Stop 3 - Rainbow Gardens
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Here the group examines a Precambrian granite mass that appears to have moved as a landslide block. The rocks are enclosed in the Thumb Member of the Miocene Horse Spring Formation (about 17 million years old by fission track method). The closest source for these granites in in the South Virgin Mtns., some 37 miles east of the outcrop. The presence of the landslide deposits in the Thumb Member helps date the initiation of uplift in the area. It is unlikely that the rocks traveled anymore than 6 miles. The granites were slowly separated from their source by lateral faulting along the Las Vegas shear zone. This stop demonstrates the importance of both catastrophic and gradual processes in earth history. (Interpretation from Stephen L. Salyards and Eugene M. Shoemaker, Landslide and debris-flow deposits in the Thumb Member of the Miocene Horse Spring Formation on the east side of Frenchman Mountain, Nevada: A measure of basin-range extension, Geological Society of America Centennial Field Guide 1987 - Cordilleran Section, pp. 49-51). |
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View to the south of Triassic (Lower Mesozoic) Moenkopi and Chinle Fms. from the Rainbow Gardens area. |
