Initial Field Study by a wannabe geologist studying at Lake Tahoe Community College. I blame it all on Professors Mark Lawler and Scott Valentine.

1. South Lake Tahoe
ADVENTURES IN TIME
Field Study
Geology 103
Prof. Mark Lawler
8/3/2014
Lake Tahoe Community College

2. Adventures in Time
The Lake Tahoe Basin is a treasure chest of evidence of all manner of geologic history. Here we can find traces
of:
 Accretion
 Magmatic Intrusions
 Uplift and Extension tectonics
 Volcanism
 Glacial scouring
 Fauna
Unless otherwise noted, all material comes from lecture notes from Scott Valentine, LTCC 2013, 2014, or, my
own observations.
Photo credits are mine, unless otherwise noted.

3. Accretion
 Accretion is a process by which material is added to a tectonic plate or a
landmass. This material may be sediment, volcanic arcs, seamounts or other
igneous features (Valentine, 2014).
 Much of the western half of North America is made up of accretionary material
that was welded or sutured onto the craton and platform.
 We find evidence of this in some of the rocks in the basin, most of which can be
found in the broken debris left on the glacial moraines.
 The following pictures will show some of the rock types discovered by this ardent
explorer. 

4. Accretionary Rocks: Chert
 Chert is a metamorphic rock formed in association
with an accretionary event.
 Hard and brittle like flint, chert fractures
conchoidally and was used by early humans for
cutting tools.
 Chert is a microcrystalline or cryptocrystalline
sedimentary rock material composed of silicon
dioxide.
 Some of the SiO2 in chert has a biological origin.
Shallow seas were home to diatoms and
radiolarians, which had a glassy silica skeleton.
When these organisms died their silica skeletons
fell to the bottom, dissolved, recrystallized and
become part of a chert nodule. Chert formed in
this way could be considered a biological
sedimentary rock.

5. Accretionary Rocks: Schist (Blue)
 High-pressure, low-temperature conditions are
most typical of subduction, where marine crust
and sediments are carried beneath a
continental plate and kneaded by changing
tectonic motions while sodium-rich fluids
marinate the rocks.
 This example seems to be made from sodium-
rich mafic rocks like basalt and gabbro.
 Schist is somewhat rare in the Sierra Nevadas,
and this sample seems to affirm the notion
that this area was once covered by a shallow
sea.

6. Magmatic Intrusions
 Due to the subduction of the ancient Farallon Plate by the North American Plate, an up
welling of magma occurred in the form of Plutons.
 As the magma came into contact with “country rock”, contact metamorphism occurred
and roof pendants were created.
 The many plutons of magma formed the Sierra Batholith (Collection of plutons).
 The magma plutons cooled slowly underground forming granodiorite “balloons”.
 The uplift of the Sierras increased the rate of erosion and eventually the granitic
“balloons” were exposed.
 We see these plutons exposed today throughout the Lake Tahoe Basin.
(Valentine, 2014).

7. Magmatic Intrusions: Tallac Medisedimentary Rock
 The “roof pendant” from magma interactions
with country rock (sedimentary) yield a
metamorphic rock called Tallac
metasedimentary.
 Metasedimentary denotes that parent
material was sedimentary.
 Note lines of dark and white material
represent layers, most likely mud and ash, or
possibly sandstone.
 The rust on the rock is oxidized ferrous
material. The mud layers are iron rich, just as
ocean mud is expected to be.

8. Magmatic Intrusions: Granodiorite
 Grandodiorite is an intrusive rock of the
granitic family, and is made up of quartz,
feldspars, micas and biotites.
 Granitic rocks are formed from slowly
cooling magma intrusions deep
underground.
 Inclusions are bits of country rock that have
fallen into the intruding magma chamber
from the top of the chamber, usually.
 Magenta arrows point to inclusions.

9. Magmatic Intrusions: Inclusions
 Andesite with inclusions was a new one for me.
 Inclusions are bits of country rock that have
fallen into the intruding magma chamber from
the top of the chamber, usually.
 This specimen comes from a bed of rock that
suffered through periods of glaciation, erosion
and thermal cracking (frost),located on a trail
between Fallen Leaf and Lily lakes.

10. Uplift & Extensional Tectonics
 The Sierra Nevada Mountains were uplifted due to compressional tectonics
approx. 100 ma, as the leading edge of the North American Plate “lurched” over
the subducting ancient Farallon Plate.
 The crust behind the leading edge was stretched, extended and caused normal
faulting to occur, forming the Basin & Range geologic province.
 The Lake Tahoe Basin was initially formed during an extensional episode as the
“block” dropped forming a graben.
 The Tahoe Basin was not formed in one singular event, thus we find evidence in
scarps and escarpments.

