A Brief Summary of the Geologic History of Northern Mexico
A library research was conducted at the Denver Earth Resources Library and the US Geological Survey library in Denver, Colorado. The purpose of this study was to identify the geologic setting of the Copper Canyon area of western Mexico. Fortunately, even though quantity of literature was lacking, a reasonable interpretation of the area can be formulated. Primarily, northern Mexico can be divided into four physiographic provinces; each with their own unique geology. The Copper Canyon area lies within the Sierra Madre Occidental province. Even though the area contains canyons deeper than the Grand Canyon, it has a limited geologic history of only a few tens of millions of years (versus over a billion years of geologic history for the rocks of the Grand Canyon). With the exception of the rocks at the very bottom of the canyons, the rocks of the Copper Canyon area consist primarily of explosive volcanic ash flows, ash falls, and mudflow breccias deposited approximately 20 to 40 million years ago.
Regional Geography and Geology
Northern Mexico can be divided into four physiographic areas (Roldan Q. and Clark, 1992, Clark, 1976, and Raisz, 1964). These areas from east to west are: the Basin and Range of eastern Chihuahua; the Sierra Madre Occidental lying along the Chihuahua/Sonora-Sinaloa border; the Parallel Ranges and Valleys and the Coastal Plain of western Sonora and Sinaloa (Fig. 1). Ages of the rocks range from Precambrian (1.75 billion years ago) through recent with the most common rock type at the surface being Cenozoic volcanics (Fig. 2).
The Basin and Range
The Basin and Range province (Cuencas y Sierras) ranges from eastern Chihuahua northward through the Trans-Pecos of west Texas and then westward across southern New Mexico into Arizona. The province also covers most of Nevada, the western half of Utah and Idaho and the eastern parts of Oregon and Washington. In Mexico, the province consists of a series of northwest trending mountain ranges created by the tilting and uplift of Tertiary igneous rocks and/or Mesozoic sediments. These sediments are usually early Cretaceous limestone, shales, and sandstones with occasional exposures of Paleozoic marine rocks. The valleys have filled with sands and shales and can be tens of kilometers long and up to 10 kilometers wide. Some of the valleys have no outlet to the sea and are referred to as bolsones. West of Chihuahua the province is referred to as Altas Llanuras since each valley is progressively higher. La Junta lies near the boundary of the Basin and Range and the Sierra Madre Occidental provinces.
The Sierra Madre Occidental
The Sierra Madre Occidental is an extensive volcanic terrain starting near the US-Mexico border and trending southeast into the states of Zacatecas and Jalisco. The Sierra is the most extensive physiographic province in Mexico covering approximately 800,000 square kilometers. Cenozoic volcanic rocks, mostly tilting to the east, characterize the province. These rocks can be divided into a lower andesitic unit (about 70 to 40 million years old) and an upper rhyolitic unit (about 40 to 20 million years old). Rhyolite is the fine-grained equivalent to granite, which has a high percentage of silica and aluminum. Andesite is of intermediate composition lying somewhere between granite and magnesium- and iron-rich basalt. Both volcanic units are up to 1 kilometer thick. The Lower Volcanic Sequence and underlying sedimentary rocks have localized ore deposits related to mineral-rich volcanic intrusions emplaced between 140 and 40 million years ago. Ore bodies are probably present throughout the province but are only visible in the bottoms of the canyons since the overlying Upper Volcanic Sequence provides a thick cover and obscures detection.
Andesitic and rhyolitic rocks tend to be deposited during explosive volcanic events resulting in fiery ash flows, thick ash falls, and destructive mudflows; similar to what was seen with the eruption of Mt. St. Helens. The resulting rock tends to have ash-sized grains with pebble- to boulder-sized fragments scattered throughout. The larger fragments are debris that the flow picked up along the way or are bomb-like projectiles ejected from the volcano with the ash. Usually the thicker layers of flows and larger clasts are found nearest the volcanic centers. For example, a 400-meter thick ash flow with fragments up to 1-meter in diameter, interpreted to have been deposited inside a volcanic caldera, can be found at Basaseachic Falls (Schmidt, 1992). Lava flows are not common with these types of volcanoes.
