New details on onset of deposition and volcanic activity in the early Permian (Asselian); Rudník member in northwestern part of the Krkonoše Piedmont Basin
Authors: Stárková Marcela , Nádaskay Roland , Rapprich Vladislav , Pour Ondřej 
Published: October 2023

A succession of the early Permian (Asselian) Rudník member exposed at the locality Škodějov – Hrádecký potok in the northwestern part of the Krkonoše Piedmont Basin (eastern Bohemia) contains light-colored clastic rocks with macroscopically conspicuous lamination, overlying lacustrine bituminous claystones and siltstones (ʻblack shalesʻ). This work aims to constrain the origin of those laminated strata. They can be macroscopically described as heterolithic sequence formed by fine- to medium-, exceptionally coarse-grained sandstone with intraclasts of mudstone, calcified fine-grained sandstone, mudstone and claystone with interlayers of organic-rich laminae. Volcaniclastic rocks are also present. The heterolithic volcanosedimentary sequence comprises cyclic (up to 4 dm), mostly horizontally laminated rocks, in places with soft- sediment deformation structures. Diagenetic phenomena, related to calcification of volcaniclastic layers and growth of pyrite crystals and framboids, are common. Volcaniclastic rocks with a predominance of albitized volcanic glass fragments are interpreted as vitritic tuffs. Fragments of volcanic glass floating in silty to fine-grained sandy matrix contain vesicules and are locally brown- colored. They are reinterpreted as being derived from contemporary intermediate (andesitic) volcanism, most likely extrabasinal. Extensive alteration of volcanic material makes characterization of the original composition of volcaniclastic deposits difficult. Trace element analysis using a portable XRF spectrometer revelaed negligible concentrations of V, Ni and Ti, which are indicative of rather basic volcanism. Results of outcrop-scale gammaspectrometry are presented here as trends of Th/U ratio of relevant lithotypes from several localities of the Rudník member. Organic-rich laminae present within the heterolithic succession at Hrádecký potok section are commonly rich in pyrite clusters. Those are the most probably related to formation of algal mats stimulated by input of fine-grained volcaniclastic material into the Rudník Lake. However, elevated siliciclastic and volcaniclastic input, particularly during periods of intensified volcanic activity, with related seismic events (that may have triggered gravity flows), may have disturbed or punctuated growth of algae on the lake floor. Our work has shown that the volcanic activity at the onset of deposition of the Rudník member in the northwestern part of Krkonoše Piedmont Basin (Škodějov – Hrádecký potok) was probably of intermediate nature and that it influenced depositional environment. Particularly, the fluctuating input of clastic and volcanogenic material could have caused an alternation of oxic and dysoxic conditions in the lake lacustrine environment over time. The intermediate character of volcanism in this locality contrasts with confirmed acidic volcanism documented in volcaniclastic layers in another lacustrine succession at Vrchlabí – roadcut (Martínek et al. 2006) that probably represent a different eruptive phase recorded within the Rudník member.