2 Hirt–Centr ains - WordPress.com · 2019. 5. 31. · 2 Hirt–Centr ains INTRODUCTION The...

Hirt–Central Klamath Mountains2

INTRODUCTION

The Klamath Mountains of northern California and southern Oregon (Fig. 1) are one of the best studied and most accessible assemblages of accreted terranes in the world. This paper introduces the plate tectonic setting, rock types, and geologic history of some of the terranes in the Orleans area as preparation for our upcoming field trip. The definitions of words shown in italics are given in a glossary that follows the references.

PLATE TECTONICS

In order to understand the origin of the Klamath Mountains we need to review a bit about the outer part of the Earth. In particular, we need to know what lithospheric plates are, how they are formed at divergent plate boundaries and consumed at covergent ones, and how plate convergence can lead to terrane accretion and the production of magmas along subduction zones.

Earth’s internal structure

Earth’s interior is divided into three major parts: the core, mantle, and crust. Of these, only the upper mantle and crust play major roles in our discussion of the Klamath Mountains. The crust and the cold, rigid upper mantle move together as a single slab of rock called the lithosphere. Earth’s lithosphere is broken into about a dozen large plates and a number of smaller ones (Fig. 2). Each of these 100 to 150 km-thick plates is composed mostly of peridotite, a magnesium-rich rock that comprises Earth’s mantle. A 4 km-thick layer of slightly less dense mafic rock (basalt/gabbro) caps oceanic lithosphere, whereas a 45 km-thick layer of even less dense felsic rocks (granite and related

rocks) lies atop continental lithosphere.

Beneath the lithosphere is a soft region of the upper mantle called the asthenosphere that extends to a depth of about 350 km. Because the rock here is hotter than that in the overlying lithosphere, it contains a small percentage of melt that lubricates the movement of its grains. As a result, the asthenosphere flows relatively easily and accommodates the horizontal and vertical motions of the overlying lithospheric plates.

Plate Boundaries

Boundaries between the plates are the sites of most of the volcanic and seismic activity on Earth. Based on the relative

Figure 1. Simplified geologic map of the Klamath Mountains. The curved, north-trending belts of similarly-colored rocks are the individual terranes that comprise the range.

Hirt–Central Klamath Mountains 3

and produces mafic magmas that rise and ultimately solidify to form new oceanic crust (Fig. 3a). Although most oceanic lithosphere is subsequently subducted (see below), some fragments have been added onto continents along convergent or shear boundaries. These fragments of oceanic lithosphere, known as ophiolites, are layered with: seafloor sediments on top, submarine “pillow basalts” next, “sheeted” basalt dikes and gabbro beneath that, and mantle peridotite on the bottom (Fig. 3b). A typical ophiolite is about 7 km thick. All of the rocks in an ophiolite beneath the capping sediments are typically altered by hot water that circulates through the lithosphere near a mid-ocean ridge. This alteration converts much of the ophiolite to greenstone (altered basalts and gabbros) and serpentinite (altered peridotite).

motions of the plates that adjoin them, these boundaries are classified as being either: divergent, where two plates move apart; convergent, where they move together; or shear, where they move horizontally past one another. Here in the Pacific Northwest, for example, the boundary between the North American and adjoining oceanic plates is a convergent boundary called the Cascadia subduction zone. To the west lies a divergent boundary, the Gorda-Juan de Fuca ridge; and to the south a shear boundary, the San Andreas fault.

Most divergent boundaries are mid-ocean ridges where two oceanic plates are separated by narrow zones of extensional faults. As the plates move apart the soft asthenosphere wells up, undergoes partial melting due to decompression,

Figure 2. Simplified map of Earth’s lithospheric plates. Divergent boundaries are shown by red lines, conver-gent boundaries by green lines with sawteeth in the direction of subduction, and shear boundaries by blue lines. From Lillie (2005).


