Download 2015 Coaches Institute Handout - North Carolina Science Olympiad

Science Olympiad Coaches Clinic
October 10, 2015 Southeast Raleigh High
School
Geologic Mapping Event
Edward (Skip) Stoddard ([email protected])
KEY WEBSITES
USGS Topographic maps: http://store.usgs.gov
National Geologic Map Database: ncgmdb.usgs.gov
USGS/NPS Geologic Map site: http://www.nature.nps.gov/geology/usgsnps/gmap/gmap1.html
NC Geological Survey GeoPDF Maps: http://portal.ncdenr.org/web/lr/geopdfs-geologic-maps
NC Geological Survey paper maps: http://nc-maps.stores.yahoo.net/
Sites shown at end of the Quartini presentation (2014 Clinic link on sonic.org site)
THE GEOLOGICAL TIME SCALE
PERIOD
APPROX. AGE
RANGE
(M.Y.B.P.)1
LIFE FORMS
Quaternary (Q)
2 - present
humans
Tertiary (T)
Cretaceous (K)
65 - 2
146 - 65
MESOZOIC
ERA
Jurassic (J)
Triassic (TR)
208 - 146
245 - 208
PALEOZOIC
ERA
Permian (P)
Pennsylvanian (P)
Mississippian (M)
Devonian (D)
Silurian (S)
286 - 245
333 - 286
362 - 333
418 - 362
443 - 418
Ordovician (O)
Cambrian (C)
490 - 443
544 - 490
Z-youngest
Y-middle
X-oldest
A
2,500 - 544
mammals and flowering plants
dinosaurs peak and then go
extinct
first birds; large dinosaurs
first dinosaurs; conifers
reptiles and amphibians; many
marine invertebrates go extinct
coal swamps; insects
crinoids
fish and marine invertebrates
coral reefs; fish; simple land
plants
graptolites; molluscs
marine invertebrates (e.g.
trilobites)
bacteria; algae; fungi, worms
toward end
ERA/EON
CENOZOIC
ERA
PROTEROZOIC2
EON
ARCHEAN2
EON
4,600 - 2,500
1 M.Y.B.P.: million years before present
2 The Proterozoic and the Archean together are referred to as Precambrian.
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algae and bacteria
INTERPRETATION OF GEOLOGICAL EVENTS
EXERCISE A.
Examine the geologic cross-section below. Use the rules described above to arrange the
rock bodies and other lettered features in the proper sequence, from oldest to youngest, by placing
the correct letters in the blanks provided below. (Note: letters are A - L.) Could you write a
geological history of this area?
OLDEST
YOUNGEST
____ ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ ____
1. In your sequence of letters, indicate with an arrow labeled "U" those periods when an
unconformity was formed. This means that instead of deposition, the land was uplifted and
eroded.
2. Indicate with an arrow labeled "T" periods when tilting (or folding) of the rocks occurred.
3. Why is contact metamorphism associated with granite A, but not granite E?
4. If fragments (xenoliths) of other rocks are found within granite A, what units might they come from?
5. What unit might contain pebbles or cobbles eroded from granite A?
6. Write a paragraph describing the geological history of the area, beginning with the formation of
the oldest unit, and continuing to the present state.
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EXERCISE B. The diagram above is a geological cross-section of a hypothetical area. The
surface of the ground, with a tree, is at the top of the diagram. Y represents an igneous rock unit,
the other letters represent various sedimentary rock units. Answer the following questions
pertaining to the figure:
1. Which of the following units could possibly contain pebbles eroded from unit Y?
A. T
B. U
C. X
D. V
E. Z
2. Assume that Middle Paleozoic fish fossils are found in Unit X, and Late Paleozoic amphibian
fossils are found in Unit T. The event responsible for the folding of Units V, X, and Z occurred
during the:
A. Precambrian
B. Early Paleozoic
C. Early Mesozoic
D. Middle to Late Paleozoic
E. Late Mesozoic
3. What is the correct sequence representing the relative time of formation of the various units (T,
U, V, W, X, Y, and Z)? List them in sequence, with the oldest first, youngest last (Note: the oldest
is the first one that formed; the youngest is the last one that formed).
