JAC - 05-3 HW - Sedimentary rocks - guided inquiry (1)
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University of North Dakota *
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101
Subject
Geology
Date
Dec 6, 2023
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docx
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Uploaded by CommodoreFalconPerson959
1
Guided Inquiry: Sedimentary Rock Photos
All photos are found at
http://www.marlimillerphoto.com/Sed.html
Go to that website
for larger photos - you will want them
to answer the questions below.
The fifteen numbered questions refer to the 15 numbered
photographs.
SOME OF THESE QUESTIONS ARE VERY
DIFFICULT WITH NO ABSOLUTELY CORRECT
ANSWER. SO, JUSTIFY/EXPLAIN YOUR ANSWERS
WHEN APPROPRIATE.
1. Look closely at the photo and you will see that the rock
contains clasts of many different sizes. (a) What kind of rock is this(name it)? (b) The caption
says that the original sediment was deposited in an
alluvial fan
. What does that mean?
This rock is a tilted conglomerate, and being deposited in an alluvial fan means that it was
deposited from material that was transported by flowing water and then deposited in a
triangular fan shape, possibly at the base of a mountain.
2. This view shows sediment (it has not been lithified and turned into a rock yet) in a mudflow
deposit. (a) What is a
mudflow deposit
? How do such deposits form? (b) What kind of rock will
this sediment become if it is lithified?
A mudflow deposit is a poorly sorted mixture of silt, boulders, organic materials and other
debris, and is a deposit that is collected when mud mixes with water and flows like a fluid
down a slope. As the mudflow flows, it will leave behind deposits of mud, and when these
deposits do eventually lithify, they can form mudstone, or shale.
3. The caption (or the photo) is upside down. Somebody goofed. This is sandstone above
conglomerate. There is a sharp line that separates the two kinds or rocks. Why? How? How did
sediment of two completely different sorts get deposited like this in distinct and adjacent beds?
These two different kinds of sediments could have been deposited by a flow of water, that
first started with a fast current and then slowed down over time. When the water was
moving fast, large particles of rock would have been deposited, and then lithified to form a
conglomerate, and then as the current slowed, only the fine material was carried and then
deposited, and then lithified to form sandstone.
4. This photo is labeled a
channel deposit
. (a) What does that mean and (b) why does the rock
contain both large (cobbles) and small (sand sized) clasts?
A channel deposit is a deposit that was moved by a channel of water, such as a stream or
river, and is typically made up of sand and gravel. This rock contains both large cobbles
and small clasts, because its possible the water wasn’t moving very quickly, and so neither
the cobbles or smaller clasts moved very far before being deposited. It is also possible that
the channel deposit occurred at a delta. At a delta, large and small clasts may be deposited
at the same time and lead to the different sized clasts in the rock.
5. The sediment that later became the rock seen (sandstone and shale) was deposited horizontally
in a
turbidite
. Later it was tilted. (a) What is a
turbidite
and (b) why do turbidites often contain
alternating layers of sand (that later become sandstone) and mud (that later become shale)?
A turbidite is sediment that originates in an underwater canyon, as it gets disturbed, it
creates dense slurries that flush down underwater canyons in a large gravity induced event.
As the sediment flows to the base of the canyon, it gets deposited in sheet like layers. This
flush transports coarser sediment to the ocean floor, and then the sediment settles based on
density, with the heavier sediments settling first, such as sand, and the less dense sediments
such as mud settling later, leading to the formation of different layers.
6. This photo shows
eolian crossbeds
in Utah. (a) What does eolian mean? (b) And how do
eolian crossbeds form?
Eolian refers to a deposit of windblown sediments, such as fine dust and sand. Eolian
crossbeds form from wind that blows the sand and fine dust particles switching directions,
leading to layers that may form at angles depending on which direction the wind was
blowing when the sediment was deposited.
7. This outcrop shows
ripple marks
in a sandstone. (a) How do ripple marks form? There are two
kinds: eolian and fluvial. They (sort of) form the same way. Explain both.