11. Uplift & Extensional Tectonics: Grabens
 Horsts and Grabens are formed
from normal faulting, and
associated with extensional
tectonics.
 The crust extends (stretches),
allowing some blocks to drop,
while others may uplift.
 The Tahoe basin is a graben.
 Locally, the Sierras and the
Carson Range would be
considered horsts.
Tahoe
Basin
Sierra
Nevadas
Carson
Range
Photo Credit: MarsSed, 2010

12. Volcanism
 The Lake Tahoe Basin is centrally located in the Walker Lane Deformation Zone,
which is active today.
 Walker Lane Deformation Zone was volcanically active throughout the Cenozoic,
producing several volcanoes in the area surrounding Tahoe.
 Mount Pluto, and extinct volcano today, erupted approx. 35 ma and it’s lava flows
blocked off the north end of the Tahoe Basin.
 The main rock evidence of the volcanic activity is Andesite and Basalt, and,
occasionally a piece of obsidian.

13. Volcanism: Andesite
 Soils of the Tahoe basin come primarily from
andesitic volcanic rocks and granodiorite, with
minor areas of metamorphic rock.
 Andesite is an extrusive igneous, volcanic rock, of
intermediate composition, with aphanitic to
porphyritic texture.
 This andesite probably originated from volcanoes
of the Walker Lane Deformation Belt ~30 Ma.
 Note that Lake Tahoe is within the Walker Lane
Deformation Belt.

14. Glacial Scouring
 The Tahoe Basin has been sculpted by several periods of glacier activity, mainly the Tioga and
Tahoe glaciations.
 The Tioga Glaciation occurred approx. 25 ma.
 The Tahoe Glaciation occurred approx. 10 ma.
 Glaciers formed after the Sierran uplift occurred as the higher mountains attracted more moisture
and cooler temperatures.
 Cirques are evidence of places glaciers first formed.
 Moraines are the debris piles left behind marking a glacier’s passage and end.
 Erratics are boulders that were “plucked” by glaciers and left behind when the glaciers
melted/retreated.
 Striations and polishing occur when a glacier moves across harder rock surfaces, leaving shallow
grooves and in some places polished or smooth surfaces.

15. Glacial Scouring: Angora Cirque
 The “bowl” carved out of the
side of Angora Mountain &
Echo Peak is a Cirque, the
original home of the young
glacier.
 The glacier would then move
down, carving and bulldozing
as it grew.
 Within the cirque are small
lakes, called tarns.
 The Angora cirque was formed
during the Tahoe Glaciation.
 The Tahoe reached its
maximum extent perhaps
about 70,000 years ago.

16. Glacial Scouring: Striations & Polish
 Glaciers leave their mark on the
local landscape in the form of
striations (grooves cut into the
rock) and polished surfaces.
 Purple Arrow indicates striations
left from glacial movement.
 Magenta arrow indicates area of
glacial polish.
 These photos are from the Glen
Alpine glacial area on the south
end of Fallen Leaf Lake, nearer to
Lily Lake.

17. Fauna of the South Lake Tahoe Basin
 The South Lake Tahoe Basin is home to several interesting species which might
include:
 Beavers
 Bald Eagles
 Chickarees
 Coyotes
 Bears

18. Fauna: Beavers
 Beavers were native in the area
until trapping and hunting
virtually eliminated them.
 Beavers were reintroduced to the
area in 1923.
 Beavers thrive along the Upper
Truckee River and several other
larger creeks and streams.
 Beaver dams are vital in helping
to return the clarity of Lake
Tahoe to its former grandeur.

19. Fauna: Bald Eagles
 Bald Egales can be
viewed at Baldwin
Beach, Taylor
Creek and several
other places along
the lake.

20. Fauna: Chickaree
 Four types of squirrels are abundant
in the South Lake area.
 Ground Squirrel
 Golden Mantled Squirrel
 Grey Squirrel
 Chickaree
 Chicakrees are somewhat elusive and
not seen as much as the others
mentioned above.

21. References
 Cenozoic volcanism in the Sierra Nevada and Walker Lane, California, and a new
model for lithosphere degradation. (n.d.). Retrieved from
http://geosphere.gsapubs.org/content/8/2/265.abstract
 MarsSed 2010 Field trip – Day 1: Guadalupe Mountains and Evaporites | The
Martian Chronicles. (n.d.). Retrieved from
http://martianchronicles.wordpress.com/2010/04/26/marssed2010-field-tri-day1/
(Photo Credit)
 Lectures by Scott Valentine, LTCC, 2013, 2014