Stopy karbonske vulkanicke cinnosti v petrovickych vrstvach vnitrosudetske panve u Debrneho

Numerous shallow exploratory boreholes were drilled in the years 2018-2019 within the geological survey and investigation of D11 motorway in section No.1109 leading from the town of Trutnov up to the Královec village on the Czech-Polish border. The drilling core of borehole PJ707 was studied in detail. This borehole being 43 m deep reached the Petrovice Member of the Žacléř Formation stratigrafically belonging to the Upper Carboniferous Duckmantian-Bolsovian of the Czech part of the Intra-Sudetic Basin. The studied beds containing various types of volcaniclastics brought a new information on volcanic activity in southern part of the Křenov Tuffs exposed in an outcrop near the village of Debrné close to the Trutnov town. The sedimentary sequence comprises alternating beds of polymictic conglomerates, sandstones and various volcaniclastics.
The basal part of the volcano sedimentary sequence consists of conglomerates overlapped by fine-grained reddish-brown sediment rich in strongly fractured euhedral crystals and sharp-edged fragments of quartz and feldspar, and possibly already altered glass in form of clusters comprising a mixture of clay and mica minerals. These deposits may correspond to the crystal-rich sandstone facies (cf. McPhie et al. 1993), representing a redeposited volcanic material. The origin of red sediment in the roof of this crystal-rich sandstone is not completely clear. This sediment locally gray-green-spotted with conspicuous horizons with round to oval dark red-brown nodules or pellets scattered in the matrix consisting of quartz and feldspar crystals both strongly fractured may represent an alluvial sediment and/or volcaniclastic horizon affected by diagenesis. It is hard to prove or refute a hypothesis of the deposition of fine-grained volcanic dust from a volcanic cloud in which ash aggregates with a visible concentric structure resembling accretion lapilli were formed. Due to the fact that aggregates saturated with Fe oxides are in the matrix locally accompanied by strongly fractured quartz and feldspar grains, the connection with the eruption is likely. Unsorted massive reddish-brown breccia with mostly sharp-edged fragments of volcanics and sediments in the vitroclastic matrix resembles deposition of debris associated with an episode of rapid sedimentation from gravitational current in an unstable environment in the basin affected by eruptions and partly effusive activity. In the roof of the breccia lies a fine-grained green-gray ash to lapilli flow. The thickest layer of volcaniclastics is formed by gray to gray-violet hardened lapilli-tuff with pumice and numerous biotite crystals which corresponds to unwelded to slightly welded ignimbrite.