San Pedro Valley - Geologic HistoryCochise College
Geology of Southeastern Arizona
Arizona Geology San Pedro River Valley
Virtual Geology Museum
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San Pedro Valley - Geologic HistoryRoger Weller, geology instructor
wellerr@cochise.edu last edited: 10/20/10
A Brief Geologic History Leading to the Formation
of the San Pedro River Valley, Cochise County, Arizona
Introduction
The geology of Southeastern Arizona is complex:
geology outcrops in southeastern Arizona (maps)
Precambrian
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.
Paleozoic Era
Cambrian Period
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.
Silurian Period
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.
Devonian Period
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.
Mississippian Period
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.
Pennsylvanian Period
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.
Permian Period
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.
Mesozoic Era
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.
Triassic Period
Mostly erosion is
occurring in southern Arizona.
Jurassic Period
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.
Jurassic Intrusives
Cretaceous Period
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.
Cenozoic Era
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.