42-11 Late Pennsylvanian Environmental Dynamics in the Morien Group of the Sydney Coalfield, Nova Scotia

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

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Abstract:

The Upper Pennsylvanian stratigraphic record of eastern North America documents a series of abrupt shifts in paleofloral composition, culminating in the widespread extirpation of many lycopsid groups near the Westphalian–Stephanian (Desmoinesian–Missourian) boundary. This biotic turnover has commonly been attributed to the progressive development of increasingly seasonal and arid climatic regimes across lowland regions of Euramerica. However, both the tempo and underlying drivers of this change remain incompletely resolved. Here, we integrate field-based sedimentological analyses, an inventory of newly documented and previously reported plant macrofossils, and stratigraphically constrained geochemical proxy data from a c. 1500 m-thick, stratigraphically complete succession of the Morien Group exposed in the Sydney Coalfield of the Maritimes Basin Complex, Atlantic Canada. The composite section, comprising the South Bar and overlying Sydney Mines formations, forms a laterally extensive, well-exposed coastal outcrop belt. These strata record predominantly coastal-plain and alluvial depositional environments, intermittently disrupted by subtle, short-lived marine incursions, and collectively span the interval leading up to and including the end-Westphalian floral turnover. The transition from the South Bar to the overlying Sydney Mines Formation is marked by an abrupt change from sandstone-dominated lithologies to thinner sandstone layers interbedded with thicker mudstone and coal intervals.  Leaf wax n-alkane ratios (n-C23/n-C31) indicate a shift toward more standing water and vegetation in wetter soils. Paleoweathering proxies, such as the Chemical Index of Alteration, and δ¹³C_org values remain relatively uniform through the South Bar Formation but become more variable in the Sydney Mines Formation, suggesting increasing fluctuations in climate and environment. Evidence for fire, as indicated by PAHs, is present throughout the succession. However, the Sydney Mines Formation shows signs of higher-temperature fires and/or more localized burning. Through integration of sedimentological, paleobotanical, and geochemical indicators, this study implicates fluctuating climatic and environmental conditions, including swings to aridity, as potential drivers of the lycopsid extirpation near the Westphalian–Stephanian boundary.

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