Marine Geology of Sohm Basin, Canadian Atlantic Margin

Abstract

Multichannel seismic reflection, seismic refraction, and magnetic anomaly data are used to interpret the Mesozoic sea-floor spreading history and subsequent sedimentary evolution of the Sohm basin southeast of Nova Scotia. Observed M-series (Mesozoic) magnetic anomalies show that oceanic crust in the Sohm basin was generated at very slow rates of sea-floor spreading (5-15 mm/yr half rates). The slow spreading created very rough crustal topography, but the crustal thickness and seismic-velocity structure are typical of normal oceanic basement. Basement depths corrected for sediment loading fall below the predicted age/depth curve in the anomaly M25-M4 interval. These depth anomalies and an associated regional negative geoid anomaly suggest that the Sohm basin is underlai by asthenosphere denser and possibly colder than normal. Correlation of seismic reflection profiles in the basin to regional seismic stratigraphy, together with age constraints from reflection pinch-outs on dated oceanic crust, provides a chronology for interpreting the seismic sequences. The Jurassic to Lower Cretaceous sequences are similar in thickness and acoustic character to sequences observed along the United States Atlantic margin, suggesting that the New England Seamount Chain had little effect on sediment distribution patterns within the early western North Atlantic basin. Upper Cretaceous sediments in the Sohm basin are thin, probably because seaward transport of sediment was restricted by eustatic sea level rise and by a sedimentary ridge uplifted because of salt tectonics along the central and upper continental rise. Progradati nal sedimentary wedges and erosional unconformities characterize the Tertiary and Quaternary sedimentary record along the lower continental rise. The prograding sequences generally correlate to sea level lowstands. Repeated episodes of erosion by abyssal currents are marked by seismic unconformities Horizon Au (lower Oligocene), LM (lower Miocene), MM (middle Miocene), and L (upper Pliocene). The deep currents also controlled local patterns of deposition and modified cross-slope sediment transport and accumulation patterns. Thick supra-Horizon L sediments cover the Sohm basin and reflect rapid sediment influx caused by glacio-eustatic fluctuations in sea level during the Quaternary.