Lab 12 - Geologic Maps
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Dec 6, 2023
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Name__________________
___________________
Section________________
Lab 12
:
Geologic Maps
Part 1. Geologic Time Scale
Geologists cite a standard geologic time scale to reference different parts of Earth’s history along a timeline extending back 4.56
billion years to the present-day.
Although the subdivisions of geologic time sound foreign to most, geologists are quite familiar with
the sequence of geologic time as referenced by the names of the geologic Eons, Eras, Periods and Epochs.
Strata are like the library
of Earth’s history – they record past environments, preserve ancient life in the form of fossils, and retain the evidence of tectonic
deformation in their orientation and structure.
The stratigraphic record for the more recent past is more detailed (Think of a library -
it would be easier to find a record of what
happened yesterday than what happened 10 years ago on this day, or 100,
or 1000 etc).
Geologists commonly use more
subdivisions for more recent times.
Most of the geologic maps you’ll interpret in this exercise lump the three pre-Phanerozoic Eons
into an informal Eon called the “PreCambrian.”
Since the Cenozoic Era is the most recent, the periods within it are more finely
divided into smaller Epochs.
For the most up-to-date internationally agreed-upon time scale subdivisions and dates, see the
International Chronostratigraphic Chart (v. 2020/01), published by the International Commission on Stratigraphy at
https://98ca4554-1361-4fb1-a4d8-a1bb16d032e6.filesusr.com/ugd/f1fc07_c4d5c28dbe474fd89cef7cc009edd825.pdf?index=true
EONS
ERAS
PERIODS
EPOCHS
___Quaternary_____ Holocene
Pleistocene
Ng
Neogene_____
Pliocene
Cenozoic
Miocene
66Ma-present
Oligocene
Pg
Paleogene_____
Eocene
Paleocene
___Cretaceous_____
Phanerozoic
Mesozoic
___
Jurassic
_____
252-66Ma
___
Triassic
_____
___Permian_____
___Pennsylvanian_____
___
Mississippian
_____
Paleozoic
___ Devonian_____
541-252Ma
___Silurian_____
___
Ordovician
_____
___
Cambrian
_____
___Precambrian
4560-541Ma
(Includes: Proterozoic, Archean and Hadean Eons together)
3
1.
Follow the web link above for the International Chronostratigraphic chart, and write-in the starting
dates for each geologic time period in the Phanerozoic Eon on the chart in the blanks provided on to
the RIGHT of each Period name. All dates are understood to be in Ma (millions of years).
2.
Why do you think it is easier to find examples of Phanerozoic –aged strata than PreCambrian strata
in
most places on Earth?
____________________________________________________________________
3.
See p. 137 of the Lab
1
2
reading assignment to find the short-hand symbol used for each of
the
Periods in the Phanerozoic Eon, and for the Precambrian Eon, and write in the letter or letter-
combination used as a symbol for each to the LEFT of each Period name.
The short-hand for the
Paleogene and Neogene Periods have been filled in for you.
4.
On the International Chronostratigraphic Chart, a different time period name is used for the
Mississippian and Pennsylvanian Periods combined. What is the name of this time period?
____________________________
The symbol for this period is “C”
5.
Why do you think the symbol for Cretaceous is “K”
instead of a “C” ?
____________________________________________________________________
Part 2.
Stratigraphic Principles
Geologists use fundamental stratigraphic principles, developed in the mid-1600’s-
early 1800’s, to determine a sequence of geologic events based primarily on logic and observation of natural processes. Although
these were discussed in the Lab Module 1
2
reading – they are worth another reading:
A.
Superposition –
in a sequence of strata older layers are on the bottom of the
sequence, and progressively younger layers are found toward the top of the sequence.
Structural overturning of strata, which may happen in areas of severe crustal deformation
close to plate boundaries, invalidates this principle.
B.
Original Horizontality –
layers are deposited horizontally;
if layers are not
horizontal, some force (often tectonic or gravitational) has acted on the rock layers after
they were deposited to change their orientation.
C.
Original Lateral Continuity –
sedimentary layers, and some lava flows and
volcanic ash layers are deposited over wide areas (i.e. marine strata deposited on floor of a
large ocean, thick sand layers across a sandy desert, volcanic tuff or flood basalts across a
continental surface, till deposits at the base
of a continental glacier, floodplain deposits
across a broad river valley).
Layers can be traced laterally until they pinch out (thin to zero
thickness) where the depositional medium ended.
If a layer abruptly disappears, some
event after deposition of the layer disturbed the original lateral continuity – erosion,
faulting, igneous intrusion, etc.
4
D.
Cross-Cutting Relationships –
any geologic feature that cuts across another feature is
younger than the feature it cuts.
This applies to unconformities (erosional surfaces), faults and
igneous intrusions (dikes, sills, batholiths).
