A Mesoproterozoic carbonate platform (lower Belt Supergroup of western North America): Sediments, facies, tides, tsunamis and earthquakes in a tectonically active intracratonic basin

Abstract

Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America include the Haig Brook, Tombstone Mountain, Waterton and Altyn formations which crop out in the eastern Rocky Mountains of northwestern Montana, southwestern Alberta and southeastern British Columbia. They record the development on the present-day northeastern side of the Mesoproterozoic Belt Basin of a carbonate platform early in the basin history while it was still relatively deep. The Waterton–Altyn succession documents a shallowing-upward, westward-prograding, broadly ramp-style platform composed of mixed carbonate–siliciclastic sediments. Five main facies types are recognized: Laminite, Ribbon, Grainstone, Oolite and Stromatolite. The first two were formed of lime muds deposited on the ramp and outer platform under low-energy conditions. The Grainstone facies consists of sand-sized peloids, aggregates and intraclasts plus admixed quartz and feldspar sand, microspar grains, radial ooids and silicified oolite and anhydrite. The Oolite facies is dominated by ooids with a concentric cortex. These are allochthonous coarse-grained particles interpreted to have been transported westward to a marginal belt and outer platform mainly by tsunami off-surge from the platform interior, coastal sand shoals and tidal-flat sabkhas, outcrops of which are not preserved. Absence of hummocky or swaley cross-stratification suggests that the platform was not affected by strong storms. Instead, flat-pebble conglomerates on the ramp are ascribed to episodic, tsunami-induced wave action which caused localized rupture and imbrication of flat pebbles. Scouring by tsunami off-surge produced intraclasts in the outer platform. There, this sediment lay undisturbed, but in the shallower marginal belt it was reworked by strong tidal currents which generated variably directed cross-lamination from dune and ripple migration and, locally, large sand bars with northwest-dipping clinoforms. Deformation features caused by synsedimentary earthquakes are common, with the various seismite types reflecting the facies-specific rheology of the sediment. Seismites in the lower Altyn Formation appear not to be linked to individual tsunami-lain dolograinstone beds in the outer platform, suggesting that these two sets of features were not generated by the same faults. The carbonate factory shut down when the platform was suffocated by siliciclastic mud sourced from the west, and tidal activity diminished as the whole basin shallowed. The interpretation of the carbonate platform presented herein is radically different from previous views that considered the Waterton and Altyn formations to be predominantly of shallow-subtidal and tidal-flat origin. These rocks are relevant to the appreciation of other carbonate platforms, especially in that tsunamis may be an under-appreciated agent of erosion and sediment transport offshore.