The southeastern Cape Breton Highlands consist of pre-Carboniferous plutonic, volcanic and metamorphosed basement rocks of the Bras d’Or Terrane, with Carboniferous sedimentary material in the adjacent valleys and coastal regions. The older basement rocks outcrop at higher topographic elevations than the younger Carboniferous sedimentary units. The structural relations between the Carboniferous sedimentary units and adjacent pre-Carboniferous basement rocks have been resolved using geological mapping with an emphasis on lithological contact configurations, structural data constructions such as cross-sections, three-point problems and stereoplots, thin section petrography to decipher a potential provenance source of many of the sedimentary units and the degree of alteration of parts of the basement material, and a comparison of these data with geophysical maps and satellite imagery.

Mapping of the area around the basement-cover contact has shown faulting, shearing and unconformities. Structural analyses confirmed the field mapping in the Goose Cove – St. Ann’s area, showing that the dip of the contact plane between the basement and sedimentary units is shallow, with an average dip angle of 14.8 degrees. Thin section petrography has revealed a wide range of basement conditions from completely unaltered to mylonitic. Interpretation of gravity data in the southeastern Highlands suggests that the basement has no root, particularly in the Goose Cove – St. Ann’s field area.

These observations have several implications for the development of the southeastern Highlands, the most significant being that the basement material appears to have been transported on a thrust plane over the younger sedimentary units in an eastward direction in this area of Cape Breton Island. Potentially a large part of the Cape Breton Highlands may be allochthonous.