HOW REGIONAL BEDROCK MAPPING REMOVED “BASEMENT” FROM THE MEGUMA TERRANE OF SOUTHERN NOVA SCOTIA, CANADA

Systematic bedrock mapping at 1:10 000-scale began in 1998 in the southwestern part of the Meguma terrane of southern Nova Scotia with the goal of better understanding its stratigraphy, tectonic setting, and economic potential. New formations and members were mapped throughout the area of 20 000 km², thus justifying upgrading of the former Goldenville and Halifax formations to groups. One of the key marker units in the region is a manganiferous locally coticule-bearing metasiltstone which forms the uppermost 100-300 m of the Goldenville Group. This distinctive formation has been mapped throughout the southwestern part of Meguma terrane, where it has been named preliminarily the Bloomfield (Digby-Yarmouth area), Moshers Island (Shelburne-Mahone Bay), or Beaverbank (Halifax area) formation. This unit has a distinctive high areomagnetic signature and can be recognized and mapped, even through areas of high-grade regional and contact metamorphism.

In 2008, mapping continued in the Trafalgar area on the northern margin of the Meguma terrane where upper amphibolite- to granulite-facies metamorphic rocks and associated plutons had been interpreted previously as a core complex (Liscomb Complex). In that area, units previously identified as pelitic gneiss are contact metamorphosed Goldenville and Halifax groups consisting of spotted hornfels to migmatite containing sillimanite, andalusite, cordierite, ± garnet, and ± staurolite superimposed on chlorite-zone regional metamorphic assemblages. The distinctive Beaverbank Formation can be traced through this area of higher metamorphic grade, and coticules occur as xenoliths in the 380–370 Ma plutonic units. The areas previously identified as mafic and quartzo-feldspathic gneiss are contaminated tonalitic to granitic plutons (Trafalgar plutonic suite) with magma-mingling textures and/or superimposed protomylonitic fabrics. These features have also been recognized elsewhere in southwestern Meguma terrane and are not unique to the Trafalgar area.

This reinterpretation demonstrates that systematic regional bedrock mapping is critical to understanding areas that appear to be anomalous when viewed in isolation.