A Brief Geologic History of Arizona
Chapter 1 Precambrian time
Introduction to this series:
The geology of Arizona is complex. In this series I will hit some highlights that, I hope, will give you an appreciation about how things change with time and some examples of the clues geologists use to figure out what happened. Because of the way things happened, this story will be told in seven chapters, so let’s begin.
Approximately 650 million years ago, the place that would become Arizona was at the South Pole and the place that would become Antarctica was on the Equator. The paleomap below shows the situation then.
Geologists call the time extending from the beginning to about 542 million years ago Precambrian; it represents about 80% of geologic time. The Precambrian is divided into two eons, the Archean (rocks older than 2.5 billion years) and the Proterozoic (rocks from 2.5 billion to 542 million years old). In the current classification, there are no Archean rocks in Arizona (although rocks at the bottom of the Grand Canyon were originally assigned to Archean time). See geologic time scale here.
Archean rocks form the craton (basement or earliest stable part) of the North American Continent. These rocks underlie Canada and the northern Rocky Mountains. Arizona formed at the southern edge of the craton by several methods.
As tectonic plates came together and separated, they produced volcanos, island arcs, intrusions of magma, and sedimentation in shallow seas, all of which would eventually build the continent. Most of these rocks became metamorphosed due to the heat, pressure, and shearing of one plate grinding against another. Precambrian metamorphic rocks crop out in a belt extending from northwest Arizona near Lake Mead, through Kingman, Prescott, and Globe, to southeastern Arizona near Wilcox. There are scattered outcrops trending west from Tucson to Yuma. See an interactive geologic map of Arizona from the Arizona Geological Survey here.
In mid-Proterozoic time (1.7- to 1.1 billion years ago), things quieted down and there was deposition of sediments and volcanic rocks in shallow seas between land masses. These rocks remain relatively undeformed and only locally metamorphosed. They are exposed principally in the mountains north of the Salt River, near Globe and Miami, Mammoth, and in the Catalina Mountains. This series consists, from oldest to youngest, of the Pioneer Shale, the Dripping Springs Quartzite (a hard sandstone), the Mescal Limestone and a capping basalt flow, which together are called the Apache Group. The Apache group is overlain by the Troy Quartzite which is intruded by Diabase along the sedimentary layers. Diabase is crystalline subvolcanic rock similar to basalt. For some great photos of these rocks, see this post from Wayne Ranney’s Earthly Musings.
The Mescal limestone, which has been metamorphosed to dolomite (magnesium carbonate rather than calcium carbonate), contains some of the oldest fossils in Arizona: calcareous algal mats called stromatolites. Oxygen isotope data indicates that certain cherty parts of the limestone originated in ocean water with temperatures of 25- to 30 degrees Centigrade (77-86 degrees Fahrenheit). After deposition and consolidation, the Mescal was lifted above sea-level and intense weathering created caves and sink holes. This process continued even as river-borne (fluvial), and wind-blown (eolian) sands of the Troy Quartzite were being deposited.
In the Grand Canyon, the Vishnu Schist is the equivalent of the earlier metamorphic rocks, and these are overlain by equivalents of the Apache Group and Troy Quartzite, but have different formation names.
Precambrian rocks, principally those in Yavapai County, contain significant mineral deposits. The best known of these is the United Verde mine at Jerome. These deposits, all about 1.7 billion years old, are called volcanogenic massive sulfide deposits. They formed from under-sea volcanic vents that erupted hot, sulfide-rich fluid into the ocean water which caused cooling and precipitation onto the ocean floor. Similar mineral deposits are forming today in the oceans. These deposits are rich in copper and zinc with lesser lead, silver, and gold.
The photos below show the copper rich vent ore and the iron-rich distal mineralization which I collected from the Jerome deposit. The Dripping Springs Quartzite contains uranium minerals along fractures and bedding planes. In other Precambrian rocks, there are vein deposits of gold, sometimes with base metals, tungsten deposits, iron deposits, and asbestos deposits (the mineral is chrysotile, the not so dangerous kind of asbestos).
The division of Precambrian time into two eons, the Archean and Proterozoic at 2.5 billion years ago marks a profound change in the atmosphere. In the Archean, bacteria were the only known life-form. Initially, the bacteria fed on hydrogen sulfide from volcanic vents. But they are good chemists that gradually worked their way to shallow water where they could take advantage of the energy from sunlight and the fuel of carbon dioxide to construct carbohydrates. They became cyanobacteria. But that change in chemistry had consequences.
The cyanobacteria take in carbon dioxide and give off oxygen. The oxygen from bacteria reacted with methane in the atmosphere, turning it into more carbon dioxide which is a much weaker greenhouse gas. As a result of that and some celestial mechanics, the planet was thrown into a profound ice age which lasted for about 30 million years. The bacteria retreated to equatorial habitats and to warm volcanic vents. Populations became isolated and some changed; they became more organized into a new life form called Eukaryotic microbes. The Eukaryotes have discrete cell nuclei and chains of protein to organize internal structure. The Eukaryotes would eventually become animals, plants, and fungi. Also, at that time, oxygen levels in the ocean reached some critical level which caused iron and manganese to precipitate. All of the world’s large iron deposits, called Banded Iron Formations, formed between 2.5 and 1.8 billion years ago, and none have formed since.
Chapter 2 will explore Cambrian and Ordovician time when Arizona was tropical.
Geologic Evolution of Arizona, Arizona Geological Society Digest 17
Proterozoic Geology and Ore Deposits of Arizona, Arizona Geological Society Digest 19
Another compilation of Arizona Geological history, with a different emphasis, is given by Dr. Jan Rasmussen’s Geologic History of Arizona.