Quaternary |
Holocene |
Modern
day Tecopa Valley is dominated by fluvial systems and deposition of alluvial |
8,000yrs.-present |
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fans. The rocks in this unit consist of gravels
to cobbles, and include present |
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day
stream flood plains, low inactive stream terraces, and flatlands in southern |
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Tecopa
Valley. Age estimates of this unit yield a date of 11 k.y. old. These holocene |
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alluvial
deposits are bounded on their lower surface by a disconformity. Tectonically
quiet. |
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Pleistocene
|
Late |
Erosional
unconformity |
1.8
mya-8,000years |
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Lithologically
similar depositional environment as in holocene time that was |
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controlled
by fluvial systems. Age constraints
place this gravel unit at ~86ky. |
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Erosional
unconformity |
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Another period
of alluvial gravel deposition truncated by erosional surfaces |
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on top
and bottom of this unit. ~110-50ky. |
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Erosional
unconformity |
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Oldest
period of gravel deposition also bounded by a basal unconformity. |
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Draining of
Lake tecopa occurs just before deposition of this unit begins. |
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Middle |
Age of
this first gravel unit is ~180-200ky. |
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Lake Tecopa Allogroup |
Wadworth
Tuff |
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The
uppermost portion of this unit is composed of a Tephra layer |
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that is
~200-155ky. This unit is called the Amargosa Alloformation |
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and at
the beginning of this period Lake Tecopa was rising to |
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water
depths of 450-485m. Deposition of lacustrine(lake) sediments |
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was occurring,
and particle size ranges from small gravels, to finer |
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sediments
such as sand, silts, and clays. The lowermost contact |
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is a tuff
layer that is related to the eruption and creation of Yellow- |
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stone caldera.
It is used to differentiate between separate units of Lake Tecopa |
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deposits.
It confines the Amargosa Fm. To ~ 665-186Thousand years ago. |
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Lava
Creek Tephra layer |
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The Shoshone
Spring Alloformation, except for its uppermost portion, was |
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deposited
in deep water. The uppermost portion
records a transgressive |
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lake
sequence. Lake and shoreline environments are the primary forces at |
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work in this
unit. It is seperated from underlying strata by another tuff layer |
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that is
attributed to the eruption of California's Long Valley caldera. The unit |
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is ~
758-665ky. |
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Bishop
Tephra layer |
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The Greenwater
Fan Alloformation is composed of 3 members. The lowest |
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consisting
of alternating layers of sandstone, siltstone, and mudstone which |
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Early |
indicates
a series of shallow lake sequences.
The second member indicates |
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a playa
environment and formation of evaporites. The uppermost appears to be |
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another
series of lake cycles. This Greenwater unit is underlain by yet |
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another
tephra layer that is attributed to an earlier eruption of the Yellow- |
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stone
volcano. |
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Huckleberry Ridge Tephra |
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Tertiary |
Pliocene |
The
Spanish Trail Alloformation is a product of a playa environment |
5-1.8 mya. |
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alternating
with shallow lacustrine deposits.
These deposits consist of |
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sediments
such as fine sand, silts, and clays.
Some researchers contend |
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that the
tectonism in Tecopa valley had quieted by this time. |
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Miocene |
Tectonically,
Tecopa valley is rapidly extending. Some have constrained this |
24-5mya. |
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extension
to a period between 11 and 7 million years ago. Rapid erosion and |
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deposition
is occurring, and China Ranch lake beds are forming to the south. |
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Information compiled from Cenozoic Basins of The Death Valley Region:
A paper by Roger Barron Morrison titled Lake Tecopa: Quaternary
geology of Tecopa Valley, California, a multimillion-year record and its
relevance to the proposed nuclear-waste repository at Yucca Mtn., Nevada.
By Chris Garner
