TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. Using the Principle of Original Horizontality, has the Valley Springs Formation undergone major tectonic change? A) Yes. The ash/tuff is tilted more than 30 degrees from the horizontal; B) No. The tuff/volcanic ash is still relatively horizontal; C) It is impossible to tell because the tuff/volcanic ash is not layered.
4. Using the Principle of Superposition determine which is older, the Valley Springs tuff/volcanic ash or the Auriferous Gravels: A) The Auriferous Gravels are younger because they overlie the tuff/ash; B) The Valley Springs tuff/volcanic ash is younger because it overlies the Auriferous Gravels; C) It is impossible to tell by observing the outcrop at this location.
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples are relevant (list)?
6. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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GEOLOGY OF AURIFEROUS GRAVELS AND THE VALLEY SPRINGS FORMATION
This EarthCache teaches more about the Valley Springs Formation and the auriferous gravels that crop out next to one another along Camp Nine Road. You will examine their physical characteristics and spatial relationships to understand more about the geologic history of this area.
Valley Springs Formation
The Valley Springs Formation is a volcanic tuff (from the Italian “tufo”, a type of rock made of volcanic ash ejected from a vent during an eruption), formed from pyroclastic flow deposits, that is thickest closer to Sonora Pass and thinner at the base of the Sierra foothills (see Figure 1). The volcanic vent system that produced the ash likely existed to the east of Sonora Pass and was active during Oligocene time (approximately 35-23 Ma (million years ago)). The ash has been age dated at 20-33 Ma. The tuff has been reworked by water and deposited in the Central Valley. It overlies the older Ione Formation, another volcanic ash and sedimentary deposit along the foothills of the Sierra and it underlies the younger Mehrten Formation (see Figure 2). The Ione Formation has been hypothesized to have formed during Eocene time (approximately 55-35 Ma). It is no younger than 37-40 million years old. The Mehrten Formation (volcanic mudflows, lava flows, and volcanic ash deposits) formed during the late Miocene to Pliocene (approximately 14-3 Ma). In describing the Valley Springs Creely and Force (2007) wrote:
“The Valley Springs Formation consists mostly of relatively fine grained sedimentary and/or pyroclastic rock. Rhyolitic detritus is generally present, and in some instances is dominant. The most characteristic rock low in the section is hard claystone with numerous sub-vertical fractures (probably the “clay rock” of the early folios). It is typically light-colored, ranging from white to light gray or light tan, generally with a faint but distinct greenish cast. A few beds of conglomerate also are found. A distinctive basal conglomerate is present locally.”
Additionally Rapp (1982) reported:
“Much of the Valley Springs Formation consists of weathered and reworked tuff. Where weathering is pervasive and intense, the Valley Springs Formation appears to have the same color and texture as the commercially important lone Formation. Clay of the lone formation is mostly kaolinite, whereas clays of the Valley Springs are montmorillonite. The kaolinite of the lone Formation was derived from deep weathering of granitic bedrock. Montmorillonite of the Valley Springs Formation was derived from the alteration of plagioclase feldspar. The montmorillonite of the Valley Springs Formation nearly negates its value as a commercial clay.” (Rapp, 1982).
In general, the Valley Springs Formation consists of: 1) white to pale yellow layers of tuffaceous siltstone deposited by water; and 2) weathered rhyodacite tuff.
Figure 1: Map showing the Valley Springs Formation in the Central Sierra Nevada.
Figure 2: Stratigraphic Column showing the Ione, Valley Springs, and Mehrten Formations (modified from Creely and Force, 2007).
So how do geologists come up with the names of volcanic rocks in order to compare and contrast them so that they can understand the geologic history of an area? After locating field samples, we first try to identify the minerals in the rock. If the rock is coarse-grained, they can use a QAP diagram (see Figure 3) to classify the rock by assessing the percentage of quartz (Q), alkali-feldspar (A), and plagioclase feldspar (P). If the rock is too fine-grained, then we use bulk chemistry, or whole rock analysis techniques. This includes taking a sample of the unknown rock, grinding it up, and running it through a machine called a mass spectrometer to obtain very accurate measurements of what elements are in the rock, and how much of each is present. This gives petrographers (geologists who specifically study rocks) valuable information on the chemical composition of the unknown sample, and, therefore, the ability to likely determine what type of rock it is. The key is to determine the percentage of alkalis (sodium and potassium) compared to silica (quartz) and the graphic result determines the name from a TAS (Total Alkali Silica) diagram (see Figures 4 & 5).
