Name:
Answers
Lab day:
Tu
W
Th
F
Geology 1023 — Lab #2, Winter 2004
Structural Geology & Interpreting Sedimentary Rocks
I.
Folds – a review
Using the appropriate terms (anticline, syncline, upright, overturned, recumbent, plunging) label
the folds below in the spaces provided. See pages 276–277 of your text for additional help.
Plunging syncline
Inclined (overturned?)
anticline
Recumbent syncline
Structure & Sedimentary Rocks – Winter 2004
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II. Faults – a review
In the following diagrams label the hanging wall and foot wall (A or B), if appropriate, and the
type of fault shown (normal, reverse, thrust, oblique-slip, or strike-slip). See page 281 of your
text for additional help.
A
B
Normal fault
B
A
Reverse fault
A
B
Oblique fault
B
A
Thrust fault
n/a
n/a
Strike-slip fault
Structure & Sedimentary Rocks – Winter 2004
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III. Strike and dip – review.
The strike of an inclined plane is the compass direction of a horizontal line on that plane. The
strike can have any value from 0º (usually written 000º) to 360º (which are equivalent). Note that
from now on we will follow the Right Hand Rule. That is, we measure the strike so that the
plane we are measuring dips to the right of the direction of strike. See the PowerPoint tutorial on
the Compass, Drawing a Strike and Dip, and the Right Hand Rule (online).
The dip of an inclined plane is the amount of inclination of that plane from the horizontal. Dip
varies from horizontal (00º) to vertical (90º). As a precaution (to ensure that the Right Hand Rule
has been followed) we also indicate the general direction of dip. A general compass direction
will serve here, e.g., N, SW, etc.
1. a Assuming that the vertical edge (right or left hand) of the page points to North, draw a
strike and dip symbol for each of the following readings. Be sure to use the correct symbol.
b
bedding
090/45S
cleavage
135/30SW
bedding
225/10NW
bedding
045/27SE
cleavage
150/76W
Give strike and dip readings for the following bedding symbols.
045/16 SE
270/72 N
– 260/31 NW
135/89 SW
–060/15 SE
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2. Draw a cross section A-B for figure (a) and name the structure. Draw a topographic profile
and geological cross-section for A-B in figure (b). Complete the block diagrams (c) and (d)
and name the structures.
Structure & Sedimentary Rocks – Winter 2004
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IV. Sedimentary rocks – some review and some new material
The two major types of sedimentary rocks are:
Clastic (or detrital)
Made up of fragments of pre-existing rocks deposited by a flowing medium. The
character of clastic rocks is influenced mostly by physical factors, which include:
distance from the source area; nature of the transport medium (ice, air, or water); speed of
the medium.
Clastic sedimentary rocks are composed of:
Grains
Discrete particles deposited by sedimentary processes.
Matrix Variable amounts of finer grained material (usually mud) between the grains.
Cement Chemically precipitated material that binds the grains and matrix together.
Pores
Any remaining open spaces.
Non-clastic (or chemical, or biochemical)
Made up of chemically or biologically derived material. Rocks made from fragments of
biologically derived material are sometimes called “bioclastic”.
3. Indicate which of the Group 1 samples are clastic or non-clastic
Clastic
AC, AD
Non-clastic
AA, AB
Four major properties are used in classifying and interpreting sedimentary rocks. These
properties are composition, texture, sedimentary structures, and colour.
Composition
Composition refers to the mineral and chemical make-up of the rock (including the composition
of the grains, matrix, and cement in clastic rocks). Clastic rocks are composed dominantly of
silicate minerals and are commonly referred to as “siliciclastic”. Carbonates are dominantly
composed of calcite and dolomite. Evaporites are dominantly composed of sulphates and
halides (chlorides). Chert is the most important non-clastic silicic rock and is often referred to as
a chemical sedimentary rock along with evaporites and chemically precipitated dolostones and
limestones. The term biochemical is used for peat, coal, and most limestone.
4. Indicate which of the Group 2 samples are carbonates, evaporites, biochemical, or chemical
sedimentary rocks. Note that some samples will be in several categories.
Biochemical:
AE, AF, AG
AH, AI
AE, AF, AJ
Chemical:
AG?, AH, AI, AK
Carbonate:
Evaporite:
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Texture
The term texture refers to the nature of the constituent particles (crystals or grains), the size of
the grains, and, in clastic rocks, the relationship between the grains, matrix, cement, and pores.
Clastic rocks show the greatest amount of textural variation and are described in terms of four
variables: grain size, roundness, sphericity, and sorting.
Grain size has 3 major divisions. The following table gives the grain size, the name for the
unconsolidated sediment and the equivalent sedimentary rock.
Grain size
> 2 mm (c.g.)
1/16 - 2 mm (m.g.)
< 1/16 mm (f.g.)
Sediment
Rock
gravel
conglomerate, breccia
sand
sandstone
mud (silt and clay)
mudrock
(siltstone mudstone, shale)
5. Circle the rocks or sediments that present among the samples in Group 3.
gravel
sand
mud (silt/clay)
conglomerate/breccia
sandstone
mudrock
Roundness refers to the degree to which sharp edges have been removed from the particles
during transportation. There is a continuum from angular to fully rounded. In most cases
particles are relatively well rounded. Even small amounts of transportation tend to knock off
sharp edges. Because finer grains tend to be transported farthest, roundness is only an issue with
the coarse-grained rocks. Conglomerate contains rounded fragments. Breccia has angular ones.
