MARITIMES BASIN EVOLUTION - KEY GEOLOGIC AND SEISMIC EVIDENCE FROM THE MONCTON SUBBASIN OF NEW-BRUNSWICK

The Maritimes Basin is a composite large (148,000 km2), post-Acadian i nternal successor basin. It comprises several early northeast- to east -trending, relatively deep, isolated subbasins which are covered by ma inly regionally derived and widely distributed fluviatile sequences. T he early basin fill in the Moncton Subbasin of southeastern New Brunsw ick is seen from stratigraphic and seismic reflection evidence to comp rise two depositional sequences (allocycles) which are separated by a basin-wide unconformity. The basal cycle, the Horton Group, is a 3 to 5 km thick coarse-fine-coarse (alluvial-lacustrine-alluvial) cycle. Th e medial lacustrine interval implies a period of rapid subsidence. The unconformably overlying allocycle, recorded by the Windsor and Hopewe ll groups, is a coarse-fine-coarse (alluvial-marine-alluvial) cycle. T he marine part of the Windsor Group indicates a medial period of tecto nic subsidence or eustatic sea level rise. The depositional history of the Moncton Subbasin sensu stricto ended following Hopewell time when the subbasin was inverted by late Namurian deformation. The Hopewell and older basin fill is unconformably overlain by quartzose fluviatile sandstones with associated inter-channel mudstones and paludal deposi ts of the regionally distributed late Namurian/Westphalian Cumberland Group. Cumberland rocks are succeeded by meandering(?) fluviatile stra ta of the late Westphalian/Permian Pictou Group. An angular discordanc e between Cumberland and Pictou strata implies a period of uplift or r egional tilting. The tectonism that initiated and terminated the early allocycles and which is recorded by unconformities following Horton a nd Hopewell deposition is seen from structural data and seismic reflec tion profiles to have resulted from dextral transpression. Evidence in cludes a network of basin-parallel northeast-trending anastomosing fau lts (many with shallowly pitching slickensides), associated en echelon folds, and geologically and seismically indentified positive flower s tructures. An interpretation of early Maritimes Basin evolution in a w rench setting is consistent with the many northeast-trending steeply d ipping terrane boundary faults in the underlying basement and with mos t recently published evidence of Variscan (Alleghenian) orogenesis as resulting fro dextral oblique collision of Laurentia and Gondwana.