SEDIMENTARY DYNAMICS OF THE GRINNELL FORMATION (BELT SUPERGROUP, CA 1.45 GA), WESTERN NORTH AMERICA: NOT ARID PLAYAS CROSSED BY CREEKS BUT SUBAQUEOUS MUDS ROCKED BY EARTHQUAKES AND SWEPT BY TSUNAMIS

The Grinnell Formation (ca. 1.45 Ga) is a thick, strikingly red- to maroon-colored mudstone unit in the lower part of the Belt Supergroup of the northeastern Belt Basin. Dominantly plane laminated, these strata also exhibit micrograding, short-wavelength symmetrical ripples, layers with thin tabular intraclasts, and bedding surfaces with halite crystal molds. The unit is famous for its abundant mudcracks—widely regarded as exemplary desiccation features. In detail, however, the vertical to inclined fissures are filled with mud and small claystone fragments that were injected upward—not from downward gravitational settling. Sporadically intercalated are single to amalgamated, thin to medium beds of cross-laminated, medium- to coarse-grained sandstone, appearing as white quartzite which typically contains subangular to rounded intraclasts of red claystone, often with sand grains adhering to them. Typically, either or both the bottoms and tops of quartzite beds show small sandstone dikes indicative of downward and upward injection respectively. Rather than being passive desiccation features, both the mudcracks and the sandstone dikes are seismites, the result of a combination of mud shrinkage and sediment fluidization due to shaking. Consequently, there is no evidence that the Grinnell Formation was deposited on subaerial mud flats, tidal flats, or playas as has been universally assumed. Rather, these muds were deposited in relatively deep water at and below the limit of ambient storm wave base. The halite was not from in situ evaporation in salinas but from precipitation out of bottom-hugging brines. The quartzite beds are not fluvial. They consist of allochthonous sediment and record both unidirectional and oscillatory currents. The strong bimodality of grain size in the Grinnell Formation and irregular stratigraphic distribution of the quartzites argue against episodic deltaic influx. Rather, these are interpreted as well-sorted, well-rounded coastal sands delivered from the east by the backwash from episodic large tsunamis generated by normal faulting in the basin center to the west, and reworked by bottom currents and wave action, likely from the passage of subsequent tsunamis. Generations of geologists have misunderstood these rocks and overlooked their depositional secrets.