Coal-forming through time in North America

Abstract Economically important coals of North America were formed principally from autochthonous peats of freshwater swamps. North American coals of different ages are generally characterized by different dominant swamp trees or occasionally by shrubs or herbs. Carbonized compressions and permineralized or vitrinized tissues within the coal, well-preserved plants in coal-ball concretions, and micro-fossils (cuticles, spores, pollen, resins and other identifiable plant dentritus), all contribute to identification of plants that formed coal. Of principal importance in coal formation are vascular plants. Interaction of bacteria and fungi have been important in degradation of peat to form most humic (banded) coals dominated by tissues and organs of vascular plants. Their alteration products and algae constitute significant amounts, by volume, of some coals, particularly sapropelic, canneloid and boghead layers. Algae have contributed to some singular coal accumulations locally, one as early as Precambrian. Compression floras from clastic sediments proximal to coal, or included in clastic partings, may or may not represent peat-swamp plants that formed coals. This needs to be corroborated by fossils from within the coal. Roof-shale floras occasionally indicate coal-bed plant assemblages, but more often, they represent different environments or habitats in which organic-rich mudstones or carbonaceous shales, but not economic coals, might have accumulated. Identification of plants comprising floras of coals and associated rocks of any age presents difficulties according to kinds of plants and plant parts present, preservational states, etc. Most peat-swamp plants represent atypical terrestrial communities adapted to hydrologically influenced succession, in habitats of low pH and low nutrient availability. Such communities are constituted of a comparatively limited inventory of tree, shrub and herb types, generally widely distributed within coal-forming peat-swamp habitats, often occurring repetitiously for long periods of time, in coal-bearing strata. Floras of localized coal seams of Late Devonian to Late Mississippian age are generally dominated by fossil remains of lycopods. Floras in locally developed, thin, Middle to Late Devonian coals of Northwest Territories are dominated by lycopod spores. A thin Late Devonian coal in West Virginia also contains meager lycopod debris transported from near-swamp habitats, but the coal is dominated by plant material from a scrambling, fern-like shrub. Five major kinds of tropical to subtropical trees contributed to the peats that formed the principal Pennsylvanian coal beds: lycopods, tree ferns, calamites, seed ferns and cordaites. The extensive diversity in the architectural framework of stems (combinations of supporting or strengthening tissues), and the nature of the wood, bark or fibrous tissues in stem and root system of these plants, greatly modified the role of each in contributing mass to the peat accumulation and ultimately to the remaining volume of each in coal. Coal-ball studies have demonstrated that the peat was comprised of much bark and root material when the coal flora was dominated by lycopods, or of roots and some foliage when dominated by tree ferns. Wood was a relatively minor component of the biomass constituting the peat except in swamps with abundant cordaites or calamites. Pennsylvanian peat swamps were dominated successively by several type of lycopod trees and later by tree ferns, following the extinction of many lycopods. Early Permian coal floras of the Applachians represent a waning of tree-fern-dominated Late Pennsylvanian floras with minor elements of callipterids and conifers. In the southwest U.S.A., the Early Permian coal floras are present only in the gigantopterid province. Those coals contain significant ferns, seed ferns, lycopods and conifers. Late Triassic floras in coals of the Atlantic margin rift basins are dominated by cycadophytes and ferns. The palynofloras of these coal beds demonstrate close relationship of these strata with those in the High Atlas, Morocco. Mid-Jurassic coals of Oaxaca, Mexico, are characterized by cycadeoids and a glossopterid and lack conifers and ginkgophytes. Late Jurassic coal floras of the Canadian and American Rocky Mountains are dominated by cycadophytes, with ferns, conifers and ginkgophytes, in descending importance. Early Cretaceous coal floras are exemplified by the Kootenai Formation of Montana and equivalents in Alberta and Saskatchewan. Great floral diversity characterizes these with several conifers present (Taxodiaceae, Elatocladus and Arthrotaxites ) that were absent from the Morrison Formation below. These represent delta-plain swamps of temperate climate. Many ferns, pteridosperms and bryophytes are present, but cycadophytes and ginkgophytes are limited. Late Cretaceous and Tertiary swamps were first dominated by woody conifers and later by mixtures of conifers and dicots, depending on paleolatitude. Wood, including that of root systems, was a major peat component. Generic diversity in peat swamps increased markedly with Late Cretaceous evolution of flowering plants. The Late Cretaceous is characterized in the Rocky Mountains and Great Plains areas by diverse floras rich in angiosperms. Cenomanian floras of Texas and Oklahoma lack the abundant angiospermous palynofloras of the coals of the eastern Gulf Coast. Late Cretaceous coal floras are well represented on the western margin of the Western Interior seaway. Several arborescent conifers ( Sequoia, Brachyphyllum, Moriconia ) and angiosperms ( Rhamnites, Platanus, Cissus ) identify peat-forming warm-temperate swamps of that period. Several types of modern peat swamps have served as models for the interpretation of Tertiary coal floras. However, decreasing similarity of these models to incresingly older Tertiary coal floras constrain the accuracy of interpretation of the kinds of plants and the depositional and paleoclimatic conditions.