Date post: | 08-Jul-2015 |
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The Great Unconformity
Frenchman Mountain Southern Nevada
http://www.geotimes.org/nov05/Travels1105.html
http://www.terraprints.com/catalog/nevada_satellite_map.jpg
Or, how I
learned
Why we need
Nevada!
• What is an
Unconformity?
• What is the great
Unconformity of
Frenchman mtn?
• What creates them?
• Where did the name
come from?
• An unconformity is a buried erosion surface. It is a surface of rock that was exposed on
the Earth's surface and was then covered by younger layers.
• Unconformities are important because they represent missing intervals of the geologic
record, like pages missing from a history book. The Great Unconformity of Frenchman Mountain represents about 1.2 billion years, which is more than one fourth of the age of
the Earth.
http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#whatis
• The concept of an unconformity arises from two of the oldest principles of geology, first stated in 1669 by Nicholas Steno:
• Layers of sedimentary rock (strata) are originally laid down flat, parallel to the Earth's surface. That's the law of original horizontality.
• Younger strata always overlie older strata, except where the rocks have been overturned. That's the law of superposition.
http://geology.about.com/od/geoprocesses/a/unconformities.htm
Frenchman Mountain
http://www.idahosummits.com/frenchman/images/frenchman%20mountain.jpg
LAS VEGAS “BOWL”
EAST- Frenchman
Mountain Range;
WEST - Spring
Mountain Range;
NORTH - Sheep
Mountain Range;
SOUTH - McCullough
Mountain Range
http://nevada-history.org/facts.html
http://geoscience.unlv.edu/pub/rowland/Virtual/hike.html
Forming the Great Unconformity
http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#name
FROM THE GRAND CANYON TO THE LAS VEGAS
BOWL…HOW A MOUNTAIN MOVES.
6-10 Million years ago western North America begins
moving west due to interaction between the North
American and Pacific lithospheric plates, which today
slide past one another along the San Andreas fault in
California.
Movement of blocks becomes tilted as secondary faults
encounter a nearly horizontal master fault causing the
blocks to tilt eastward while moving westward.
Continuous sliding on the master fault causes
Frenchman Mountain to move about 50 miles from
original position.
http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html
Precambrian granite (about 1.7 billion years old) and beginning with the white rocks exposed along the slope of the low hill, are the Cambrian
Tapeats Sandstone (about 500 million years old).
• Above the Great Unconformity we
see the sedimentary Tapeats
Sandstone with recycled large, light
gray, angular, quartz pebbles.
These rocks have a dark coating
called desert varnish or rock varnish
which is a thin manganese- and iron-
rich coating that develops on exposed
rock surfaces and is most common in
desert regions.
Bacteria, using chemicals from dust
particles,[produces this varnish over
thousands of years
http://geoscience.unlv.edu/pub/rowland/Virtual/hike.html
FRENCHMAN MOUNTAIN WAS PARTLY CREATED BY
MOVEMENT ALONG THE FRONT RANGE FAULT
SHOWN BELOW
http://www.wheaton.edu/ACG/trip/jpgs/35b.jpg
http://img.geocaching.com/cache/log/423d19f3-5359-49b0-97ea-25560519bb36.jpg
Granite intrusion in the schist.
http://img.geocaching.com/cache/log/577d4037-28e0-4cc6-8aaa-9433f8aaf221.jpg
Precambrian granite and schist / Tapeats sandstone
Some old guy and The Great Unconformity
CROSS SECTION OF FRENCHMAN
MOUNTAIN.
Vishnu granites and the overlying Tapeats Sandstone
http://www.wheaton.edu/ACG/trip/stop2a.html
•Clarence Dutton in his 1882 book Tertiary History of the Grand Cañon District.
•People assume that the Great Unconformity was so named due to the large
interval of time it represents in the Earth’s history.
•The truth is Dutton had no idea of the age of the rocks on either side of the
Unconformity.
•Basically, he knew the importance of this find and that something was
missing but had no idea of the extent of time that had passed between the
Cambrian age Tapeats sandstone and the granite and schist that lie beneath it.
WHERE DID THE NAME “GREAT
UNCONFORMITY” COME FROM?
http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#name
Great Unconformity Geologic Interpretive Site and Trail
http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html
http://www.flickr.com/photos/kitkattt9/2061774736/
http://www.flickr.com/photos/kitkattt9/2061775502/
West face
Igneous laccolith, Lava Butte south of Frenchman Mtn, Alluvial fans and Lake Mead.
http://www.idahosummits.com/frenchman/images/lake%20mead.jpg
Eastern Face of Frenchman Mtn. with Las Vegas in the foreground.
Peak of Frenchman mtn.