Overview of the Grand Canyon Supergroup
| Sixtymile Formation | Kwagunt Formation | Galeros Formation | Nankoweap Formation |
| Cardenas Lava | Dox Formation | Shinumo Quartzite | Hakatai Shale | Bass Limestone |
by Bob's Rock Shop
The most ubiquitously exposed geologic formations in Grand Canyon are the Kaibab, Toroweap, Coconino, Hermit, Supai Group, Redwall, Muav, Bright Angel, and Tapeats (all sedimentary rocks of Paleozoic Era origin) and the highly altered basement rocks of the Vishnu Group underlying them, which was deposited and subsequently metamorphosed during early Proterozoic time. These Paleozoic formations cover the contorted, crystalline Vishnu rocks like layers of a cake and are stacked one upon the other in an orderly and continuous fashion. A tremendous span of geological time exceeding a billion years lapsed between the metamorphosis of the Vishnu Group rocks approximately 1750 million years ago and the deposition of the Tapeats, lowermost of the Paleozoic strata, approximately 550 million years ago.
During this lengthy Precambrian interval, from approximately 1250 million to 700 million years ago, sediments of primarily marine origin and volcanic extrusives accumulated in horizontal layers over the highly eroded Vishnu Group rocks to a depth of several miles. These Precambrian formations, collectively known as the Grand Canyon Supergroup, were subjected to extensive faulting and tilting after their deposition. In most places in Grand Canyon the Supergroup was completely removed from the underlying Vishnu by erosion prior to the beginning of the Paleozoic Era, and the Vishnu further eroded prior to encroachment by the Cambrian sea depositing the Tapeats.
However, wedge shaped grabens of Supergroup survived where they were downfaulted and tilted below the late Precambrian erosion surface between five major, northwest trending extension faults, from east to west: Butte, Phantom, Crystal, Muav and the 137 Mile Faults. These faults exhibit a combined vertical displacement of approximately 20,000 feet and the hinging and tilt of the blocks between these faults suggests they rotate towards the horizontal as their depth increases.
The surviving Grand Canyon Supergroup rocks have not been extensively metamorphosed and altered like the underlying Vishnu Group, and Supergroup remnants, where exposed, are generally well preserved. Geologists have broadly divided the Supergroup into the Unkar Group, the Nankoweap Formation, the Chuar Group and the Sixtymile Formation. The Unkar Group is further subdivided into the Bass Limestone, Hakatai Shale, Shinumo Quartzite, Dox Formation and the Cardenas Lava. The Chuar Group is further subdivided into the Galeros Formation and the Kwagunt Formation. Most of these formations are yet further subdivided into up to 4 or 5 constituent members based on their lithologies. The earliest record of life in Grand Canyon rocks are stromatolite fossils found in the Bass Limestone of the Unkar Group, the most ancient of the Supergroup formations.
With the exception of the Cardenas Lava, the Supergroup formations are sedimentary in origin and as such cannot be directly dated using radiometric techniques. Deposition of the Unkar Group ended with a period of volcanic activity depositing repeated, thin basalt and andesite flows over the Dox Formation which interfinger with the upper Dox at their conformable horizon. During Cardenas time eastern Grand Canyon was at or near sea level. Features in the lava flows at contacts with the Dox suggest outpourings on wet or shallow water Dox sediments. After Cardenas time the region was tectonically uplifted and slightly tilted towards the northeast.
An unknown amount of the Cardenas Lava was removed by erosion prior to Nankoweap time, but outcrops of the Cardenas Lava up to 985 feet in thickness are exposed in eastern Grand Canyon. Sills and dikes of igneous intrusives similar in radiometric age and composition to the Cardenas Lava occur in the underlying Chuar Group formations. A network of these structures probably fed eruptions and outflows of the lava to the surface during Cardenas time.
Radiometric dating using the Rubidium-Strontium method places the age of the Cardenas Lava at approximately 1100 million years ago. Radiometric dating of the underlying Vishnu Group places its metamorphosis at about 1750 million years ago, so the ages of the sedimentary formations of the Unkar Group must be younger than the Vishnu and older than the Cardenas. Data from studies of paleomagnetic pole positions and polar wandering indicates the Unkar Group was deposited over a period from approximately 1250 million to 1070 million years ago, which correlates with radiometric dating of the Cardenas for the end of Unkar time.
Various Unkar Group members are exposed in seven vicinities, from east to west: the "Big Bend" region in eastern Grand Canyon; Clear Creek; Bright Angel Creek; Phantom Creek/Phantom Ranch; Crystal Creek; Shinumo Creek/Hakatai Canyon; and Bedrock Creek/Tapeats Creek.
However the Cardenas Lava, youngest of the Unkar Group rocks, and the overlying Nankoweap, Galeros, Kwagunt and Sixtymile Formations outcrop only in an eastern region of Grand Canyon that is roughly bounded between the East Kaibab Monocline (Nankoweap Creek) to the north, the Butte Fault and Palisades segment of same to the east, Seventyfive Mile Fault (Seventyfive Mile Creek) to the south, and the Vishnu Fault, which roughly parallels the eastern border of the Walhalla Plateau, to the west. Due to their pronounced dips towards the northeast, erosion into Supergroup preserving grabens tends to expose the deepest and oldest Supergroup formations towards the southwest, with younger formations progressively surfacing and cropping out towards the northeast.
The uplift and erosion of the Cardenas Lava was followed by the deposition of the Nankoweap Formation, which intervenes between and unconformably contacts both the Unkar Group's Cardenas Lava below and the Chuar Group's Galeros Formation above. Paleomagnetic evidence provides a provisional age of approximately 1050 million years for the Nankoweap Formation, which also correlates with Rb-Sr radiometric dating of the underlying Cardenas Lava.
Deposited atop the Nankoweap are thick sedimentary sequences of the Galeros and Kwagunt Formations of the Chuar Group. The lithology and structures of the Chuar Group overall suggests quiet marine environments of deposition. Stromatolitic limestones and pisolite horizons in the Chuar Group indicate periods of marine regression and shallower depositional environments, and ripple marks and mud cracks on surfaces throughout the Chuar Group suggest intermittent desiccation.
Deposition of the Unkar Group and the overlying Sixtymile Formation is believed to have ended more or less contemporaneously with the Grand Canyon Disturbance, which has been dated by various geologists at between approximately 825 to 725 million years of age. Long distance lithologic correlations, paleomagnetic data and limited radiometric dating suggest the Nankoweap Formation, Unkar Group and Sixtymile Formation were deposited over the approximate interval of 1000 to 700 million years ago.
As more paleomagnetic data is accumulated and more microfossils are discovered and documented in the Supergroup, the estimated ages and periods of deposition for the various Supergroup formations may be further refined by geologists correlating them with Proterozoic formations elsewhere.