Dragoon Gulch and the Don Pedro Terrane EarthCache

This EarthCache teaches more about the geology of the metamorphic rocks in the Western Sierra Nevada Metamorphic Belt, in Tuolumne County, California. The rocks observed are in the Don Pedro terrane and earthcachers will hike to an outcrop, make measurements of the attitude of the fracture feature observed, just like a geologist might do, working for a mining company.

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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. Measurement/estimate of: 1) the horizontal orientation of the fracture feature (number of degrees between 0 and 360); and 2) the vertical orientation of the fracture feature (number of degrees between 0 (horizontal) and 90 (vertical), including the dip direction).
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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INTRODUCTION

This EarthCache is one of a series in Tuolumne and Calaveras Counties, intended, primarily, to inform EarthCachers about the geology and geologic history of the Central Sierra Nevada Mountains. Additionally, the science of geology, the thinking skills geologists use, and the types of career opportunities are also part of the series.

The series consists of approximately 20 EarthCaches in both Tuolumne and Calaveras Counties (~10 in each county). EarthCachers will visit locations highlighting the main geologic evidence supporting claims made by geologists, who have constructed the geologic history of this area, telling the amazing story that has unfolded over the past 400 million years. This EarthCache ("Dragoon Gulch Geology") is the only one with a geologic history of the Sierra section. All others have a link back to this page if anyone wants to review it.

Enjoy learning more about the Geosciences and the geology of the Central Sierra Nevada!

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BACKGROUND

Central Sierra Geologic History

The geology of the Central Sierra Nevada Mountains consists mainly of igneous and metamorphic rocks with lesser amounts of sedimentary rocks. The first geologists to research this region grouped the rocks into 2 main categories: 1) the Subjacent Series; and 2) the Superjacent Series. The Subjacent series included the granitic rocks most people think of when traveling through the high Sierra, plus the metamorphic rocks of the Mother Lode gold country. Superjacent series rocks included the younger volcanic rocks mainly found deposited upon some of the granitic rocks of the high country. We can still understand the geology of the Sierra in terms of these general groupings.

Some of the oldest rocks in the Sierra Nevada are the metamorphic rocks in the foothills. They contain the gold. The oldest of these rocks, known as the Shoo Fly and Calaveras Complexes, were initially sediments deposited up to 400 million years ago, onshore and offshore, along the edge of the continent that existed at the time. These sediments were changed into metamorphic rocks due to convergence of tectonic plates, along a subduction zone (see Figure 1), that created heat, pressure, and the presence of rock-altering fluids, including those that contained gold. The supercontinent, Pangea, formed about 270 million years ago with the Shoo Fly and Calaveras Complex metamorphic rocks along its western margin. When Pangea started breaking up around 200 million years ago, the Shoo Fly and Calaveras Complexes were deformed and metamorphosed even further. Volcanic islands, called island arcs, were slowly swept into the trenches filled with sediments, by compressive tectonic forces that existed along that margin through geologic time. This process has caused the continent to grow laterally westward as the islands have docked, or accreted (see Figure 1) onto the edge of the continental plate, mashing them into the Shoo Fly and Calaveras rocks. This process primarily occurred during the Jurassic time period (approximately 200-145 million years ago), forming the Western Sierra Nevada Metamorphic Belt and most of the rocks seen in the foothills of western Tuolumne and Calaveras counties (see Figure 2).

Figure 1: Subduction zone showing accretion process at the “Accretionary prism/wedge”.

Figure 2: This is a tectonic map (modified from Rich Schweickert, 2006) of the Sierra Nevada and the Western Sierra Nevada Metamorphic Belt. The inset map on the left shows nomenclature of major faults.