Many convergent boundaries are subduc-tion zones where one plate of oceanic lith-osphere sinks into the mantle beneath an overriding plate. Only oceanic lithosphere is able to subduct because continental lithosphere, with its thicker, less-dense felsic crust is too buoyant to sink into the mantle. Here in the Pacific Northwest, the Gorda and Juan de Fuca plates are sink-ing eastward beneath the North American plate along the Cascadia subduction zone. Volatiles bound into the sinking plates are released as these plates are heated within

the Earth (Fig. 4). This water causes the hot rock of the asthenosphere to partially melt and produce mafic magmas that rise slowly towards Earth’s surface. Only a fraction of this magma reachs the surface and erupts, however. The rest stalls in the crust where it crystallizes, releases heat that partially melts the surrounding rocks, and then mixes with these crustal melts to produce intermediate magmas. Some of these magmas reach the surface to build volcanoes like Mount Shasta but most accumulate and slowly crystallize underground to form bodies of diorite and granite called plutons. Even though they form deep underground these plutons dot the Klamath Mountains (Fig. 1) where they have been exposed by later uplift and ero-sion. The Castle Crags pluton along I-5 just south of Dunsmuir is a good example of on such body.

Finally, although much of Earth’s oceanic lithosphere is formed at mid-ocean ridges, it can also be formed behind subduction zones where extensional stresses related to circulation in the asthenosphere pull the lithosphere apart and enable rising magmas to produce narrow back-arc

Figure 3. (a) Block diagram of a mid-ocean ridge. (b) Schematic cross-section of the oceanic lithosphere. If such lithosphere is accreted to a continent it is referred to as an ophiolite.

Figure 4. Block diagram of a continental-margin suduction zone like the Cascadia subduction zone here in the Pacific Northwest.


basins (Fig. 5). The Sea of Japan is a modern example of a back-arc basin and the Josephine Ophiolite, which crops out west of Orleans, is thought to have formed in such a basin during Jurassic time.

Terrane Accretion

Terranes are large blocks of lithosphere that (1) contain rocks record similar geologic histories, and (2) are typically separated from one another by faults that mark ancient subduction zones or shear boundaries.

Many of the terranes found in the Klamath Mountains actually originated far from North America. Some may have been frag-ments of continental lithosphere that were rifted away from their parent landmasses, just as Baja California is being rifted away from North America today. Most, however, were once offshore island chains similar to the modern Japanese archipelago. These collided with western North America when the intervening ocean basins were closed by subduction. These continental fragments and island arcs were carried across the Pacific Ocean basin as parts of subducting oceanic plates and collided with North America when subduction con-

sumed the intervening oceanic lithosphere (Fig. 6).

As a result of repeated accretion events that stepped westward—thrusting young-er more westerly terranes under older, more easterly ones—the Klamath Moun-tains have a subsurface structure that has been described as being like “shingles on a roof”. In this case, each terrane (“shin-gle”) is separated from adjacent terranes by eastward-dipping thrust faults along which older, deeper rocks were pushed up over younger, shallower ones (Fig. 7). Each of these thrust faults marks the approxi-mate location of a former subduction zone.

KLAMATH GEOLOGIC HISTORY

Although we do not know precisely when the individual subterranes that comprise the Eastern Klamath Terrane were as-sembled, we do know that this terrane had begun to “backstop” subsequent accretion events by 400-380 Ma (Irwin and Wooden, 1999). During approximately the next 250 million years a series of eight collisions built the Klamath Mountains by suturing a succession of terranes to the western mar-gin of the North American plate (Fig. 1).

Rocks in each terrane have been dated by identifying fossils from the sedimentary rocks they contain or by measuring the radiometric ages of the volcanic rocks they contain. These dates provide maximum ages for the time at which each terrane was accreted. After a terrane collided, sub-duction “jumped” westward and magmas rose from the new subduction zone and in-truded into the freshly-accreted terrane or across its boundary with adjacent terranes to form “stitching plutons”. The radiomet-ric ages of these latter plutons provide lower limits on the ages of the accretion-

Figure 5. Cross-section of a back-arc basin develop-ing by extension of the overriding plate behind a subduction zone due to convection in the astheno-sphere. From Wikimedia commons.


