Oceanography, bathymetry and syndepositional tectonics of a Precambrian intracratonic basin: integrating sediments, storms, earthquakes and tsunamis in the Belt Supergroup (Helena Formation, ca.1.45 Ga), western North America

The carbonate-dominated Helena Formation of the Mesoproterozoic Belt Superg roup of western North America provides an instructive example of how a rang e of regional depositional and environmental characteristics of an ancient sea can be deduced on the basis of micron- to metre-scale features. Particu larly revealing is the window opened by the presence of abundant molar-toot h structure onto the paleoceanography, paleobathymetry, paleoclimate and te ctonic regime of this intracratonic Precambrian basin. The facies hosting molar-tooth structure is composed dominantly of lime mud with substantial subangular quartz and feldspar silt and clay derived from the western and southwestern side of the basin. These are low-energy tempe stites deposited on a remarkably flat sea bottom at the limit of storm-wave base, at about 50 m. Sporadic domical, stromatolite patch reefs confirm th at the sea bottom was normally within the photic zone. The ubiquity of molar-tooth structure suggests frequent, near-field seismic activity during subsidence, which generated ground motion sufficient to li quefy granular lime mud and terrigenous silt. Sporadic tsunamis from major submarine faults far to the west pounded the shallow-water platform to the east. Tsunami off-surge swept ooids and rounded, coarse-grained, feldspathi c quartz sand westward into deeper water, and created strongly erosive curr ents that left gutter casts composed of lags of preferentially cemented mol ar-tooth structure in otherwise relatively low-energy facies. Mineralogical and geochemical evidence, confirms that the Belt basin was ma rine. Organic matter was essentially fully oxidized in the water column. Or iginal high-Mg composition and cementation of lime mud in molar-tooth struc ture indicate that calcite precipitated above the thermocline in supersatur ated seawater under tropical conditions. Scattered bimineralic ooids in all ochthonous grainstones indicate that shoals on the platform to the east wer e intermittently above a shallow aragonite compensation depth in warm water . Rare vestiges of halite and gypsum demonstrate occasional, temporary sali nity stratification for periods of up to a few years. Unaltered feldspar an d mica grains point to a generally arid climate, but the large volume of cl ay is suggestive of protracted weathering and transport. (C) 2001 Elsevier Science B.V. All rights reserved.