Download Lab 5

Geology 101
Name(s):
Lab 5: Sedimentary rocks
Needed: Weathering samples W1 through W6 (Tub 18), sedimentary rock samples
M15, 16, 17 (Tub 19), R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1 (Tub 36)
Sedimentary rocks are those that are formed either by direct deposition of material
by crystallization of minerals in aqueous solution or by organisms, or through
sediment settling under gravity. The settled sediment or biological material will
lithify, in which it will be compacted and then chemically cemented into a rock.
To make sediment, a rock of any type must be weathered (either physically or
chemically) on the Earth’s surface, then erosion will transport the sediment to
where the sedimentary rock forms.
Weathering
1. Samples W1 and W2 are igneous rocks; you should verify that W1 is granite and
W2 is gabbro. Sample W3 is a partially weathered igneous rock. Sample W4 is
beach sediment.
a. Identify the minerals in the clasts breaking off of W3.
b. Did sample W3 start off as W1 or W2?
c. Look at sample W4 with a hand lens. Did this sediment come from W1 or W2?
d. When sample W4 compacts and lithifies, what will its basic rock name be? Hint:
Would it still be an igneous rock?
e. What would a beach composed of sediment from the weathering of a pluton
made of W2 look like? (Hint: think of Hawaiian beaches)
Physical weathering
2. a. Imagine freezing a sealed bottle full of water. What will happen to the bottle?
What type of physical weathering is this?
b. Examine the section of four-inch diameter concrete drain pipe from my backyard.
What type of physical weathering caused the holes in the pipe?
Chemical weathering
3. Examine the penny. What the green material on it? How did it form? If you were
to remove all of the green material, would the penny weigh the same as it did before
or less?
Soils and soil classification
Soils comprise the weathered bedrock (regolith) and organic material at the
surface of the Earth. Most soil investigations begin with an analysis of the particle
(grain) size distribution, a measurement of the percentage of each category of
primary particles by weight. Most scales used to measure particle sizes (including
the one used here) identify four categories — gravel (cobble, pebble or larger), sand,
silt and clay.
If you classify soils by their grain size distribution, then the triangular soil
texture diagram (on the next page) can be used. This kind of classification, as
shown in the diagram, relies only on the percentages of sand, silt and clay. Note that
this information will be useful for some endeavors (construction) but less useful for
others (agriculture).
Fertility, drainage and stability of the soil are some very important soil
properties that are assumed to be correlated with the particle size distribution, or
texture, of the soil.
4. Use the triangular soil texture diagram provided to classify the following soils
(note: these are not the soil samples on the side display cabinet):
Soil
% sand
% silt
% clay
Texture
A
15
51
34
B
72
14
14
C
47
32
21
Although precise measurement of the particle size distribution is an involved
laboratory procedure, a good first approximation can be made on site with no more
equipment that your hand and a little water. Instead of determining exact
percentages, this field test (called the three-test technique) determines the
textural class of the soil. The textural classes each have specified ranges of sand, silt
and clay percentages (they are the thick-outlined areas on the triangular soil texture
diagram). The boundaries between two classes of soil are not as distinct as shown;
one soil class grades into the adjacent classes.
5. Examine each of the soil samples on the side display cabinet and determine its
textural class using the three-test technique flow chart shown on the next page. Use
the Munsell soil color chart to determine the soil color; use the color notation
given in the chart.
Soil
Ribbon SandiSmoothSoil texture
Color (using
sample
test
ness
ness/
the Munsell
stickiness
chart
notation)
1
2
3
4
THREE-TEST TECHNIQUE FOR SOIL TEXTURE ANALYSIS
This field technique comprises three tests determining the relative amounts of clay, sand and silt in a
soil, and classifying the soil TEXTURE accordingly.
Ribbon test — gather a small handful of the soil and moisten (not saturate) it. Clench that handful of
moist soil and see if a continuous ribbon of wet soil is extruded from between your thumb and bent
forefinger.