Figure 3: QAP (Quartz, Alkali Feldspar, Plagioclase Feldspar) Diagram for classifying volcanic rocks.
Figure 4: Volcanic Rock Classification Chart showing where the rhyodacite volcanic ash flows from the Valley Springs Formation plot.
Figure 5: Rhyodacite tuff (volcanic ash), from the Miocene Valley Springs Formation, Camp Nine Road, near Vallecito, CA. The largest (dark) crystals are approximately 1 mm in diameter.
Tertiary Rivers and Auriferous Gravels
Auriferous (from the Latin “aurum” meaning gold and “aurifer” meaning gold-bearing) gravels are gravel-sized sediments containing gold. There was great interest in finding and understanding these gravels due to their “rich” nature. Gravels without gold were called “barren”. These gravels were deposited by rivers that can be traced back to Tertiary time (from approximately 66-2.6 million years ago). The Tertiary is no longer recognized as a formal unit (it has been divided into the Paleogene and Neogene units), but the term is still widely used. Tertiary rivers flowed from the eastern part of the Sierra down to the Central Valley, but along different pathways than today. They left behind auriferous gravel deposits along ridges, the sides of hills and mountains, and in valleys. Miners were very interested in the auriferous gravel gold resources and geologists spent time studying and mapping the old channels from their deposits (see Figure 6).
During the Eocene, auriferous gravels were accumulating along the slopes of the Sierra Nevada while Ione Formation sands and clays were being deposited into a gulf that existed in the Central Valley at the time. Valley Springs volcanic ash was later eroded from volcanoes to the east, then deposited in stream channels along the slopes and at the base of the Sierra Nevada on top of the Ione Formation. As volcanic eruptions continued over geologic time near the crest of the Sierra, stream channels filled with their volcanic debris and streams were forced to find new courses downhill. Older channel deposits were then left high and dry in places, later to be exploited for their placer gold. Most of the gold extracted from Tertiary channels was smaller, finer gold, the size of wheat grains or smaller. The largest gold pieces found in the Sierra were found in veins, such as the 195 pound (Troy) chunk that came out of the Carson Hill mine in the 1850’s. The larger nuggets from stream deposits most likely came from smaller tributary streams or gulches. The gold probably had not been transported far from its source in a quartz vein in bedrock nearby. Columbia basin had several large (approximately 50 pound) pieces of gold that were reported discovered lying on the ground in abandoned river channels.
“In 1858 a nugget, weighing over fifty pounds and containing over eighty-five hundred dollars worth of gold [@ $20/oz], was found by a Mr. Strain about half a mile east of Columbia in Tuolumne county on a trail leading up the slope of a hill, where it had been repeatedly passed by others.” (Hettell, 1898).
Much gold was eroded from veins by Tertiary rivers in the Sierra Nevada. Additionally, Quaternary streams have reworked some of those deposits and redeposited them at the base of the foothills. Though this EarthCache is not directly related to gold dredging, it is notable that the fine gold that was deposited near Jenny Lind in Calaveras County, by LaGrange in Stanislaus County, and at Merced Falls, in Merced County, all had large gold dredges operating up until the 1950’s pulling significant quantities of fine gold from the cobbles, gravels, sands, silts, and clays deposited by Quaternary rivers (See Figure 7).
Figure 6: Map of estimated Tertiary River locations in the Central Sierra Nevada.
Figure 7: Diagram and image showing imbrication, a sedimentary feature, where some rocks (those longer than they are wide) show a tiling pattern in the downstream direction, much like how dominoes can align (from Rowan, 2008). Imbrication helps geologists infer the direction the water flowed (the paleocurrent) when the rocks were deposited. Knowing the paleocurrent direction of auriferous gravel deposits has helped geologists to map the Tertiary river channels in the Sierra Nevada.
