Silicified wood from the Czech part of the Intra Sudetic Basin (Late Pennsylvanian, Bohemian Massif, Czech Republic): systematics, silicification and palaeoenvironment

Silicified trunks, colloquially called araukarity, are plentiful plant fossils of the Late Pennsylvanian in the Czech part of the Intra Sudetic Basin (ISB) in NE Bohemia. They are predominantly embedded in Zaltman Arkoses, a unit of fluvial sediments deposited during Barruelian, Late Carboniferous. This unit is a part of the Odolov Formation with the richest outcrops in the area of Jestrebi hory (Hawk Mts.). Since GOEPPERT (1857) firstly described these fossils as species Araucarites brandlingii and A. schrollianus, interpreting both as conifer woods close to the Araucariaceac, they have later never been re-examined or studied as a whole by any modern analytical methods. As the original material of GOEPPERT Was unavailable to our Study, we re-evaluated the previous taxonomical assignments on the basis of newly collected material and supplemented a detailed description of their mineral matter (petrography, mineralogy). Following the modern classification, A. brandlingii (= Dadoxylon brandlingii) describes the wood of cordaites, and Araucarites schrollianus (= Dadoxylon saxonicum syn. Dadoxylon schrollianum) is a name of conifer wood, but our systematical study proves only the presence of cordaites. The pycnoxylic stems were silicified in alluvia without apparent influence of volcanic material. Data from sedimentary structures were used for reconstructions of palaeostreams. The weathering of feldspars is presumed as a source of silicification amplified by the oscillation of water table under seasonally and periods within Late Pennsylvanian/Early Permian long climate cycles. This mode of permineralization is responsible for frequently poor preservation and high recrystallization of these fossils. Their mineral mass consists of pure highly crystalline quartz without other SiO2 phases. Cathodoluminescence (CL) microscopy and spectroscopy revealed a polyphase process of silicification including influence of thermal fluids which healed cracks in previously silicified mass. It is possible that all these facts responsible for poor preservation of anatomical features of Dadoxylon wood type have precluded its more detailed taxonomical study for more than one century.