LATE MESOPROTEROZOIC BASIN FORMATION AND DEFORMATION OF THE UNKAR GROUP OF GRAND CANYON, AZ: NEW INSIGHTS FROM INTRACRATONIC GRENVILLE-AGE DEPOSITS

The Late Mesoproterozoic Unkar Group records protracted deformation during basin formation and sedimentation, and preserves a complex depositional and deformational history. Older deposits record early NW-directed shortening across NW-facing monoclines and younger deposits record NE-SW extension across NW-striking normal faults. A combination of previously reported and new field observations indicates that lower formations of the Unkar Group were being deposited in a structurally active and dynamic near-shore depositional environment.

The mixed carbonate and siliciclastic beds of the Bass Fm. are involved in the development of Precambrian monoclines and exhibit abrupt thickness and facies changes across monoclines. They also preserve disharmonic folds and potential seismites consistent with down-slope movement and provide a rare snapshot of early Unkar Group time. Carbonate beds are intimately interbedded with very coarse grained arkosic sandstone and conglomerate beds. Planar laminated stromatolitic beds reflect relatively low energy conditions, large oncolites and encrusting stromatolites occur in association with coarse siliciclastic beds in higher energy settings. Depositional analogs may include marine margins of alluvial fans in the Red Sea region or lacustrine settings like the modern Great Salt Lake or Eocene Green River Fm.

Overlying carbonate and siliciclastic beds, the Hakatai Fm. represent a gradational transition from mixed carbonate deposition to coarser clastic deposition. Field observations and petrographic analysis indicate that the Hakatai Fm. records relatively rapid deposition in a shallow depositional environment, perhaps as near-shore deltaic deposits. The Hakatai Fm. is also folded by Precambrian monoclines and in a few localities preserves liquefaction pipes that form aligned, weather-resistant columns. The clastic pipes cross-cut siltstone beds of the upper Hakatai Fm., are composed of medium to coarse sandstone consistent with derivation from the lower Hakatai, and are commonly tilted with bedding. Collectively, the observations suggest that liquefaction was penecontemporaneous with deposition of the Hakatai Fm. and predated tilting of late Precambrian deposits.