South Lake TahoeADVENTURES IN TIME

Field StudyGeology 103Prof. Mark Lawler8/3/2014Lake Tahoe Community College

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.

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.

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.

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.

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).

Magmatic Intrusions: Tallac Medisedimentary Rock

The “roof pendant” from magma interactions with country rock (sedimentary) yield a metamorphic rock called Tallacmetasedimentary.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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

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.

Fauna: Bald Eagles

Bald Egales can be viewed at Baldwin Beach, Taylor Creek and several other places along the lake.

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.

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