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| This zoomed-in view shows the Colorado River in the Inner Gorge near Phantom Ranch from along the South Kaibab Trail near where it begins its descent through the Redwall Limestone. In this view of the canyon, the Precambrian-age crystalline Vishnu Basement Rocks form the core of the gorge. In this portion of the canyon, Precambrian-age sedimentary rocks are preserved below the Great Unconformity below the Tapeats Sandstone. Precambrian-age sedimentary rocks include the cliff-forming Bass Formation (mostly limestone), the red Hakatai Shale, and overlying, cliff-forming Shinumo Quartzite. Collectively, the Bass Formation, Hakatai Shale, and Shinumo Quartzite are part of the Unkar Group and were deposited about 1200 to 1100 million years ago. These Precambrian-age sedimentary rocks are locally steeply dipping and are broken by a series of faults in this vicinity. The Great Unconformity is locally an angular unconformity where the flat-lying Tapeats Sandstone is in contact with the dipping and truncated sedimentary layers of the Unkar Group. Younger Precambrian-age sedimentary rocks are preserved (and now exposed) below the Great Unconformity farther east in the Grand Canyon. In contrast, the Tapeats Sandstone began to accumulate about 525 million years ago (Mathis and Bowman, 2005, Beus, 1987). |
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