ABSTRACT

Lithofacies analysis of the Middle Precambrian Omarolluk and Loaf Formations demonstrates that these two units represent prograding submarine-fan and fluvial-marginal marine environments respectively. Within the Omarolluk Formation a number of facies have been identified and include: i) a black shale facies, representing a starved-basin environment; ii) a turbidite facies, which composes 65-75 per cent of the Omarolluk Formation; iii) a composite bedded facies; iv) a thin-bedded siltstone-shale facies representing distal turbidite or interchannel deposits; v) a channel-fill conglomerate facies, and vi) a transitional facies. A comparison with models developed for other ancient turbidite sequences suggests that the Omarolluk represents a mid- to outer-fan setting. Sediment dispersal trends and lateral facies changes that occur in the Omarolluk (i.e., changes in bed thickness and sandstone/shale ratios) also show that the submarine fan prograded toward the southeast.

The Loaf Formation consists predominantly of trough and planar cross-bedded arkoses and minor red shales that possess desiccation cracks, and is tentatively interpreted as a braided-river succession. Three shoaling-upward cycles also have been identified, each cycle representing the transition from an environment influenced by weak tidal currents to a fluvial environment. Sediment dispersal trends in the Loaf parallel those observed in the Omarolluk Formation; namely, toward the southeast.

The Omarolluk and Loaf Formations are considered to have been laterally associated, and comparisons are made with modern examples of fan-delta systems such as the Bengal Fan and Nile Cone.

Petrographic analysis indicates that two major source terrains supplied detritus to the Omarolluk and Loaf Formations; a cratonic source north of the Belcher Islands supplied sediment to the turbidite basin via the Loaf fluvial system; basaltic debris, on the other hand, was derived from a volcanic ridge (Flaherty Formation) that was situated along the western margin of the turbidite basin. Dispersal of the volcanic sediments occurred only within the turbidite basin, although they were mixed with the cratonic component before resedimentation by turbidity currents.