A Brief Geologic History of Arizona
Chapter 2 Cambrian and Ordovician time
In Chapter 1, the Precambrian, Arizona was at the South Pole and had just emerged from the sea. Plate tectonic movement was carrying what would be North America northward, and by Cambrian time (542- to 488 million years ago), North America spanned the equator. But the orientation was 90 degrees clockwise from its current setting so that the equator ran from Mexico through Canada.
As time progressed, the continent rotated counterclockwise, and by Ordovician time (488- to 444 million years ago), the equator traced from near Washington State to Greenland.
By the way, positions of the continents are inferred from paleomagnetic studies and correlation of rock formations.
Magnetic minerals in igneous rocks were oriented parallel to the magnetic lines of force on the planet during the time the rocks solidified, and thus point toward the position of the magnetic poles at that time. By 1950, accumulated magnetic data from all the continents appeared to show that the magnetic poles were wandering around the globe. But the apparent position of the poles at any given time was different as measured on different continents.
Geophysicists finally concluded that the poles were not moving; the continents were.
In the following, I will describe the geological interpretation as if Arizona were in its current orientation relative to the poles.
The sandstones, shales, and limestones deposited during Cambrian and Ordovician time represent deposition on a continental shelf, near shore, and on dry land. Much of the deposition was in an inter-tidal environment. Deep sea was toward the west and the continent toward the east. At times, parts of Arizona and New Mexico may have been an island. The sequence of rocks suggest that sea level was rising and falling (or the continent was doing the same).
The shoreline progressed across Arizona from west to east as sea levels rose. In general, the sandstones were deposited on the shore and in shallow water. They show cross-bedding that would form from sand dunes and near-shore reworking. The shales were deposited in deeper water beyond the sands, and limestones in even deeper water.
Apparently, the land emerged again at the end of Ordovician time and remained emergent during the subsequent Silurian Period (444- to 416 million years ago) since there are no rocks of Silurian age recognized in Arizona, though they are recognized in New Mexico.
The position of the continent near the equator implies it was subject to trade winds. Some of the rocks hold evidence of major storms and erosion of islands of Precambrian rocks.
In northern Arizona, the basal Cambrian sandstone is called the Tapeats Sandstone. It was deposited on eroded Precambrian rocks, and soil developed from those rocks. In southern Arizona, the basal sandstone is called the Bolsa Quartzite. Above the basal sandstone is a series of shale, sandstone and carbonates (Bright Angel Shale north, Abrigo Formation south). These rocks are interpreted to represent mainly shallow water deposits, tidal flats, and sand dunes, both of which were subject to scouring and erosion in a near-shore environment. The sequence is capped by limestone (Mauv Limestone north and El Paso limestone south) which means the western sea rose once again to cover Arizona. In Southern Arizona, the Abrigo Formation and the overlying Devonian Martin Formation are the best host rocks for mineral deposits because of their structure and chemistry.
Cambrian time was when life became abundantly obvious. Invertebrate fossils include trilobites, mollusks, soft-bodied worms, jellyfish, and sponges. The Ordovician added corals, snails, clams, and the first vertebrates: fishes, as well as primitive terrestrial plant life. The development of terrestrial plants greatly reduced the rate of erosion. Marine fauna recycled the carbon contained in Precambrian algal mats.
Ordovician time ended with a major ice age and the extinction of about 20% of marine species. The Ordovician ice age occurred even though atmospheric carbon dioxide was more than 10 times the current concentration.
Chapter 3 covers Devonian to Permian time and the mass extinctions.
Middleton, L.T., 1989, Cambrian and Ordovician Depositional Systems in Arizona, In Geologic Evolution of Arizona, Arizona Geological Society Digest 17.
Hayes, P.L. 1978, Cambrian and Ordovician Rocks of Southeastern Arizona and Southwestern New Mexico, in New Mexico Geological Society Guidebook 29.
Credits: Global paleomaps by Christopher R. Scotese (http://www.scotese.com/)
North American paleomap by Ron Blakey, Colorado Plateau Geosystems.com
Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.