SYNDEPOSITIONAL FAULTING IN THE BELT BASIN: JUST ONE DAMNED TSUNAMI AFTER ANOTHER

The Belt Basin was tectonically very active, as shown by its high subsidence rate, and dramatic thickness changes across normal faults, as was demonstrated by Don Winston. The strata are also riddled with synsedimentary earthquake-induced deformation structures specific to the original sediment type, including arrays of small dikes (‘syneresis cracks’), molar-tooth structure, microfolds, clay vein arrays, ball-and-pillow structure, wrinkled surfaces, microfaults, and breccias.

The Belt Supergroup on the eastern side of the basin is dominated by laminated to thin-bedded strata, both siliciclastic and carbonate, which record low-energy conditions under an arid tropical climate. Abruptly intercalated coarse-grained beds and scoured surfaces, however, indicate episodic high-energy events and delivery of allochthonous sediment from landward and coastal areas.

The Waterton Formation contains flat-pebble conglomerate that is typically imbricated by wave action. The Altyn Formation contains sandy grainstone with micritic intraclasts and ooids, as well as silicified anhydrite eroded from sabkhas. Mudstone of the Appekunny Formation is interrupted by massive beds of coarse-grained sandstone, whereas the Grinnell Formation contains thinner beds of sandstone with mud chips. In the Helena Formation, common are sandy oolite, small gutter casts filled with fragments of molar-tooth structure mixed with ooids, and deeply scoured surfaces containing flat-pebble conglomerate and encrusted by stromatolites.

The abrupt and irregular stratigraphic occurrence of these anomalous features shows that they are not related to the ambient climate, that is, generated by storms. Given the evidence for syndepositional tectonic activity, however, the alternative explanation is that erosion and transport were caused by tsunami backwash. It is likely that these features varied due to amount, length, and orientation of fault displacement, bathymetry, seafloor sediment type, composition of landward sediments, distance to shore, and coastal topography, plus possible development of seiches. There is much uncertainty about the precise effects of tsunamis in shallow epicontinental seas, owing to the lack of modern analogs, but they were undoubtedly more common than is currently appreciated.