The Proterozoic Chuar Group (<1100 Ma, >535 Ma) is exposed in a 6 x 24 km area in the eastern Grand Canyon, northern Arizona. The three middle members (~720 m thick) of the Chuar Group are composed of interbedded mudstone, siltstone, sandstone, and dolomicrite. The lowermost Carbon Canyon Member is the most variable and is composed of variegated shale, variegated siltstone, ripple-laminated and cross-bedded sandstone, and laminated dolomicrite facies. The overlying Duppa Member exhibits similar facies, yet is generally lacking dolomicrite. The overlying Carbon Butte Member also has similar facies, yet is lacking the dolomicrite facies entirely and the ripple-laminated and cross-bedded sandstone facies is more abundant.

Environmental interpretation of these facies has been the subject of debate, in part due to the lack of megafossils and bioturbation. However, the ripple-laminated and cross-bedded sandstone facies exhibits epsilon cross-beds, bidirectional (180°) trough cross-beds, interference ripples, convolute bedding, load casts, and mudcracked mud drapes over wave ripples. These sedimentary structures collectively suggest a marine tidal environment. The mud drapes are interpreted to reflect a relatively deeper marine environment than that of the immediately underlying wave-rippled sandstones. Mudstones similar to these mud drapes are found interbedded with the dolomicrite facies, suggesting that the dolomicrite facies is also of marine origin.

A marine origin for these three members implies that the supercontinent of Rodinia had rifted and a continental margin developed by the time of deposition of the middle Chuar Group. Ongoing geochemistry and radiometric dating (of rare interbedded tuff) will test the marine hypothesis and attempt to determine the absolute age of the middle Chuar Group. This will lead to a better understanding of the timing and nature of continental margin formation (i.e., timing of Rodinian breakup).

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas