A DEPTH AND ENVIRONMENTAL MODEL FOR CORDAITEAN COAL-BALL FORMATION BASED ON PLANT CONSTITUENTS

Using peat composition and taphonomy, we model the depth of coal-ball formation in cordaitean swamps. Coal balls with flat-lying leaves and horizontally disposed cordaitean roots and rootlets preserve surficial leaf mats. Leaf-mat peat contains ephemeral plant organs such as pollen and seed cones, pollen with pollen tubes, and a diverse array of arthropod coprolites. In some leaf-mat coal balls, individual leaves are separated by layers of magnesian calcite, suggesting formation in porous peat near the surface. In others, leaves are closely packed with no intervening carbonate, suggesting formation near the bottom of the leaf mat. About 18% of all cordaitean peat formed in the surficial leaf mat; the thickest cordaitean leaf mat is 16 cm thick. In modern freshwater and marine forested peats, the surficial leaf mat is seldom more than a few centimeters thick and like cordaitean leaf-mat peat, consists of coherent and fragmented leaves, rootlets, aerial roots, and reproductive organs. A decomposition gradient links coal balls containing pristine leaf mats with coal balls that consist of cordaitean, lycopsid, and tree-fern roots and large pieces of lignitic debris (i.e. wood and the outer root zone of tree ferns) surrounded by peat matrix (particles ≤100 μm), interpreted as decayed leaf-mats. About 4% of cordaitean peat is decayed leaf-mat peat, which has the lowest porosity of all cordaitean peat types. The matrix-rich peat below the leaf-mat in freshwater swamps might be the closest modern analogue of cordaitean decayed leaf-mat peat, which may be rare in coal balls due to its low porosity. A gradient in root percentage links cordaitean leaf-mat peat with equal percentages of leaves and roots to cordaitean root peat with >90% roots and no leaves. Both peat types can be quite porous, with abundant magnesian carbonate separating plant fossils. About 36% of cordaitean peat is root peat. Mangrove peats from below the surficial leaf mat in fringe sites, which have high root percentages (>85%) and low matrix frequencies (47 – 57%) may be the closest modern analogue of cordiatean root peat. About 43% of cordaitean peat consists of large pieces of wood, which preserve the root/stem transition of small trees, suggesting formation near the peat surface. Due to the rapid decomposition of wood in modern wetlands, this peat type has no obvious modern analog.