Group A: Ancient Continental Slope
GeoSite Group A is the Ancient Continental Shelf of North America. This is present in the east central part of planned Geopark (e.g., Humber River Gorge and Goose Arm of the Bay of Islands) and parts of Gros Morne National Park. The Port-au-Port Peninsula, located southwest of the present Park proposal, contains little-disturbed sedimentary rocks and superb original relationships, but most other areas are structurally complex.
Rock types are Precambrian to middle Ordovician and of sedimentary origin. They include clastic sequences of the early rift zone, overlain by a thick and diverse carbonate section including rich fossil assemblages, which is overlain at the top by deep-water clastic sequences derived from tectonically transported sequences represented by Site Groups B and C. They record the initiation of the Iapetus Ocean and the development of its continental shelf sequence. Variations in rock assemblages (facies) within this sequence record changes in global sea level and climates. The uppermost part of the sequence records the drowning of the continental shelf in deep water as it foundered, due to loading by approaching thrust sheets, including ophiolites.