Figure 6. Block diagrams of subduction in the Pacific Northwest showing the approach (a) and accretion (b) of a terrane to the westrern margin of North America. In part (b) notice that subduction has “jumped” to the western edge of the first terrane and that a second terrane is approaching outboard of the first. From Orr et al. (1992).

ary events. Table 1 summarizes the ages of the eight accretionary events that built the Klamath Mountains based on mapping and dating by Irwin and Wooden (1999). The last accretionary event occurred about 145-140 Ma and corresponds, ap-proximately, to the time of the Nevadan mountain-building event farther south in California. Figure 9 shows the time scale of Earth’s history and illustrates that most of the terrane accretion events that built the Klamath Mountains occurred during the Mesozoic Era. The oldest rocks exposed in the Klamath Mountains, however, formed during the Paleozoic Era more than 400 million years ago!

After the accretion of the Pickett Peak terrane about 145-140 Ma uplift created a large, gently-dipping fault in the East-ern Klamath terrane (Cashman and Elder, 2002). This shallowly-dipping extensional fault caused the forearc of the old Trinity subduction zone (Yreka subterrane) to move northwards and the backarc (Red-ding subterrane) to move southwards as Trinity subterrane rose. Ongoing plate convergence has lifted the Klamath Moun-tains, and erosion by streams and glaciers has deeply incised the range during Ceno-

zoic time.

ROCKS OF THE KLAMATH MOUNTAINS

Geologists classify rocks into three fami-lies—igneous, sedimentary, and meta-morphic—based on how they are formed. Igneous rocks are created by the solidifica-tion of magmas that result from the partial melting of older rocks in Earth’s interior. Sedimentary rocks, on the other hand, are formed by the deposition of sediments produced by weathering of rocks exposed at the planet’s surface. Finally, metamor-phic rocks are formed from older rocks that are exposed to elevated temperatures and pressures within the Earth and grow new minerals in the solid state, without melting. Rocks of all three families are found in the Klamath Mountains, although igneous and metamorphic rocks are the most common.

Igneous rocks

Melting of Earth’s crust and upper mantle take place at both divergent and conver-gent plate boundaries. At mid-ocean ridges (divergent boundaries) spreading of the lithosphere allows hot asthenosphere to rise towards the surface, partially melt,


and produce mafic magmas that are rela-tively rich in magnesium and iron and poor in silicon. These magmas yield dark, fine-grained basalts if they erupt at Earth’s surface and dark, coarse-grained gabbros if they crystallize slowly underground (Fig. 10).

At subduction zones (convergent bound-aries) water added to the asthenosphere causes it to partially melt. As the resulting magmas rise they melt the surrounding crust and mix with it to produce interme-diate magmas with moderate amounts of magnesium, iron, and silicon. These yield fine-grained gray andesites if they erupt and coarse grained, “salt and pepper”-colored diorites if they crystallize under-ground.

In the Klamath Mountains, basalts, gab-bros, and andesites are found in the accreted oceanic terranes whereas coarse-grained diorites occur as younger, cross-cutting plutons. The fact that we see these plutons at the surface today means there has been a lot of uplift and erosion in the Klamath Mountains.

Figure 7. Geologic cross-section of the Klamath Mountains showing the major accreted terranes, the faults that separate them, and the plutons that intrude them or cut across the bounding faults. From Irwin (1981).

Table 1. Chronology of accretionary events that assembled the terranes of the Klamath Mountains from Irwin and Wooden (1999). Ages are given in millions of years ago (Ma).

Accretionary episode Age (Ma)Central Metamorphic 380-400Fort Jones 240-260North Fork 193-198Eastern Hayfork ≈180Western Hayfork 168Rattlesnake Creek 164Western Klamath 150-152Pickett Peak 140-145

Sedimentary Rocks

True sedimentary rocks are uncommon in the Klamath Mountains because, during terrane accretion, almost all of them have undergone at least some metamorphism. If the growth of new minerals is not too pronounced, however, we can often make out the texture of the original sedimentary rock and describe the rock as a “metasedi-ment”. Sedimentary rocks made of pieces of older rocks (Fig. 11a) are classified


down along faults. This causes some of the original minerals in these rocks to break down and the compounds that formed them to reorganize into new minerals that are stable at higher pressures and tem-peratures. Subsequent cooling and uplift “lock in” these new minerals so that we find them at the surface today.