The volcanic centers of the andesitic Lower Volcanic Sequence were located near the oceanic trench that was lying just off the west coast of North America. Through time, as the North American continent overran the Pacific tectonic plate, the volcanic centers moved farther to the east (Fig. 3). About 40 million years ago, the mid-Pacific spreading center was finally subducted beneath the continent and compression of the continent ceased. Three things happened during this time. First, the continent started to stretch out from east to west forming the Basin and Range, the Parallel Ranges and Valleys, and the Gulf of California. Secondly, the volcanic centers, now spewing out massive quantities of rhyolite, rapidly regressed back toward the west coast. And finally, the entire area started to be uplifted. With uplift and tilt, erosion began. Dissection of the volcanic sequences was especially severe along the western margin of the Sierra Madre Occidental. This removal of section eventually formed the deep canyons of the Barrancas Del Cobre. The canyon walls are almost exclusively composed of the stacked layers of rhyolitic ash and debris of the Upper Volcanic Sequence.
The Parallel Ranges and Valleys
The Parallel Ranges and Valleys Province is similar in structural style to the Basin and Range to the east. This province contains Mexico's oldest rocks, 1.75 to 1.675 billion year old Precambrian intrusives of northwestern Sonora (Anderson & Silver, 1979). Younger Precambrian rocks in the Parallel Ranges and Valleys Province include dolomite, limestone, sandstone, and shale (Stewart et al., 1984). Unlike the Sierra Madre Occidental province, ore-bearing volcanic intrusives are not covered by Late Tertiary volcanics and are more readily identified at the surface.
The Coastal Plain
This 75 kilometer wide geographic province between San Blas and Topolobampo is characterized by a mostly flat section of Tertiary and Quaternary gravels and alluvium. In a few places, most notably around the deep water harbor of Topolobampo, the flat plain is punctured by low hills of basalt breccia and intrusive volcanics.
Anderson, T.H., and Silver, L.T.,1979, The role of the Mojave-Sonora megashear in the tectonic evolution of northern Sonora, in T.H. Anderson, and J. Roldan Q. (eds.) Geology of Northern Sonora: Geological Society of America Annual Meeting guidebook (Trip #27), p. 59-68.
Clark, K.F., Dow, R.R, and Knowling, R.D., 1979, Fissure-vein deposits related to volcanic and subvolcanic terranes in Sierra Madre Occidental province, Mexico: Nevada Bureau of Mines, Geological Report 33, p. 189-202.
Clark, K.F., 1976, Geologic section across Sierra Madre Occidental, Chihuahua to Topolobampo, Mexico, in Tectonics and Mineral Resources of Southwestern North America, L.A.Woodward and S.A. Northrop (eds.), New Mexico Special Publication, no. 6, pp. 26-38.
Raisz, E., 1964, Land forms of Mexico: Cambridge, MA, Office of Naval
Roldan Quintana, J., and Clark, K.F., 1992, An overview of the geology and mineral deposits of northern Sierra Madre Occidental and adjacent areas, in K.F. Clark, J. Roldan Quintana, and R.H. Schmidt (eds.) Geology and Mineral Resources of Northern Sierra Madre Occidental, Mexico, El Paso Geological Society guidebook, p. 39-65.
Schmidt, H.H., 1992, El Parque Nacional Cascada de Basaseachi, in K.F. Clark, J. Roldan Quintana, and R.H. Schmidt (eds.) Geology and Mineral Resources of Northern Sierra Madre Occidental, Mexico, El Paso Geological Society guidebook, p. 197-201.
Stewart, J.H., McMenanin, A.M., and Morales R., J.M., 1984, Upper Proterozoic and Cambrian rocks in the Caborca region, Sonora, Mexico: Physical stratigraphic, biostratigraphy, paleocurrent studies, and regional relations: U.S. Geological Survey, Prof. Paper 1309, 36 p.
C. Elmo Brown is an independent and consulting geologist located in Denver, Colorado. He presently explores in many of the basins of the western United States in search of hydrocarbons. Education includes a BA in Geology and extensive graduate studies in Geology from the University of Texas at Austin and a Teaching Certificate from Metro State College of Denver. Having traveled to Cerocahui and having stayed at the Paraiso del Oso, Elmo was asked to write a little summary of the area. It was his pleasure to do so. This summary is for the sole use of the Paraiso del Oso and its owner, Doug "Diego" Rhodes.
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