Ashford University Geology Lab

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Geol 103
Prelab for Lab 19 – Geologic Maps
The above is a geologic map showing a cross section of the line A-A’. You’re looking at the top or map
view in the large image, and what is close to the side view in the cross-section. On the right is the key,
or legend, which identifies ages of each rock by color and symbol (e.g. Pe1 is the name, and yellowgreen the color). Use the above to answer the following questions.
1) What are the oldest and youngest units on this map, according to the key? (Hint – geologic map
keys always arrange units in a certain age order – think law of superposition)
2) Notice the map has dip and strike marked on the layers. Which unit on the map do most of the
layers appear to be dipping towards?
3) Where are the youngest layers on the map located? Where are the oldest?
4) What geologic structure appears to be at unit J2 on the geologic map (excluding the faults and
T1)?
5) What geologic structure appears to be in the lower right-hand corner of the map?
1
Geol 103
Lab 19 – Geologic Maps
INTRODUCTION
Geologic maps show the distribution of geologic features, including rock units (i.e., formations)
and geologic structures (faults, folds…), which are exposed on the surface on the earth. These
features are often superimposed on a topographic map. Rock units are shown using a variety
of colors. The age and name of individual rock units are also shown using a combination of
letters. An upper case letter is used to show geologic age (for examples, “O” for Ordovician)
and lower case letters are then used to show the name of the rock unit (for example “h” for the
Harding Sandstone); the Ordovician Harding Sandstone is shown on a map using the letter
combination “Oh”. Geologic features are shown using lines and/or symbols. For example,
normal faults are shown using a solid line that traces the surface expression of the fault, with a
lower case “u” used to identify the up-thrown block (also known as the foot wall) and a lower
case “d” used to identify the down-thrown block (also known as the hanging wall). Each
geologic map contains a key to explain all the symbols shown on the map. The key also includes
a description of the rock units, including the lithology. Geologic maps also include a scale, to
relate distance on the map with distance on the earth’s surface (i.e., an inch on the map
represents a given distance on the ground).
READING GEOLOGICAL MAPS
Questions #1 – #6 will be answered using the Douglas County Geologic Map. Use this link to
access geologic map:
1) What is the oldest map unit outcropping in Douglas County? What is the geological age
(give the period) of that map unit? (3 pts)
2) Click different parts of the geologic map and explore the different types of lithologies
found in Douglas County. What kinds of rocks (lithology) make up the Pennsylvanian
rocks of Douglas County? (3 pts)
3) Click on the upper part of the white diamond on this map, this should take you to a part
of the map that has Lawrence marked with a target symbol. What is the unit that covers
most of Lawrence, and what is the geological age (give the period, series, and group) of
this map unit? (3 pts)
Geol 103
Lab 19 – Geologic Maps
Dip and strike of Pennsylvanian rocks in Douglas County.
The outcrop expression of rock units is a function of topography and the attitude (strike and
dip) of the beds. The outcrop expression of beds that are flat lying is controlled entirely by
topography. The formations exposed in eastern Kansas are relatively flat lying (they have a very
low angle of dip), and extend from Iowa to Oklahoma, and from eastern Kansas to eastern
Colorado. Think about the topographic variations across the outcrop extent.
4) In which part of Douglas County are the oldest units exposed? (2 pts)
5) In which part of the county are the youngest Pennsylvanian units exposed? (2 pts)
6) What is the relationship between the formation contacts and the contour lines? You will
need to click around the map to see the contour lines in relation to the formation
contacts. (3 pts)
MAP PATTERNS OF FOLDS
Contour Lines:
The shape of a set of contour lines depicts the shape of the
horizontal sections through the folded surface. Cylindrical folds give
structure contour patterns consisting of contours of similar shape
and size. Non-cylindrical folds, however, have more complex
structure contour patterns. Concentric circular contours indicate
domes or basins.
Geol 103
Lab 19 – Geologic Maps
Anticlines and Synclines:
In map view, anticlines appear as parallel beds of the same
rock type that dip away from the center of the fold. The
oldest beds are in the center, along the axis of the fold (an
imaginary line that marks the center of the fold). In map
view, a syncline appears as a set of parallel beds that dip
toward the center. The youngest beds are at the center,
along the axis of the fold.
