Grand Canyon Supergroup

The sediment deposited makes up the Grand Canyon Supergroup. The oldest section in the supergroup is the Unkar Group, and the oldest formation of the group is the Bass Formation. Gravels in river valleys eventually formed basal conglomerate that is now called the Hotauta Member. Sediments over time formed dolomite. This 1250 million year old section is the oldest to contain fossils in the canyon. Hakatai Shale is next and was formed from the recession of seashores that left mud flats. It is a bright orange. Bits of tough quartzite remained in the face of erosion to form small islands in the Cambrian time. Eventually, they were reburied with sediment. These cliffs of sandstones and quartzites formed many of the “temples” or peaks known in the canyon today. These Mesoproterozoic formations, originally islands, were termed monadnocks or inselbergs. Their primary characteristic is their isolated and abrupt features that emerge abruptly from gently sloping plains. Above the Shinumo Quartzite is the Dox Formation, which forms broad sloping hills and makes up most of the Unkar Group. Stromatolites and algae can be found in this formation. The youngest of the Unkar group is the Cardenas Basalt which was formed from dark basaltic lava. Though not a part of the group, the Nankoweap Formation, primarily comprised of coarse sandstone, is found on top of the Cardenas Basalt only in the eastern canyon. There is an unconformity after the Nankoweap. On top of the Nankoweap is the Neoproterozoic Chuar Group, which makes up the upper half of the Grand Canyon Supergroup. Most of this is mudrock with many fossils and appears black because of its high organic content. The Chuar Group is made of the Galeros and Kwagunt Formations. The Galeros Formation is made of all types of sediments and is green in color. The Kwagunt is mainly shale and limestone and runs black to purple. Both are abundant with fossils. During the Grand Canyon Orogeny (a period of time when the earth’s crust folds and compresses to form mountain ranges). The fault block mountains formed then exposed the sedimentary layers of the Grand Canyon Supergroup and eroded much of it in the canyon, exposing the Shinumo Quartzite and some basement rock. The uppermost section of the Grand Canyon Supergroups is the Sixtymile Formation. This 200’ layer is tiny in comparison to the rest of the canyon and was likely much larger before erosion during the Great Unconformity and before the Tapeats Sandstone. This formation is a mix of sandstone, siltstone, and breccia.