Original paper

Über den Zusammenhang zwischen Vulkanismus und Kieselschiefer-Bildung im Harz

[About the correlation of volcanism and chert genesis in the Harz Mountains.]

Zellmer, Henning

Kurzfassung

Die Kieselschiefer des Harzes sind aus unterschiedlichen Ausgangs-Sedimenten hervorgegangen. Das häufigste primäre Sediment war Radiolarien-Schlamm. Außerdem wurden radiolarienarme, siltig-sandige Schlämme und Pyroklastika in Kieselsedimente umgewandelt. Die unterkarbonischen Adinole werden als verkieselte pyroklastische Abfolge mit einem hohen Gehalt an Hyaloklasten gedeutet. Als Begleitgesteine der Kieselschiefer kommen Pyroklastika, Grauwacken, allodapische Kalke, Quarzarenite, Rotschiefer, Schwarzschiefer und Alaunschiefer vor. Die Verkieselung erfolgte frühdiagenetisch. Wichtigste SiO2-Quellen waren Radiolarien, Schwammnadeln sowie Glasscherben und andere Vulkanoklasten. Als wesentlicher Verkieselungs-Faktor wird die periodische Versiegelung des Sediments gegenüber dem Meerwasser durch bentonitische Aschelagen interpretiert, welche, in dünne Tonminerallagen umgewandelt, die Bankung der Kieselschiefer-Abfolgen bedingen.

Abstract

The paleozoic cherts of the Harz Mountains are derived from various primary sediments, changed by drastic diagenetic alteration. The most common primary sediment was radiolarian ooze. Next frequent were silty muds with scarce radiolarians and pyroclastic detritus. Early Carboniferous Adinol-type cherts consist almost completely of silicified vitric tuff. The main silification took place during early diagenesis. Radiolarian skeletons, sponge spicules, glass-shards and other volcaniclastic material were the most important sources of SiO2. Frequent sealing of the sediment surface by bentonitic layers was the most important premise for this early chertification. Thin argillaceous ash-layers, which cause the typical rhythmic bedding of chert sequences, prevented the contact between SiO2-subsaturated sea water and concentrated interstitial waters. The Harz cherts are classified according to increasing silification into biogenetic (diagenetically altered radiolarian / spongean oozes), volcanogenetic (chertified pyroclastic rocks) and terrigeneous (altered siliciclastics) sediments. The biogenetic sediments are termed radiolarian, respective spongean chert (= "Radiolarit", "Spiculit"). Slightly silicified non biogenetic sediments, that can be scratched by steel are classified as cherty tuffs ("kieselige Tuffe", volcanogenetic) or cherty shales ("kieselige Tonschiefer", terrigeneous). Cherts that bear scarce identifiable components and can not be scratched by steel are termed "Kieselschiefer" (= chert s.s.). "Adinol" is used as a synonym for silicified tuffs (= pyroclastic cherts). Completely chertified rocks with no identifiable particles are termed "Hornstein", which includes paleozoic cherts termed "Lydit". Common sediments intercalated in chert sequences are red shales, quartzarenites, greywackes, allodapic limestones, black shales and various pyroclastic rocks. The water depth was up to a few hundred meters, but not comparable with recent deep sea basins. In contrast to recent shallow basins with pelagic conditions no carbonate oozes were formed, because there did not exist any plankton with carbonatic skeletons.