San Pedro Valley
- Geology
San Pedro Valley
- GeologyRoger Weller, geology instructor
wellerr@cochise.edu last edited: 6/4/08
Late Tertiary Surfaces and the Valley Fill Deposits of the San Pedro River
The portion of the San Pedro River Valley in Cochise County contained a large
freshwater lake between about two and four million years ago. The lakebeds
characteristic of that environment are seen well exposed along the road from
Benson nearly to Tombstone, and can be seen lapping up onto the flanks of the
Rincon Mountains to the north. The lakebeds contain abundant fossils of such
large animals as ground sloths, camels, large bears, mammoths, bison, turtles,
early horses, many rodents, and numerous plants. A few miles south of Saint
David , on the east side of the road are exposed some white beds which are
marls, composed of a mixture of limestone and clays. These suggest a fairly
large body of quiet water with not much drainage by way of a river from the
lake. In this same area is exposed a record of the lake drying up before the
last major glaciation, because a thick paleosol (fossil soil) is contained in
the section which is interpreted as forming during a time with a temperate or
moist climate with probably rather lush grasslands or scrub oak forests growing
in the area. The lake's maximum depth is hard to estimate because old
shorelines are destroyed, but it could have been 200 or 300 feet deep.
Following the deposition of the lakebeds, a whole series of alluvial sediments
was washed down to the valley floor from the highlands. These came first from
the Dragoon Mountains to the east, and then at a slightly later time, from the
newer mountains, which lie to the west- the Whetstones. The gradual
accumulation of these sediments with very shallow dips leaves behind alluvial
surfaces that can in this case be correlated from the area of Tombstone around
to the north side of the Santa Rita Mountains. This surface, which is called
the Whetstone surface, is now slowly being dissected by erosion, but remnants
are easily visible from I-10 between Tucson and Benson.
Following this alluvial
deposition in the San Pedro Valley, the land, which had been rising slowly for
quite some time, was elevated to a point where the through-flowing San Pedro
River began cutting away and removing the deposits in the center of the valley,
and thus exposing them to the eyes of the curious geologists. The actual age of
the lakebeds has been determined by studies of the fossils contained in them,
and by potassium-argon dating of volcanic as beds found mixed in with the
lakebeds.
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