13.2: Geologic Overview and Evolution of the Mojave and Colorado Desert Provinces

Introduction to a Geologic Overview

The bedrock geology of this region is virtually identical to that of the Basin and Range Province, encompassing a broad range of geologic time and environments. The sequence of events that impacted this region is similar to that of most of the Western US and is described in more detail in A Brief Geologic History of California. Readers are encouraged to refer to Basin and Range to review the geologic similarities of these areas.

The Oldest Rocks of the Mojave Region

Basement and overlying sedimentary rocks of the Mojave and Colorado Desert Provinces are similar in ages and characteristics. As in the Basin and Range Province to the north, the oldest rocks exposed in the Mojave and Colorado Desert regions have ages of 1.7-2.5 billion years ago (early Proterozoic in age). They consist of metamorphic rocks derived from pre-existing sedimentary, volcanic, and igneous intrusive rocks (Figure \(\PageIndex{1}\)). Some of these rocks contain high-grade metamorphic minerals and textures consistent with having experienced pressures and temperatures typical of the lower crust between 20 and 40 km (12 and 25 miles) below the Earth's surface. About 1.4 billion years ago, magmas intruded these older rocks (including the magmas that created the mineral deposits at Mountain Pass). Rocks similar to these form a basement complex throughout the eastern Mojave region and throughout the Great Basin and beyond. They formed in association with a long period of mountain building as smaller land masses were gradually assembling to form the core of the modern continental landmasses.

Figure \(\PageIndex{1}\): A typical outcrop of ancient metamorphic rocks consisting of poorly foliated amphibolite schist and gneiss with granitic dikes and intrusions exposed in the Providence Mountains of California. The rock hammer in front of the outcrop is about 30 cm for scale. "Gneiss" by USGS, is in the public domain.

Passive Margin Sedimentary Rocks & Fossiliferous Limestones

After about 1.4 billion years, erosion gradually wore down the landscape of this region to a nearly level plain, similar in character to the modern continental core of Australia or the Canadian Shield region. The large "supercontinent," Rodinia, that had assembled earlier in time began to break apart, and a proto-Pacific basin began to develop. The edge of the North America continent gradually sank beneath the ocean surface, and a thick sequence of sedimentary rocks began to accumulate on the continental margin. This passive continental margin setting persisted for nearly 800 million years, starting in the late Proterozoic and lasting through all of Paleozoic time (570-245 million years ago).

Rocks containing sediments laid down in this passive margin environment are preserved throughout the Mojave and Colorado Province today in uplifted fault-block ranges. The oldest of these sedimentary deposits formed from sand, mud, and limey sediments deposited in shallow marine conditions, similar to the modern continental shelf around the Gulf of Mexico. In these environments, limey (Ca-rich) sediments consisting of the calcareous skeletal remains of algae and invertebrate shell material or precipitates directly from agitated, warm seawater. Extensive precipitation produced a thick sequence of carbonates (limestone and dolostone) that contain abundant fossils. Ancient carbonate rocks like those in the Mojave region tend to be enriched in the mineral dolomite (CaMg(CO₃)₂). Dolomite is typically a secondary mineral replacement of original calcite (CaCO₃) material. For more information, refer to Rocks and Minerals.

While the older carbonates in southeastern California formed from planktonic algae, several of the ranges within the Mojave Desert contain outcrops of Cambrian fossiliferous carbonates with abundant and varied fossils. These units include the lower through middle Cambrian Latham Shale, Chambless Formation, and Cadiz formation, which are correlated to the Carrera Formation of the Great Basin region, as well as the Tapeats Sandstone and Bright Angel Shale of the Grand Canyon region. These units contain a range of marine organisms including Cambrian trilobites (Figure \(\PageIndex{2}\)) and algal structures (Figure \(\PageIndex{3}\)), and Devonian sponges (stromatoperoids) and corals. In late Paleozoic time, coralline reefs became significant producers of carbonate sediments in this area.

Figure \(\PageIndex{2}\): An Olenellus gilberti trilobite, Order Redlichiida, Family Olenellidae, 15 mm along the axis, collected from the Pioche Shale, California, USA, from the late Lower Cambrian (Toyonian). “Olenellus gilberti lowlight CRF'' by Dwergenpaartje, is licensed under CC BY-SA 3.0.

