Reconstruction of Oligocene environment on the České středohoří volcanic complex northern periphery – integration of volcanology and paleontology

Abstract

Fossilised remains of plants and animals can be found at many sites of volcaniclastic deposits and intravolcanic sediments (limestones, diatomites, lignite) of the České středohoří Volcanic Complex. Many of these localities have been known and studied since the 19th century. Some of these localities even gained international recognition due to abnormal abundance of fossil material. Collected and carefully stored paleontological material is still intensively studied with an aim to better understand the evolution of life but also environmental changes in Central Europe during Cenozoic. Combination of paleontology and volcanology has a potential to better constrain the paleo-environmental interpretations, as the two geological branches use different methods and gathered results can be confirmed by independent techniques. For this reason, our research focused on less-known paleontological locality at Starý Šachov, which provides interesting exposure documenting relations between volcanic activity, surrounding environment and biosphere during early stage of the České středohoří Volcanic Complex formation. The site Starý Šachov, also referred to as Malý Šachov (Radoň 2001), belongs to the basanitic Lower Oligocene Ústí Fm. of the České středohoří Volcanic Complex (sensu Cajz 2000) and is located at the northern margin of the České středohoří Mts. (Fig 1a). The Starý Šachov gorge exposes a sequence of pyroclastic rocks, capped with basanite lava (Fig. 1b), which was dated to 30.70 ± 0.45 Ma (K-Ar, bulk- rock). The pyroclastic sequence overlays redeposited volcaniclastic and intravolcanic sedimentary rocks (Fig. 1c). The volcaniclastic layers in the lower part of the sequence, where they alternate with intravolcanic sediments, consist of a mixture of basaltic epiclasts, and redeposited pyroclasts mixed with quartz and K-feldspar grains (Fig. 2a). This association suggests a short fluvial transport of volcaniclastic material from the growing volcanic complex, mixed with clastic material of the poorly solidified Merboltice Fm. sandstones representing the intimate bedrock of the locality. The pyroclastic sequence starts with deposits dominated by glassy non-vesiculated lapilli (Figs 1e and 2b, c) later replaced by deposits dominated by highly vesiculated lapilli (Figs 1d and 2d). The light-coloured sediments from the lower part of the succession contain abundant paleontological material. Clusters of white strongly flattened and crushed mollusc shells (Figs 3a, d, f, g) occur in whitish to light grey thinly bedded claystones. Together with molluscs, small cracked ostracod shells (Figs 4a–c) and caddisfly (Trichoptera) larval cases (Figs 4e, f ) are very abundant. Very rarely, twigs of plants from the Taxodiaceae family (Radoň 2001) and indeterminable leaf fragments were found. In dark coaly clays, undercut ostracod valves are abundant as well as in light coloured clays. Small rounded seeds of water lily-like plants (Fig. 4h) and fragments of frog bones (Fig. 4i) occur in larger numbers in the coaly clays. Gyrogonites of charophytes (Figs 4d, g) and only three shells of freshwater gastropods were found in the redeposited volcaniclastic layer underlaying the light clays. Unlike the compressed and crushed gastropod shells of the overlying clays, these three shells are spatially preserved. They belong to the species Planorbarius blazkai (Klika 1891) and Stagnicola sp.Based on the described taphocenosis, sedimentary and pyroclastic deposits we may reconstruct the paleoenvironment of this locality in Early Oligocene (Fig. 5). The landscape prior to the volcanic activity can be characterized as alluvial plain on the northern periphery of a growing volcanic complex. In this plain, braiding rivers were transporting and depositing volcaniclastic material as well as quartz grains of the poorly solidified Merboltice Fm. sandstones. Numerous oxbow lakes from abandoned meanders represented static water bodies. Such environment explains the observed biotic assemblages, dominated by flowing water preferring like trichoptera, with sporadic finds of static water elements like water lily-like plants, which may be either redeposited or reflect presence of minor static water bodies (oxbow lakes). The water rich environment is reflected also in the character of the subsequent volcanic activity. The phreatomagmatic pyroclastic deposits suggest that the eruption started in water- saturated environment (Surtseyan style eruption in a shallow lake or marsh) and later turned into the drier style (Strombolian) as the volcano emerged above the water body. The volcanic activity was terminated by lava emission.