Tectono‐sedimentary analysis of alternate‐polarity half‐graben basin‐fill successions: Late Devonian‐Early Carboniferous Horton Group, Cape Breton Island, Nova Scotia

Abstract

Abstract The Devono‐Carboniferous Horton Group of Cape Breton Island was mostly deposited in two fault‐bounded asymmetric sub‐basins which were part of a large intracontinental rift system. This system lay at a palaeolatitude of about 10–15o S–a warm, semi‐arid climate. The half‐graben sub‐basins had opposed polarity, were approximately 100 times 50 km in size and were separated by a narrow zone of elevated Acadian basement. These features are common to the adjacent structural segments of known rifts, and are unlike those of transtensive pull‐apart systems. Sedimentation occurred in four successive depositional systems which reflect a tectonic evolution of increased and then decreased extensional subsidence through the 8–12 Myr interval represented. Post‐Acadian sedimentation began with System 1 bimodal volcanics and grey distal braided fluvial sediments deposited in a slowly subsiding broad linear sag basin. System 2 consists of reddened braidplain sediments near fault‐bounded margins and mudflat/playa sediments in sub‐basin centres, deposited in two discrete asymmetric sub‐basins with a general upward‐fining trend. Gradual expansion of the mudflat setting and confinement of coarse marginal fades is interpreted as a response to increasingly rapid and deep fault‐bounded subsidence. Depositional System 3, is a complex of grey lacustrine offshore, shoreline and fan delta facies deposited in two adjacent half‐graben segments with opposed polarity of asymmetry. An increased rate of tectonic subsidence allowed a large standing body of water to accumulate lacustrine sediments along the axis of each sub‐basin during this phase of maximum subsidence. System 4 consists of reddened proximal alluvial fan, medial fluvial and distal grey meandering fluvial/floodplain sediments which accumulated in sub‐basins with fault‐bounded margins and asymmetry identical to those of earlier systems, indicating a continuation of tectonic style. However, an overall coarsening‐upward trend indicates waning of active fault‐related subsidence and consequent progradation of marginal coarse wedges to fill the sub‐basins. Rapid marine transgression and deposition of Windsor Group carbonates, evaporites and elastics continued within a more extensive rift basin during renewed fault‐bounded subsidence.