A GLIMPSE WITHIN THE GREAT UNCONFORMITY: PARSING THE MESOPROTEROZOIC RECORD OF EROSION, SEDIMENTATION AND DEFORMATION PRESERVED IN THE GRAND CANYON UNKAR GROUP

The Great Unconformity of Grand Canyon and the western U.S. represents, at its maximum, a 1.3 billion year gap in the rock record reflecting the cumulative effects of at least 3 major erosional episodes in the Mid-to-Late Proterozoic to earliest Cambrian. For the late Mesoproterozoic, the Unkar Group preserves an important glimpse within the Great Unconformity and of the events that formed the unconformities above and below it, between 1350 Ma and 775 Ma.

The Unkar Group was deposited nonconformably on exhumed mid-crustal basement rocks of 1840 Ma-1375 Ma. It is approximately 2000 m thick and made up of 5 formations, from bottom to top: the Bass, Hakatai, Shinumo, Dox and Cardenas basalt. New high-precision CA-IDTIMS zircon dates from a re-worked volcanic tuff in lower Bass yields a refined age of 1250.21 ± 0.44 Ma. The Cardenas basalts and associated intrusive rocks are 1104 Ma (⁴⁰Ar/³⁹Ar). Depositional environments within the lower Unkar Group vary from shallow marine transitioning to marginal marine and fluvio-deltaic deposits. Carbon and oxygen isotopes from carbonates in the Bass Formation yield primary values of 0 to 4 ‰ and -2 to -15 ‰ respectively. ⁸⁷Sr/⁸⁶Sr values of 0.706595 to 0.707467 in stromatolitic dolostones of the Bass Formation are consistent with a near-primary marine signal. An unconformity (~100 Ma) occurs at the top of the Hakatai Shale and below the ultra-mature Shinumo Sandstone. Dox Formation (950 m) records a dramatic influx of fine grained, distally derived fluvial sands and muds.

Unkar Group detrital zircon and mica indicate a mixed provenance from proximal basement terranes (lower formations) to the more distal Grenville orogen (Dox). Abrupt facies changes and soft sediment deformational features indicate NE-striking, NW-vergent monoclines were developing during deposition of the lower Unkar Group. Unkar Group was tilted and broken into symmetric and asymmetric NW-striking normal fault bounded blocks, creating an angular unconformity with overlying Neoproterozoic and Paleozoic strata. Field relationships suggest that deposition and normal fault development were dominant in late Unkar time consistent with NW-directed shortening during the Grenville. The post-Unkar unconformity is marked by an angular discordance with the overlying Chuar Group and represents a 325 million year gap in the rock record.