So any rock that has a fault running through it is older
than the fault itself.
Or any rock intruded by a dike, sill, batholith or other igneous intrusion is older
than the intrusion itself.
6.
Why do you think the geologic time scale is depicted with the Phanerozoic Eon on the
t
op and the
PreCambrian Eon on the bottom?
Why not the other way around?
____________________________________________________________________
Examine this block diagram showing both a geologic map view (top of block) and a cross-section view of a
sequence of strata affected by faulting (at A and F), intrusion (at B and D), and erosion (squiggly line separating
formation C from strata below).
Answer the following using the information provided above.
7.
Which principle tells you that
fault F is younger than intrusion B
and layer C?
______________________
8.
Which principle tells you that
the folded layers intruded by
batholith B were truncated by
erosion AFTER they were
deformed?
____________________________
9.
What principle tells you that
layer C is older than layer E?
___________________________
10. Referring to the geologic time scale, and using the principle of superposition, if intrusion B, and the
layers it intrudes, date to the very end of the Ordovician Period, and layer E dates to the very beginning
of the Mississippian Period, what time period does formation C and Intrusion D
likely belong
to?____________________________
11.
Only two formations would show up in a map view of this area – which two?
B
C
D
E
Fig. 1.
Source
www.opengeology.org
Part 3.
Geologic Maps and Cross-Sections
–
Geologic maps depict the distribution of strata of
different ages and rock types on the Earth’s surface.
As such, they represent something like a birds-eye view
of the geology of an area.
Geologic maps also include interpretations of what the shallow crust would look
like in side view based on the orientation of strata and structure types found on the surface.
These are called
cross sections.
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5
Examine this photograph of a piece of decorated birthday cake.
Imagine what it looks like in birds-eye view (looking straight down on it)
12.
Draw what you would see in top-down view below.
Use different
colors for different materials (frosting, flowers, writing, candle, etc.)
This is what geologists would call a “map view”
Now focus on what you see in the cake in side view only.
13.
Draw what you would see if looking at the cake from the side.
Again, use different colors for different
materials, but be consistent with color scheme you used in the above question (use same color for
frosting in both views)
This is what geologists call a “cross-section”
What you’ve done is very much like what geologists do when making a geologic map.
They plot out different
rock types at different places on the earth’s surface, and make a “map view”, using different colors and
symbols for rocks of different ages and types.
They also often provide a cross-section to help the reader
visualize what layers are present below the surface, their orientation, and their structure.
Part 4.
Geologic Map Symbols for orientation of strata, faults and folds.
Carefully examine Fig. 2 on the next page, which depicts symbols used for orientation of strata with respect to
geographic north and the horizontal plane, degree of certainty or type of contacts between strata, different
types of folding and different types of faulting, in both cross-sectional and map views.
6
Figure 2 – Common Geologic Map Symbols.
Block diagrams provide map view (top of block) as well as cross-sectional
views (sides of block)
Note symbols used for
•
orientation of strata:
strike, dip, dip angle, vertical beds, horizontal beds
•
contacts between stratigraphic units:
certain, inferred, inferred beneath sedimentary cover
•
folds:
dome, basin, anticline, syncline, plunging anticline, plunging syncline, monocline
•
faults:
normal, strike-slip, reverse, thrust
Source:
https://gotbooks.miracosta.edu/earth_science/images/map_symbols.jpg
Refer to Fig. 2 to answer the following:
14.
What is the relationship between the strike of a layer and the dip direction of the layer?
a.
They are parallel to one-another
b.
They are perpendicular to one-another
15. What would be the appropriate way to report the strike of the strata in the upper-left-hand block
diagram?
a.
N40W
b. W40N
16. What would be the appropriate way to report the dip angle and direction of the strata in the upper-
left-hand block diagram?
a.
N40W
b.
W40N
c.
SW 30 °
d.
NW 40°
7
17. What would the dip angle of horizontal strata be?
a.
0 degrees
b.
90 degrees
18. What would the dip angle of vertical strata be?
a.
0 degrees
b.
90 degrees
19.
Examine the strike-slip fault depicted.
Is it a left-lateral fault or a right-lateral fault?
Circle one.
20. Describe how the map view of a regular anticline/syncline pair differ from the map view of a plunging
anticline/syncline pair:
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
21.
On the block diagram of a geologic dome, are the oldest strata found on the edges of the structure in
map-view, or in the center of the structure?
a.
Edges
b.
Center
22. On the block diagram of a geologic basin, are the oldest strata found on the edges of the structure in
map-view, or in the center of the structure?
a.
Edges
b. Center
23. On the block diagram of a regular anticline, are the oldest strata exposed in the center of the anticline
or away from the center of the anticline?
a.
Center
b.