No ribbon/poor ribbon — go to A
Fair ribbon (short stretches with many breaks) — go to B
Good continuous ribbon — go to C
A. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — go to D
Negative result (not sandy) — go to E
B. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — soil is SANDY CLAY LOAM
Negative result (not sandy) — go to F
C. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — soil is SANDY CLAY
Negative result (not sandy) — go to G
D. Finally, perform the aggregate test — in a dry sample of the soil, look for aggregates (dirt clods).
No aggregates — soil is SAND
Some aggregates — soil is SANDY LOAM (or LOAMY SAND)
E. Finally, perform the smoothness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers.
Soil feels smooth (no graininess) — soil is SILT (or SILTY LOAM)
Soil feels even slightly coarse — soil is LOAM
F. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers.
Soil feels sticky but not slick — soil is CLAY LOAM
Soil feels sticky and slick — soil is SILTY CLAY LOAM
G. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers.
Soil feels sticky but not slick — soil is CLAY
Soil feels sticky and slick — soil is SILTY CLAY
6. Which of the samples (#1, 2, 3 or 4) would have the greatest permeability?
Why?
7. Which of the samples (#1, 2, 3 or 4) would most likely have been the B horizon
of a soil? Why?
Making sedimentary rock
8. Lithification (making a sedimentary rock) involves both compaction and
cementation. W5 and W6 are glacially deposited sedimentary rocks of the same
composition. Which has undergone more compaction? How can you tell?
9. a. Sedimentary rocks are held together by cement, a non-mineral chemical
compound, which forms bonds (though not as strong as chemical bonds) between
mineral grains. The three common cements are silica (SiO2), calcite or iron
oxide (rust). How would you identify each cement (think of a test for each)?
Iron oxide —
Calcite —
Silica —
b. Look at rock samples R18 and R19. What is the cement that holds each together?
R18
_________________
R19 _________________
10. How does the cement get into these rocks?
11. Of course, like all rocks, sedimentary rocks are made of minerals. The most
common minerals were given on the second page of Lab 3. Using the Mineral ID
sheets from the lab manual, identify the following minerals:
Mineral #
Distinguishing features (color, cleavage,
hardness, magnetism, density, etc.)
Mineral name
M-15
M-16
M-17
12. Now consider the actual mineral grains in rock samples R18, R19, R24 and
R25. Because sand-sized dark minerals are very hard to identify, sedimentary
petrologists (much to the horror of igneous petrologists) use the term “dark lithic
fragments” to categorize lots of little dark minerals.
Sample
Hint to ID the mineral
R18
Use glass plate
R19
Well, look at it
R24
Acid bottle?
R25
Carefully, get it moist and see
what happens to its size
Most common mineral in
the rock
Identifying sedimentary rocks
Sedimentary rocks can be classified in a number of ways. For our purposes, the first
division to be made is between clastic sedimentary rocks (those that are made of
weathered and eroded grains) and non-clastic or other sedimentary rocks (these
include sedimentary rocks of biological and chemical origin).
Flow chart for identifying sedimentary rocks — If the rock is made of grains or
other materials which have been deposited by wind, water or ice, or else was
generated by biological or surface chemical activity, it's a sedimentary rock.
First step.
If the rock is made of broken up bits of
rock (including extremely fine grains)
→ GO TO Second step alternative
A. (Clastic rocks)
Else → GO TO Second step
alternative B.
(Chemical or biological rocks)
Second step alternative A. Consider the most common grain size in the rock from
the following list.
cobble or
pebble
sand
silt
clay
> 2 mm
0.062 — 2 mm
0.005 — 0.062
mm
< 0.005 mm
easily visible to naked eye; "grains" may
contain identifiable minerals
visible to naked eye
not visible but can be felt between fingers or
across teeth
not visible; cannot be felt between fingers or
across teeth
If the most common grain size is cobble or pebble → conglomerate
If the most common grain size is sand → sandstone (arenite)
If the most common mineral is quartz → quartz arenite
If the rocks is medium gray to red and well-sorted → arkose
If the rock is dark-colored and has much fine grain-size material in its
matrix → greywacke
If the most common grain size is silt or clay or a combination of both:
If the rock splits into irregular or regular layers → shale
If the rock is massive (no layering) → mudstone
The terms siltstone and claystone are also used on occasion.