Ripple marks are created as sediment grains pile up on top of a plane bed and are
determined by flow type. Eolian ripple marks can occur when wind blows sediments such
as sand into wave like or ripple like forms, such as sand dunes. Fluvial ripple marks are
formed in much the same way, however they occur underwater, when the water currents
flow and form fine sediment deposits such as sand into wave like or ripple like patterns.
8. This photo shows
mudcracks
in a kind of limestone called
dolomite
. How do mudcracks form?
Mudcracks form from clay rich sediment that is submerged underwater and then dries out.
As the clay’s crystalline structure fills with water, the grains of sediment will swell, and as
they lose water, they shrink back up leaving behind deep polygonal cracks that taper to the
surface.
9. This photo shows
graded bedding
. (a) What is graded bedding? (b) How does graded bedding
form?
Graded bedding refers to a sequence of coarse- or fine- grained sediment layers. It develops
when sediment deposition occurs in an environment where the energy is decreasing, as in a
stream that begins flowing very rapidly, leaving behind deposits of only larger sediments,
and then slowing down over time to eventually leave behind deposits of finer sediments.
10. This photo shows sandstone with
cross bedding
. (a) What is cross bedding and (b) how do
crossbeds form? There are two kinds: eolian and fluvial. They (sort of) form the same way.
Explain both.
Cross bedding occurs when ripples or dunes pile on top of one another, cutting into the
underlying layers, usually in a different direction. Fluvial cross beds are formed in water,
and are formed when the water that is carrying the sediment deposits changes direction, as
in when the waves in the ocean form ripples of sand on the ocean floor, and eolian cross
beds are formed with wind, but like fluvial cross beds, they are formed when the wind
carrying the sediment deposits changes direction and blows the sediment in a different
direction than it was previously settling.
11. This photo shows the Grand Canyon. The major rock formations include sandstone,
limestone, and shale. Some form cliffs and some do not. Which ones do you think form the
cliffs? You may not know for sure, but choose one and explain your reasoning.
The cliffs of the Grand Canyon are most likely formed by sandstone. A reason being, that
limestone and sandstone are pretty resistant to weathering, so as the other rocks making up
the canyon weather and erode, the rocks that would remain left behind and atop all the
other rocks would be those that are the most resistant to weathering (such as the
sandstone).
12. The photo is labeled
quartz sandstone
. I think that is redundant. Because almost all
sandstones are made of quartz. Why? Why are most sandstones made of quartz?
Most sandstones are made of quartz because quartz is extremely resistant to weathering
and typically keeps its form long enough to lithify and form new rock.
13. Here we see some sand-sized sediment. It contains the usual quartz but also some feldspar
(pinkish and not clear) and some dark colored
lithic fragments
. (a) What are
lithic fragments
?
(b) Why do some sandstones contain more than just quartz grains compared with normal
sandstones that are entirely quartz.
Lithic fragments are pieces of finely grained bed-rock such as mud chips and volcanic
clasts that have been eroded down to sand grain size. Some sandstones contain more than
just quartz because they formed before the lithic fragments could fully break down, or
because there were large amounts of these other deposits in the same location as the deposit
of sand that ended up forming the sandstone.
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(c) We often classify sediment by its
sorting
. Is this sediment well-sorted or poorly-sorted?
Explain your answer.
This sediment is poorly sorted because there are clasts of many differing sizes present. In
this photo there is the very clear presence of very small clasts, more medium sized clasts,
and larger clasts, and so this sediment would classify as poorly sorted.
14. Here is a photo of three different clastic sedimentary rocks. I cannot think of a good question
for this photo. So, you come up with a good question. What is a good question to ask about this
photo so that by answering it you are learning something? (I don’t need the answer,
just the
question.)
What can the properties of each of these three clastic rocks tell you about the origins of the
sediments that these rocks formed from?
15. The huge cliff in the background is shale. The formation is hundreds of feet thick and covers
a large part of west Texas. Other shale formations cover several states. How does shale form and
why can it covers such large regions?
Shale forms from deposits of clay left behind a very long time ago in large marine basins
that covered large areas. As the water dried up, what was left behind were shale formations
that ended up covering large areas of land that make up several states.