Phantom Ranch Tour -Greatest Angular Unconformity & Great Unconformity

The Tipoff is the location where the South Kaibab Trail begins its descent into the Inner Gorge. This image provides a view of two ancient erosional surfaces in the lower Grand Canyon: the Greatest Angular Unconformity and the Great Unconformity. On the opposite side of the Colorado River, crystalline Vishnu Basement Rocks (mostly schist in this view) are exposed in the Inner Gorge. These rocks formed about 1700 million years ago when pressure and heat at mid-crustal depths metamorphosed pre-existing rocks, changing the crystalline structure. Then, they were uplifted and overlying rocks were eroded away. The Greatest Angular Unconformity is the erosional surface at the top the Vishnu Basement Rocks. In late Precambrian time, between about 1200 and 1100 million years ago, marine sediments accumulated on this surface when shallow seas covered the region. These strata are called the the Unkar Group, which, at this locality, includes the Bass Formation (dark, ledge-forming beds, mostly limestone), the Hakatai Shale (red shale slope), and Shinumo Quartzite (massive, cliff-forming rock that forms the promontory). This sequence of Precambrian-age sedimentary rocks is visible to the left of the center of the image. In the eastern Grand Canyon, lava flows (the Cardenas Lava) and other younger Precambrian-age sedimentary deposits are also preserved. Another period of erosion stripped off some of the cover of the Unkar Group and formed another erosional surface, the Great Unconformity. At this locality, the Great Unconformity is at the top of the Shinumo Quartzite. The Cambrian-age Tapeats Sandstone (about 525 million years old) was deposited on the eroded surface of the quartzite. The Tapeats Sandstone forms a thin cap on top of the promontory (Beus, 1990; Billingsley, 2000; Billingsley and others 1997; Mathis and Bowman, 2005).