Second step alternative B. Identify the most common mineral in the specimen
(use mineral ID chart if necessary).
If the most common mineral is quartz → chert
If the most common mineral is halite → rock salt
If the most common mineral is gypsum → rock gypsum
If it is black-colored, not very dense and flaky → coal
(also look for plant fibers)
If it fizzes, the most common substance is calcium carbonate, usually in the form
of the mineral calcite (be careful you are not fizzing the cement)
If the rock is not very dense and pure white → chalk
If the rock is made of broken-up shells → coquina
If the rock is dense and white, gray or black → limestone
One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If
the fossil is literally the body of the organism (or parts such as skeleton or shell), it is
called a hard parts fossil; if the fossil merely records the shape of an organism (like
a leaf impression in silt) or the passage of an organism (like preserved footprints),
then it is called a trace fossil. For any sedimentary rock, if it contains any fossils,
use the adjective fossiliferous in front of the rock name.
Needed: sedimentary rock samples R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1
(Tub 36)
13. Fill in the following table for clastic sedimentary rocks. Begin by determining the
average grain size of the clasts in the rock (use the grain size terms in the flow
chart), then the grain sorting (the choices are: well-sorted, moderately sorted,
poorly sorted and unsorted) and the grain roundness (the choices are; wellrounded, sub-rounded, sub-angular and angular). See the diagrams to determine
which type of rounding and sorting the grains have. Under fossils, your choices are
none, hard parts or trace fossils. Finally, identify the rock, using the flow chart.
Grain sorting:
Grain rounding:
Clastic sedimentary rocks
Sample #
R18
R19
R20
R21
R22
Grain
size
Grain
sorting
Grain
roundness
Fossils
Rock name
14. Fill in the following table for “other” sedimentary rocks. Begin by determining the
rock’s mineral composition. Then add any other details that help identify it. Under
fossils, your choices are none, hard parts or trace fossils. Finally, identify the
rock, using the flow chart.
“Other” (chemical and biological origin) sedimentary rocks
Sample #
Mineral
composition
Other
defining
details
Fossils
Rock name
R23
R24
R25
R26
R27
Sedimentary rock properties and depositional environments
15. Return to R18 and R19 and circle the correct answers:
a. Which rock contains the most stable mineral clasts?
(at the Earth's surface)
R18
R19
b. Which rock is composed of rounder grains?
R18
R19
c. Which rock is more well-sorted?
R18
R19
d. Based on a-c, which sample was deposited furthest
from its source (and thus is called mature)?
R18
R19
16. The energy of the system (how much force is behind the medium of
transport (air or water)) can be characterized by the size of the particles the
system can carry. For instance, high-energy systems can carry large grains; lowenergy systems can carry small grains. Examine and rank rocks R18, R21 and R20
in order from highest energy to lowest energy depositional system.
17. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5).
What test can you perform to tell them apart, and how does each behave in the
test?
18. Which rock (R33 or R5) has fossils? By the way, in general, why didn’t you
worry about fossils in igneous rocks?
The place in which the sediment is deposited or the organisms lived is called the
depositional environment. Examples of depositional environments include
terrestrial environments (like lakes, deserts and rivers), transitional
environments (like beaches and tidal flats) and marine environments (like
continental shelves and the abyss). Note that, over time, a beach area may be
uplifted by plate tectonics so that you will find a transitional depositional
environment quartz-rich sandstone deep in a mountain range!
19. In what depositional environment did rock R25 form (see diagram above)?
Hint: these kinds of rocks are called evaporites. Explain how they form.
20. Look at sedimentary structure S1, which is an example of preserved ripple
marks. Are they symmetrical or asymmetrical? Based on that answer and on the
wavelength of the ripples, is it more likely that these ripples were originally
deposited in a desert, a river, or a tidal flat? How are they preserved so that you
can see them today?