Bass Formation
In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone.
This photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.
The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.
Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.
-JBB
Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/
Reference: https://bit.ly/2REaVCr