THE DEPTH OF COAL-BALL FORMATION

Coal balls are carbonate and pyrite concretions that preserve permineralized peat, occuring in Pennsylvanian and early Permian coals. During coal-ball formation, void spaces within and between plant organs were filled with carbonate cement; these cement-filled spaces in coal balls thus indicate former void spaces. The organ composition, distribution and shape of void space in coal balls indicates the depth of coal-ball formation within the peat. Within cordaitean-dominated peat, coal balls composed of cordaitean leaves and roots with large amounts of bedding-parallel void space (cordaitean leaf-mat peats) formed at the peat surface. In these coal balls, most roots and rootlets grew horizontally between accumulated leaves. Cordaitean leaf-mat peats without bedding-parallel void spaces formed at the base of the leaf mat. Coal balls that consist of large pieces of cordaitean wood or other duroparticles, surrounded by small particles of comminuted peat that have almost no cement-filled void space (duroparticle-matrix peat) formed below the leaf mat. In these coal balls, most cement-filled spaces (i.e. former void spaces) are associated with aerenchymatous roots that grew into the peat, perpendicular to the bedding plane, void spaces within plant organs (mostly seed cavities and empty sporangia), and burrows. These duroparticle-matrix coal balls have more secondary pyrite replacing carbonate cement than other types.

Cordaitean coal-ball peat consists of 60 - 100% framework particles (particles with one dimension ≥ 100 µm). Within framework particles, about 50% are roots and 40% are wood. These values contrast with those of modern mangrove peat, where framework particles contribute about 54% of the peat, with almost all of them being roots, and large pieces of wood rare. Overall, these insights suggests that Pennsylvanian peat swamps dominated by cordaiteans likely had thick surficial leaf mats penetrated by cordaitean roots that became increasingly root-rich with depth as leaves decayed. Most cordaitean coal balls are wood- or root-dominated, suggesting a taphonomic bias for coal-ball formation within the subsurface of the swamp. Relatively few cordaitean coal balls contain duroparticle-matrix peat. Decreasing peat particle size and loss of pore space at depth may limit coal-ball formation in peat swamps.