THE PENNSYLVANIAN SPRINGHILL MINES FORMATION: MEANDERING FLUVIAL SYSTEMS IN A CYCLIC, COAL-BEARING SUCCESSION

The Pennsylvanian Springhill Mines Formation (Langsettian-Duckmantian) is located in the Cumberland Basin of Nova Scotia and crops out along the Bay of Fundy. The 714-m-thick type section can be subdivided into three cycles, each of which records the transition from lacustrine/wetland deposits to redbeds deposited in a seasonally-oxidizing floodplain.

Cycle 1 (0- 240 m) consists of 17 m of lacustrine deposits overlain by redbeds with scattered drab mudrocks and thin coals. Fluvial deposits within this interval consist of fixed-channel bodies (9-16 m thick; width: thickness (W:T) ratios of ~5) and meandering-channel bodies with pronounced lateral accretion surfaces (3-7 m thick; W:T ratios of ~35). Cycle 2 (240-417 m) commences with 75 m of dominantly drab mudrock, thin coals, and rare channel bodies. The overlying 100 m of redbeds contain fixed-channel bodies (4.5-9 m thick, W:T ratios of ~24) which overlain by sheet-channel bodies (4.5-18 m thick; W:T ratios of ~38) with a complex internal architecture dominated by low-angle, cross-cutting erosional surfaces. The basal 52 m of Cycle 3 (471-714 m) contains thin packages of drab floodplain deposits with scattered, poorly exposed channel bodies. The upper part of this cycle consists of redbeds with meandering-channel bodies (3-10 m thick; W:T ratios of ~40) that increase in thickness and abundance toward the top of the cycle.

These poorly developed cycles may record a glacioeustatic signature muted by high subsidence rate and sediment supply. Coarsening upwards via an increase in the abundance of channel bodies occurs within individual cycles and at the formation scale. Although widely cited as an example of an ancient anastomosed river system, the majority of the channel bodies in this unit have pronounced lateral accretion surfaces and were likely deposited in association with a meandering river system.