_____
_____
_____
_____
_____
3
_____
_____
EXERCISE 8: GEOLOGIC STRUCTURES
Synopsis:
• The orientation of bedding and foliation planes is indicated by strike and dip.
• Measurements of strikes and dips show the presence of faults and folds.
• Faults result from breaking and movement of rocks; folds from bending of rocks.
• Faults are normal, reverse, thrust, or strike-slip; folds are synclines and anticlines.
Importance:
• Geologic structures, especially anticlines, are common sites for oil deposits.
• Earthquakes occur on active faults; they indicate the type of motion.
PART I. STRIKE AND DIP
The orientation, or attitude, of a
non-horizontal rock layer is indicated by
two measurements, called strike and
dip.
Strike refers to the compass
direction of a horizontal line lying within
the plane of the rock layer. Dip is the
downward angle of the rock layer,
measured from a horizontal plane. The
direction of dip must be at right angles to
the strike, but it can be either to one side
or the other; for example, a layer that
strikes due north must dip either to the
east or the west (right angles to north-
south). The angle of dip lies between 0
and 90 degrees.
Geologists routinely measure
strike and dip on rock outcrops in the
field; it is the basis for the construction
of geologic maps, and ultimately for the
interpretation of the geological history of
an area.
Strike and dip may be
abbreviated with compass directions
and angles, and specific symbols are
used to plot strike and dip directly on
maps.
See the figure below.
In the figure, the dipping rock layer
(shown with stippling) strikes 45° east of
due north, as measured with a compass.
If viewed parallel to the N45°E strike
direction, the bed dips toward the right,
or southeast (actually toward S45°E to
be precise, but this is unnecessary to
specify, since dip must be at right
angles to strike, so for this particular
strike, dip must be either southeast (SE)
or northwest (NW)). The map symbols
of strike and dip are plotted directly on
the location of the outcrop from which
the attitude was measured; when they
are all plotted up, they reveal the types
of rock structures present in an area. In
plotting strike and dip symbols, use a
long line segment to indicate the
direction of strike. The orientation of the
strike line segment should be measured
carefully with a protractor (be sure you
know which way north is on the map!);
therefore, it is normally unnecessary to
write the strike angle on the map. For
the dip, draw a much shorter line
segment to indicate which side of the
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strike the dip direction is in, and write
the (downward) angle of dip next to this
short line segment. For vertical and
horizontal beds, there are special
symbols, and there is no need to write
any angles; however, note that a vertical
bed does have a strike, which must be
measured, whereas a horizontal bed
has no specific strike direction.
1. For each of the following strike and dip examples, write them in abbreviated
form (e.g. N50°E, 65°SE) and plot their symbols, oriented correctly, in the space to the
right. Assume north is toward the top of the page. If you are unsure how to use your
protractor, ask for help!
Abbrev form Map symbol
A.
strike: north thirty-two degrees west
dip: twenty-four degrees northeast
B.
strike: north seventy-six degrees east
dip: fifty degrees northwest
C.
strike: north ten degrees east
dip: ninety degrees (vertical)
2. For each of the following map symbols, determine and fill in the abbreviated form of
the strike and dip. Again, you will need to use your protractor.
surface. A block diagram combines a
geologic map with two perpendicular
cross-sections, to give a threedimensional view of a "block" of the
Earth. In this part of the lab, you will
work with such views of simple geologic
structures. It is expected that you will be
familiar with the terminology and
classification of the various types of
faults and folds.
The following
paragraphs are for quick reference, but
will not substitute for a careful reading of
the relevant pages in your textbook.
Diagrams on the next page illustrate
these structures.
