and anoxia was restricted to the sediment in the “Mosel Slates” and to dysoxic episodes in the Spanish slates.