What is an Unconformity? What is the great Unconformity of Frenchman mtn? What creates them? Where did the name come from?

1. The Great Unconformity
Frenchman Mountain Southern Nevada

2. http://www.geotimes.org/nov05/Travels1105.html
http://www.terraprints.com/catalog/nevada_satellite_map.jpg
Or, how I
learned
Why we need
Nevada!

3. • What is an
Unconformity?
• What is the great
Unconformity of
Frenchman mtn?
• What creates them?
• Where did the name
come from?

4. • 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

5. • 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

7. Frenchman Mountain
http://www.idahosummits.com/frenchman/images/frenchman%20mountain.jpg

9. 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

10. http://geoscience.unlv.edu/pub/rowland/Virtual/hike.html

11. Forming the Great Unconformity
http://geoscience.unlv.edu/pub/rowland/Virtual/geology.html#name

12. 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

13. 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).

14. • 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

15. FRENCHMAN MOUNTAIN WAS PARTLY CREATED BY
MOVEMENT ALONG THE FRONT RANGE FAULT
SHOWN BELOW
http://www.wheaton.edu/ACG/trip/jpgs/35b.jpg

16. http://img.geocaching.com/cache/log/423d19f3-5359-49b0-97ea-25560519bb36.jpg
Granite intrusion in the schist.

17. http://img.geocaching.com/cache/log/577d4037-28e0-4cc6-8aaa-9433f8aaf221.jpg
Precambrian granite and schist / Tapeats sandstone

18. Some old guy and The Great Unconformity

19. CROSS SECTION OF FRENCHMAN
MOUNTAIN.

20. Vishnu granites and the overlying
Tapeats Sandstone
http://www.wheaton.edu/ACG/trip/stop2a.html

21. •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

23. Great Unconformity Geologic
Interpretive Site andTrail
http://geoscience.unlv.edu/pub/rowland/Virtual/virtualfm.html

24. http://www.flickr.com/photos/kitkattt9/2061774736/

25. http://www.flickr.com/photos/kitkattt9/2061775502/

26. West face

27. Igneous laccolith, Lava Butte south of Frenchman
Mtn, Alluvial fans and Lake Mead.
http://www.idahosummits.com/frenchman/images/lake%20mead.jpg

28. Eastern Face of Frenchman Mtn. with Las
Vegas in the foreground.

29. Peak of Frenchman mtn.