If a metamorphic rock is so changed that we cannot readily recognize its original igneous or metamorphic parent, we give it a new name (Fig. 12). Metamorphic rocks that contain lots of flaky or elongate minerals tend to develop a sheet-like or layered texture, called foliation, during metamorphism. Foliated metamorphic rocks are further classified according to how coarse their grains are (e.g., slate is very fine grained whereas schist is coarse.) Metamorphic rocks that lack flaky or elongate grains are said to have nonfoli-ated textures and are often recognized by the dominant mineral they contain (e.g., calcite yields marble and serpentine yields serpentinite.)

REFERENCES

Barnes, C.G., 1987, Mineralogy of the Wooley Creek batholith, Slinkard pluton, and re-lated dikes, Klamath Mountains, northern California: American Mineralogist, v. 72, p. 879-901.

Cashman, S.M., and Elder, D.R., 2002, Post-Nevadan detachment faulting in the Klamath Mountains, California: Geological Society of America Bulletin, v. 114, no. 12, p. 1520-1534.

Irwin, W.P., 1981, Terrane accretion of the Klamath Mountains, in Ernst, W.G., ed., The Geotectonic Development of California: Englewood Cliffs, New Jersey, Prentice-Hall, p. 29-49.

Figure 9. Geologic time scale showing the corre-spondence of named Eons, Eras, and Periods with ages in millions of years. Note that the scale on the right expands the last 542 million years of Earth’s history (Phanerozoic Era), the time during which animals with hard parts have been abundant.

according to the sizes of the particles they contain (e.g., gravel yields conglomerate and sand yields sandstone.) On the other hand, sediments made of a single mineral precipitated from ions dissolved in sur-face waters (Fig. 11b) have special names (e.g., calcite yields limestone and quartz yields chert.) Organisms like single-celled radiolaria and multicellular coral are often involved in producing these chemical sedi-mentary rocks.

Metamorphic Rocks

Most of the rocks that comprise the accret-ed terranes of the Klamath Mountains are metamorphic rocks. During terrane accre-tion pre-existing rocks (mostly ophiolitic crust, seafloor sediments, and occasional oceanic islands and reefs) are squeezed, sheared, and heated by being pushed


Irwin, W.P., and Wooden, J.L., 1999, Plutons and accretionary episodes of the Klamath Mountains, California and Oregon: U.S. Geo-logical Survey Open-File Report 99-374.

Orr, E.L., Orr, W.N., and Baldwin, E.M., 1992, Geology of Oregon, 4th ed.: Dubuque, Iowa, Kendall/Hunt, 254 p.

Wallin, E.T., and Metcalf, R.V., 1998, Suprasu-bduction zone ophiolite formed in an ex-tensional forearc: Trinity terrane, Klamath Mountains, California: Journal of Geology, v. 106, p. 591-608.

GLOSSARY

Asthenosphere: Layer of Earth’s upper mantle that lies between depths of about 100 and 350 km and is relatively “soft” or weak because of the presence of a small amount of melt along the boundaries of mineral grains within the peridotite.

Basalt: Volcanic rock with a low silica con-tent (about 47 to 52 wt. %) that typically has a fine black groundmass and contains coarser crystals of olivine, plagioclase, and pyroxene.

Crust: The uppermost solid part of the Earth. It consists of a 7 kilometer-thick lay-er of mafic igneous rocks (mostly basalt) beneath the oceans, and a 45 kilometer-thick layer of mostly felsic igneous rock (granite) beneath the continents.

Felsic: A family of igneous rocks—in-cluding coarse-grained granite and fine-grained rhyolite—that contain 65-75% silica and consist mostly of light-colored minerals such as quartz and feldspar.

Lithosphere: The rigid outer layer of the Earth, which includes both the crust and the cool, stiff uppermost mantle. Litho-sphere is up to about 100 km thick be-

Figure 10. Classification of igneous rocks. Compositions range from felsic (silicon rich; iron and magnesium poor) to ultramafic (silicon poor; iron and magnesium rich). Intrusive rocks are coarse grained due to slow cooling underground. Extrusive rocks are fine grained due to rapid cooling at or near Earth’s surface.


iron-nickel core. The mantle is 2900 km thick and makes of most of the planet’s volume.

