Roger Weller, geology instructor
firstname.lastname@example.org last edited: 10/20/10
A Brief Geologic History Leading to the Formation
of the San Pedro River Valley, Cochise County, Arizona
The geology of Southeastern Arizona is complex:
geology outcrops in southeastern Arizona (maps)
What was to eventually become Southern Arizona started out as offshore deposits of sand and silt adjacent to a landmass called Laurentia (which eventually became the North American continent). Due to early plate tectonics (continents moving around and banging into each other), these sediments were compressed, deformed, and metamorphosed by mountain building forces. Radioactive age dates suggest major events at 1.7 and 1.4 billion years ago. These rocks today are known as the Pinal schist.
During the Precambrian the uplift of mountains and erosion of these mountains happened repeatedly. Just prior to the end of the Precambrian a major world- wide period of extensive erosion occurred, grinding off thousands of feet of overlying rock and creating a nearly flat surface. Some geologists theorize this erosion was created by an ice age that lasted tens of millions of years.
During the Cambrian period, the land slowly subsided and was covered by a shallow sea. As the shoreline advanced across the land, a thick layer of sand was deposited that eventually became the Bolsa quartzite. In the Bisbee area this layer was 340 feet thick.
As the land continued to sink below the shallow ocean, a thick sequence of thin-bedded siliceous, calcareous sands were deposited on top of the Bolsa quartzite. These beds are known as the Abrigo formation. Fragments of fossil trilobites and brachiopods have been found in the Abrigo formation.
Although flooding occurred in other parts of the early North American continent, southern Arizona was above sea level at this time and therefore did not receive any Silurian sediments.
During the Devonian era, the region that was to eventually become southern Arizona again subsided and was covered by a warm shallow ocean. A dark-colored limestone, the Martin formation, was deposited that was rich in fossil life forms such as brachiopods and corals. Exposures of Martin limestone are often slope-formers. Castle Rock in old Bisbee is a chunk of Martin limestone.
The shoreline moved further away from the Cochise County area and a light-colored, thick-bedded limestone, known as the Escabrosa limestone was deposited. This limestone is made up of the crushed remains of crinoids. Where the Escabrosa limestone is exposed, it is a bright light-colored cliff-former.
The land briefly rose up above the waves and the settled back down again for a third period of major flooding. During the Pennsylvanian, more limestone was again laid down. The layers of this new limestone, the Horquilla formation, has a step-like appearance on the side of hills. Large brachiopods, sea urchin spines, and corals are often found in the Horquilla limestone.
Additional layers of limestone from the third period of flooding continued to be deposited during the Permian. Some of these layers contain small gastropods (snails).
Summary of the Paleozoic Era
The total thickness of sediments laid down during the Paleozoic era add up to approximately 5000 feet of mostly limestones! As the Paleozoic era ends, the deposition of shallow ocean in a quiet tectonic setting also ends. The continent of Laurentia (North America) has collided with Europe Africa, and South America to become Pangaea, a giant continent.
The Mesozoic era is a time of change. Mountain building starts up along with major tectonic activity. The land is lifted up and is subjected to faulting, folding, igneous intrusions, and major volcanism. Eventually Pangaea breaks up and the North American continent moves away westwards.
Mostly erosion is occurring in southern Arizona.
Tectonism and igneous activity start up big time. Broad folds start to form from southwest tending forces and thrust faults soon follow from compressive forces. Large volcanic eruptions from several calderas occur in southern Arizona. The igneous event that produces the Juniper Flats granite in the Mule mountains and the Sacramento igneous stock that produces the copper deposits in Bisbee are Jurassic in age.
Major erosion follows these tectonic and igneous events and large amounts of the thick Paleozoic sedimentary rocks are ground off the uplifted fault blocks.
During the Cretaceous period another major world-wide flooding event occurs. To explain this event it has been proposed the the floor of the Pacific ocean was uplifted by a major heating event taking place in the mantle of the Earth. As the floor of the Pacific ocean bulged upwards the water in the Pacific ocean had to move elsewhere and so sea levels rose dramatically around the world.
