Stratigraphy and sedimentology of the Lower Triassic Montney Formation, Peace River Arch area (Alberta)

The Lower Triassic Montney Formation in the Peace River Arch area forms a sedimentary wedge up to 350 m thick, deposited on the craton margin. This study, based on 127 cores and about 1300 well logs, shows that the Montney Formation is characterized by overall southwesterly-dipping clinoform deposits with a maximum dip of about 0.3°. Based on prominent well log markers, the Montney Formation has been divided into seven depositional units lettered A through G.; Deposition of the Montney Formation was strongly influenced by tectonic activity, and two major basin reorganizations occurred: one between Unit A and Unit B and the other between Unit F and Unit G. The overall sediment source for the Montney Formation was to the northeast, but at least for Unit B and Unit C, the main source was to the southeast.; In the deeper part of the basin (southwest), the Montney Formation commonly contains turbidite deposits. Two styles of turbidite depositional systems developed: an unchannelized sheet-like system and a channel-related system. Unchannelized sheet-like turbidite systems occur in several stratigraphic units and constitutes the bulk of turbidite sedimentation, whereas the channel-related turbidite system developed only in Unit C. Here, an incised channel cuts down as much as 40 m into underlying sediment. It is relatively straight, trending southeast to northwest for a distance of 70 km. At the distal end of the channel, it turns sharply to the southwest, following a downthrown fault block. Upslope, the width of the channel is about 1 km, but farther downslope (northwestward) it widens to 3–5 km. The turbidites are mostly thick and structureless fine-grained sandstones. Upslope the sandstones are completely confined within the incised channel, but farther downslope they spread out of the incised channel for a distance of about 10 km. Synsedimentary faulting controlled the thickness of accumulated sediment and sediment facies. Thick accumulations of sand occur in downthrown fault blocks whereas thinner and/or muddier sediments were deposited in uplifted blocks.; The turbidite systems of the Montney Formation were developed in relatively shallow environments, not deeper than a few hundred meters. The development of the Montney turbidite systems may not have been related to global sea level fluctuations, but local tectonic activity.