Name ________________________
Siliciclastic Rock Identification
Geology 103
Directions: Spend some time looking at the display specimens that are spread around the back
and side counters. These rocks are grouped by grain size. As you look at each group, try to
apply appropriate names or modifiers, and practice naming the rocks.
When you are ready: Answer the following questions.
Shale
Sample PSR:
What environment (oxygen content) would this green shale
be formed in?
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Sample MSO:
What depositional environment (oxygen content) would you
expect for this red, silty shale?
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Sample OBS:
What does the black color indicate in this shale?
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Sample 52:
This shale has fine laminae, but it does not break along the laminae.
What term would you use to describe the way the shale breaks?
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Sample MSP:
These shale chips break alone laminae. List two terms that are
used to describe this weathering texture.
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Sample FWS:
This silty shale contains leaf imprints and fish bones, and is from a
lake (fresh water environment). Since the organic content of shale is
related to color, what estimate would you give for organic content in
this sample? (high = > 10%, medium = 2-10%, low = <2%)
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1
Sample MSH:
This marine shale effervesces with dilute hydrochloric acid. What adjective
would you use to describe a shale that is almost a limestone?
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Sample PMR:
This silty shale has no identifiable bedding. What is
the rock name when no bedding is visible?
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Siltstone
Sample RSS:
This siltstone has green spots, but is predominantly red.
What geochemical environment might be indicated by
the green spots?
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Sample MSI:
This micaceous, sandy siltstone contains marine fossils (brachiopods).
Look closely, and describe the bedding:
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Sample BSI:
This blocky siltstone has laminae and is thinly bedded. Which are
finer-grained: the laminated or the thinly bedded intervals?
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Sandstone
Sample GVG:
This sample is from is a continental margin of NY, and is matrix-rich,
has abundant chlorite, and may be slightly slightly metamorphosed.
What informal name is often used for this type of rock?
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Sample NGA:
This sample is Triassic in age, and is from the base of the Newark
Group (New Jersey). What informal name is used for a sandstone
that has abundant feldspar grains?
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2
Sample LAS:
This sample has abundant lithic clasts, and low matrix content.
What is the name of this sandstone, using Gilbert’s classification? __________________
Sample AA:
There are many granule-sized lithic clasts in this sample, but the
average grains is still sand-sized. Salmon-colored feldspar is the
most common grain type. What name would Gilbert use for this
feldspar and matrix-rich sandstone?
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Sample OSA:
This sample is from the Oriskany Formation in Cuyahoga, NY.
It marks a major systemic boundary, and has abundant quartz grains
in addition to the large brachiopod shells. What is the name of this
sandstone, using Gilbert’s classification?
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Sample KSS:
This sandstone is from the Great Valley Sequence, near Lake Beryessa
It is from the Sites Sandstone member of the Cortina Formation, and is
Late Cretaceous in age. Lithic clasts are very common. If you look closely,
you can see larger clay pieces up to 10 mm in diameter that were ripped up
from the ocean floor during turbidity flows. These are called “rip up clasts”
What name would Gilbert give to a sandstone with high matrix and
high lithic content?
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Sample JSC:
This sample is from the Jurassic Stony Creek Formation, and forms
the base of the Great Valley Sequence near Lake Berryessa. The large,
dark grains are basalt (volcanic rock fragments). Using Gilbert’s
classification system, is this rock an arenite or a wacke?
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Conglomerate
Sample KVS:
This sample is Late Cretaceous in age, and is from the
Venado Sandstone member of the Cortina Formation. It was
collected near the Lake Berryessa spillway. Larger clasts are
clay rip-up clasts from the nearby ocean floor, and basalt.
Sandy matrix approaches 30-40% of the sample. Assuming
that the average grain size is 5-15 mm, what is the Wentworth rock name?
(be specific- 2 words!!)
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3
Sample UJM:
This sample is Upper Jurassic in age, and is a stream channel
deposit from the Morrison Formation in central Utah. The
Morrison Formation is famous for spectacular dinosaur discoveries.
Estimate the average grain size for this sample. What is the
rock name? (be specific- 2 words!!)
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Sample KPC:
This sample is from the Cretaceous Pigeon Creek Formation
in central Utah. Is it grain supported, or matrix supported?
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Sample F:
What informal name would we give to this conglomerate
with angular clasts?
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Sample UPC:
This sample has a sandy matrix, but the granules are generally
touching each other. What name do we give to a conglomerate
with grain-to-grain contact?
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Sample KBC:
This sample has a sandy matrix, and the cobbles are made of
less resistant metamorphic and limestone clasts. What name do
we give to a conglomerate that is made of unstable clasts?
(Hint- think oligomict, polymict or petromict)
4
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