A SYNTHESIS FROM THE UNKAR GROUP OF GRAND CANYON, AND INFERENCES ON LATE MESOPROTEROZOIC INTRACRATONIC SEDIMENTATION AND DEFORMATION IN THE WESTERN U.S

The Unkar Group of the Grand Canyon Supergroup records a period of intracratonic basin formation. Recent 40Ar/39Ar age determinations indicate that basement rocks below the Unkar Group cooled through 150 C by 1250 Ma. New U/Pb age determinations from tephra deposits at the base of the Unkar Group date the onset of deposition at ca. 1.23 Ga. The Unkar Group is intruded and capped by ca. 1100 Ma diabase and basalt. The Unkar Group records two styles of deformation that are both kinematically compatible with Grenville-age NW-directed shortening. Unkar Group rocks are preserved in half grabens and full grabens, which record NE-SW extension on NW-striking faults; also preserved are NE-striking monoclines that record NW-directed shortening. Both structural grains seem to be mutually crosscutting, affect all Unkar Group deposits, and are not observed in the Neoproterozoic Chuar Group or the overlying Phanerozoic sedimentary rocks. Intraformational faulting in the Cardenas Lava and an angular unconformity between Unkar Group and younger strata, suggest faulting was concurrent with the ca. 1100 Ma mafic magmatism. Deformation may have begun earlier as shown by sedimentary and deformational evidence (conglomerates interlayered with carbonate, soft sediment deformation of sandstone beds, and at least 1 unconformity). The Unkar Group is divided into the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Lava. The Dox Formation is further divided into the basal Escalante Creek, Solomon Temple, Comanche Point and Ochoa Point Members. Unkar rocks are interpreted to record regression from shallow marine to marginal marine depositional environments. The Unkar Group has been correlated to other Late Mesoproterozoic sequences in the Southwest, suggesting widespread, but perhaps structurally isolated, extensional basins oriented perpendicular to Grenville shortening. We speculate that discrete intracratonic basins were developing at or near sea level during much of the interval from 1.25 to 1.1 Ga, and in response to far-field stresses related to Grenville orogenesis (NW shortening and orthogonal NE-SW extension).