Stratigraphy, sedimentology, tectonics, and resource potential of the Lower Carboniferous Mabou Group, Nova Scotia

Abstract

The upper Lower Carboniferous Mabou Group of Nova Scotia was deposited as the final sedimentary fill into the nearly replete subbasins of a large, intracontinental rift system during the waning stages of tectonic subsidence. Deposition occurred immediately after the short-lived marine carbonate and evaporite realm of the underlying Windsor Group receded. These subbasins lay at a paleolatitude of about 15° in a warm, semiarid, seasonally wet climate. Sedimentation occurred in distensive, fault-bounded subbasins characterized by large total depositional area, and near-vertical stacking of laterally intertonguing formations of moderate thickness. Deposition was influenced primarily by gentle subsidence, and fault-bounded margins were generally located outside the study area. Most detritus was derived from previous sedimentary cover (Horton and Windsor groups). Extraformational pebbles are present in only a few outcrops located close to interpreted subbasin margins. The Mabou Group consists of two lithostratigraphic formations: the lower Hastings Formation, and the upper Pomquet Formation. However, Mabou deposition comprises a single, but internally complex, overall third-order coarsening-upward succession about 500 m thick. This succession begins with shallow-lacustrine muddy deposits (Hastings Formation), which intertongue upward with red, subaerial playa-mudflat-floodplain deposits (Pomquet Formation), which gradually become dominant. The Hastings Formation (of late Viséan - early Namurian age) comprises thin-bedded grey mudstone with interbedded sandstone and limestone of shallow-lacustrine origin. These lithologies are arranged into fourth- and fifth-order, asymmetrical, transgressive-regressive shallowing-upward sequences, deposited in broad, shallow lakes (150 to 200 km long by 100+ km wide in size). Four facies assemblages, which reflect a distal-proximal continuum, are identified in this lacustrine system: dark grey mudstone and sandstone and minor limestone (open lacustrine), grey, very fine- to fine-grained sandstone (lacustrine nearshore and shoreline), minor, red, pedogenic siltstone (subaerial coastal floodplain), and minor, thick, scour-based, very fine- to medium-grained sandstone (high-sinuosity fluvial channel). The intertonguing and overlying Pomquet Formation (of early Namurian age) comprises thin-bedded red siltstone and fine- to medium-grained sandstone, with abundant evidence of vertisol and calcisol pedogenesis. The strata are of subaerial playa-mudflat-floodplain-floodout origin and were deposited as the final fill of the rift subbasins. Four facies assemblages, which reflect a distal-proximal continuum, are identified in this floodplain system: minor thin grey mudstone (very shallow lacustrine); red, massive, pedogenic siltstone (subaerial playa-floodplain-floodout); thick, red, very fine- to medium-grained sandstone (high-sinuosity fluvial channel); and minor, thin, quartz-pebble conglomerate to pebbly sandstone (alluvial fan-braidplain). In comparison to similar deposits of the earlier Horton Group, the Mabou was deposited in less well-defined subbasins that were less dominated by fault-bound subsidence, where carbonate deposition and frequent exposure were more important in the very shallow-lacustrine phase, and fine-grained deposition and pedogenesis were more important in the subaerial phase. Although little direct evidence was found in this study for economic potential, there are some clues suggesting potential for hydrocarbon and redbed copper deposits in the Mabou Group, warranting further investigation.