Arizona Geological History: Chapter 2, Cambrian and Ordovician Time

Tropical Oceanfront Property Available, but then there is an ice age.

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 it current setting so that the equator ran from Mexico through Canada.

 Paleomap 514

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. See a geologic time scale here.

By the way, positions of the continents are inferred from paleomagnetic studies and correlation of rock formations. Magnetic minerals in igneous rocks are oriented parallel to the magnetic lines of force on the planet at 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.

Tapeats sandstoneThe 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. 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 level 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, but there are 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.

Tapeats worm burrows USGSIn 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 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. 

 Bolsa quartzite BensonCambrian 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 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.

Principal references:

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.