SEISMITES, STORMS AND SEDIMENTS: SEDIMENTARY DEFORMATION AND MISS OF THE LOWER BELT CARBONATES (WATERTON AND ALTYN FORMATIONS, BELT SUPERGROUP, CA. 1.47 GA, WESTERN NORTH AMERICA)

Carbonate rocks of the lower Belt Supergroup comprise the Waterton and Altyn formations which crop out along the eastern Rocky Mountains of northwestern Montana and southwestern Alberta. They represent the development on the present-day northeastern side of the Belt Basin of a ramp-style carbonate platform that displays lateral environmental changes over relatively short distances. Now dolomitized, the succession records a shallowing-upward sequence, with somewhat deeper water, argillaceous and silty equivalents belonging to the Haig Brook and Tombstone formations exposed to the northwest. These rocks exhibit striking primary sedimentary structures as well as those from syndepositional deformation. The former include such features as plane- and cross-laminated lime mudstone, hummocky cross-stratified sandy grainstone, sigmoidal bedding in oolite, and columnar stromatolite patch reefs. The latter include convolute bedding, ball-and-pillow structures and small-scale folding of laminae from soft-sediment deformation. Brittle deformation is recorded by intraclasts from brecciated microbial laminite, cracked and brecciated stromatolites, and up to 1 m long redeposited rafts of once-continuous thin grainstone beds. The particular nature of deformation depended on the specific rheology of the various facies, including when deformation took place relative to early cementation. Curiously, only one bed with molar-tooth structure was observed in the Waterton Formation, which suggests that the carbonate sediments were fundamentally different from those of the younger Helena Formation in which this feature abounds. Integrating all these sedimentary attributes shows that this part of the Belt Basin was the site of strong storm events, episodic and sometimes powerful seismic activity, and likely tsunami impact. The overlying Appekunny Formation records a transgression and abrupt deepening with a shutdown of the carbonate factory. The same sedimentary and deformation phenomena undoubtedly continued to occur but will have been recorded differently in these sediments.