Geologic Map of Bloomsburg, PA
The geological map of Bloomsburg, PA consists of plunging anticlines and synclines and faults,
all of which are illustrated within the map. Notice on the map, the distinct patterns of the
geologic units that are evident of plunging synclines and anticlines (repeating beds, U- and Vshaped “bending” of the units).
Strike and dip
Strike is the orientation of a horizontal line formed by the intersection of a geologic surface
with a horizontal surface (See Geologic Structures lab). A geologic surface can be the top or the
bottom of a rock unit, a thin bed, a dike, or a sill, or the contact between two units, or a fault.
Geologists measure the strike and dip of rocks in the field. However, it is possible to do
determine the strike and dip of a unit on a geological map as well. To determine the strike,
draw a line connecting two points on the same geologic surface that are at the same elevation,
i.e., find two points on the same geologic surface that cross the same contour line, and connect
these two points. The line you have drawn in the strike of that geologic feature. The direction
of that line is the strike of the bed, fault, or other geological surface.
Questions #7 – #10 will be answered using the Geologic Map of Bloomsburg, PA.
Geologic Map Link:
7) There are two major folds present in Bloomsburg, PA: one located in the top half of this
geologic map and the second one located in the bottom half. Is the fold located on the
top of the map a plunging syncline or anticline? Besides the key present on the bottom
of the geologic map, how did you come to this conclusion (think about the Principle of
Superposition)? (2 pts)
Geol 103
Lab 19 – Geologic Maps
8) Move around the fold located in the top half of the geologic map, what do you notice
about the strike and dip symbols located on this fold? (2 pts)
9) Now look at the fold located in the bottom half of the geologic map. Is this fold a
plunging anticline or syncline? Besides the key present on the bottom of the geologic
map, how did you come to this conclusion (think about the Principle of Superposition)?
(2 pts)
10) Move around the fold located in the top half of the geologic map, what do you notice
about the strike and dip symbols located on this fold? (2 pts)
MAP PATTERNS OF DIPPING ROCKS
As we saw before, map patterns of flat lying beds follow contours. Dipping beds, or dipping
tabular bodies such as dikes, make a different pattern with respect to topography.
Geological Map of the Princeton Area
Rocks that dip at an angle have variable outcrop patterns, and at times, can be quite
complex. Rocks that dip 90 degrees (i.e., rocks that are oriented vertically) will crosscut all
contour lines and its outcrop pattern is independent of topography.
Four Jurassic sedimentary formations and two Tertiary dikes are exposed in the areas near
Princeton, ‘West Dakota’ (Fig. 1). The topography of this area is rolling hills. Notice the outcrop
pattern.
Strike and dip
Strike is the orientation of a horizontal line formed by the intersection of a geologic surface
with a horizontal surface (See Geologic Structures lab). A geologic surface can be the top or the
bottom of a rock unit, a thin bed, a dike, or a sill, or the contact between two units, or a fault.
Geologists measure the strike and dip of rocks in the field. However, it is possible to do
determine the strike and dip of a unit on a geological map as well. To determine the strike,
draw a line connecting two points on the same geologic surface that are at the same elevation,
i.e., find two points on the same geologic surface that cross the same contour line, and connect
these two points. The line you have drawn in the strike of that geologic feature. The direction
of that line is the strike of the bed, fault, or other geological surface.
Geol 103
Lab 19 – Geologic Maps
Questions #11 -#17 will be answered using the “GeoMap Figures” document on Blackboard
11) What is the strike of the Jurassic sedimentary sequence on Figure 1? (2pts)
12) What is the strike of the older Tertiary dike on Figure 1? (2pts)
13) What is the strike of the younger Tertiary dike on Figure 1? (2 pts)
Dip is the maximum angle between a geological surface and the horizontal surface. Dips
commonly are between 0˚ and 90˚. Normally you can tell the direction of dip on a geologic map
and can estimate the angle of dip:
A) Unless a section is overturned, a sequence of sedimentary beds dips toward younger
beds, and the rule of superposition tells us the overlying beds are younger. This
generalization may not apply to dikes. Dikes are formed when igneous material is
injected through sedimentary layers, after the layers were deposited. Dikes are younger
than both the units through which they cut.
B) The outcrop pattern of a layer, as shown on a map, is determined by the shape of that
layer and by the topography of the map area. By examining the relationship between
the layer and topography, one can determine how steeply, and in what direction, the
layer dips.