Peridotite: Coarse-grained igneous rock that forms Earth’s mantle and consists mostly of peridotite, augite, and hyper-sthene.

Ophiolite: A piece of the oceanic litho-sphere that has been thrust up onto a continent rather than subducted.

Subduction zone: A dipping surface along which a plate of oceanic lithosphere is overridden by another plate and sinks into the mantle.

Terrane: A piece of relatively thick or low density lithosphere that has been trans-ported by plate motion from where it formed and accreted (added) to another terrane or a continent.

Volatiles: Chemical elements and com-pounds, such as H2O, CO2, Cl and SO2 that occur as gases at relatively low tempera-tures.

neath the oceans and 150 km thick be-neath the continents.

Lithospheric plate: One of several dozen independent pieces of the lithosphere that move about relative to one another across Earth’s surface. Most seismic and volcanic activity occurs at the boundaries between these plates.

Mafic: A family of igneous rocks—in-cluding coarse-grained gabbro and fine-grained basalt—that contain 45-55% silica and contain significant amounts of dark iron and magnesium-rich minerals such as olivine and pyroxene.

Magma: Partially-molten rock; typically a mixture of melt, mineral crystals, and gas bubbles.

Mantle: Earth’s middle layer, which lies beneath the crust and above the planet’s

Figure 11. Classification of sedimentary rocks. (a) Clastic sedimentary rocks are named based on the average size of the particles they contain. (b) Chemical sedimentary rocks, precipitated from compounds dissolved in water, are named based on the most abundant mineral they contain.


FIELD TRIP ROAD LOG

We’ll visit six stops during today’s field trip so that you can check out representa-tive exposures of the rocks found here in the central Klamath Mountains. Several of these stops are either along Highway 96 or the Salmon River Road, so please be care-ful when parking and watch for cars while standing along or crossing the highway. Also, please do not climb on steep slopes and watch out for rattlesnakes and ticks in the dry grass. Note that the UTM coor-dinates given for each site are for NAD 27, zone 10T.

We’ll start by setting our trip odometers to 0.0 and driving out from the Sandy Bar Ranch along Ishi Pishi Road. Turn left when you reach Highway 96 and drive eastward to about trip mile 5.5. Watch for traffic and park in the cleared area on the left-hand side of the highway.

Stop 1: Gabbro and Landslide Scar (458125, 4578017): The outcrop on the south side of the road exposes a body of gabbro that is part of the Rattlesnake Creek terrane. This terrane was thrust un-der the adjacent Western Hayfork terrane about 164 Ma along the Salt Creek fault (Irwin and Wooden, 1999). Remember, gabbro forms the lower oceanic crust, so we’re looking at a part of the oceanic litho-sphere that has been added to the conti-nent. You can cross the road to examine the rock in place if you wish (be careful) but there are plenty of pieces to look at around the edge of the parking area (Fig. 13). Notice how dark this rock is (mafic composition) and that it lacks quartz (no clear, glassy grains).

From the northern edge of the parking area look across the Klamath River to the dramatic landslide scar on the far slope (front cover photo.) Landslides (a type of mass movement) are a major erosional

Figure 12. Classification of metamorphic rocks. Foliated rocks contain platy or elongate minerals that are aligned to produce planar or layered fabrics and are classified by grain size. Nonfoliated rocks lack platy or elongate minerals and have “sugary” textures; they are classified primarily by the main mineral they contain.


feature in the Klamath Mountains. Sev-eral factors tend to promote landslides: steep slopes; weathered or sheared rocks; excess water; and an absence of veg-etation. At this site, erosion by the river likely undercut the base of the slope and over steepened it. Rocks in the Klamath Mountains are pervasively sheared (cut by faults), contributing to their instability. Finally, most landslides occur during the winter when rain saturates the slope ma-terial and destabilizes it. Since the failure that produced this scar occurred vegeta-tion has not regrown on this slope, so the anchoring effect of their roots is absent and another slide is likely.

Merge carefully back onto Highway 96 and continue northeastward, past the junction with the Salmon River road (trip mile 7.8) to the Junction School at trip mile 9.0. Park on the left side of the road and carefully cross the road to the outcrop on the right (eastern) side of the road.

