The Lower Cretaceous Chaswood Formation in southern New Brunswick: provenance and tectonics

Abstract

The most northwesterly outcrop of Lower Cretaceous Chaswood Formation is in a pit at Vinegar Hill, south of Sussex, New Brunswick. New mapping and boreholes show thick, fluvial, loosely lithified conglomerates and lesser sandstones unconformably overlying 12 m of mudstone in a 1 km2 basin bounded to the northwest by the Clover Hill fault. Sparse paleocurrent indicators to the southwest parallel this fault. The tectonic setting is similar to that of the Chaswood Formation in the fault-bounded Elmsvale basin in Nova Scotia. In both cases, a basal unit is paraconformable on underlying upper Mississippian rocks, was folded into a syncline within which a middle unit accumulated and was further deformed, and is capped by thin flat-lying sandstone and conglomerate. The tectonic style of the Chaswood Formation at Vinegar Hill demonstrates that early Cretaceous deformation was widespread in the southern Maritimes. Gravel clasts consist overwhelmingly of vein quartz, but sparse lithic clasts match source rocks in south-central New Brunswick. Heavy minerals are mostly ilmenite (40%–70%) and staurolite (20%–40%), with monazite, zircon, and andalusite more abundant than at other Chaswood Formation localities. Heavy mineral chemistry and monazite geochronology suggest a provenance from Silurian metasedimentary rocks and tourmaline granites in central New Brunswick. Different mineral assemblages from the Chaswood Formation in Nova Scotia suggest that an ancestral St. John River drained western New Brunswick and supplied sediment to the Shelburne delta of the Scotian basin.