42-12 Evaluating climatic cyclicity in a Pennsylvanian coastal plain succession: the lower Sydney Mines Formation of the Sydney coalfield, Nova Scotia, Atlantic Canada

Session: Paleozoic Events and Processes: Sedimentary Geology, Paleontology, and Geochemistry (Posters)

Presenting Author:

Authors:

Abstract:

The Sydney coalfield of eastern Cape Breton Island preserves a 1500 m + thick succession of coastal alluvial plain strata. The Morien Group comprises a lower unit (1000 m) of sandstone-dominated strata (South Bar Formation - SBF) overlain by a more heterolithic unit that preserves numerous mineable coal seams (Sydney Mines Formation - SMF, 500 m + thick). Previous work has posited that stratigraphic cyclicity based on repetitive vertical stacking of environmentally sensitive lithologies (coals, red beds, calcretes, paludal limestones) was forced by cycles of climate and sea-level change during the late Paleozoic Ice Age, but the cyclicity is somewhat enigmatic and inconsistently developed. This work aims to test the hypothesis that the SMF records a series of climatic and/or sea-level-forced cycles. A transect of detailed stratigraphic sections was measured from Great Bras D’Or to Long Beach (NW-SE, perpendicular to regional paleoslope), focusing on the lower SMF from its base up to the Backpit coal seam. Variations in depositional environment were assessed on the basis of a facies analysis. Mudrock samples were collected from each location and subjected to elemental and organic geochemical analysis. This facilitates a re-evaluation of cyclicity by establishing whether coherent patterns of change are recognizable, and whether patterns of change are consistent among the seven measured sections. Chemical Index of Alteration (CIA) values from mudrocks within stratigraphic packages between laterally correlative coal seams (Emery, Stony, Phalen, and Backpit), show a cyclical pattern with values ranging from 60 to 86, but most values fall between 75 and 85. Some inconsistencies exist as a result of low sampling density in areas of poor exposure, yet, trends can be identified in packages of greater resolution. CIA values in the succession between the Emery and Stony seams show a decreasing upward trend from 80-85 to 75. This can be seen laterally throughout the cross-section. d¹³C stable isotopes show similar variability within a range of -22 to -25 per mil. The geochemistry of these successions confirms cyclical patterns of chemical weathering intensity, however, climatic changes are relatively stable within consistent ranges of variability, with no major perturbations.

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