Stop 2: Serpentinite (458658, 4582234): This outcrop exposes a per-vasively sheared body of serpentinite that is also part of the Rattlesnake Creek ter-

rane. Serpentinite is a metamorphic rock derived from peridotite, the olivine-rich rock that comprises the lowermost oce-anic crust and the entire mantle. Fractur-ing of the lithosphere at mid-ocean ridges enables seawater to penetrate the lower crust and upper mantle where it reacts with hot peridotite to form serpentine. Serpentine is a relatively soft green or black mineral that forms shiny curved surfaces along fractures (Fig. 14). Fibrous serpentine, which commonly occurs in fractures in outcrops like this one, is one form of asbestos.

After examining the outcrop (be care-ful, it’s steep and loose) return to the cars, carefully turn around, and retrace the route back to the Salmon River road (about trip mile 10.2.) Turn left and con-tinue about 3.6 miles up the Salmon River road to where a dirt road labeled “Wooley Creek Trail/Corral” enters on the left at about trip mile 13.8. Follow this road up and to the left, parking on the wide bench. Walk carefully through the dry grass (watch for snakes and ticks) to examine the outcrops on the northern side of this bench.

Figure 13. Hand sample of the gabbro found at stop 1. The small white “stripe” is a fracture that has been filled with calcite or plagioclase.

Figure 14. Hand sample of the serpentinite found at stop 2. The small white “stripe” is a fracture that has been filled with calcite or plagioclase.


Stop 3: Metasedimentary Rocks of the Hayfork terrane (463975, 4580454): This outcrop exposes metamorphic rocks that were probably once sandstones, shales, and cherts deposited on the sea-floor west of North America. These rocks were subducted beneath those of the North Fork terrane (which crops out far-ther east) and underwent metamorphism about 180 Ma (Irwin and Wooden, 1999). Note the presence of small internal beds (crossbeds) in the sandstone that lie at an angle to the overall trend of bedding preserved in the outcrop (Fig. 15). This indicates the presence of a current during the deposition of these sandy beds.

After examining the metasediment out-

crop drive back to the Salmon River road, turn left, and continue east about 1.1 miles to where a white outcrop occurs on the left side of the road in a roadcut (trip mile 14.9.) There is no place to park directly across from this outcrop, so proceed a little farther up the Salmon River road un-til you can park safely along the right-hand side. Carefully cross the road and walk back to the white outcrop.

Stop 4: Marble (465256, 4579738): This small white outcrop on the northern side of the road exposes a body of marble (metamorphosed limestone) that was probably once part of a reef surrounding a volcanic island west of North America (Fig. 16). Today reefs only grow in the tropics, within about 200 N and S of the equator. The present location of this marble at more than 400N indicates that the rocks

Figure 15. Outcrop photograph of metasedimen-tary rock at stop 3. The original beds of sediment have “pinched” and “swelled” due to the deforma-tion that likely accompanied terrane accretion and metamorphism.

Figure 16. Hand sample of marble found at stop 4. This rock is derived from a limestone that may have originally been a reef surrounding an offshore island and tells us the Hayfork terrane formed well to the south and west of its present location in late Paleozoic or early Mesozoic time.


of the Hayfork terrane must have moved northward as well as westward before they were accreted to North America. It appears that metamorphism has erased any fossils that may have once been pres-ent in this outcrop, but fossils preserved in other marbles from the Hayfork terrane indicate that the reefs grew during Late Paleozoic and Early Mesozoic time (Perm-ian and Triassic Periods), well before the terrane was accreted during the Jurassic Period.

Marble and limestone can be confused with other rocks but it’s easy to recognize them because they fizz (break down and release carbon dioxide) when a small drop of dilute hydrochloric acid is applied. This outcrop continues to the north and is well exposed on the hillside on the opposite side of the Salmon River (Fig. 17). Please resist the temptation to climb up on the outcrop, however, as it is very steep on the other side and I don’t want anyone to fall.

After studying the marble outcrop please return to your cars and carefully turn

Figure 17. Outcrop of marble on the northern side of the Salmon River, just north of stop 4. Note that there is less vegetation on the marble than on the surrounding metasedimentary rocks because marbles weather to relatively nutrient-poor soils.

