Thrust And The Polyphase Laramide Orogeny
The Keystone Thrust is a famous as geological curiosity which has it's
strata turned upside down.
Normally we would expect that younger strata are found on top of
The Keystone Thrust defies common sense. The dark red streaks
that run through light gray limestone can be seen from the far reaches of
the Las Vegas Valley. These streaks or strata are topped by the older
with many of the exposed strata in this region, and much of mountainous Western North America
the Keystone Thrust was 'constructed' by a process
whereby huge tectonic plates shifted, ground into each other and did all
kinds of things to mess up the clean geological sequence.
For most of the Paleozoic Era, this part of the Southwest was under a
deep ocean. Sediments accumulated over the vast time span resulting
in the limestone formations we see today.
Around 225 million years ago (MYA), during the Mesozoic Era, the
movements of the Earth's crust caused this now deep sea-bed to rise
slowly. As the 'plates' which make up the Earth's crust continued to
move and the sea-bed continued to rise until it became a set of huge
lush plains. Streams criss-crossed these plains eroding them in some
places and depositing debris in others. Today we see this debris as
petrified logs and other fossilized matter. The 'Chinle' formation is
what we call this layer or 'strata'. It is seen at the base of the
Wilson Cliffs and elsewhere throughout the Southwest.
The tectonic plates upon which the surface of the earth floats on,
continued to move, push
and pull the western North American continent. By 180 MYA, this area had
become a vast red, desert. It continued that way until about 66 MYA. At
that time the shifting of the continental plates again began to lift,
stretch, compress and pull the Earth's surface. This activity continued
in earnest until approximately 23.7 MYA. Since then there has been
isolated activity. That and the process of erosion has created the
Red Rock Canyon that we see today.
The term 'orogeny' refers to the process of mountain building. The
shifting and collision of tectonic plates causes strata to be a) pushed
against other strata , causing some of the strata to rise; b) pushed
under other strata, causing the top strata to rise, or volcanic
In western North America the set of tectonically related events that
created much of the topography from Northern Mexico to Alaska is
referred to as the 'Laramide'. The effects are visible today from
Southern Nevada to the Northern Rockies. The most significant activity occurred
in Early Tertiary and late Cretaceous times.
The continental plates were basically pushed pulled and shoved around.
Once level strata were pushed up making them angular and tilted. At
other times the strata were pulled apart causing whole valley's to
slowly drop. The result today is the north - south orientation of the
mountains and valleys in the Great Basin.
As the various strata were pushed together, then pulled apart and then
shoved back together again, we get the Keystone Thrust. Through all this
movement over the countless years, the older Paleozoic limestone's which
were created from ocean bottoms, was folded over the younger Mesozoic
sandstones during the Tertiary or Cenozoic Era.
It was originally thought that the Laramide occurred only at the break
between the Tertiary and Cretaceous Era's. Now it is considered to have
been active at different places at different times since the Jurassic to
Oligocene times, 36.6 to 23.7 MYA.
Thus the 'Polyphase Laramide Orogeny' is the mountain building,
resulting from shifting tectonic plates, that went on in Western North
America from Jurassic times to as recently as 23.7 MYA. The result of
this symphony of events is the Great Basin topography and places like
the Keystone Thrust and Red Rock Canyon.