Steno’s Principles
The Ione Formation can be partly understood by using 3 principles developed by Nicholas Steno in his work titled: “De Solido Intra Solidum Naturaliter Contento — Dissertationis Prodromus” (“Provisional Report on Solid Bodies Naturally Embedded in Other Solids”). They are:
1. Principle of Superposition
"At the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." After observing local stream behavior, Steno concluded the oldest layers of sediments will be at the bottom of the sequence and the youngest will be on top.
2. Steno's Principle of Original Horizontality
". . . strata either perpendicular to the horizon or inclined to it, were at one time parallel to the horizon." Steno thought layers of sediment are originally deposited horizontally under the action of gravity.
3. Steno's Principle of Lateral Continuity
"Materials forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno believed that layers of sediment initially extend laterally in all directions (until something stops them).
For this part of the EarthCache, use Steno’s Principles to answer the following question: Has the Valley Springs Formation undergone major tectonic change? A) Yes. The volcanic sediments are tilted more than 30 degrees from the horizontal; B) No. The sediments are still relatively horizontal; C) It is impossible to tell because the sediments do not show layering.
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EARTH SCIENCE BIG IDEAS
The Earth Science education community put together 9 “Big Ideas” for the Earth Science Literacy Initiative (ESLI), shown below. Their purpose was to highlight the main concepts and ideas a person should understand to be literate in the earth sciences:
An Earth-science-literate person:
• understands the fundamental concepts of Earth’s many systems
• knows how to find and assess scientifically credible information about Earth
• communicates about Earth science in a meaningful way
• is able to make informed and responsible decisions regarding Earth and its resources
Which of these Big Ideas below do you think are most relevant to this EarthCache?
Earth Science Literacy Project:
1. Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
2. Big Idea 2: Earth is 4.6 billion years old.
3. Big Idea 3: Earth is a complex system of interacting rock, water, air, and life.
4. Big Idea 4: Earth is continuously changing.
5. Big Idea 5: Earth is the water planet.
6. Big Idea 6: Life evolves on a dynamic Earth and continuously modifies Earth.
7. Big Idea 7: Humans depend on Earth for resources.
8. Big Idea 8: Natural Hazards pose risks to humans.
9. Big Idea 9: Humans significantly alter the Earth.
For more details see: Earth Science Literacy Initiative
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GEOPRINCIPLES
There are several fundamental principles, developed over time, that guide geological reasoning and critical thinking, listed below. Read each short description, then use your best judgement to determine which principle, or principles, best relate to this EarthCache.
7 Principles in Geology:
1. Superposition – the oldest strata are at the bottom of the sequence
2. Original Horizontality - layers of sediment are originally deposited horizontally
3. Lateral Continuity - layers of sediment initially extend laterally in all directions
4. Faunal Succession - fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances
5. Law of Intrusive Relationships - the geologic feature which cuts another is the younger of the two features
6. Uniformitarianism - the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe
7. Catastrophism - the theory that the Earth has been affected in the past by sudden, short-lived, violent events, possibly worldwide in scope
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LOGISTICS AND SAFETY
This site is on Camp Nine Road, approximately 1 mile from Parrotts Ferry Road in Calaveras County. Please note that the road is a narrow, one lane road in many places so be careful while driving, parking, and walking along the road to complete this EarthCache.
Visitors to this site should plan ahead and prepare by:
• Knowing the regulations and special concerns for the area you are planning to visit (obeying laws that prohibit collection or destruction of artifacts);
• Carrying a map and a GPS unit and/or compass;
• Staying on existing roads and trails;
• Staying away from any/all mine shafts and adits;
• Planning for extreme weather, hazards, and emergencies;
• Being aware that cell phones DO NOT usually work in the rural areas away from the major highways;
• Leaving your travel plans with a responsible party, including the date and time of your return;
• Being aware of any natural hazards associated with the region (e.g. poison oak, rattlesnakes, mosquitoes, cliffs/steep slopes, etc., etc);
• Carrying a full-size spare tire, extra food, water, and warm clothing;
• Following the “Tread Lightly" and "Leave No Trace” philosophy.
