Radiolarian cherts and associated siliceous rocks of the Rhenish Massif and Harz Mountains, lower Carboniferous (Mississippian), Germany

Abstract

Siliceous sedimentary rocks are major constituents of the rock succession in the Central European part of the Kulm Basin (Mississippian) and crop out in the eastern Rhenish Massif and the western Harz Mountains. These rocks occur in five lithostratigraphical units: Oberrödinghausen Formation, subdivided into Kahlenberg Subformation (“Untere Alaunschiefer”) and Hardt Subformation (“Schwarze Kulm-Kieselschiefer/-Lydite”), Laisa Formation (“Helle Kulm-Kieselschiefer”) and coeval Hillershausen Formation (“Kulm-Kieselkalke”), as well as Bromberg Formation (“Kieselige Übergangssschichten”); upper Hastarian to Asbian. Biostratigraphically, the formations can be dated by means of radiolarians and, in the upper part of the Kulm succession, also by ammonoids. Various siliciclastic, calcareous and, locally, mafic volcanic and pyroclastic rocks are associated with these strata. Four palaeogeographical facies zones have been identified. Radiolarian chert, spiculitic chert, homogenous chert, and silicified tuff are the main siliceous rock types. These mostly form current-laminated grey, black, greenish or reddish beds rhythmically alternating with layers of siliceous mudstone and are, in places, variably intercalated with grey and black mudstones and siltstones, phosphorites, metabentonites, turbiditic limestones, greywackes and quartz arenites. The depositional area of the Kulm Basin was situated at the northwestern margin of the elongate, relatively narrow tropical Palaeotethys Ocean, a shallow-bathyal sea strait between Gondwana and Laurussia that was successively closed during the Variscan Orogeny. Westbound nutrient-rich currents favoured high fertility of siliceous plankton (radiolarians). The latter gave rise to the formation of relatively pure biosiliceous oozes and muds under temporarily anoxic conditions at the seafloor, when, contemporaneously, terrigenous detrital input and sedimentation rates became low (approx. 2 mm/1000 a) due to relatively high sea level and dry climate. Biosiliceous sedimentation terminated, when, due to the Gondwana–Laurussia collision, terrigenous detrital input multiplied, the basin narrowed and oceanic circulation was restricted resulting in lowering of radiolarian productivity.