Great Unconformity Granite and Sandstone
May 30, 2006
Provided and copyright by: Rob Sheridan
Summary authors & editors: Rob Sheridan
An “unconformity” is gap of time in a rock formation. Usually this means that exposed rock has been weathered and then buried. The eroded portion is missing from the local geologic record, creating an “unconformity.” The “Great Unconformity” is a 1.2 billion year gap between Pre-Cambrian igneous and Cambrian sedimentary rock layers in the southwestern United States. Basement granite and schist crystallized slowly beneath what was to become the southwestern United States about 1.7 billion years ago and was then uplifted and exposed to weathering for the next 1.2 billion years. This region was subsequently covered by a series of shallow salt seas over the next 500 million years. These seas left sequential sand and calcareous sediments over the eroded basement rock that subsequently metamorphosed into sandstone and limestone strata. A 1.2 billion year gap exists in the geologic record where the eroded basement rock is covered by the resulting sedimentary sandstone and limestone. As the region was then uplifted, the Colorado River cut down through the sediments, exposing the “Great Unconformity” in the Grand Canyon, where it was first described by Clarence Dutton in 1882.
The “Great Unconformity” can also be seen in one other place. Just a few million years ago, the crust in the Las Vegas, Nevada area began to rift apart, creating basin and range topography. The faces of these tilted block ranges expose the strata that compose them. Just northeast of Las Vegas lies Frenchman Mountain, one of these tilted fault-block mountains. The 50 degree tilt of the block has exposed the Great Unconformity, where 1.7 billion year old granite and schist is topped by 500 million year old Tapeats Sandstone (which contains dark deposits of manganese oxide that crystallized from water percolating through the sandstone after it formed) and then multiple layers of limestone, leaving a 1,200 million year gap in the geologic record.
In this photo, taken at the Great Unconformity site northeast of Las Vegas, the two rocks are shown;1,700 million year old granite and 500 million year old Tapeats sandstone. In the background there is a glimpse of the limestone that overlies both. The two rocks were gathered within inches of each other, as the person (me) holding the rocks hiked along the exposed unconformity. Between the rocks, there's a gap of 1,200 million years, the “Great Unconformity.”
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