In the region of Cochise County, the first material to be deposited are pebbles derived from the exposed Precambrian Pinal schist. This maroon-red red layer of angular pebbles is known as the Glance conglomerate. As the shoreline transgressed and the water got deeper, sands were laid down that eventually became the Morita formation. At its deepest extent, a very fossil-rich layer of limestone was laid-down, the Mural limestone. This limestone layer received its name from the large number of fist-sized fossil snails and clams exposed on cliff surfaces; the early inhabitants of Bisbee thought that these fossils were Indian carvings. As this last ocean slowly withdrew from the area, additional sands were deposited (Cintura formation).
The Cretaceous flooding event deposited an additional 5000 feet of sedimentary rocks, approximately the same amount as the three major periods of flooding during the Paleozoic.
The Cretaceous was not quite finished with surprises. A tectonic event marked by compressional folding and igneous intrusions started about 80 million years ago and continued locally to about 55 million years ago, overlapping the Mesozoic and Cenozoic eras. The cause of this event is tied to plate tectonics. North America had started moving to the west about 190 million years ago during the Mesozoic. Hundreds of miles of the Pacific plate had already been subducted beneath the western edge of North America. So why the sudden increase in compression? Probably North America had just run into a portion of the East Pacific rise, a spreading ridge, that the continent could not quite climb over. Consequently the collision lead to compression and thrust faulting. This event is known as the Laramide orogeny and is responsible for many copper deposits such as Twin Buttes south of Tucson and Cananea in Sonora, Mexico.
The Cenozoic starts out with a bang. A large asteroid has just slammed into the Yucatan peninsula in Mexico and the cataclysm has ended the reign of the dinosaurs. A green glass tektite, perhaps formed by rock being melted by the impact and splashed outwards, was found in the Huachuca Mountains.
After the Laramide orogeny calms down, erosion takes over again, The area that is to become the Huachuca mountains is flatted by erosion between 50 and 30 million years ago.
Another period of tectonic and volcanic activity starts up in mid Tertiary times. From about 35 to 25 million years ago the volcanoes making up the Tucson mountains erupt, the Santa Catalina mountains are uplifted, and the Turkey Creek caldera in the Chiricahuas blows its top.
The geologic event that produces much of the current topography in southern Arizona, basin and range faulting, starts abut 15 million years ago and continues to about 6 million years ago. This is a period of crustal extension, probably caused by a spreading ridge now sitting directly under southern Arizona. During this crustal extension, steeply dipping faults form and the continental crust is broken into large blocks. Some of the blocks are down dropped (grabens), forming valleys, while others are uplifted to form mountain ranges (horsts). Many of the blocks are tipped, so that the flat beds of Paleozoic and Mesozoic sedimentary rocks are now longer horizontal. Although most of basin and range tectonic activity ended about 5 million years ago, small displacements of the blocks continued. In 1887, the eastern side of the San Bernardino graben, southeast of Douglas dropped about 7 feet suddenly creating a major earthquake. Small basaltic lava flows and more than 20 small volcanic cinder cones in the San Bernardino Valley with ages ranging from 5 million to 300,000 years indicate that there is still some restless geologic forces still in the area.
Scenery is slowly becoming more familiar. Erosion of the mountain ranges is covering the down dropped graben blocks, such as the San Pedro valley with hundreds of feet of gravel, sand, silt, and clay.
San Pedro Valley
From 5 million years ago to 1 million years ago, the Upper San Pedro Valley had lakes, playas, and streams. From 4 to 2 million years ago there was a large lake in the Benson area that was between 200 and 300 feet deep. Sedimentary deposits at the edge of the lake can be seen in the St. David area as eroded badland deposits. The lake dried up before the last period of Ice Age glaciation (Wisconsin).
As the climate changed, the nature of the San Pedro River valley changed. Lake deposits were prominent fro 5 to 3 million years ago. Channels and flood plains were prominent from 3 to 1 million years ago. Downcutting by the San Pedro river started about 600,000 years ago. The deposition and downcutting were not a continuous process; there are three distinct pediments that have been identified.
The last 11,000 years have been a warm interglacial period, marked by valleys being incised in older gravels by streams.
The Quiburis formation, consisting of mudstones, siltstones, and volcanic tuffs has been dated from 6.5 to 4.5 million years ago.
The St. David formation, mostly mudstones, has been dated from 4.5 million years ago to about 500,000 years ago.
The Lehner ranch, located adjacent to the San Pedro river, is a Clovis period early man mammoth kill site.
Link to great website on the San Pedro River Valley