ABENAKI CARBONATE PLATFORM IN RELATION TO THE JURASSIC-CRETACEOUS SABLE ISLAND DELTA, OFFSHORE NOVA SCOTIA, CANADA

Abstract

The offshore Nova Scotia Late Jurassic to earliest Cretaceous continental scale shelf-margin Sable Delta succession is juxtaposed with the extensive kilometre-thick Abenaki carbonate platform. This thesis addresses how that unusual association could have occurred and lasted 15 Ma and the effects it had on the margin carbonates. Several carbonate morphologies and two separate systems will be discussed – ramps near the delta and a platform to the southwest. Highs produced by allochthonous salt beneath the Sable Delta localized oolite shoals and possibly atolls. Carbonate production on abandoned, flooded lobes and on prograding ramps generated carbonate oolite-quartz sandstone proximally and slope thrombolites distally. Sediment loading, bypass onto the deeper slope and faulting on the delta front, and flexuring and faulting on the near-delta platform margin, may have helped generate a proposed bathymetric separation between the delta and the near-delta platform margin. This long-continued bathymetric separation maintained a carbonate platform relatively clean of siliciclastic influx. However, prodelta sedimentation did affect two of the three major Late Jurassic reef/mound types – siliceous sponge and thrombolitic-microbial slope mud mounds – but not coral reefs nor the oolitic shoals of the shallow platform. Lateral changes in the coral-stromatoporoid shallow reefs and oolite shoals are identifiable only relative to their quartz content. Carbonate transitions influenced by deltaic sedimentation are revealed in two cores and give major insights – within the Sable Delta by a thin ramp limestone series and on the carbonate platform margin by deepening reeflets on a pinnacle slope. The deeper foreslope mounds reveal a gradient with increasing shale interbeds and distinct colour changes distal to the delta. The uppermost Abenaki and sections above show the diachronous development of sponge-rich beds and reef mounds at the toe of the expanding Sable prodelta succession. Condensed and palimpsest shelf sediment packages are identified further southwest by coated-ironstone redbeds in a marine setting, and more distantly by continued shallow carbonate sedimentation on the margin and nearshore ridges into the Early Cretaceous. Descriptions and interpretations from cuttings and sparse core from all available margin wells are presented using an updated Abenaki carbonate facies association template supported by seismic and biostratigraphic data. Major facies such as oolite, oncolite and all three mid-Mesozoic reef/mound types are illustrated using cored intervals. The currently producing Deep Panuke reefal and Venture shelf deltaic gas fields are both shelf margin accumulations of contemporaneous age. This thesis highlights the effects produced by the Sable Delta on the regional petroleum systems relative to the margin carbonate, and in particular to the reservoir/seal pairs and gas-prone source.