Erica+-+Death+Valley+schist

=Natural Bridge - Death Valley=



GPS - 36.2812°N 116.7689°W (in minutes) 36° 16' 52.3194" N 116° 46' 8.04"W Elevation - 374 ft. "Death Valley Natural Bridge is an old waterfall natural bridge cut by a deeply incised runoff channel draining part of the western slopes of the Black Mountains in Death Valley National Monument, California. The sedimentary rock that forms the natural arch is geologically very young, perhaps only a few million years old. It is an example of how the age of the natural arch may be quite different from the age of the rock it formed in..." (http://www.naturalarches.org/db/nabs/water4.htm) Named for this natural bridge, the arch of tertiary conglomerate spanning the canyon, Natural Bridge Canyon is located 4 miles north of Badwater and 15.4 miles south of Furnace Creek in Death Valley California. The canyon itself was created by the erosion of poorly cemented Pleistocene alluvial fan gravel deposits that are interbedded with basaltic lavas. The major rock units are related to Mesozoic thrusting of the predominantly Paleozoic carbonates and other clastics. The metamorphic rock collected from this site is most likely a greenschist, associated with the greenschist facies, which of the quartz-carbonate type, usually fall into districts that contain large scale folding. This site is located on the Furnace Creek Fault line, a strike slip, right lateral fault, slipping to the SE. This fault line is not related to the presence of rocks in the greenschist facies as strike-slip faults can not generate enough heat to cause a regional metamorphic event. Most likely, the cause of the creation was an orogeny in the past.

As seen in the map above Published by the California Geological Survey in 1966, the green lines indicate the GPS points of the Natural Bridge Canyon where the rock was collected. As seen in the map, the site falls not only on the Furnace Creek fault line, but also falls on a boundary where the rocks transition between Cenozoic volcanic rocks and Cenozoic nonmarine (continental sedimentary rocks and alluvial deposits). Based upon this information, it makes it difficult to determine the protolith of the rock collected as both igneous as well as sedimentary rocks were present before the Barrovian metamorphism took place.



Regional or Barrovian metamorphism is typically associated with mountain ranges, particularly subduction zones or the remnants left from previously eroded mountains. When extreme compressional forces are exerted like those exhibited in regional metamorphism, the result is metamorphic changes in the substrate. The deformed rocks are then later exposed when the orogenic rocks are later eroded. A subduction zone can also cause the same kind of metamorphic effects. In Barrovian or Regional Metamorphism there are 3 facies which are listed below in order of increasing metamorphic intensity.
 * 1) Low grade - greenschist facies
 * 2) Medium grade - amphibolite facies
 * 3) High grade - granulite facies

The facies that we will concentrate on for the rock in question is that of the low grade Barrovian metamorphism, or the greenschist facies. The greenschist facies is one that occurs in a low temperature/high pressure setting as can be seen in the above and below diagrams. Greenschist can be created at temperatures as low as 400 - 500 ° C, and at pressures as moderate as 8km below the surface. Greenschist is also associated with major orogenies when mafic igneous rocks are metamorphosed through depth of burial and proximity of batholiths (large emplacements of igneous intrusive rock that forms from cooled magma deep in the earth’s crust).



The rock of interest collected at this site is seen below. Thin sections were made to determine the type of rock.

Figure 8: Images taken under the microscope in PPL. The green color of this sample is caused from the decomposing tuff-derived micas ie: Chlorite. The arrow is pointing at the pleochroic nature of the green mineral in the sample. As seen below under XPL, this same mineral has an anomalous blue color which would indicate that it is chlorite, which would be considered a mica. The green schist is dominated by the mineral chlorite. Quartz, feldspar, and muscovite, also present in the sample are typical of greenschists but are also relatively universal minerals that do not tell us anything about the degree of the metamorphism that the rock has undergone. Figure 9: Images taken under the microscope in XPL. It is important to locate the highest pressure/temperature index mineral in the sample because it is the indicator of the highest grade of metamorphism the rock has undergone. Biotite is also present in the sample which is an additional indicator of metamorphism. The chlorite in the indicator that tells us that metamorphism has begun, then biotite also becomes prevalent in the sample as an indicator that the degree of metamorphism progressed past chlorite. The next indicator mineral would be amphiboles and garnet which are not present in the sample. As seen below in the diagrams, the presence these minerals would indicate a higher degree of metamorphism signifying that the rock would have moved into the amphibolite facies, as in not the case. We can be relatively certain because of the indicator minerals that the rock in in fact a greenschist.