PART II. FAULTS AND FOLDS
These structures are the most
common in regions of deformed rocks,
such as mountain belts. They represent
the way that the Earth has responded,
and continues to respond, to its internal
stresses. The two most important tools
for depicting and understanding such
structures are geologic maps, which
show the distribution of rock types and
the orientation of rock layers (using
strike and dip symbols), and geologic
cross-sections, which show a view of
the geologic units below the Earth's
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fencelines, etc. are offset to the right
from the perspective of someone
standing on one side of the fault and
looking across to the other side, then it
is called a right-lateral strike-slip fault.
Conversely, if things are moved to the
left, it is a left-lateral strike-slip fault.
Oblique-slip faults combine some dipslip and some strike-slip movement; you
can think of these as having diagonal
movement, with a horizontal and a
vertical component. For example, the
earthquake of 1989 in California
involved both right-lateral and reverse
movement on the San Andreas fault.
Faults can indicate the types of
stresses that existed at the time of the
movement. Normal faults indicate that
tension, or pulling apart forces, existed.
The stresses were oriented at right
angles to the fault plane, and the pulling
apart tends to lengthen the Earth's crust
in the region of the faulting. Reverse
and thrust faults indicate compression,
or squeezing together, forces existed, at
right angles to the fault plane. These
types of faults shorten the Earth's crust.
Faults are planar breaks in the
Earth's crust along which movement has
occurred. Faults are usually recognized
by the offset of rock units and the
contacts between them.
Faults are classified on the basis of the
dip direction of the fault plane and the
type of movement that has occurred.
Because a fault is a plane, it has a strike
and dip, just like a rock layer. As long
as the fault is not vertical, the rocks on
one side of the fault (above the fault
plane) make up the hanging wall, and
the other side (beneath the fault plane)
is referred to as the footwall.
Understand this, and you can learn the
classification of faults. First, faults are
divided into dip-slip faults and strike-slip
faults. Dip-slip faults are those in
which the movement has been in the
direction of the dip of the fault plane
(either down or up); in strike-slip faults,
the movement has been parallel to the
strike of the fault plane (sideways). Dipslip faults are further divided into two
basic types: normal faults and reverse
faults. A normal fault is one in which
the hanging wall side has moved down,
and a reverse fault is one in which the
hanging wall has moved up. A thrust
fault is just a reverse fault that dips less
than about 15 degrees.
Strike-slip
faults are subdivided into two types,
depending on the sense of sideways
movement. This is best visualized from
above, as in a geologic map or an aerial
photograph.
If rock units, streams,
Folds are bent rock layers; they
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are first divided into anticlines and
synclines. An anticline is an upfold, or
arched-up layers of rock; a syncline is a
downfold, a trough-like fold. Understand
that, after erosion has leveled off the top
of a folded region, the types of folds are
recognized not by arches and troughs,
but by two things: (1) changes in the
direction of dip of the rock layers (beds
dip away from the middle in anticlines,
toward the middle in synclines); and (2)
the fact that anticlines always occur with
older rocks in the center, flanked
symmetrically by younger rock layers,
whereas synclines have the youngest
layers in the middle, flanked by
progressively older layers. Folds are
caused by compression in the Earth,
and both types of folds produce crustal
shortening.
Folds have an axial plane, which
divides the fold into two symmetrical, or
nearly symmetrical halves, called limbs.
The axial plane can be thought of as
containing the line of maximum
curvature of each rock layer; the
direction of these lines of maximum
curvature is called the fold axis. Folds
whose axes are horizontal are called
non-plunging folds; on a geologic map
these folds are easy to recognize as a
series of stripes (representing rock
units) that are bilaterally symmetrical. In
plunging folds, the fold axes plunge
down into the ground; these folds make
horseshoe-shaped patterns on geologic
maps.
The horseshoes of plunging
synclines open in the direction of the
plunge; those of plunging anticlines
open away from the plunge direction.
As if this isn't enough, there are also
doubly-plunging
anticlines
and
synclines, and domes and basins!
