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The evolution of a ground-water-influenced (Westphalian B) peat-forming ecosystem in a piedmont setting: The No. 3 seam, Springhill coalfield, Cumberland Basin, Nova Scotia

By
J. H. Calder
J. H. Calder
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Published:
January 01, 1993

The basis of modern fresh-water peatland (mire) classification, namely ground-water influence and source of ionic input, has been adopted in this study for ancient mire analysis. Trends that result from a modern mire’s evolution from a rheotrophic (ground-water influenced) planar to ombrotrophic (solely rain-fed), raised status, under decreasing influence of ground water, include decreasing pH levels, nutrient/ionic supply, ash content, species diversity, and ratio of arboreous to herbaceous vegetation. These attributes are inferred to give rise to the following upward trends within a coal seam: enhanced preservation and reduced biochemical geliflcation within similar tissues; decreasing abundance of liptinite macerals of aquatic affinity, sulfur (especially pyritic) content, and waterborne ash; and decreasing floral diversity. Reversals in these trends may signal change in the trophic status of the ancestral mire (e.g., deflation). The identification of such trends relies heavily upon the description of vitrinite in terms of relative geliflcation. The significance of Eh and the historical use of inertinite in paleomire analysis is questioned.

The ancestral mire of the Westphalian B No. 3 seam of the Springhill coalfield, Cumberland Basin, Nova Scotia, formed between a piedmont of coalesced, retreating alluvial fans and the medial reaches of a basin-axis channel belt. The lithologically distinct piedmont, inner mire, and riverine zones of the seam reflect this geomorphic setting. Modeling of a maceral-based index of ground-water influence (strongly gelified tissues and mineral matter versus well preserved tissues) led to the deduction that the paleomire originated as a rheotrophic, and presumably planar, ecosystem that evolved progressively toward a less ground-water-influenced (mesotrophic) state, and possibly to an ombrotrophic, weakly domed system within the inner mire. This maceral-based method suggests a succession of mire types from swamp to fen (and questionably to bog) representing the classic hydroseral succession that forms by the autogenic process of terrestrialization. Contrary to the maceral-based evidence of progressive, albeit weak, raising of the mire surface, ash, sulfur, and miospore diversity increase, and lithotypes become duller upward within the upper third of the seam in the inner zone, suggesting that the mire may have ultimately reverted to a more ground-water-influenced state. A decrease in pH, inferred from an upward increase in tissue structure and decrease in geliflcation, accompanied inner mire development; elsewhere conditions were less acidic.

The paleomire flora was dominated throughout by the arboreous lycopsids Lepidodendron hickii and Anabathra (cf. Paralycopodites), confirming the rheotrophic nature of the ecosystem and the prevalence of flooded conditions. Floral succession of these arboreous lycopsids is evident within the inner mire. Groundwater discharge from alluvial fans at the piedmont margin favored conditions for the colonization of the forest flora. The feedback mechanism of lateral or upslope paludification was aided by the rapid, noncompetitive growth strategy of the arboreous lycopsids. At the riverine margin, autogenic evolution of the ecosystem was stymied by allogenic fluvial processes and by differential compaction about entombed multistory sandstone bodies. Lithotype trends record a general, but similar history of mire development. The ultimate demise of the mire is ascribed to allogenic change, potentially involving precession-induced climate change in concert with basin subsidence and sediment supply.

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GSA Special Papers

Modern and Ancient Coal-Forming Environments

James C. Cobb
James C. Cobb
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C. Blaine Cecil
C. Blaine Cecil
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Geological Society of America
Volume
286
ISBN print:
9780813722863
Publication date:
January 01, 1993

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