Deformation of Upper Carboniferous coal measures in the Sydney Basin: evidence for late Alleghanian tectonism in Atlantic Canada

Abstract

Upper Carboniferous (Stephanian) coal measures of the Sydney Basin are deformed by reverse faults and folds with dips of up to 50° on the limbs. These structures represent compressional or transpressional events (Donkin Episode) of unknown sequence and timing. Offshore swath-bathymetry data indicate that northwest-striking reverse faults are folded by prominent northeast-trending folds. Vitrinite reflectance patterns of oriented coal samples from the fold limbs are uniaxial negative, with minimum reflectance axes (Rmin) near-normal to bedding and fanning across fold axes. This pattern implies that the bulk of coalification took place through burial compaction without directed horizontal stress, prior to the deformation that produced the folds. Slight divergence between Rminaxes and poles to bedding may reflect slight tilting, associated with differential subsidence or early reverse faulting, before or during coalification, or may lie within analytical error. One moderately biaxial sample suggests that some deformation accompanied coalification locally. Available apatite fission-track data for the Sydney Basin indicate that cooling of the coal measures to below 100 ± 20°C was underway by the Middle to Late Triassic or earlier. Because cooling implies that coalification was largely complete, the fission-track results are compatible with latest Paleozoic or early Mesozoic deformation. We infer that the Donkin Episode represents late Alleghanian (Permian?) compression within the Appalachian Orogen. If so, the Sydney structures are manifestations of a little-known tectonic episode in Atlantic Canada. However, Mesozoic–Cenozoic deformation cannot be ruled out.