Figure \(\PageIndex{3}\): Oblate oncolites, a type of stromatolite, the size of a quarter in a carbonate matrix of the Chambless formation. These rounded structures are defined by concentric rings surrounding a core of rock or shell material and are preserved in a homogeneous grey limestone and dolomite matrix "Pisolites" by USGS, is in the public domain.

Mesozoic Deformation and Igneous Rocks

As with other regions throughout North America, Mesozoic units in the Mojave and Colorado Desert Provinces record evidence of a changing geologic environment. Beginning around 250 million years ago, the western margin of North America went from being a passive to an active continental margin (Figure \(\PageIndex{4}\)). As depicted in this image, the western margin of the continent began to override and subduct the adjacent oceanic crust beneath the Pacific Ocean. Along with subduction of oceanic crust, uplift of the continental margin began, and erosion began to strip away the exposed rocks and sediments. Magma generated by subduction processes began to intrude upward, some of which reached the surface and erupted to form volcanoes. Beginning in the Jurassic Period, an extensive volcanic arc developed across the greater Mojave region. Deep below the surface, a series of great igneous intrusions (called batholiths) were gradually intruded throughout the region. These igneous rocks (consisting mostly of granite) form the cores of many of the ranges throughout the Mojave region. Great granitic intrusions formed in the Jurassic (170-140 million years ago) and again in mid-Cretaceous time (about 100 million years ago).

Figure \(\PageIndex{4}\): Simplified geologic history of the Mojave region in three panels: A) Prior to 250 million years ago, the Mojave was a passive continental margin with sediment deposits. B) From 250 to 60 million years ago, volcanic arcs formed due to plate convergence. C) Starting 30 million years ago, extensional faulting created alluvium-filled basins, with strike-slip faults and erosion active today. “Generalized geologic history of the Mojave Desert region” by USGS, is in the public domain. Access a detailed description of this image.

Late Cenozoic Volcanism, Faulting, and a Wetter Climate

The history of late Tertiary and Quaternary faulting and tectonism in the Mojave and Colorado Desert Provinces is unlike any other geologic province in California. The tectonic framework is most similar to the basin and range style structures to the north and east. While crustal extension was important here, this region is lacking deep basinal deposits and large normal faults like the Basin and Range Province to the north. Rather than basin and range style faulting, fault systems active in the region today are associated with regional right-lateral shearing forces associated with the San Andreas Fault System, the Garlock Fault, and the Eastern California Shear Zone all of these fault systems display measurable right-lateral displacements in the range of hundreds of kilometers, along with a component of normal faulting.

Throughout the Late Tertiary and Quaternary periods, large volcanic eruptions occurred fairly frequently in association with extension of the crust (for more information, see Basin and Range. Volcanic ash blanketed the landscape, and many of these ash beds are preserved in the alluvial deposits that accumulated in the basins. Eruptions associated with cinder cones and lava beds within the Mojave National Preserve began in the Late Tertiary (around 7 Ma) and have continued episodically through late Quaternary time (in the past one million years). The last volcanic episode in this area occurred roughly 8,000 years ago.

Late Cenozoic Basins

Sedimentary rocks that fill the basins of the Mojave, like those of the Basin and Range province to the north, document the age of crustal extension and the erosional responses to uplifting ranges. Across the region, sediments seem to begin accumulating between 22-16 Ma, at the same time as early volcanic activity.

Rainbow Basin, near Barstow is a good place to practice field mapping and is therefore visited by many geology programs. The Barstow Beds, which are found in this area, are examples of fossiliferous units that provide insight into past climate in the region. The Barstow Beds are a 650 meter thick section of sedimentary units of Miocene age that are folded and faulted by subsequent tectonic activity. Mudstone, sandstone and conglomerate are interbedded with ash layers, which allow these units to be dated (16-13 Ma). Layers contain a range of mammal fossils including dogs, bears, cats, mammoths, horses, and camels. To the north, similar units are found at Red Rock Canyon State Park, indicating that these fauna were widespread at this time.

The Salton Trough in the southern region of this province is home of the Salton Sea. This transtensional basin (see the Basin and Range chapter) formed because of crustal stretching and subsidence caused by the meeting of the right lateral San Andreas Fault with the northern limit of the East Pacific Rise. This geologic interaction produced a large "graben" or rift valley that filled with sediments deposited by the Colorado River over millions of years. The Salton Sea was created in 1905 when the Colorado River breached an irrigation canal and flooded the low-lying region.

Video \(\PageIndex{1}\): California's Red Rock Canyon State Park

This short video discusses the geologic and other features found at Red Rock Canyon State Park.

References

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