The Lewis thrust, which is >225 km long and has a maximum displacement of >80 km, is a major Foreland belt structural element in the southeastern Canadian Cordillera. We use low-temperature thermochronometry in the preserved Lewis thrust sheet stratigraphic succession to constrain variations in both paleogeothermal gradient and Lewis thrust sheet thickness immediately prior to motion on the Lewis thrust fault. Fission-track and vitrinite reflectance data combined with stratigraphic data suggest that maximum Phanerozoic burial and heating occurred in the Lewis thrust sheet during a short interval (<15 m.y.) in late Campanian time immediately prior to thrusting (ca. 75 Ma). The data suggest that the late predeformational Lewis thrust sheet paleogeothermal gradient was between ∼18 and 22.5 °C/km, which is higher than that inferred for subsequent syn- and postdeformational intervals by other studies. The inferred paleotemperatures and geothermal gradients indicate that the preserved Lewis thrust sheet stratigraphic succession was overlain by ∼4–5.5 km of additional Late Cretaceous strata that were subsequently removed by erosional denudation. We estimate that the Lewis thrust sheet was ∼12–13.5 km thick when thrusting commenced. Deposition of the Late Cretaceous succession was terminated by the onset of displacement on the Lewis thrust (ca. 75 ± 5 Ma) and was followed by intervals of erosional denudation that are constrained stratigraphically by both early Oligocene and current erosion surfaces on the Lewis thrust sheet.

Abstract As a foreword to a revision of the paper on this subject prepared early in 1941 a brief review of developments since that time is in order. During 1941–1945 in Alberta chief attention continued, as previously, to center on the southern plains, the east-central plains, and the foothills. One major company was engaged in an intensive exploration program in south-central Saskatchewan and there was some activity in northeastern British Columbia. Inauguration of the Canol project in 1942 brought immediate development of the far north Norman Wells pool, discovered in 1920, and extensive exploration of the Northwest Territories in general. Steady progress was made in application of geophysical methods in Western Canada, especially the seismograph, in both the plains and foothills, and considerable shallow-structure test drilling was done. Surface geological mapping and subsurface studies were widely extended. On the southern plains minor oil pools were discovered at Taber and West Taber in basal Blairmore (Lower Cretaceous) sands and at Conrad in the Ellis (Jurassic) sand. Discoveries of oil in the Princess-Steveville area in basal Blairmore sand and Rundle (Mississippian) limestone were somewhat disappointing, although considerable gas reserves have been indicated. However, the discovery of a small pool of light crude in the Jefferson dolomite of Upper Devonian age at Princess in 1944 was highly significant. The field itself was small, but its effect on geological thinking and as a spur to deeper wildcat drilling to prospect the Devonian formations was most important. Furthermore, it had been demonstrated by this time that