These are easily recognized on geologic
maps by a series of concentric ovals
(doubly-plunging folds) or circles in a
bull's-eye pattern (domes and basins).
These
structures
sometimes
are
produced by two separate periods of
folding with compression in different
directions, but more often, domes (and
doubly-plunging anticlines) are produced
by the pushing up of a block from below,
and basins
(and doubly-plunging
synclines) are produced by a localized
sinking.
Map Symbols for folds and faults
On geologic maps and crosssections, faults are shown with a letter
or arrow on each side of the fault to
indicate the direction of movement. On
the map view only, several tick marks
similar to dip symbols can be used to
indicate the direction of dip of the fault
plane. This, combined with a "D" for
down and a "U" for up, is mandatory in
order to distinguish reverse faults from
normal faults on geologic maps.
On geologic maps, folds are
indicated with a line that traces the axis.
For plunging folds, this line has an arrow
at the end toward the direction of
downward plunge (both ends for doublyplunging folds).
In addition, fold
symbols are decorated with short paired
arrows that point inward for synclines, or
outward for anticlines.
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8
U
D
Normal
fault
D
U
Reverse
fault
(East-) plunging
anticline
Strike-slip
fault
Syncline
Anticline
Doubly plunging syncline
EXAMPLES OF MAP SYMBOLS FOR FAULTS AND FOLDS
QUESTIONS
1. For three block diagrams below (B-D), you are asked to (1) Identify the type of
structure, and (2) Sketch a geologic map of the area in the empty box. Note that in the
perspective of the block diagrams, north is toward the upper right, as indicated by the
arrows; in the geologic maps, however, north should be directly toward the top of the
page. For the strike and dip symbols, there is no need to measure or include the dip
angle, just place the strike and dip directions in their correct orientations. Include on
your map views the correct fault and fold symbols. A is completed as an example.
Block Diagram
Map Sketch with Symbols
Name of Structure
A.
Block Diagram
Map Sketch with Symbols
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Name of Structure
N
B.
N
C.
N
D.
2.
What types and orientations of stresses are suggested by each of these
structural diagrams from above?
Diagram A.
Diagram B.
3.
The following block diagram is incomplete. Assuming that the youngest bed is in
the middle of the bull's eye pattern, complete the diagram by sketching in the two
missing cross section panels. What is this structure called?
_____________________________________________________________
10
N
What might cause the formation of such a structure? _________________
______________________________________________________
4. This block diagram shows two different structures combined. Describe the two
structures, and indicate the order in which they occurred.
N
First structure
to form:
Later structure to form :
2.
A. Examine the Geologic Map of Pennsylvania. What sort of geologic structures
dominate the central portion of the state, in a band running from south-central to
northeastern Pennsylvania? _____________________________
What type and orientation of stresses must have caused these structures to form?
________________________________________________________
B. Examine the Geologic Map of North Carolina, and the inset "Map Showing
Major Litho-Tectonic Features." Find two different structures that suggest stresses
similar to those in Pennsylvania. ___________________________ and
_______________________________.
EXERCISE 9: GEOLOGIC MAPS AND CROSS SECTIONS
Synopsis:
• Geologic maps show the distribution of rock types and geologic structures in an area.
• The patterns that rock units and their contacts make indicate the structure and history.
• Cross sections show the geology below the surface; they are made from geologic maps.
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Importance:
• Geologic maps are widely used for locating sites for landfills and waste disposal, for
planning construction projects, and for evaluating mineral and groundwater resources.
A geologic map shows the distribution of rock
units in an area. Even where outcrops of each
unit may not be visible at the surface at every
point, the map shows what unit exists beneath
the thin surface cover of vegetation, soil, and
loose sediment. The geologic map is made by
locating and identifying exposed rock outcrops,
determining and plotting their strike and dip,
and plotting contacts between rock units directly
on a base map (usually a topographic map).