True sedimentary breccias are relatively rare and were deposited very close to the source.
Sphericity refers to the degree of equidimensionality of the particles. The more equidimensional
the particles are, the greater the sphericity. This means that a cubic particle has high sphericity
whereas a rod-shaped or pancake-shaped particle has lower sphericity.
Do not confuse roundness with sphericity. Note that a cigar and a ball bearing are both well
rounded but differ markedly in sphericity. However, a dice and a ball-bearing differ markedly in
the degree of roundness but have high sphericity.
6. a
Rank the samples in Group 4 according to roundness.
Least
b
AS
AR
AQ/AP
AP/AQ
Most
Rank the samples according to sphericity from least (left) to most (right).
Least
AP
AR/AS
AS/AR
AQ
Most
Sorting refers to the degree of variability in the particle size and composition. A rock/sediment
with particles of the same size/mineral is well sorted. Rock/sediment with a wide range of grain
Structure & Sedimentary Rocks – Winter 2004
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size/composition are poorly sorted. Sorting by size and composition are described separately,
but, as a rule, sorting in one way is accompanied by sorting in the other.
7. a
Rank the samples in Group 5 according to sorting by size.
Least
b
AU
AT
AV/AW
AW/AV
Most
Rank the samples in Group 5 according to sorting by composition.
Least
AU/AV
AV/AU
AT
AW
Most
Sedimentary structures
Sedimentary structures in a rock result from processes that occurred during deposition or shortly
thereafter. These processes may be primary (either inorganic or organic) or secondary (physical,
or chemical). Primary structures form during sedimentation. Secondary structures form after
deposition but prior to (significant) lithification.
Primary sedimentary structures
Inorganic
Bedding at all scales (thick bedded to laminar)
Ripples (current and wave)
Cross-laminations or cross-bedding
Graded bedding
Sole markings (flutes, grooves)
Cracks
Raindrop prints
Evaporite mineral pseudomorphs
Organic
Tracks and trails
Burrows (visible discrete structures)
Bioturbation (bedding disturbed by biological activity)
Stromatolites (small mound structures up to 30cm across, algal mats)
Reefs (large mound structures from meters to kilometres across/long)
Secondary
Physical
Fissility in shales (dewatering?)
Cone in cone (dewatering)
Load structures
Chemical
Nodules and concretions
Dendrites
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8. Look at the display of structures at the back of the lab then examine the samples in Group 6.
Indicate which of them show the following structures. Some samples may show more than
one structure and some of the listed structures will not be represented at all.
bedding AX, AY, AZ, BA, BC, BD
BA
concretions
cross-lam. or bedding
BC
graded bedding
BC
raindrop prints
AY
ripples (wave)
sole markings
burrows
cracks
dendrites
load structures
ripples (current)
stromatolites
tracks/trails
AZ
BB
AX
Colour
The colour of a rock is the result of many of the above features. Particularly in fine-grained rocks
it may be useful in determining composition and the environment of deposition. Common
colours in clastic rocks include grey to black, nearly white, green, red, and various shades of
brown (yellow to reddish shades). The following are simplifications will help you make your
first interpretations of environment of deposition.
• Red (hematite-bearing) – subaerial, non-marine fluvial environments.
• Dark grey to black (usually shales) – strongly reducing, often deep water environments.
• Plant fossils are indicative of non-marine environments.
• Pale shades (white, pale green, yellow) – oxidising to mildly reducing, shallow to
relatively deep water environments.
9. Using colour (and any other features you think appropriate) indicate which of the samples in
Group 7 were deposited in oxidizing, reducing, marine, or non-marine environments.
Oxidizing
BF (BG?)
Reducing
BE, BH, BI
Marine
BG (BH?)
Non-marine
BE, BF, BI
Maturity
“Mature” sediments or rocks contain well-rounded and highly spherical grains of chemically
stable minerals (usually quartz). They are well sorted, and contain little or no matrix (mud).
10.
Rank the samples in Group 8 according to maturity.
Most
BL
BK
BJ
Least
Texture (where applicable)
m.g.
Mod.
Mod.
Well
Mod/poor
Bedding
f.g.
Not vis.
Not vis.
Not vis.
Not vis.
Bedding
Raindrip
imprints
Environment
Non-mar.
Oxidiz.
Mar?
Oxidiz.
Any
Sandstone Chert
Mudrock
(siltstone) (arenite)
Name
Greyish
Cavities
n.a.
n.a.
n.a.
n.a.
n.a.
SiO2
Reddish
Pale grey
Sand
Silt
1023-R4
Non-mar.
oxidiz.
Arkose
Pink/red
Porous
Poor/mod
Mod/well
Mod.
Mod.
m.g.
Sand
Non-clast. Clastic
1023-R3
Colour
Structures
Grain size
(c.g./m.g./f.g.)
Rounding
(poor/mod/well)
Sphericity
(low/med/high)
Sorting – size
(poor/mod/well)
Sorting – comp.
(poor/mod/well)
Composition
Clastic
clastic
1023-R2
Mar.
oxidiz.
Limestone
Brownish
Fossils
n.a.
n.a.
n.a.
n.a.
n.a.
Calc (shel)
Non-clast
1023-R5
11.
Clastic / Nonclastic
1023-R1
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Describe the rocks R1 to R5 from the “Rock and Fossil” drawers at the back of the lab.