Medullosalean fusain trunk from the roof rocks of a coal seam: Insight from FTIR and NMR (Pennsylvanian Sydney Coalfield, Canada)

Abstract

Reported for the first time from the Sydney Coalfield, Canada, is a fragmentary fusain (Ro = 2.51%) specimen, 41 cm long, of a medullosalean trunk or massive petiole that originated from the roof rocks of the banded bituminous Hub Seam (Ro ∼ 0.65%). Megascopic characteristics of the flat-preserved specimen include an irregular-cracked fusain surface with secretinite–rodlet structures and sclerenchymatous strands some of which still embedded in the shaly matrix, and locally preserved vitrain (Ro = 0.69%). Co-occurrence with a compression flora composed virtually of only the seed-fern taxon Macroneuropteris scheuchzeri (Hoffmann) is noted. The goal of this study is to provide a framework for the phytophysicochemical taphonomic history which includes the perspective on vitrinite/fusinite relationship, formation of fusinite, and on the significance the only fusinized identifiable plant-fossil specimen in the Sydney Coalfield. We use state-of-the-art solid-state FTIR, 13C, 1H NMR CP/MAS techniques, and standard reflected-light microscopy and SEM methods as investigative tools. Results indicate that the fusinite is characterized by long and narrow xylem fibers, without fungal signals, and cell structures infilled with pyrite and carbonate. FTIR spectra of the fusinite and secretinite are similar particularly in respect to high absorbance of aromatic and low absorbance of aliphatic compounds, and absorbance of Si–O functionalities relating to kaolinite. 13C NMR experiments with direct carbon excitation quantify the aromatic to aliphatic ratio as being 20 ± 3:1. As part of the taphonomic history, the evidence favors a hot, > 400 °C, directional surface-fueled flame palaeofire of local extent that charred trunks of growing arborescent seed ferns on only one side, and that these trunks were transported to comprise part of the roof rocks of the Hub Seam.