The locations of contacts in areas of sparse
outcrop exposure must be inferred and
interpolated between areas of exposure. A
geologic
map
always
involves
some
interpretation, so the ability to describe rocks in
detail and a good knowledge of structural
geology are indispensable to the field geologist.
As we discovered last week, the
patterns made by units on a geologic map
indicate the types of structures present in the
area.
Non-plunging folds are shown by
symmetrical parallel stripes, plunging folds by
horseshoe-shaped
symmetrical
patterns.
Doubly-plunging folds, domes, and basins have
circular or elliptical bull's-eye patterns. Faults,
intrusions, and unconformities all cut across
older units, but faults offset the contacts,
unconformities are parallel to contacts of
younger rock units, and intrusions are igneous
rock bodies that commonly cause contact
metamorphism.
In this lab, we will examine two geologic
maps and interpret the geologic histories of the
regions they represent. In addition, we will
construct geologic cross-sections of the two
map areas. First it is necessary to review some
characteristics of geologic map patterns.
GEOLOGIC CONTACTS
On a geologic map, a line separates
different units of rock. Such a line is called a
contact. Understanding the significance of
contacts is very important in interpreting
geologic maps.
There are basically four
different
types
of
geologic
contacts:
conformable, unconformable, intrusive, and
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fault.
Conformable contacts are boundaries
between rock units that were deposited in layers
one on top of another in a continuous sequence.
These are typically sedimentary rocks, but volcanic
units may also be part of a conformable sequence.
Unconformable contacts are the result of
unconformities, or periods of erosion, as described
in Lab 4. Unconformities are especially important
when they truncate the older units (angular
unconformity), or when the older rocks are igneous
or metamorphic, and the younger are sedimentary
(nonconformity).
Why are these especially
important types of unconformities?
Intrusive
contacts are present between older rocks and
younger igneous rocks that intruded them. Fault
contacts show where faults have moved rock
bodies. Fault contacts have the effect of displacing
or offsetting older geologic contacts on either side.
By studying the offsets, it is often possible to
determine the type of fault.
OUTCROP PATTERNS OF LAYERED ROCKS ON
A TOPOGRAPHIC BASE MAP
Up until now, the geologic maps we have
looked at have been simplified, and they have
ignored the effects of topography. For example,
unlike the Earth, the block diagrams and maps from
last week's lab all had perfectly flat surfaces.
However, when units cross hills and valleys, the
resulting geologic map is usually not as simple as
in the idealized flat case. Nevertheless, the outcrop
patterns of layered rocks on a topographic base
map provide important clues to the structure of the
area. Let's look at two of these effects: the width
of a unit, and the "rule of V's". Note that the
contacts between units in this discussion are
conformable contacts.
Figure showing map patterns, on a perfectly flat surface, of horizontal, vertical, and dipping sedimentary
beds. Note that the width of a unit on the map is greater than or equal to (case B) the true thickness.
(In (C), the strike of the dipping beds is north-south.
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What controls the width of a rock unit on a geologic map? Obviously, the thickness of the unit is
important, but it is seldom the same as the unit's width on a map. See the figure above. Consider a
horizontal layer (case A above) underlying our perfectly flat region: on a geologic map this unit would
cover the entire region. Only if the unit were vertical (had a dip of 90°; case B above) would the
outcrop width on the map be the same as the true thickness of the unit. Again assuming a horizontal
surface, for dipping beds (case C), the width of the unit on the map always exceeds the true thickness
of the unit. (The simple trigonometric relationship between the width of a unit on the map (W), the true
stratigraphic thickness of the unit (T, which must be measured perpendicular to the unit's contacts as
seen in cross-section), and the angle of dip (d) is: sin d = T/W. If you know two of the three quantities
(W, T, and d), you can easily calculate the third.)
But what if we allow for the effects of topography? In this case, things are not so simple. In most
cases, the map width is still greater than the unit's thickness. The major exception is when the slope of
the land surface is steep. In the extreme case, a unit exposed on a vertical cliff face would be represented
by a pencil line with no thickness on a map. The diagram below illustrates the case for horizontal and
vertical beds.
