A brief geologic history of Arizona Chapter 5: Jurassic Time
Jurassic time, dinosaurs and birth of Atlantic Ocean
Jurassic Time, the age of dinosaurs, was from 241- to 145 million years ago. See geologic time chart. The super-continent of Pangea was breaking up and the Atlantic Ocean was born along a spreading axis.
During the Jurassic, there were no Rocky Mountains. The ancestral Rockies of the Paleozoic had eroded away and the current Rocky Mountains were yet to be born. Northern Arizona, and all of what is now the Colorado Plateau, was a featureless desert of blowing sand, much like the Sahara Desert today. These sands became the Wingate Sandstone, Kayenta formation, Navajo Sandstone, and Entrada Sandstone that form the arches and cliffs of parks in southern Utah such as Arches National Monument and Zion National Park.
The real action was in southern Arizona. Magmatism begun in the Triassic Period continued and moved inland, so that southern Arizona and California contained a magmatic arch and subduction zone where one tectonic plate is drawn under another, with development of many volcanoes on the western edge of the continent. (See the hatched line in the global map, first figure above.) This subduction zone still exists along the west coast of North and South America. The figure below shows a cross-section of a subduction zone, magmatic arc, and spreading axis. This mechanism continued through Cretaceous and Tertiary time to build the Rocky Mountains.
In Jurassic time, southern Arizona was a volcanic field, and some of the volcanoes collapsed into calderas. Remnants of these calderas are recognized in the dragoon mountains near Courtland-Gleeson, in Tombstone, at the southern end of the Huachuca Mountains, in the Canelo Hills, and in the Santa Rita Mountains. Gold, silver, and copper is associated with the subvolcanic intrusions of these calderas. Many of the historic mining camps of southern Arizona were founded on these deposits. The Juniper Flat granite just north of Bisbee has been dated at about 180 million years, and the copper deposit at Bisbee is presumed to be about the same age.
The Jurassic was also a time of other structural complications. According to Tosdal et al., “In southeastern Arizona, movement along northwest-striking fault systems broke the area into elongate structural blocks, forming topographic highs and basins in which … clastic and volcanic rocks accumulated.” The Canelo Hills volcanics are some of the rocks deposited at this time. Tosdal continues: ” In northwestern Sonora, southern Arizona, and southeastern California, a system of sinistral [left] strike-slip faults, the Mojave-Sonora megashear, cut obliquely across the magmatic arc, as much as 800 km of aggregate displacement along these faults may have occurred in Jurassic time.” This megashear hypothesis is controversial.
At the end of Jurassic time, and extending into the following Cretaceous period, the style of tectonism changed from strike-slip shearing to normal faulting (one side down relative to the other side). This formed basins which received sediments and volcanic deposits, and eventually formed the basin which held the Cretaceous-age Bisbee Sea.
Glance Conglomerate, up to 2,000 meters thick, is the youngest Jurassic deposit in southern Arizona, and it spans the time division to form the base of the Cretaceous Bisbee group of rocks. The Glance represents high-energy deposition of alluvial fans by debris flows and rivers along a mountain front.
For most of Jurassic time, global temperatures are estimated to have been 15 -to 20 F warmer than today, the same as in the preceding Triassic Period. Most of the land area was hot and steamy, but in southwestern North America, it was arid. Plant life consisted mainly of conifers and palm-like cycadeoids. Flowering plants had not yet evolved. On land, this was the age of dinosaurs, including flying reptiles. There were some primitive mammals and abundant insects.
Mid-Jurassic volcanism caused atmospheric carbon dioxide to rise from about 1,500 ppm to about 2,500 ppm (vs. 400 currently) by late Jurassic time. But while carbon dioxide remained high, Jurassic time ended with an ice age. There is evidence of glaciation on some continents, but apparently temperatures did not get as cold as in the previous ice age in late Paleozoic time nor as cold as the glacial epochs of the current ice age.
Next time, the Cretaceous Period: bad news for dinosaurs.
For more of the story see:
Lipman, P.W., and Hagstrum, J.T., 1992, Jurassic ash-flow sheets, calderas, and related instrusions of the Cordilleran volcanic arc in southeastern Arizona, GSA Bulletin, v.104.
Tosdal, R.M., Haxel, G.B., and Wright, J.E., 1989, Jurassic Geology of the Sonoran Desert Region, Southern Arizona, Southeastern California, and Northernmost Sonora, in Arizona Geological Society Digest 17.
Credits: Global paleomaps by Christopher R. Scotese (http://www.scotese.com/ )
Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.