ABSTRACT

Seismic data and well control released to the end of 1976 permit preliminary modelling of the 900 to 1200 m (3000 to 4000 ft) thick Abenaki Formation of Middle Jurassic to earliest Cretaceous age. The lowest member (Scatarie), which overlies the deepening-upward Mohican Formation clastic rocks, shows a cyclicity of oolitic, then oncolitic to muddy limestones, and culminates in the neritic middle shale member (Misaine). These regionally developed cycles are interpreted as transgressive pulses resulting from initial rise of the North Atlantic mid-oceanic ridge system. The thick upper limestone member (Baccaro) was deposited along the paleocontinental-shelf-edge and adjacent to the early Sable Island delta. A neritic 'moat' zone separates the nearshore sandy basement ridge zone from the shallow-water Baccaro platform. Oolitic-bar, mud-shoal, shelf-edge stromatoporoid-coral-algal reef and possible channel depositional environments were recognized. Both oolite beds and lagoonal loferite cyclothems occur in correlative, mainly terrigenous sediments of the Orpheus basin. Interpreted eustatic sea-level changes affected both deposition and diagenesis. During lowered sea levels paleo-highs, whether reefal, salt- or basement-cored, may have been subjected to freshwater diagenesis in a humid, subtropical climate. The only significant dolomitization appears to have occurred in freshwater-marine phreatic mixing zones beneath subaerially exposed reefs. Porosity is associated with the dolomite when present and with limestones altered in the overlying freshwater and vadose paleo-environments. Most of the Abenaki has little porosity and shows mainly submarine and deep subsurface diagenesis. Stromatactis with multiple fibrous calcite druse occurs in pelleted mudstones interbedded with minor, algal?-coated, skeletal grainstones beneath the shelf-edge reefal beds and may represent thin mud mounds with flanking platform sands. The shallow-water Baccaro carbonates were finally buried by coarser MicMac or Missisauga Formation siliciclastics near the Sable Island delta or drowned by Verrill Canyon Formation shale and locally by a deeper-water, slightly phosphatized, sponge reefal limestone and shale, the uppermost member (Artimon).