Either side of center
24. On the block diagram of a regular syncline, are the oldest strata exposed in the center of the syncline,
or away from the center of the syncline?
a.
Center
b.
Either side of center
25. From the block diagrams provided, describe the difference between the dip angle of a reverse fault,
and the dip angle of a thrust fault:
____________________________________________________________________
Part 5.
Reading Geologic Maps and Integrated Google Earth Overlays
For the following questions, refer to Fig. 3 (p. 8)
26.
What time period do the oldest rocks in this map area belong to?____________________________
27.
What time period do the youngest rocks
in this map area belong to?
___________________________
28.
Assuming for this and the remaining questions that geographic north is at the top of the map area,
what is the strike direction of most of the strata in this map area?
a.
N-S
b.
E-W
c.
NE/SW
d.
NW/SE
29.
What kind of fold is depicted in
the map area from A-A'
?
a.
Monocline
d.
Plunging Anticline
g. Basin
b.
Regular Anticline
e.
Plunging Syncline
c.
Regular Syncline
f. Dome
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8
left-lateral
3
0
.
Circle the type
of fault is depicted in the SW1/4 of the map area
:
right-lateral
31. a)
Find the strike and dip symbol in far SW corner of the map. Circle the correct answers:
Strike:
N/S
E/W
NW/SE
NE/SW
Dip angle in degrees:
5
8
14
21
Dip direction:
N
S
E
W
NE
NW
SE
SW
b
)
Is this fold an anticline or a syncline?__________
Figure 3:
geologic
map, cross-section,
and list of geologic
units
https://
www.geol.umd.edu/
~jmerck/geol100/
images/21/toymap.gif
For the following, refer to Fig. 4 – Geologic Map of Red Rock Conservation Area
in the Vicinity of Calico Basin, Blue Diamond, Keystone Thrust. (p. 9-10)
32.
Draw the symbol
(heavy, black)
used to depict the Keystone Thrust:
33.
In the west-central portion of the map area the Keystone Thrust brings Jurassic strata in fault contact
with Cambrian Strata. Which strata are located on the hanging wall block of the thrust fault, Cambrian
or Jurassic?
Circle one.
34.
Locate the La Madre Fault in the extreme NW corner of the map area. Which is younger, the La
Madre
Fault or the Keystone Thrust?
Circle one.
Explain how you reasoned your answer to this question:
9
35.
Look to the central map area an exposure of the Permian Kaibab and Toroweap formations (map
symbol Pkt, shown in light blue coloration).
This area is known as "Blue Diamond Hill."
What is the
strike and dip reported in the north-central
outcrop area of this formation as shown?
Strike ______
Dip Angle_______ Dip Direction
________
.
36.
At the south end of Blue Diamond hill, there are two
small folds.
Which
fold has an exposure of
Monte Cristo Limestone (map symbol Mm) sitting on its
axis?
Anticline
Syncline
37.
Look at the cross section provided below the map excerpt: what kind of geologic structure is depicted
on the footwall block of the Keystone Thrust?
a.
Overturned anticline
b.
Overturned Syncline
c. Monocline
Fig. 4
Geologic Map and Cross-Section of the Keystone Thrust, Calico Basin, and Blue
Diamond area of RRNCA & Vicinity.
Excerpted from
: Page, et al, 2005.
Geologic and geophysical maps of
the Las Vegas 30' x 60' quadrangle, Clark and Nye Counties, Nevada, and Inyo County, California.
USGS Scientific
Investigations Map SIM-2814 20005.
Full Map and Google Earth .kmz overlay available at the National Geologic Map
Database link:
https://ngmdb.usgs.gov/Prodesc/proddesc_71661.htm
10
Fig. 4, continued – Geologic cross-section of Keystone Thrust
For the following, examine the geologic map and accompanying Google Earth satellite image of this region
of Canyonlands National Park in Utah.
Fig. 5.
Geologic Map and Google Earth satellite images of the Upheaval Dome
area
of Canyonlands NP
Source:
https://geology.utah.gov/map-pub/survey-notes/geosights/upheaval-dome/
38.
What is the age (time period) of the strata in the center of the structure? ________________
39.
Are the strata surrounding the core of the structure younger than, or older than, the strata exposed in the
center? Circle one.
40.
Does the Triassic Kayenta Formation dip toward the center of, or away from the center of this structure?
Circle one.
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11
Visit the story of Upheaval dome as depicted in the Utah Geological Survey’s field guide at the link provided
above.
41.
Explain, IN YOUR OWN WORDS, how salt can form a dome:
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
42.
What other process has been suggested as a possible mechanism of dome formation at Upheaval Dome?
__________________________________________________________________________
43.
What evidence exists that this is NOT a salt dome, and rather something much more…extraterrestrial?
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________