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
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
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
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
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
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
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. Down cutting 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.