The Late Cretaceous and Cenozoic Geological History of the Outer Continental Margin off Nova Scotia, Canada: Insights into Margin Evolution from a Mature Passive Margin

Abstract

The continental margin off Nova Scotia (the Scotian margin) forms the northern edge of the North American Basin. The Cenozoic stratigraphy and geological history of the outer margin is not well known. This study examines aspects of the Upper Cretaceous-Cenozoic geological history of the outer Scotian margin addressing the following objectives: 1) determine the geological history of a large deep-water depocenter, 2) investigate processes that led to deep-water unconformity formation in the study area, 3) determine the role of deep-ocean circulation in margin evolution, 4) examine the effects of morphological heritage on subsequent depositional patterns. High quality 2-D and 3-D seismic reflection data along with lithostratigraphic and biostratigraphic data from hydrocarbon exploration wells provide the basis for this investigation.

The seismic stratigraphy of a large deep-water depocenter along the western Scotian margin was broadly divided into four units. Unit 1 (Upper Cretaceous-Upper Eocene) is attributed to repeated, widespread erosion events interspersed with periods of hemipelagic and pelagic, carbonate-rich sedimentation. Unit 2 (Lower Oligocene-Middle Miocene) consists of a variety of seismic facies overprinted by dense, small-offset faults. Unit 3 (Middle Miocene-Upper Pliocene) is dominated by sediment drift deposition. Unit 4 (Upper Pliocene-present) is characterized by channel development and gravity flow deposition. The processes that led to regional seismic stratigraphic horizons were complex. Both large mass-wasting events and along-slope bottom currents contributed to the formation of unconformities in the study area. Most of the succession preserved in the depocenter belongs to seismic units 2 and 3. These deposits are mainly confined to the area seaward of the Abenaki carbonate bank and landward of shallow salt structures below the slope. Locally, however, modification of the slope profile through mass-wasting and bottom current processes greatly influenced subsequent depositional patterns. The Cenozoic geological evolution of the study area was strongly affected by northeast-to-southwest flowing bottom currents. The earliest indication of bottom current activity was in the Eocene. Upper Miocene and Pliocene sediment drifts represent >50% of the preserved stratigraphic section in the thickest part of the depocenter. It is clear that along-slope sedimentary processes were far more important in shaping the margin than previously understood.