Stratigraphy, structure, and argon-40/argon-39 geochronology of the Lochaber-Mulgrave area, Nova Scotia

Detailed mapping in the Lochaber-Mulgrave area of northern mainland Nova Scotia has shown that most rocks belong to the Horton Group. They are in faulted contact with older rocks of the Guysborough Group and Knoydart Formation and younger rocks of the Windsor and Mabou groups. The Horton Group is subdivided into four formations on the basis of lithology and sedimentary structures: Clam Harbour River, Tracadie Road, Caledonia Mills, and Steep Creek formations. These formations have a total thickness of at least 4000 m, and were deposited in varied braided fluvial and shallow to deep lacustrine environments. Sparse paleontological data from macrofossils and spores indicates a Famennian to late Tournaisian age for these rocks. Compared to the Horton Group in other areas of Nova Scotia, the rocks in the Lochaber-Mulgrave area appear to be somewhat older, exhibit higher thermal maturity indicating deeper burial, and are more deformed and metamorphosed, especially in the southern part of the area near the Roman Valley Fault. Whole-rock 40Ar/ 39Ar dating as part of this study indicates that the rocks had undergone low-grade regional metamorphism, including new muscovite growth in cleavage by ca. 350–340 Ma. These data require the rocks to have undergone rapid burial, deformation, and cooling through the argon retention temperature in muscovite by ca. 350–340 Ma. A possible explanation is overthrusting of the Horton Group from the south by older rocks of the Guysborough block as a result of transpression at a restraining bend along the Chedabucto-Roman Valley fault system. Uplift and subsequent deformation that also involved younger units was probably the result of on-going movement on the fault system.