Figure showing the effect of topography on outcrop pattern for horizontal beds (A) and vertical beds (B). Note in A that the
width of a unit on the map is greater than the true thickness of the unit, except when the slope is very steep.
The Rule of V's describes what happens when a geologic unit crosses a stream valley. Remember
that topographic contour lines make "V's" that point upstream, when they cross a valley. Let's consider
three cases to see what happens when geologic contacts cross stream valleys.
Case 1: Horizontal rock units. The contacts of horizontal beds are horizontal surfaces, just like specific
elevations (i.e. contours) are. So, like contour lines, contacts between units that are horizontal are
everywhere at the same elevation. They do not cross contour lines, but remain parallel to them, and
like contour lines, they make V's that point upstream.
Case 2: Vertical rock units. The contacts of vertically dipping units cross contour lines, but do not
make V's when they cross stream valleys.
Case 3: Dipping rock units. The contacts of dipping (but not vertical) rock units cross contour lines,
and make V's where they cross stream valleys. The V's of dipping units almost always point in the
direction of the dip of the unit. This is very useful in determining the structure of an area!
MAP I. Use a geologic map of the East Mesa Quadrangle, New Mexico to answer these questions.
Download this map at: http://ngmdb.usgs.gov/Prodesc/proddesc_1071.htm
15
The map shows the distribution of rock units superimposed on a topographic map. The
topographic contours are shown in brown, and each rock unit is represented by a different color.
The Explanation along the right-hand side of the map lists the rock units in order of their age, with
the oldest at the bottom and the youngest at the top. The color representing each unit is shown.
Each unit has an abbreviated symbol, such as Jw (Wingate sandstone). The first letter
stands for the age of the unit (in this example, Jurassic), and the second for the formation name (in
this example, Wingate). Brief descriptions of the rock types are often included with each symbol.
The positions of unconformities within the stratigraphic sequence are shown as well in the
Explanation.
At the bottom of the Explanation are other symbols used in the map, such as contacts,
faults, folds, and locations of mines in the area. Refer to the table below, and use the Geologic
Time Scale in order to answer the questions concerning the geologic map.
1. What is the attitude of the Mesozoic sedimentary formations depicted on this map (e.g.
horizontal, vertical, dipping 45° W, etc.)?
_____________
How do you know?
_________________________________________________________________
2. Where the contacts between these units run across a valley, they form a "V." Describe the location
of a place where this happens: _________________________
Which way do these V's point? ___________ Is this upstream or downstream? ____________
3. In sequences of layered rocks, those units that are more resistant to weathering and erosion tend
to form very steep to nearly vertical slopes, and those that are more easily weathered and eroded form
gentle or flat slopes. The resistant units are referred to as "cliff-formers" and the softer units are
"bench-formers." Name a unit is this quadrangle that is a good cliff-former: __________ What rock
type is it mostly composed of?
______________________________________________________________
Now name a bench-former and its rock type: _______________________________
MAP III. A. Wetterhorn Peak Quadrangle, Colorado
Download this map at: http://ngmdb.usgs.gov/Prodesc/proddesc_10559.htm
1. Find examples of the unit labeled Qs. What type of material is this, and what types of places does it
occur in?
2. Find examples of the unit labeled Tql. What is the general shape of these rock bodies in the westcentral portion of the map? ____________________________
How about in the northeastern portion of the map? ________________________
Explain the origin of these different forms. ______________________________
3. What is the oldest rock unit shown? ____________________________
4. What do units Qr and Qd tell you about the area's geologic history?
5. Look at the red dots on Bighorn Ridge. What do they mean?
_____________________________________________________ Why might this area be of interest to
a mining company? ________________________________
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6.
What type of igneous rock is unit Tan in the southwest corner of the
_____________________ What type of rock bodies does it form? ____________________
map?