The main pattern observed in the tectonic map in Figure 2 shows long, thin “belts” of similar rock types in orange, purple, blue-green, brown, and green, which are called terranes. These are the island arc complexes (volcanic islands like Japan or the Philippines), separated by long thrust faults (with “teeth” on the upper plates).  On the west side of each fault, pieces of island arc crust were thrust under the continent, toward the east, as subduction occurred (see Figure 3). The orange belt is the Northern Sierra terrane containing the Shoo Fly Complex; the blue-green belt is called the Calaveras Complex; the brown belt is called the Don Pedro terrane; and the green belt is named the Foothills terrane.

While subduction was occurring, magma chambers were created under the continent (see Figure 1). The granitic rocks of the Sierra Nevada formed from this magma as it cooled several miles beneath the earth’s surface. Once subduction stopped (at approximately 70 million years ago), the crust was uplifted and eroded, exposing the granitic rocks that we see today in the high country up highways 88, 4, 108, and 120.

Figure 3: In this model (modified from Ted Konigsmark, 2002) of the Western Sierra Nevada Metamorphic Belt, the Shoo Fly Complex (1) went into the subduction zone first, followed by the Calaveras Complex (2), then followed by the Don Pedro (island arc) Terrane (3), and finally followed by the Foothills (island arc) Terrane (4). This occurred mainly during Jurassic time (~200 to 145 million years ago) and continued until approximately 70 million years ago, when subduction stopped and the rocks were uplifted and eroded. Rocks that had been buried at depths of 30 miles or more were eventually exposed at the surface.

The timing of the uplift has long been debated and recent geologic research suggests that the Sonora Pass region is an active tectonic plate boundary that started forming 12 million years ago and continues to form even to this day. Figure 4 shows younger volcanic rocks (that cover granitic rocks in the high Sierra) mapped in the Sonora Pass region. The Little Walker Center (~11 to 9  million years old) and the volcanic center near Ebbetts Pass (~5 to 4 million years old) on Highway 4, were formed by major faults that opened up, releasing magma in vents at the surface. These faults resulted from the earth’s crust pulling apart as a new tectonic plate boundary began to form roughly 12 million years ago. The tip of that boundary has been migrating northward, from Sonora Pass, to Ebbetts Pass, and is now located near Lassen Peak National Park at the south end of the Cascade Range. The Table Mountain Latite lava flow resulted from pulling apart of the earth’s crust along one of these fault structures near Sonora Pass 10 million years ago. The lava flowed down the paleo Stanislaus River channel before much uplift had taken place, hardened, then was left standing above the landscape when the surrounding rocks eroded away over the past 10 million years. This is a world class example of inverted topography – the former low lying topography is now the high feature on the landscape.

Figure 4: This map (by Cathy Busby, 2012) shows the distribution of volcanic rocks (colored) in the central Sierra Nevada Mountains. The gray zones are paleochannels down which volcanic deposits flowed when volcanoes were recently active in the central Sierra (~14 to 3 million years ago), near Sonora and Ebbetts Passes. The 10 million year old Table Mountain Latite lava flow is shown in tan and stretches from its origin near the Little Walker Center to Knights Ferry.

Figure 5: Tectonic setting of the Sierra Nevada microplate showing major faults in blue, volcanic centers as red or green stars, and location of the Mendocino Triple Junction (MTJ) over the past 12 million years (modified from Busby, et al, 2016). (Note: M = Mono Lake; T = Lake Tahoe).

The last part of the geological story is one of: 1) glaciation and ice ages; 2) earthquakes and uplift; and 3) mass wasting and landslides. During the past 2 million years (Pleistocene time), there have been at least 5 major glacial periods in the central Sierra Nevada Mountains. These events have been identified by the deposits and landforms they have left behind. They include, from oldest to youngest, the: 1) McGee (c. 1.5 Ma); 2) Sherwin (c. 1 Ma); 3) Tahoe (c. 70-150 Ka); 4) Tioga (c. 19-26 Ka); and 5) Recess Peak (c. 14-15 Ka). Other smaller events have been identified, as well as the “Little Ice Age”, when the climate began to cool at around 1350 C.E. and glaciers began to grow, persisting until roughly 1850 C.E., when glaciers in the Sierra are thought to have reached their maximum extent during that time. Since then, Sierran glaciers have been receding and continue to shrink today.

