Abstract
Fossil flora imprints have been found during remediation works on the road no. 262 Děčín - Benešov nad Ploučnicí, running through Soutěsky Pass and cutting through a lahar sequence of the České středohoří Volcanic Complex by the remediation company employee. The exposed lahar sequence belongs to the Děčín Fm. (sensu Cajz 2000), where Oligocene flora has been recently found at a nearby locality (Teodoridis et al. 2020).
Subsequent field campaign at the Soutěsky outcrop focused on documentation of the lahar deposits and collection of permineralized wood fragments. The outcrop was mapped in detail and photographed for later interpretations.
Samples from selected wood pieces were thin-sectioned for anatomical studies and prepared for powder X-ray diffraction and micro-probe analyses. Late Oligocene woods at the Soutěsky locality were mineralized with a mixture of zeolites belonging to the Ca-K mineralization series [predominantly phillipsite-(Ca) and chabazite-(Ca)] representing the alkaline to intermediate members of the zeolite group.
The zeolites are present mainly in the cell fillings, whereas a mineral of the smectite group (belonging to the K mineralization series) was identified on the cell walls. Scarce analcime was identified by powder X-ray diffraction analysis together with a slightly more abundant thomsonite (Na-Ca mineralization series).
The observed mineral assemblage corresponds to the low temperature of hydrothermal fluids at very low pressures, which suggests that the mineralization proceeded very close to the surface, probably only in the presence of meteoric water which permeated the lahar sequence. Such zeolite mineralization series are common in the continental rift environment.
However, complete zeolite permineralization of fossil wood in the alkaline volcanic rocks is less common than carbonate permineralization. The observed association corresponds to the low temperature of hydrothermal solutions at very low pressures.
Two taxa were identified among the collected samples of permineralized woods. The predominant type is represented by Taxodioxylon gypsaceum.
A single fragment was identified as Platanoxylon sp., which represents the very first find of fossil wood of this species in the České středohoří Volcanic Complex. In addition to these two wood taxa, imprints of deciduous trees were also found at the outcrop, but their mode of preservation did not allow any closer species identification.