HALITE CUBES IN MUDSTONES OF SHALLOW-MARINE EPEIRIC SEAS: NOT PLAYA EVAPORITES BUT PRECIPITATED FROM BOTTOM-HUGGING HYPERSALINE BRINES IN THE OFFSHORE SETTING

Molds and mud-filled casts of millimeter- to centimeter-size halite cubes are common in some laminated, organic-poor mudstones in ancient epicontinental marine successions. These occurrences are routinely interpreted as sediments deposited in coastal playas or salinas where evaporation of seawater in an arid climate reached halite-precipitating hypersalinity. This interpretation is commonly supported by associated mudcracks assumed to be desiccation features. They have also been taken as evidence that Precambrian seawater was sulfate-poor. Examples from the Mesoproterozoic Belt Supergroup of northwestern Montana and southwestern Alberta (Grinnell, Helena, and Gateway formations) and the middle Cambrian of southern Alberta (Arctomys Formation) show that, rather, the mudcracks are intrastratal deformation features caused by synsedimentary earthquake-induced shrinkage and injection in low-energy, subtidal deposits. Thus, the halite precipitated at the sediment–water interface under submerged conditions, from warm, bottom-hugging brines that were formed in shallow coastal areas and then flowed into deeper water where cooling led to precipitation. Zoning evident from mold and cast surfaces suggests successive growth pulses during the hypersaline event, whereas variable size of molds and casts suggests varying lengths of time that the salinity stratification was stable and being fed until that stopped and the brines became diluted by the ambient seawater. The shallow areas where evaporation and concentration took place and where gypsum had to have precipitated if seawater had a composition broadly similar to modern seawater, are not preserved in most basins. However, in some associated units there is evidence in allochthonous particles for sulfate precipitation, at least in tidal flats. This model not only challenges prior sedimentological interpretations of crack- and halite-bearing successions, it also removes a cornerstone of evidence employed for pinning ancient sea level in the stratigraphy.