Uplift of the Sierra Nevada is generally thought to be caused by motion of an ancient oceanic plate, called the Farallon plate, that once subducted under this part of western North America and is now breaking up and sinking under Nevada and western Utah. This area is experiencing higher than average geothermal heat flow in response to magma generated by subduction of the remnants of the  Farallon plate under central Nevada. This is a classic example of “back arc spreading” and, while it is an oversimplified model, it may help to explain much of the uplift of the Sierra and the rest of the Basin and Range. In fact, GPS measurements show the crust between Reno, Nevada and Salt Lake City, Utah is rifting apart, or widening. Mountain building continues today as demonstrated by earthquake activity along the Sierra Nevada Frontal Fault system, which runs along the east side of the Sierra crest. On March 26, 1872, the Owens Valley Earthquake occurred on this fault system  uplifting the crust 15-20 feet and moving it horizontally for 35-40, demonstrating there are both vertical and horizontal forces at play. From these data, geologists infer that the Farallon Plate continues to influence the region causing vertical motion, while the Pacific Plate, in frictional contact with the North American Plate along its western edge, is causing the horizontal motion observed with the Sierra Nevada microplate. We can expect to experience earthquakes at any time in the seismically active central Sierra Nevada region, as well as more volcanic activity in places like Mammoth Lakes, which sits inside the Long Valley Caldera, a super volcano, which has magma a mile below the surface.

Mass wasting (rockfalls, mud and debris flows, and landslides) also help to shape the mountains today, mostly during periods of higher rainfall or during earthquakes (or both). There have been significant, damaging debris flows in both Tuolumne and Calaveras Counties during the 1997 El Nino event. Therefore, we can expect mass wasting to occur in the future.

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DRAGOON GULCH GEOLOGY

The Dragoon Gulch Trail is maintained by the City of Sonora (please follow the rules and regulations posted on their entry signs). As you hike to the GPS location of this EarthCache, you will see evidence of past gold mining operations during the gold rush era. There are piles of overturned cobbles, called tailings, along the stream. You may also see an old remnant ditch system that conveyed water to the area (likely during summer when the stream was dry and for processing gold away from, or high above, the stream). There are also “diggin’s” where miners tried to establish pocket mines to extract gold. There are also pieces of milky quartz visible a few places along the trail; since gold tends to be found along with quartz, keep your eyes open!

Once you have arrived at the GPS location for the EarthCache, observe the rock outcrop. You should see reddish colored rocks with a crude form of wavy layering called foliation present. These rocks, mostly phyllite and schist, were metamorphosed from pre-existing volcanic rocks in the Don Pedro terrane. Most likely they were once deposits of pyroclastic flow materials – volcanic ash - and/or pillow lavas that were erupted, then deposited on the flanks of a volcanic island arc system offshore of the western coast of North America during Jurassic time (approximately 200-150 million years ago). The volcanic rocks were buried, deformed, and metamorphosed by tectonic compression and accretion in a subduction zone. After being uplifted, they were exposed by erosion of the overlying materials and you can see them today, protruding from the crust, fractured and foliated. The major fault system along which the subduction occurred, called the Sonora Fault, lies at the base of the hill, where the stream flows through the gulch. The other side of the creek is mapped as the Calaveras Complex and the Sonora Fault separates that major group of rocks from the Don Pedro terrane and the EarthCache site. Quartz veins occur in the metamorphic rocks and formed when the rocks were buried deeper in the earth’s crust, under high pressure. Gold and quartz are associated with one another and recent research indicates they both form from “flash vaporization” along fault zones during earthquakes. The Sonora Fault that runs through Dragoon Gulch would have been forming gold when it was active. The fluids in the fault zone, under tremendous pressure, would have crystallized into quartz veins, laden with gold, here, earthquake by earthquake.

