Cellular preservation and maceral development in lignite and wood from the Brassington Formation (Miocene), Derbyshire, UK

Abstract

Fossil wood is well known from the uppermost part of the Brassington Formation of Miocene age from two localities in Derbyshire, central England, UK, but its preservation has not been previously studied. Likewise, lignite is also present but has not been studied hitherto. This study examines preservation of selected wood samples using a combination of organic petrography and organic geochemistry. The best conserved exposure of the Kenslow Member of the Brassington Formation, representing the majority of the lower Kenslow Member, occurs at Bees Nest Pit. Disseminated woods occur throughout the Kenslow Member, with the greatest concentrations, and largest specimens, occurring around a meter below a thin lignite at Bees Nest Pit. The lignite is composed primarily of wood and charcoal, as well as organic-rich clay and leaves. Wood in the lignite and in the disseminated wood samples appear unaltered or charred in hand sample. Larger pieces typically have numerous cracks and pockets in the surface, highly suggestive of desiccation and white-rot prior to burial, but appear to be solid mummified wood overall. Drying in acetone and epoxy impregnation permitted the wood and lignite to be examined using reflected light microscopy, while air-dried wood samples were coated with gold‑palladium and examined using SEM and prepared for organic geochemistry. All wood samples examined herein appear to be softwoods. Members of the “charred” group have high-reflecting exteriors and low-reflecting interiors in reflected white light illumination. Members of the “unaltered” group of tissues are consistently low-reflecting unaltered to slightly gelified textinite and ulminite A which display different reflectivity in transverse and radial sections. Random reflectance places the unaltered wood in the upper range of peat or lowest range of lignite B. Low-reflecting cells are primarily fiber cells and are strongly fluorescent. In most cases the three-ply fiber cell wall has begun to delaminate, a key feature of white-rot decomposition of the wood. Rays appear largely unaltered, and bordered pits show no evidence of fungal hyphae. Annual rings and vessels, by contrast, have undergone significant humification and have the appearance of ulminite and textinite, respectively. Near the wood margins, extensive pockets of rot show strong gelification, with the result that the cellular framework is gelified and brightly reflecting with isolated fibers present which still fluoresce. Huminite reflectance suggests that during very early stage lignification, reflectance varies depending upon cellular orientation, with cells in transverse section being more highly reflective than those in radial section. Organic geochemistry indicates all the fossil woods are chemically altered relative to modern counterparts. The elemental ratios suggest these sit between modern wood and brown coal. The changes are probably due to loss of polysaccharide as compared to lignin structures. The spread in O/C and H/C is probably due to variable microbial alteration (e.g. white, soft or brown rot fungal decay), as also indicated by the organic petrography. This type of wood preservation is consistent with early coalification in aerial settings, and consistent with fungi recovered during palynological studies which are indicative of softwoods decaying in wet forest settings.