Karst-related outliers of the Cretaceous Chaswood Formation of Maritime Canada

Abstract

The Lower Cretaceous Chaswood Formation is a terrestrial deposit preserved as scattered outcrops across Maritime Canada. Here we describe newly recognized outliers of the Chaswood Formation near Windsor, Nova Scotia. A Cretaceous age is confirmed only in Bailey Quarry, where sediments are provisionally assigned a Valanginian–Hauterivian (140–130 Ma) age based on palynology, making them among the oldest known deposits of the Chaswood Formation. At three nearby sites, putative Cretaceous sediments are recognized based on similar geological context, facies, and petrography; however, their age cannot be confirmed because sediments either lack palynomorphs or contain equivocal assemblages. Although the Chaswood Formation has been previously documented mainly in small tectonically generated basins, these new-found deposits are fluvial sands and gravels, and lacustrine or floodplain clays associated with a karstified gypsum surface developed on the Carboniferous Windsor Group. Deposits are preserved in karst valleys, sinkholes, and fissures, locally up to 36 m below the paleosurface. Although occupying a karstic setting, sediments were evidently deposited in a through-flowing drainage system because they are quartz-rich and show petrographic similarity to basinal deposits elsewhere. Abundant plant material, including lignite, charcoal, cuticles, and palynomorphs, implies that the surrounding landscape was covered by fire-prone forests of conifers, ginkgos, bennettites, cycads, ferns, and lycopods — typical pre-angiosperm Mesozoic vegetation. Analysis of growth patterns in fossil woods, combined with lithological indicators, suggest a humid, tropical climate, punctuated by aperiodic droughts that may have been accentuated under a karstic hydrological regime.