THE PENNSYLVANIAN JOGGINS FORMATION OF NOVA SCOTIA: NATURE AND ORIGIN OF CYCLIC SEDIMENTATION IN A RAPIDLY SUBSIDING DEPOCENTER

The Joggins Formation is spectacularly exposed along the shore of the Bay of Fundy and is widely considered one of the world's best outcrops of coal-bearing Carboniferous strata. This 915.5 m thick unit accumulated in <1 Ma during the Langsettian and records one of the highest subsidence rates amongst Pennsylvanian coalfields. Syndepositional withdrawal of underlying evaporites and motion along basin-bounding faults restricted deposition of this unit to a 25 by 75 km depocenter.

The Joggins Formation contains fourteen major transgressive-regressive cycles which typically commence with open water limestones and shales that record widespread drowning of the floodplain; the presence of brackish faunas suggest communication with the sea. Overlying drab mudrocks with standing lycopsids, channel and sheet sandstones mark the advance of coastal wetlands into the basin. Continued progradation resulted in the deposition of red mudrocks, channel and sheet sandstones deposited on a well drained alluvial plain. Many cycles are capped with poorly drained floodplain deposits, which represent retrogradational deposits formed as a precursor to the next flooding event.

The Joggins Formation was deposited during a known glacial episode and exhibits cyclicity in the Milankovitch band. Cycles within this unit may record glacioeustatic events, but the overwhelming tectonic signature masks the influence and magnitude of eustatic and climatic change. Well developed paleosols, deeply incised “paleovalleys” and limestones typical of Carboniferous cyclothems are absent from the Joggins Formation. Instead, rapid subsidence enhanced base-level rise and suppressed base-level fall, while abundant sediment supply kept the sediment surface within a few meters of base level throughout deposition of this kilometer-thick succession. Paleosols abound, but high floodplain aggradation rates precluded the development of useful paleoclimatic indicators within these immature soils. The uncertain influence of climate and eustasy on the Joggins floodplain highlights the problems in using paleoequatorial cyclic successions as a proxy for high-latitude glacial events, and illustrates the need to further refine the timing, magnitude, and duration of late Paleozoic glaciation and link the Gondwanan record with that of Euramerica.