SALT TECTONICS AND STRUCTURAL GEOLOGY IN THE MCCULLY GAS FIELD AREA, SOUTHEASTERN NEW BRUNSWICK

The McCully gas field area is located in the Late Devonian-Carboniferous Moncton Subbasin, immediately to the north and east of the town of Sussex. The area is of significant economic importance because of the presence of potash deposits in the Viséan Cassidy Lake Formation (Windsor Group) and the presence of natural gas associated with source rocks in the Tournaisian Albert Formation (Horton Group). A structural study of this area, incorporating field, seismic and borehole data, and supported by the New Brunswick Geological Surveys Branch and Potash Corporation of Saskatchewan, is currently being undertaken by the author.

Field observations and seismic evidence reveal that rocks of the Albert Formation have undergone significant fold/thrust deformation, while in overlying rocks the structural geology is dominated by brittle faulting and salt tectonics. The Berry Mills fault is associated with a fault-bounded wedge structure which appears to have been facilitated by the presence of potential décollement horizons within the Windsor Group evaporites. Salt has flowed towards the basin centre, leaving salt welds near the basin margins and a pair of elongate salt walls in the central part of the basin. Seismic and borehole evidence shows that the Cassidy Lake Formation (rock salt and potash) is separated from the underlying Upperton Formation (anhydrite) by a low-angle décollement surface marked by the presence of mylonitic salt, and cataclastic limestone and anhydrite. Within the salt body itself flow has been complex, resulting in structural repeats of the salt stratigraphy through recumbent to inclined isoclinal folds and sheath folds.

Field observation coupled with seismic evidence shows that salt-linked thrust faults cut Cumberland Group rocks, indicating that salt movement occurred in post-Westphalian B time; an upper limit for the age of salt movement has not been established. Salt movement has therefore not significantly affected patterns of sediment deposition within the basin, but has modified the structure of the basin in important ways.