LOWER DEVONIAN COALY SHALES OF NORTHERN NEW BRUNSWICK, CANADA: PLANT ACCUMULATIONS IN THE EARLY STAGES OF TERRESTRIAL COLONIZATION

摘要

We describe two Early Devonian occurrences of coaly shale composed mainly of the compacted remains of early plants, such that they resemble coal. Among the earliest thick phytodebris accumulations known, the occurrences lie within Pragian strata of the Val d'Amour Formation and Emsian strata of the Campbellton Formation of northern New Brunswick, and were deposited in low-energy wetlands where they were rapidly buried. Although the coaly shales did not yield recognizable plant taxa, numerous taxa are present in adjacent beds. Petrographic analysis revealed an average of 80.7 vitrinite (predominantly telovitrinite) and 18.7 liptinite (predominantly sporinite) on a mineral-matter-free basis, with a higher vitrinite content than most Devonian coals. Low sulfur content and atomic C/N ratios that vary from 44.3 to 82.1 in organicrich samples indicate terrestrial derivation. Vitrinite reflectance in organic-rich samples (21.4-35.6 wt total organic carbon) ranges from 0.48 to 1.00, indicating a low degree of thermal alteration that is supported by cross-polarization spectra from studies of 13C nuclear magnetic resonance; two carbon-poor samples were thermally altered to anthracitic rank adjacent to an intrusive body. Although plants at this time were still comparatively primitive, the presence at both sites of specimens with recognizable lignified cellular structures in vitrinite and particularly thick and resistant cuticle may represent an important step in peat development. In these Early Devonian formations, diverse assemblages of small vascular plants are present in a range of environments, and the plants were sufficiently abundant to form coal-like accumulations under suitable burial conditions, serving as a proxy for plant biomass and vegetation cover in the early stages of terrestrial colonization.