Grand Manan Island - Geology

Geology

Grand Manan has a “split personality” regarding its geology. The western ⅔ of the island shows thick lava flows (basalt) of Late Triassic age, which appear little changed from when they cooled around 201 million years (m.y.) ago. They are part of a massive “flood basalt” that underlies most of the Bay of Fundy, although the Grand Manan Basalt was later separated into its own rift basin by block faulting . The Fundy basalts are themselves only a small portion of the enormous Central Atlantic Magmatic Province or CAMP, which was formed in a volcanic event preceding the breakup of the supercontinent Pangaea in Early Jurassic time. These lava flows also crop out along the western shores of Nova Scotia, where they are known as the North Mountain Basalt. Both there and on our island, many interesting minerals have filled the cracks and bubbles left by gases boiling out of the cooling lavas. They include zeolite minerals such as chabazite, mesolite, stilbite, and heulandite, plus attractive quartz-related amethyst, agate, jasper, and many others. Good collecting areas include Seven Days Work, Indian Beach, and Bradford Cove. A few meters of siltstone are exposed under the basalt along the western shoreline, which by analogy with the Blomidon Formation in Nova Scotia must include the Triassic-Jurassic boundary .

Inhabitants of the island live mainly around harbors along the eastern shoreline, which were created by the erosion of complex fault and fold structures in ancient metamorphosed sedimentary and volcanic rock formations. A major north-south fault is exposed at Red Point, and it divides these older eastern rocks from the much younger basalts that form the western 2/3 of the island. The metamorphic formations are organized into groups called Castalia, Ingalls Head, and Grand Manan, and there are also metamorphosed plutonic masses such as Stanley Brook Granite, Rockweed Pond Gabbro, and Kent Island Granite . These rocks have recently been dated between 539 and 618 million years old and are now considered to correlate with the New River and Mascarene terranes of southern New Brunswick, Canada. Although originally they were igneous and sedimentary rocks such as basalt, sandstone, and shale, the eastern formations have been metamorphosed into greenstone, phyllite, argillite, schist, quartzite, and other foliated types. In addition, many folds and faults have bent and broken the formations in rather tortured-looking outcrops. One such fault can be seen at the north end of Pettes Cove, where it separates metabasalt of Swallowtail Head from schist of North Head.