Earth and Environmental Science

Evaporite tectonics in Nova Scotia: salt, subsidence, sinkholes

Huggins Science Hall 336 - 4.00 p.m.

John W.F. Waldron

Earth & Atmospheric Sciences, University of Alberta.

Harrison McCain Foundation Visiting Professor, Earth & Environmental Science, Acadia University.

The Maritimes Basin of Atlantic Canada is a large and deep sedimentary basin underlying large parts of Atlantic Canada. The basin fill is predominantly late Paleozoic (Devonian-Permian) non-marine clastic sedimentary rocks, but the Viséan Windsor Group, and the correlative Codroy Group of Newfoundland, contain substantial evaporites, including gypsum and anhydrite, halite, and potash. Laterally correlative limestone-evaporite-shale cycles have been traced throughout the middle and upper parts of the Windsor Group.

The role of Windsor evaporites in the tectonics of the Maritimes Basin has long been recognized. In addition to diapiric features generated by primarily vertical tectonics, there are extensive low-angle deformation surfaces characterized by anomalous breaks in the basin-wide stratigraphic succession. These breaks were originally interpreted as thrust faults, but later investigations, noting substantial omission of stratigraphy, led to their re-interpretation as a single low-angle detachment - the Ainslie Detachment. The availability of industry seismic reflection data allows these structures to be again reinterpreted as salt welds, in the light of recent advances in evaporite tectonics on passive margins.

For example, the famous Joggins Pennsylvanian succession was rapidly deposited in accommodation space created by salt expulsion, showing that Windsor Group salt remained in place until the Pennsylvanian before rapidly moving into diapiric salt walls.  In contrast, in the eastern Cumberland sub-basin, evaporite expulsion was already controlling sedimentation during Mississippian deposition of the Windsor and Mabou groups. Field relations in other parts of the Maritimes Basin, suggest that this history of early evaporite expulsion is more usual.

These observations suggest a new interpretation, in which movement of the thick lower Windsor evaporites began within a few million years of their deposition. Feedback between halokinesis and sedimentation occurred from mid-Viséan onward. Multiple minibasins were simultaneously flooded by eustatic sea-level rises, related to glacial cycles on Gondwana, accounting for the laterally continuous limestones. Differences in the overlying stratigraphic successions are best explained, therefore, by deposition above a changing configuration of moving evaporite bodies that culminated in complete expulsion of salt beneath some minibasins.

The tops of evaporite diapirs have probably remained near the surface, producing areas of subsidence and karst development, throughout much of Nova Scotia's subsequent history. The distribution of near-surface evaporites continues to be marked by widespread development of sinkholes at the present day.