麻豆传媒

During the early Westphalian (Early to Middle Pennsylvanian), tropical peatlands (mires) flourished along the southern margin of the intermontane Cumberland Basin in the area of the present day Springhill coalfield. The resulting bituminous coal seams and associated sideritic mudrocks and multistorey channel sandstone bodies (Springhill Mines Formation) lie basinward of, and progressively onlap, alluvial fan conglomerates (Polly Brook Formation) derived from the adjacent Cobequid Highlands. Within the basin-fill sequence, coals occur in association with grey mudrock and multistorey sandstones (cyclothems, 20-30 m thick). These strata are constrained within a 600 m-thick "coal window" within the fining and reddening upward basin-fill sequence (2-5 km thick). The Springhill coals, up to 4.3 m thick and spanning a transverse distance of 5 to 8 km, exhibit a distinct areal zonation. The piedmont zone is marked by ill sorted partings and is lowest in ash and sulphur. The riverine zone is characterized by splits with locally entomb sandstone bodies and basin-axis rivers.

The precursors to the Springhill coals were rheotrophic peatlands, fed by nutrient-rich groundwater emanating from the fans. Evidence includes the consistent relation between coals and basin-margin conglomerates, the incursion into the mires of distal fan sheetfloods, trends in lithotype and maceral composition of the coals, especially gelification of vitrinite macerals, and the domination of mire flora by arboreous lycopsids suited to areas of nutrient-rich groundwater influx. The rheotrophic mires, and their reliance on fan discharge, indirectly suggest a sub-humid tropical paleoclimate.

A number of controls interacted to influence the development of the peat-forming ecosystems. Tectonism exerted a major control at the basin-fill sequence level (106 yr.) through both extremely rapid subsidence and orogenesis. The resulting alluvial fans discharged nutrient-rich groundwater supply which in modern sub-humid settings is crucial to the maintenance of wetlands in drier periods. Progressive denudation of the Cobequid Highlands probably resulted in a decreased catchment area, orographic rainshadow and fan size and may have contributed to the progressive decline in peat accumulation. The coal window within the basin-fill sequence is inferred to reflect a coincidence of optimal groundwater supply and net subsidence. Cyclothems indicate that conditions favouring peat accumulation recurred with a cyclicity in the order of 104yr. The similar timeframe of Milankovitch cycles of obliquity and precession suggest that climatic or related (eustasy, geoid) change was influential in cyclothem and mire development.

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