Figure 10 & Figure 11 can be located at : [|http://images.google.com/imgres?imgurl=http://csmres.jmu.edu/geollab/fichter/MetaRx/Images/gradechrt.gif&imgrefur l=http://csmres.jmu.edu/geollab/fichter/MetaRx/Barrovian.html&usg=__zlZq_jH0hjWNOs5fyQNkhB7R8yw=&h=306& w=580&sz=13&hl=en&start=23&um=1&itbs=1&tbnid=HRGh-AJcXLDEM:&tbnh=71&tbnw=134&prev=/images%3Fq %3DBarrovian%2Bmafic%26start%3D18%26um%3D1%26hl%3Den%26sa%3DN%26ndsp%3D18%26tbs%3Disch:1]

The foliation in this rock can be seen in the following images. Figure 12: Same image taken under the microscope in PPL and in XPL. Figure 13: 100x magnification of Figure 11 (40x), inspecting the quartz phenocryst in PPL, and in XPL with the stage rotated. This is a large quartz phenocryst. Again, the thin section is too thick and is giving a second order color. But this phenocryst has wavy extinction and is uniaxial.

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Figure 14: Three images are full of plagioclase. Polysynthetic twins can be viewed. Although the birefringence colors are second order, this is due to the thickness of the this section, as the 2V angle is biaxial and denotes that they are in fact plagioclase. Light layers are fine grained quartz filling in around the huskier plagioclase grains. Quartz is more easily deformed that plagioclase.======

Figure 15: Images taken under the microscope in PPL. Images are beautiful examples of foliation. Same image with the stage rotated 90 degrees to show pleochroic nature of the chlorite phenocrysts.

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Figure 16: Images taken under the microscope in XPL. The same images in Figure 14. The micas are growing in the direction of the least tension. They are flattening and subsequently lining up. All of the micas are growing in one direction. The leucocratic minerals (plagiosclase and quartz) are lining up. A ductile transormation begins to take place at half the melting point of quartz (MP is around 1600 ° C). A t surface temperatures and pressures, quartz is the most stable form of silicon dioxide, remaining stable up to 573 °C at 1kb of pressure. As the pressure increases the temperature at which quartz will lose stability also increases. When quartz begins to lose stability, the ductile transformation also begins causing the foliation. ======

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In the above images and also as seen throughout, the dark micas, chlorite (green in PPL, dark blue/purple in XPL) and biotite (dark brown in XPL, light brown in PPL), layers together with the muscovite (clear in PPL, 2nd order colors in XPL). The leucocratic minerals (quartz and plagioclase) then also layer all together. So the minerals within the thin secions examined take on the foliation formation of a layer of felsic minerals, then a layering of micas, then a layering of felsic minerals, then a layering of micas, and so on.======

Figure 17: Images taken under the microscope in PPL as well as in XPL. Here fine grained quartz can also be seen filling in around the huskier plagioclase grains. A small omount of muscovite and chlorite layering can be seen at the bottom of the slide.

Figure 18: Thin sections in PPL. Pleochroism can be seen in the chlorite micas as well as in the biotite present. Large leucratic portions can be seen which are a mixture of quartz, plagioclase and muscovite conglometates, better seen in the XPL images below. Figure 19: Thin sections in XPL.

Figure 20: Foliation between the leucratic and darker minerals can be seen beautifully here in PPL.

Figure 21: These are same images as figure 19 in XPL although orientation is slightly different in the first image. The white arrow is indicating the presence of a large chlorite phenocryst easily identified by the anomalous blue seen in XPL light.

Small Figure 22: Plagioclase phenocrysts can be seen denoted by their polysynthetic twinning along with smaller phenocrysts of quartz. The minerals in the sample are seen in higher order colors because the thin section is slightly thicker than 30 microns. Foliation is also notable in the images.

