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

Abstract

<p>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.</p>