Alluvial and volcanic pathways to silicified plant stems (Upper Carboniferous–Triassic) and their taphonomic and palaeoenvironmental meaning

Abstract

Petrographic imaging, in combination with qualitative and quantitative instrumental analyses of mineral mass, allow us to obtain material signatures of silicified plant stems that are relatively common in sediments of continental basins since the late Palaeozoic. These fossils can be found in their original strata but commonly have been removed from their environmental and stratigraphic context, redeposited, and scattered on the recent land surface after erosion of the parent rocks. Analytical data gathered from X-ray diffraction analysis, hot cathodoluminescence (CL) imaging and spectroscopy, electron microprobe analysis, Raman spectrometry, and polarised light microscopy serve to characterise material signatures of samples from basins in Brazil, Germany, the Czech Republic, Sultanate of Oman, Mongolia, Antarctica, France, and the USA. This collection includes silicified Pennsylvanian and Permian plant taxa (and a few from the Triassic) found in fluvial environments and sites influenced by volcanism with the purpose to discern fundamental material characteristics formed under particular environmental circumstances. Late Pennsylvanian and Permian silicified stems in fluvial rocks include the presence of well-crystalline quartz (α-SiO 2), sometimes with a trace of kaolinite, showing weak CL (mostly blue or dark reddish), occasional mosaic or patchy preservation of anatomical details, and other signs of pressure distortion of wet trunks in fluvial deposits and subsequent diagenetic recrystallisation. The presumed silica source for the initial stage of silicification was weathering of labile minerals, mostly feldspars in the alluvium. In wood from aeolian deposits, moganite in combination with goethite was detected. Based on our results, we propose that the stems were silicified in sandy or gravelly fluvial deposits, most frequently in arkoses and arkosic sands, indicators of relatively warm climate with pronounced seasonal distribution of precipitation. Excluded from this interpretation are stems silicified primarily by volcanic material; these are distinguished by a higher species diversity, silicification close to the site of growth, miscellaneous mineralogy, usually with very colourful CL shades, and the presence of metastable forms of SiO 2, opal-CT or moganite. This volcanic influence on silicification mode is less clearly controlled by seasonality of precipitation or palaeoclimate itself.