Figure 23: XPL: The top arrow in the lower left image is pointing out a muscovite phenocryst dark in XPL which can be seen in high 2nd order colors in the following image and can be seen as colorless in the above PPL images. The lower arrow is indicating a plagioclase phenocryst noted by its polysynthetic twinning.

=   = Figure 24: This portion of the thin section is dominated by the leucocratic minerals, quartz and plagioclase. There are larger phenocrysts of plagioclase surrounded by smaller phenocrysts of quartz. This band of light minerals is then sandwiched between the darker minerals chlorite and biotite. =Interpretation:=

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There are very different looking rocks produced from the basalt parent as different as the sequence of metamorphic rocks produced from the shale parent so it is very important to distinguish what the sounce rock is. In this case of this hand sample, it is plausible that the protolith is a sedimentary rock. A force large enough to generate a regional metamorphic event in the Death Valley Region near Natural Bridge would be the Lemoigne Thrust. All of the faults in the area (Last Chance Thrust, Racetrack Thrust, Clery Thrust, Schwab Peak Thrust, and the Lemoigne Thrust) thrust Cambrian/Late Pre-Cambrian sedimentary rocks eastward over Paleozoic rocks.======

It has not been proven, but it is speculated that the thrusting in this region occurred in the late Permian to late early Jurassic but most probably Middle Triassic to Early Jurassic.

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Structurally, there is interesting bending on Furnace Creek Fault line in the Amargosa Valley which would indicate that at one point in history P/T conditions were met for the rocks to undergo a Barrovian metamorphism. This bending coincides with the projected trace of a major transverse structural zone of oroclinal bedding and right lateral faulting (Furnace Creek Fault). Oroclinal bedding refers to bedding planes that are thick-skinned, regional, and have a significant margin-parallel displacement. They require either subduction or substantial crustal thickening along the inside of the buckles, and extension along the outsides.======

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The northern Death Valley-Furnace fault zone is a major fault system in the region. There is a Northeast trending Mesozoic back-fold within the east directed fold and thrust fault. Simple pre-extensional geometry exists for the Mesozoic orogen in this location. Over time, intense Cenozoic tectonism has dismembered the Mesozoic fold and thrust belt within the Death Valley terrain. These circumstances of the mesozoic thrust belt prove to be quite analogous to that of the Canadian Cordillera which provides a better understanding of this region, as the Canadian Cordillera is well studied.======

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Funeral Mountains region consist of a metamorphic core complex. They are a Northwest trending range in which metamorphic grade increases northwestward from biotite grade in the Southeast to kyanite grade in the Northwest.======

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In conclusion, the sample that was collected in the Natural Bridge Canyon was from a sedimentary Cambrian/late Pre-Cambrian protolith that was thrust up and eastward over younger Paleozoic rocks during the Mesozoic period. It is believed that Triassic rocks were already deposited at the point of thrusting so more closely, the thrusting that generated the low temperature/high pressure conditions needed to metamorphose the sedimentary protolith into its modern form probably occured between the Middle Triassic to Early Jurassic periods of the Mesozoic. The sample displays gneiss-like foliation where the leucocratic minerals have separated themselves from the darker micas. Because both Chlorite and Biotite minerals are present, and it was collected in the Southeast region of the Funeral Mountain Range, it can be assumed that this rock is in fact a Biotite-grade Greenschist.======

=**Sources:**=

Burchfiel, B.C., P.J. Pelton, and J. Sutter. 1970. “An Early Mesozoic Deformation Belt in South-Central Nevada-Southeastern California.” Geological Society of America Bulletin, vol. 81: 211-215.

Snow, J. Kent, and Brian Wernicke. 1989. “Uniqueness of geological correlations: An example from the Death Valley extended terrain.” Geological Society of America Bulletin, vol. 101: 1351-1362.

Kolm, Daniel K., and Roy K. Dokka. 1991. “Major Late Miocene Cooling of the Middule Crust Associated with Extensional Orogenesis in the Funeral Mountains, California.” Geophysical Research Letters, vol. 18, no. 9: 1775-1778.