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TO LOG THIS EARTHCACHE INCLUDE
1. The name of this EarthCache on the first line.
2. The number of people in your group.
3. Using the Principle of Original Horizontality, has the Valley Springs Formation undergone major tectonic change? A) Yes. The ash/tuff is tilted more than 30 degrees from the horizontal; B) No. The tuff/volcanic ash is still relatively horizontal; C) It is impossible to tell because the tuff/volcanic ash is not layered.
4. Using the Principle of Superposition determine which is older, the Valley Springs tuff/volcanic ash or the Auriferous Gravels: A) The Auriferous Gravels are younger because they overlie the tuff/ash; B) The Valley Springs tuff/volcanic ash is younger because it overlies the Auriferous Gravels; C) It is impossible to tell by observing the outcrop at this location.
4. Which Big Ideas (1-9) are connected (list)?
5. Which GeoPrinciples are relevant (list)?
6. Include a photo or 2 if you're so inclined (optional).
Note: In order to manage email volume, you may assume your responses are accurate if you do not get an email after logging this EarthCache. If a response is grossly inaccurate, you will not receive credit for the cache.
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Note: For a brief summary of the geologic history of the Central Sierra, see this EarthCache:
Dragoon Gulch EarthCache
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REFERENCES
1. Bartow, J. Alan, 1992, “Contact Relations of the Ione and Valley Springs Formation in the east-central Great Valley, California”, Open File Report 92-588, U.S. Department of the Interior, United States Geological Survey.
2. Busby, Cathy J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., 2016, “Progressive derangement of ancient (Mesozoic) east-west Nevadaplano paleochannels into modern (Miocene–Holocene) north-northwest trends in the Walker Lane Belt, central Sierra Nevada”, Geosphere 12, p. 135-175, 2016, http://www.geosphere.gsapubs.org.
3. Busby, Cathy J., Koerner, Alice, Hagan, Jeanette, and Andrews, Graham, 2012, “Sierra Crest graben: a Miocene Walker Lane Pull-apart in the Ancestral Cascades Arc at Sonora Pass”, in, N. Hughes and Garry Hayes (eds), “Geological Excursions, Sonora Pass Region of the Sierra Nevada”, Far Western Section, National Association of Geoscience Teachers field guide, p. 8-36.
4. Creely, Scott and Eric R. Force, 2007, “Type Region of the Ione Formation (Eocene), Central California: Stratigraphy, Paleogeography, and Relation to Auriferous Gravels”, Open-File Report 2006-1378, U.S. Department of the Interior, United States Geological Survey.
5. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.
6. Gamez, J., Gini, R., Fau, E., Hinch, L., Orlanda, H., 2016, “Ione Formation”, Geology 103 Sedimentology and Stratigraphy, CSU Sacramento, Professor Tim Horner, poster presented during Spring Semester 2016, http://www.csus.edu/indiv/h/hornert/geol_103_spring_2016/group%20project/ione%20poster.pdf.
7. Garside, Larry J., Henry, Christopher D., Faulds, James E., and Hinz, Nicholas H., 2005, The upper reaches of the Sierra Nevada auriferous gold channels, California and Nevada, in Rhoden, H.N., Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005.
8. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.
9. Putnam, Roger, (pers. comm.), May 2017, Professor of Earth Science, 11600 Columbia College Drive, Sonora, CA, 95370.
10. Rapp, John S., October 1982, “The Valley Springs Formation in the Sonora Pass Region”, Volume 35, Number 10, California Geology, California Division of Mines and Geology, p. 211-219.
11. Schweickert, Richard, 2006, “Accretionary Tectonics of the Southern Part of the Western Sierra Nevada Metamorphic Belt” (modified from a 1999 guidebook article by Schweickert, Girty, and Hanson), in J. Tolhurst (ed), “Geology of the Central Sierra”, National Association of Geoscience Teachers Far Western Section Fall Conference field guide, p. 55-95.
12. Waggoner, B. (n.d.), “Nicholas Steno (1638-1686)”, retrieved June 18, 2017, from http://www.ucmp.berkeley.edu/history/steno.html.
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