Fossil wood in coal-forming environments of the late Paleocene–early Eocene Chickaloon Formation

The early Cenozoic Chickaloon Formation is the main coal-bearing sequence in south-central Alaska. Coal and associated plant macrofossils are well exposed in the Eska, Premier, and Jonesville Coal Groups in the Wishbone Hill District. There is a striking variation in the degree and mode of preservation of fossil trees in a vertical sequence through these coal groups. We examined the wood taxonomy, mineralogy, and geochemistry to elucidate potential cause(s) for these differences. The wood anatomy indicates that the wood is all assignable to the Cupressaceae (Taxodiaceae s.str.) and assignable to the form genus Taxodioxylon Hartig emend. Gothan. However, we found no significant variation in the taxonomy of the wood between stratigraphic layers. The wood generally becomes better preserved in the Jonesville Coal Group. Wood from the stratigraphically lower Premier Coal Group exhibits greater plastic deformation and contains less well-preserved wood anatomical features. The wood from the overlying Jonesville Coal Group retains most of its original morphology, but has suffered from brittle fracture. The geochemistry and mineralogy of the wood follow a pattern that corresponds to the degree of preservation. The less well-preserved wood tends to have a higher concentration of iron carbonate minerals (e.g., siderite and ankerite), whereas the better preserved wood contains more calcium carbonate or silica. All evidence points to rapid permineralization of the original wood, although the least well-preserved wood shows evidence of greater organic matter decomposition than the better preserved wood. The wood taxonomy and mode of preservation of wood from the upper part of the Chickaloon Formation is similar to wood from coeval fossil forests that developed at higher (polar) paleolatitudes.