The Geology of Gold Butte National Monument
by Steve Rowland
Emeritus Professor of Geology, UNLV
The most common and conspicuous rocks exposed in Gold Butte National Monument can be lumped into three groups: (1) Jurassic windblown sandstone, (2) Paleozoic marine limestone, and (3) pre-Cambrian granite and metamorphic rocks. Each of these groups is briefly described below.
The Jurassic sandstones (belonging to the Aztec Sandstone formation) are the reddish rocks that are dramatically exposed where the pavement ends at Whitney Pocket, and at other places in the national monument, such as “Little Finland.” This is the same formation exposed in Valley of Fire State Park and Red Rock Canyon National Conservation Area. These sandstones were deposited in a huge desert approximately 180 million years ago. It is the same deposit of cliff-forming sandstone that forms the high cliffs of Zion National Park, except in Utah it goes by a different name―Navajo Sandstone. Exposed surfaces of the Aztec Sandstone typically develop a black coating called desert varnish. Native American petroglyphs were made by scraping through this black patina to expose the light-colored sandstone below.
Paleozoic limestones form most of the ridges in the northern portion of the national monument. They were deposited on the shallow sea floor during the Paleozoic Era (between 540 and 250 million years ago). Many rock formations are included; generally speaking these are the same sedimentary formations that are exposed in Grand Canyon. The key difference between Gold Butte and Grand Canyon is that in Grand Canyon the layers are horizontal while in Gold Butte they are tilted. This tilting occurred when this region of North America was pulled apart by tectonic forces roughly 10 to 15 million years ago, during the Miocene Epoch.
The pre-Cambrian granite and metamorphic rocks are mostly exposed in the southern portion of the national monument, south of Gold Butte town site. Radiometric dates of these rocks cluster around 1.7 billon years. Generally speaking, these are the same dark rocks that occur in the inner gorge of Grand Canyon, where they are overlain by Paleozoic strata. In Gold Butte, these pre-Cambrian crystalline rocks became exposed as prominent high peaks (in contrast to their low position in Grand Canyon) when the Paleozoic strata that used to lie above them were pulled off and dragged to the west (also in the Miocene Epoch). When this heavy pile of overlying limestones was removed, the pre-Cambrian rocks of Gold Butte floated upward to their present elevation because they are less dense than the underlying mantle.