Figure 18. Simplified geologic map of the Wooley Creek batholith (includes Wooley Creek and Slinkard plutons) from Barnes (1987). Rock ab-breviations are: gr = granite; gd = granodiorite; qd = quartz diorite; dio = diorite; and ton = tonalite. Gray “stripes” trending NNW are contours labeled with the pressure at the time of emplacement in kilobars. Numbered dots are sample localities.


around. Drive back down the Salmon River road about 0.6 miles (trip mile 15.5) to the dirt road labeled “Steinacher Trail” on the right. Turn right onto this road and pro-ceed about 0.1 miles to the turnaround at the end of the road. Park carefully (we all must be able to turn around and get out) and then walk a few hundred meters up the trail to where a rockfall has brought down pieces of the outcrops above.

Stop 5: Wooley Creek pluton (464958, 4580631): The outcrops above us ex-pose the southwestern part of the large Wooley Creek pluton. This complex body (Fig. 18) was built by several batches of magma that rose from a subduction zone and were emplaced into the rocks of the Hayfork terrane about 162 Ma. It has been tilted by uplift of the Klamath Mountains and gradually changes from diorite near

its base in the northeast to granodiorite and granite near its top in the southwest (where we are.)

If you examine a sample under a hand lens, you’ll spot small, clear glassy grains of quartz which identify this as a “granitic” rock. It has a composition about halfway between diorite and granite and so is called a granodiorite. The darker bodies of fine-grained rock in the Wooley Creek plu-ton (Fig. 19) are mingled “blobs” of mafic magma that were injected into the grano-diorite while it was molten; such enclaves are common in plutons associated with subduction zones.

After studying the Wooley Creek grano-diorite return to your car, drive back to the Salmon River road, turn left, and proceed to the junction with Highway 96 (trip mile 19.8). Turn left and continue about 7.2 miles back to Ishi Pishi Road (trip mile 27.0). Turn left, drive about 0.2-0.3 miles, and park in the wide area on the left near the bridge. Walk carefully north about 0.1

Figure 19. Hand sample of granodiorite from the Wooley Creek pluton at stop 5. The dark mass is a diorite enclave that was formed when more mafic magma was injected into the molten granodiorite, cooled, and broke up into pieces.

Figure 20. Hand sample of Galice phyllite from near stop 6. This metamorphic rock is derived from mud deposited in a Jurassic back-arc basin that was floored by the Josephine Ophiolite. Metamorphism during terrane accretion gave the rock its pro-nounced foliation and the growth of tiny mica crys-tals gives it the “sheen” characteristic of a phyllite.


miles to study outcrops of the Galice For-mation along the road.

Stop 6: Galice Formation (455383, 4572627): The outcrops along the west side of the road expose the strongly foli-ated and folded phyllite of the Galice For-mation (Fig. 20). This rock was deposited as a clay-rich sedimentary rock (shale or mudstone) in a back-arc basin that sepa-rated a volcanic arc to the west from the coast of North America to the east. Fos-sils indicate the shale or mudstone was deposited between about 163-152 Ma (Irwin and Wooden, 1999) Subsequently, the sedimentary rocks of the Galice For-mation were thrust beneath the rocks of the Rattlesnake Creek terrane during the Western Klamath accretion event between about 152-150 Ma. Fluids produced as the clay in the original sediments recrystal-lized to mica dissolved silica, carried it to local areas of lower pressure in the hinges of folds, and deposited it as pods and

stringers of white quartz (Fig. 21).

After you’ve finished studying the Galice Formation walk back to your vehicle and drive carefully back the remaining 0.2-0.3 miles to the Sandy Bar Ranch. This con-cludes our field trip through the rocks of the central Klamath Mountains. I hope it gives you a better understanding of the origin of this complex and beautiful coun-try. Thanks for joining us!

Log ends. Last updated 31-May-2019.

Figure 21. Outcrop of the folded phyllite of the Galice Formation at stop 6 along the Ishi Pishi Road. Fluids produced during metamorphism carried dissolved quartz and deposited it (white mass just above the ham-mer) in the relatively low-pressure area near the center of the fold.


NOTES:

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