A Brief Geologic History of Arizona
Chapter 3: Silurian to Permian time
Arizona becomes a tropical island, gets flooded by the ocean, suffers another ice age and a major extinction of life.
In this chapter we will complete the Paleozoic Era with five periods: Silurian (444-416 million years ago, mya), Devonian (416- to 359 mya), Mississippian (359-318 mya), Pennsylvanian (318- 299 mya), and the Permian (299-251 mya). In the European classification, the Mississippian and Pennsylvanian are, together, called the Carboniferous period because it was during this time that major coal deposits were formed. During this time, Arizona cycled between being completely covered by ocean, a tropical island, and a tropical peninsula of the mainland to the northeast.
After recovery from the Ordovician ice age (about 440 mya), Arizona was apparently a highland on the southwest edge of a continental mass, about 30 degrees south of the equator. I say apparently, because there is no Arizona record from the Silurian period (444- to 416 mya ), so Arizona may have been dry land that was subject to erosion.
During the last four periods of the Paleozoic, Arizona was mainly under water. The rocks deposited during this time represent deposition on a continental shelf environment. There were several episodes of transgression (encroaching) and regression of the sea from the west. Only what is now the northeastern corner of the state remained above sea level for most of the time. The rise and fall of the sea was due to both tectonic shifting of land and changes in water volume from the glacial epochs.
Limestone was the principal rock deposited during this time along with relatively minor shale and sandstones. All the formations contain fossils. These limestones currently make up most of the mountain ranges south of Tucson.
Mississippian rocks rest unconformably (not at the same angle or with evidence of erosion) on Devonian and older rocks. This means that there was some tectonic adjustment and erosion between the two Periods. (And by the way, the geologic Periods are usually defined by their distinct fossil assemblages). The principal formation of the Devonian is called the Martin Formation with type area in Bisbee. The principal Mississippian limestone is called the Redwall Limestone near the Grand Canyon and the Escabrosa Limestone in southern Arizona. Kartchner caverns are in the Escabrosa Limestone, but the caves formed recently.
Pennsylvanian and Permian rocks represent complex cycles of transgression/regression by the sea, caused by changes is water volume due to glacial epochs, and by tectonic uplift and sinking of the continent. This tectonic shifting was the result of the collision of Gondwana on the south with Pangea on the north. Carbonate rocks dominate in the northwest and southeast, while sandstones and conglomerates dominate in central and northeast Arizona.
The fossil photo is Omphalotrochus (a snail) from the Permian Colina formation, collected about 2 miles southeast of the Tombstone airport. Notice also the pits made by rain drops differentially eroding the limestone.
Many coal deposits of the world developed during the Carboniferous period. However, Coal in Arizona on the Colorado Plateau was formed much later during the Cretaceous (65-145 mya). It is mined in Navajo county, and, according to the Arizona Department of Mines and Mineral Resources, ranks second only to copper in economic importance. But, I get ahead of the story.
Coal is mostly carbon accumulations from fossil plant material deposited in swamps so devoid of oxygen that bacteria and other critters couldn’t survive to feed on their remains. This implies that climate was warm and wet, and that the cyclic transgressions/regressions of the sea were relatively quick enough to bury the swamps before the luxuriant plant life could be destroyed.
Worldwide coal formation stripped the atmosphere of carbon dioxide. Beginning in mid- Devonian time, about 380 mya, through early Mississippian time, atmospheric carbon dioxide dropped from around 4,000 ppm to near current levels of 400 ppm by 340 million years ago. Temperature, however, remained high (about 68 F world average vs 57 F today) for about 50 million years in spite of the reduced carbon dioxide. But near the Pennsylvania-Permian boundary time, about 270 million years ago, the planet was plunged into another ice age. By the end of the Permian, temperatures rose again to an average of about 63 F, soon followed by a rise in carbon dioxide to just under 3,000 ppm. (Rising temperature causes more carbon dioxide to be exsolved from the oceans.)
Volcanism contributed to the rising carbon dioxide. For a better visualization of the relationship between temperature and carbon dioxide, see the second chart from the article: “Climate change in perspective.”
The first known land vertebrates, amphibians, appeared in late Paleozoic time. Devonian rocks contain fossils of amphibians called stegocephalians (roofed head) because of flat, broad heads. Most were one- to two inches long, but later forms became as large as a crocodile and most were probably carnivorous judging by the teeth.
Reptile fossils appear in Pennsylvanian-age rocks. The first were small like amphibians, but later Permian reptiles got up to eight feet long. One group, the Therapsids, had teeth differentiated into incisors, canines, and molars similar to present-day mammals.
The Permian ended with a mass extinction in which about 90% of species disappeared, including marine fauna, plants, and terrestrial animals. The reason for this extinction is unknown although there are many speculative theories. This extinction happened over a period of several million years and is coincident with the coalescing of continents and extensive volcanism.
When Pangea and Gondwana collided is reduced marine habitats and brought deep, oxygen-poor ocean water to near surface environments. Major volcanism, in what is now Siberia, lasted for about one million years and annually spewed billions of tons of sulfur dioxide and carbon dioxide into the atmosphere. These two events are probably contributory to the extinctions.
But, with the dawning of the new Mesozoic era, life rebounded and became more diverse and more robust.
P.S. A note on ice ages.
The term “ice age” is often misused by the press; they really mean glacial epoch. What’s the difference? An ice age, which lasts several million years, consists of several glacial epochs separated by interglacial periods. Ice ages have occurred regularly every 145 million years or so. That periodicity is related to the position of the solar system within our galaxy. When the solar system enters one of the spiral arms containing a denser cluster of stars, the increased cosmic ray flux tends to produce more clouds and cooler temperatures on Earth.
The frequency and duration of glacial epochs within an ice age are related to the position and orientation of the Earth with respect to the sun. The location of the continents also influences the severity of glacial epochs because continents confine ocean currents. For the last 500,000 years of our current ice age, the glacial epoch-interglacial cycle has had a periodicity of 100,000 years. Prior to 500,000 years ago, the glacial-interglacial cycle was 41,000 years. We are now enjoying an interglacial period.
Chapter 4 will cover the Triassic period, the age of dinosaurs and Arizona crocodiles.
Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.