14) What is the direction of dip of the Jurassic sedimentary beds in Figure 1? (Remember,
the dip must be perpendicular to the strike.) Explain your reasoning for the direction
you choose. (2 pts)
15) What is the direction of dip of the older of the two Tertiary dikes? (2 pts)
Geol 103
Lab 19 – Geologic Maps
It is possible to estimate the degree of dip from an outcrop pattern. We learned from the
Douglas County map that contacts between gently dipping beds of rock approximately follow
the contour lines. The contacts between beds of the Jurassic sedimentary formations on Figure
1 cross contour lines, but at a low angle, indicating they dip at angles of a few to several
degrees.
16) Describe the pattern made by the outcrop of the older dike compared in relation to the
contour lines. Is the position of the outcrop influenced by topography? (see Figure 2) (3
pts)
17) Does the older dike cross contour lines at a high or low angle (e.g. high is near
perpendicular, low is near parallel)? (2 pts)
Summary of points on the Princeton Area geologic map.
A) Strike can be determined on geological maps by constructing lines that connect points
where geological surfaces (for example, contacts between units) intersect a horizontal
surface at the same elevation.
B) Sedimentary beds dip in the direction of younger beds (unless the whole succession is
overturned).
C) The outcrops of planar features make a pattern of V’s where they cross valleys. If the
slope of the valley is opposite of the dip of the beds, the apices of the V’s point down
dip. If the slope of the valley is in the same direction as the dip of the beds, but at a
gentler slope, the apices of the V’s also points down the dip.
D) Beds that dip at angles of a few degrees or less approximately or exactly follow contour
lines. Beds that dip at intermediate angles have outcrop patterns that are less and less
influenced by topography as the angle of their dip increases.
E) By observing these principles, it is possible to determine the strike of beds or other
planar features and estimate the amount and direction of their dip reliably.
Geol 103
Lab 19 – Geologic Maps
INTERPRETING GEOLOGIC MAPS
South-Central Texas Exercise
The geologic map of South-Central Texas shows several sedimentary units separated by
unconformities. Discordance of dip above and below the unconformity show that these are
angular unconformities, but the all the rocks in the area dip at low angles and the angular
difference is not great between units. However, on a regional scale, the discordance is obvious.
Questions #18 – #22 will be answered using the South-Central Texas Geologic Map. PDF on
Blackboard.
18) What is the dip direction of Cretaceous rocks in the west-central part of the area (Sutton
and Edwards counties)? How did you determine the dip direction? Is this dip direction
the same or different than the dip direction of Lower Cretaceous Rocks in Bosque or
Hamilton counties (upper center of map)? (3 pts)
Texas is the site of many oil fields. Both the Midland basin, northwest of the map area, and the
Gulf of Mexico basin, which begins at the zone of faults that run in a curving band NE-SW across
the center of the map and extends off shore to the east and southeast of the map, are major
oil-producing areas. Suppose you are an oil geologist.
19) If you were to drill a deep well at the NW edge of the map at the ‘h’ in Fisher County,
what would be the first five units you would encounter as you went down? List them in
the order you would encounter them. (5 pts – 5 units!)
20) If you were to drill a deep well south of the east center of the map, just east of
LaGrange, at one of the ‘t’s in Fayette County, would you encounter Quaternary rocks at
depth? (3 pts)
For Questions 21 and 22, find the Llano area, the outcrops of Proterozoic (Precambrian Y),
Cambrian, and Ordovician rocks just west of the center of the map.
21) Is the Llano area a structural dome or structural basin? How do you know? (3 pts)
Geol 103
Lab 19 – Geologic Maps
22) Several faults cross the Llano area. What are the youngest beds they cut? What are the
oldest beds that are not cut by them? What can you say about the age of these faults?
(3 pts)
a. Generally speaking, what is the orientation of the faults (what two compass
directions do many of the faults run)? (1 pt)
b. Though not marked, these are reverse faults. What kind of stress creates reverse
faults? (1 pt)
c. Referring to the type of stress you answered above, what sort of geographic
features (landforms) would you expect to be created? (1 pt)
d. Thinking back to geologic principles of relative dating (not using numbers),
explain how you would assign the oldest age possible to a fault that cut through
multiple geologic units. (3 pts)

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