For this EarthCache, you will measure the 3D geometric orientation of the rocks, at the EarthCache site. Geologists measure the attitude – called strike and dip – of rocks in order to try to infer what happens to them underground. This is a significant skill in mineral and oil exploration when trying to locate important resources such as metals or hydrocarbons (oil, natural gas, coal, etc.) trapped beneath the earth’s surface.

To complete this part of the EarthCache you will need to observe the fracture features in the rock and measure their horizontal and vertical orientations. To do this, look carefully at the diagram below (see Figures 6 & 7). Note that for this outcrop you will need to locate what appears to be an old quartz vein that may have been removed by miners in hopes of finding gold (see Figures 8, 9, 10, and 11). Record the 2 measurements when logging the EarthCache.

Figure 6: Strike (horizontal direction of the rock feature) and Dip (vertical direction of the feature).  The value for the strike of the layers in this diagram is east-west, which is 90 degrees. North is always 0 (or 360) degrees, while east = 90 degrees, south = 180 degrees; and west = 270 degrees. The approximate dip of the layers in this diagram = 45 degrees toward the north.

Figure 7: Using the right hand rule: 1. Place your right hand on the surface (if the feature you are interested in is visible). 2. Rotate your hand so that your four fingers point downward, toward the dip direction. 3. Finally, extend your thumb on the same plane. The direction which your thumb is pointing to is the direction of the strike.

Figure 8: Fracture feature to be measured. Measure the horizontal and vertical dimensions of this fracture. Note that the high Sierra Nevada Mountains are off to your right (east) as you face the outcrop and the sun rises in the east and sets in the west. Horizontal = zero degrees; vertical = 90 degrees (straight downward).

Figure 9: Fracture feature to be measured (view showing approximate location of hand placement).

Figure 10: Fracture feature to be measured (zoomed in).

Figure 11: Fracture feature strike and dip measured with free smartphone apps (compass and clinometer).

Figure 12: Fracture feature with geologist's hand in correct position.

Figure 13: Close up of last photo showing geologist's hand in correct position to estimate strike and dip of fracture feature.

Geoscientists have to be skilled in several different areas. They need to be somewhat proficient in: math, physics, chemistry, biology, surveying, geospatial technologies (GPS, GIS, remote sensing, and webmapping), lab technologies (microscopes, spectrometers, etc.), computer use, and more. Career opportunities in the earth sciences include: geologist, hydrologist, mapping/surveying and geospatial technologist, watershed analyst, mineralogist/mining engineer, petroleum geologist, environmental scientist, natural resources scientist, and more. If you are interested in learning more, take a course at your local community college to get started.

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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

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 (no need to go off trail for any measurements at this site);
• 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. Measurement/estimate of: 1) the horizontal orientation of the fracture feature (number of degrees between 0 and 360); and 2) the vertical orientation of the fracture feature (number of degrees between 0 (horizontal) and 90 (vertical), including the dip direction).
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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REFERENCES

1. Busby, Cathy J., Andrews, G.D.M., Koerner, A.K., Brown, S.R., Melosh, B.L., and Hagan, J.C., “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.

2. 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.

3. Earth Science Literacy Initiative (ESLI), 2010, http://www.earthscienceliteracy.org/.

4. Hill, Mary. 2006. Geology of the Sierra Nevada. University of California Press revised edition, Berkeley and Los Angeles, California 468 pp.

5. Konigsmark, Ted, 2003, “Geologic Trips: Sierra Nevada”, GeoPress.

6. Portland State University, “Glaciers of California”, last updated 08-04-2011, http://glaciers.research.pdx.edu/glaciers-california.

7. Rohlen, Ginger, (pers. comm.), June 2017, Teacher, Sierra Waldorf School,19234 Rawhide Rd., Jamestown, CA., 95327.

8. 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.

9. Senanayake, Sanuja, “Right Hand Rule in Geology”, 04-01-2013, http://sanuja.com/blog/right-hand-rule-in-geology.
 

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