Dating Thrust Faults in the Canadian Rockies to Assess Play-Element Timing

Peter J. Vrolijk, Michael C. Covey, David R. Pevear, Allan Lariviere

Trap formation must predate maturation and migration in successful plays. In fold-and-thrust belts thrusting can cause simultaneous trap formation and maturation, but deep burial of a source rock before deformation may cause maturation to precede trap formation. If a fold-and-thrust belt is deeply eroded, there may be little stratigraphic evidence to define the age of thrusting, thus increasing risk that oil or gas yield preceded trap formation. We have developed a method to date clay-rich fault rock, interpret age of fault displacement, and infer the timing of trap formation. We have applied and tested the method in the Front Ranges of the Canadian Rockies between 49° and 51°N where faults range in age from 100-55 Ma.

Fault ages are determined by K/Ar dating of three size fractions of illite and illite-smectite from clay-rich fault rocks; we describe the detailed methodology in our poster. For most samples, the measured K/Ar ages of medium and fine fractions are identical, detrital mica in the medium and coarse fractions show evidence of Ar-loss, and there is textural evidence of deformed neoformed illite. We interpret concordant K/Ar ages in these samples as fault ages.

Fault ages in the Front Ranges are: Sulphur Mtn thrust - 96 Ma; Rundle thrust - 86 Ma; McConnell thrust - 77 and 53 Ma; Lewis thrust - 92 and 65-55 Ma; and Prairie Mtn thrust - 66-53 Ma. The 77 Ma age for the McConnell thrust represents main fault slip; the 53 Ma age may reflect fault reactivation near Mt. Yamnuska. The Lewis thrust is interpreted to be 92 Ma near its northern terminus between the Sulphur Mtn and Rundle thrusts. Further south (Crowsnest Pass) where much of the 100 km displacement is out-of-sequence, we interpret an age of 65-55 Ma. Fault ages increase from hinterland to foreland (west to east) and agree with all known stratigraphic data for fault ages. Accuracy of the method is ±10 m.y., although it may be better in some cases.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995