Geology Lab Field Trip
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What is western North America
Passive margin The edge if the continent would have been Nevada
When did the passive margin swith to a series of active convergent subduction zones?
Middle of Paleozoic to the end of the Mesozoic 251-65mya. complex terranes (carbonate and silicate marine sediments, basalts, volcanic island arcs were accreted to the edge leading to hot ground water metamorphism. Nevada, Washington, Oregon, and much of CA were added
What did the subduction zones cause?
uplift of a large mountain range (similar to andes), during the mid-late Mesozoic. Rocks were heavily deformed (folded) and matamorphosed and the remnants are called the Western Metamorphic Belt. As it was uplifted the sediments filled the Great Basin and our underneath Sacramento.
What is the Sierra Batholith?
heated rock that make thier way to the surface and decompress and melt to form magma chambers. Igneous plutons formed in Sierra Nevada. Most intruded 150-80 mya when oceanic Farallon plate was subducted underneath the North American plate. A lot of tectonic uplift with them formed 6-20 miles under the earth
If the batholith is now exposed at the surface, what happenened to the volcanic and sedinmentary rocks that originally covered the batholith?
They were eroded away and were eventually deposited elsewhere
What happened at the end of the Mesozoic?
most of the subductions had seased and the Andes like mountains eroded. Large rivers carried the sediments to the coast. Volcanism resumend burying the lowlying proto Sierra in volcanic rocks. 5 mya tensional stresses formed a series of normal fault blocks (eastern CA across NV)The Sierra Nevadas were tilted up like door on hinges along a normal fault system. Western side hing, and eastern side the open door.
What did glaciers do to the Sierra Nevadas 2mya - 10000 ya?
scoured, scraped, and carved by glaciers furing the last ice age. Yosemite resulted
Where did the gold come from?
Mined out of Eocene river gravels (not original source). It was disolved in seawater, which was subducted along with the ocean floor rocks during the Paleozoic/Mesozoic; overtime it was precioitated in the rocks and concentrated into deposits.
Granite, intrusive, cooled underground, coarse grained. Red color is from oxidized iron
Stop 2 Cool, CA
Medisedimentary: slickensides, formed when mass slides over another fault. Stra deforemd into folds. Chert, hematite, Quartz (hardness 7 with reaction to acid) Depositional enviroment: Accretion at a convergent PB caused this chert (which formed as silicate sediments in the deep ocean) to get scraped up onto the continent.
mottled green metamorphic rocks with waxt luster. Parent rock: basalt formed along MOR in the oceanic crust, then was accreted during subduction of the oceanic plate up onto the edge of the continent
Stop 3 cool cave quarry, big hole in ground
limestone or dolomite or melange or crustalline or limestone: palezoic 542-251 mya, modter than glass, reacts with acide, calcite. Likely formed as deep ocean sediments that were deposited near passive margin in the palezoic
Stop 4 Confluence, north and liffle forks of the American River
Large basketball sized gravel. Jurassic/Triassic volcanics/matamorphics or palepzoic Melange. 1 metavolcanic (early metazoic lighter in color, felsic, granite, colled slowly undeground, coarse grained igreous rock composition, quartz, feldsbar, formed in magma chambers then cooled and the volcano erroded away.Parent rock is slate 2. Slate was formed by the sedaments of muds into the ocean floor, scraped off during subduction, of this oceanic derived rock. Metavolcanic early metozoic,
Did the parent rocks form there or travel from somewhere else?
Deep ocean shales are accreted along a subduction zone and metamorphosed to slate in the process, later, the slates are intruded by magma, later still, erosion and uplift bring the rocks to the surface.
Stop 5 really really cool igneous rocks on applegate road
Rock shapes are blobby, rounded masses. Fine grained, cooled quickly. The non weathered side is blue-grey, grey, green tinted, mafic darkish brown Oceanic origin has Fe/Mg. It was formed Pillow Basalts! Erupted as lava onto the ocean floor, likely along the MOR then carried to continent on tectonic plate, then accreted onto the continent during subduction in the mid-end of Meszoic
Stop 6 Hapshire rocks road trailhead to loch leven
Igneous rocks, coarse grains, cooled off slowly underground (Phanertic) as magma chamber intrusive. Color: mostly light with dark speckles (salt & pepper) mostly felsioc, but range from felsic to intermediate (high silicates) The rock is granite. Formed during subduction, friction heats riks, which rist toward the surface some cool underground and form granite which is uplifted and eroded at the surface. There are darker colored blobs included with the main rock called xenoliths that have different mineral comps then the main rock.
What is the reason why the rocks are rounded blobby and smooth?
Glaciation scoured them, acting like sandpaper to make them smooth round polished until 10000 ya. Erosion has removed the smoothness off the pluton/batholiths from the last glacier 10000 ya, the xenolioths are eroding slower than the granite.
Stop 7 Gold run rest stop
Graded beds, high energy to low energy. Formed in the Eocene 55 mya. River cobbles, gravels at the confluence of the N and Mid Dorms of the American. These sediments must also have been deposited in the Rover systems. Human interested in the hills because of gold. Big metal cannon like object is the hydro mining which led to sediments raising the bed of the sac river, flooding downtown.
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