Abstract

The Upper Cambrian Deadwood Formation of north-central Montana is composed mainly of fine-grained calcareous sandstone and intercalated silty shale, and contains scattered intraclastic flat- pebble conglomerate. These rocks, typical of many mixed carbonate- siliciclastic, subtidal shelf successions of Proterozoic through Early Ordovician age, are distal storm deposits in which the conglomerates are conventionally thought to record occasional, very high energy events that scoured incipiently cemented layers. In the Deadwood Formation, most sandstone beds have a linear to crudely reticulate pattern of vertical cracks filled with injected silt and clay. The cracks exhibit a consistent northwest-southeast trend, and are very shallow burial deformation structures caused by the passage of seismic waves possibly originating from strong earthquakes in Idaho. Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. “Tsunamites” are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.

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