Geology of the Muleshoe Preserve

by Lisa Ely

The Muleshoe Ranch, managed as a Nature Conservancy preserve, is located in the Galiuro Mountains, a remote southern Arizona mountain range. This range forms part of the Basin and Range Province, a vast sweep of land including nearly all of Nevada and portions of California and Arizona. The characteristic basin and range topography consists of a series of parallel, north-south trending ranges separated by wide linear valleys.

The geologic history exposed in the cliffs and canyons of the Galiuro Mountains reaches from the Precambrian Era, over 1.7 billion years ago, up to the present, as geologic forces continue to alter the mountains and surrounding valleys. The oldest formation in the Galiuros are the Precambrian Pinal Schist and Apache Group sedimentary rocks. These rocks are not seen by most visitors to the Muleshoe Preserve as they form a relatively small exposure in the northwestern portion of the mountain range. The Pinal Schist is composed of metamorphic rocks that were buried and deformed during an ancient mountain building episode, or orogeny, that is unrelated to the present day topography of the Galiuro Mountains.

Later in the Precambrian (still over 570 million years ago), layers of sedimentary rocks were deposited by the encroaching sea on top of the partially eroded Pinal Schist. these sedimentary formations, known as the Apache Group, consist of layers of sandstone, limestone, and conglomerate. Both the Pinal Schist and the Apache group are found in several mountain ranges throughout southern Arizona.

Throughout the Paleozoic and Mesozoic Eras this region was covered or intruded by various sedimentary, volcanic and igneous geologic formations. These rocks have been largely buried by the locally dominant Tertiary Galiuro Volcanics. The Galiuro Volcanics form the bulk of the Galiuro Mountain Range. During the Mid-Tertiary Period, from about 28 to 15 million years ago, southern Arizona was a hot spot of geologic activity. Regional forces broke and folded the earth's crust into new mountain ranges, resulting in violent explosive volcanic eruptions that spread thick layers of ash and lava over hundreds of miles. The Galiuro Mountains were one of many centers for this volcanic activity. Many other mountain ranges were built up during this period, including the Santa Catalina, Rincon and Tortolita Mountains near Tucson, and the Chiricahua Mountains to the southeast.

The geologic formations near the Muleshoe Preserve headquarters are all part of the Galiuro Volcanics. The buff-colored rock along Bass Canyon is a series of ash-flow tuffs. The tuffs form when volcanic ash and steam fall to the earth and flow across the land in a thick layer. If the ash is still hot enough, it melts slightly upon settling and reconsolidates into a welded ash-flow tuff. A close look at the tuffs along Bass Canyon will reveal a variety of rock fragments that were catapulted into the air during the volcanic explosions and incorporated into the ash flow.

In several locations, layers of large cobblestones separate the ash units. these conglomerates were deposited by streams during the periods between volcanic eruptions. Although we speak of the Mid-Tertiary orogeny as a single episode in the geologic history, it actually consisted of short bursts of volcanic activity spread over millions of years. For most of this period the surface of the land lay exposed to the erosional action of wind and water. Modern processes of erosion are responsible for the undulating cliffs and spires that one can see while looking out over Bass Canyon. This cliff and slope topography is caused by the relatively soft and easily eroded welded tuffs.

The large black basalt boulders in the streambed of Bass Canyon contrast with the soft buff-colored tuffs of the canyon walls. The basalt originates from a lava flow that outcrops near the headwaters of Bass Creek. Basalt is an igneous rock comprised of very small tightly interlocking crystals which make it extremely resistant to the physical rolling, pounding and grinding of the swiftly flowing stream. By contrast, the loosly consolidated chunks of ash-flow tuff are quickly ground to sand-sized particles by the stream action.

The basalt flow in the upper reaches of Bass Creek formed in the early Pleistocene, about 2 million years ago. This volcanic flow may have been caused by the dwindling effects of the Basin and Range period in southern Arizona, which peaked between 15 and 8 million years ago. The Basin and Range disturbance involved an east-west tension that stretched the earth's crust in the western part of the North American continent. The land gave way in a series of north-south trending faults, which caused the basins to drop drastically in elevation relative to the elevation of the adjacent mountain ranges. Evidence of some of these nearly vertical faults can be seen in the steep western flank of the Galiuro Mountains.

The last two million years witnessed dramatic changes in the Galiuro Mountains and the adjacent San Pedro Valley. Thick beds of sediment were deposited in the valley. Erosion has now cut vast badlands out of these sediments where scientists found the bones of hundreds of prehistoric mammals, such as the forerunner of the modern horse. About 11,000 years ago, at the end of the last ice ago, hunmankind first entered this region. In recent decades archaeologists discovered the butchered remains of mammoth, bison and camel around the campfires of these ancient peoples, called the Clovis culture.

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