The impact of climate change, tectonism and hydrology on the formation of Carboniferous tropical intermontane mires: the Springhill coalfield, Cumberland Basin, Nova Scotia

Abstract

Both climate and tectonism are influential in determining basin and mire hydrology. In the rapidly subsiding intermontane basins of Euramerica which formed in response to the Variscan-Hercynian-Alleghenian orogeny, however, the record of climate change has been muted and may therefore go unrecognized. The complex interplay of climate and tectonism in a tropical continental setting is recorded in the > 4 km thick Westphalian A-B fill of the Cumberland Basin. Thick peat accumulation was favoured at the southern basin margin where rheotrophic forested mires 4–9 km wide flourished between basin-axis rivers and coalesced alluvial fans derived from the Cobequid highlands. As a result of their geomorphic setting the thicker coals (⩽ 4.3 m) exhibit distinct areal zones: piedmont, inner mire and riverine. Evidence that the forest swamps were nourished both by rainfall and by groundwater discharge from alluvial fans includes the stratigraphic affinity of thick coals and basin-margin conglomerates, interfingering of distal fan sheetflood and mire deposits, petrographic evidence of elevated pH (partial gelification of tissues), and predominance of the arboreous lycopsids Lepidodendron and Paralycopodites. Whereas such groundwater supply is crucial to maintenance of modern wetlands in drier periods, a subhumid climate is implied. Coals at Joggins, further removed from groundwater discharge at the basin margin, are less well developed. A 600-m thick “coal window” within the fining and reddening upward basin-fill sequence at Springhill is inferred to represent a coincidence of optimal groundwater supply and net subsidence. The ultimate decline of peat accumulation in the basin may reflect decrease in catchment area and fan size and loss of rainshadow as the Cobequid Massif was denuded and the basin filled. The coal window [ca. (6−9) × 10 5 yr] is punctuated by non-marine cyclothems of coal, grey sideritic mudrock and multistorey sandstone that share similar time frames with astronomic cycles of axial precession [ca. (1−4) × 10 4 yr]. The stress of related climate change is inferred to have caused peat accumulation to fall into disequilibrium with sediment flux and basin subsidence. The Late Quaternary history of certain African mires illustrates the dramatic impact of climate change (especially temperature and seasonality) and orographic effects on tropical continental peat accumulation. Aspects of the groundwater-recharge model may be applicable to the Westphalian-Stephanian intermontane mires of Spain and Mesozoic coal basins of northern China.