B. Woronoco Quadrangle, Massachusetts.
Download this map at: http://ngmdb.usgs.gov/Prodesc/proddesc_1100.htm
1. What type of contact is there between JTRs and Dg? ________________________________
What is the strike direction of this feature? ____________________________
What can you say about the age of this feature, and why? ____________________
2. Look at the lines representing fold axes in the northeastern portion of the map. How do you get two
fold axes that cross each other, and which one is older? How do you know?
3. What is the oldest unit in the area, and how do you know?
4. What type of fault exists in the eastern half of the map? (Hint: look at the cross-section on the back
if you are having problems with this one.)
5. What is the general rock type in the eastern part of the map (unit JTRs)?
6. What is the general orientation of these beds, and how can you tell from the map?
7. What is the orientation of the rocks west of JTRs, and how can you tell from the map?
8. What type of structure or feature does the oval-shaped purple unit (in the middle of the
quadrangle) form? _______________________ Do you expect the light blue Dg unit to the
west to be younger or older? ____________ Why? _________________
9. Referring to the cross-sections, what type of faults separate Dg from Ocd, Ocbr, and Ocb, in
the western part of the map? _______________________________
EXERCISE 10: GEOLOGY AND GEOLOGIC RESOURCES OF NORTH CAROLINA
Synopsis:
• North Carolina's geological history goes back two billion years and continues today.
• Piedmont and mountains are older igneous and metamorphic rocks.
• Coastal Plain is younger sedimentary rocks.
• These rocks are divided into "belts" that run NNE, and record the construction of the
Appalachian Mountains, which is understood using plate tectonics.
• North Carolina has a great variety of geological resources, which correlate with its geology.
Importance:
• Appreciation of the geology of an area makes travel and outdoor activities more interesting.
• Geological resources play an important role in our state's history and economy, even today.
• Our society is dependent on geological resources: "If it can't be grown, it has to be mined.
PART I. GEOLOGY
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For this exercise, refer to the "Generalized Geologic Map of North Carolina.” You can
download this map at the following website:
http://portal.ncdenr.org/web/lr/earth-science-outreach
North Carolina has an unusually great variety of rock types and an interesting and
complex geologic history. The state is divided by geologists into a number of "belts", each of
which is composed of rocks of different types and ages. Because of this geologic diversity,
North Carolina is also blessed with a great variety of mineral deposits. North Carolina was the
leading gold-producing state prior to the California Gold Rush of 1849, and mining continues to
be an important part of the state's economy.
Despite the fact that much of it is not mountainous today, geologically speaking, the
Appalachian Mountain belt extends all the way from central Tennessee to the North Carolina
Coastal Plain.
In the western part of North Carolina, the Blue Ridge and Inner Piedmont belts
consist of very old rocks that have been deformed and metamorphosed. These mainly
Precambrian rocks were pushed more than 100 miles westward over younger, flat-lying and
undeformed Paleozoic sedimentary rocks at the end of the Paleozoic Era, as part of the platetectonic interactions that produced the Appalachian Mountains. Though the sedimentary rocks
that the Blue Ridge was thrust upon are now covered, they are related to rocks that are
exposed farther to the west, in the Valley and Ridge and Appalachian Plateau of central and
eastern Tennessee. The boundary between the Blue Ridge and Valley and Ridge belts is
defined by this major fault, the Blue Ridge thrust, which is exposed at the earth's surface in
eastern Tennessee.
Running between Raleigh and Greensboro is the Carolina Slate belt. This belt is
made up of mildly metamorphosed volcanic and sedimentary rocks that were formed in an
ancient volcanic island arc around 600 million years ago. With respect to North America, this
chain of island volcanoes was at the other side of an ocean that no longer exists. The closing
of that ocean by subduction caused the volcanoes to slam into and become welded to North
America.
Toward the end of the Paleozoic era, the squeezing forces that thrust the Blue Ridge
westward evolved gradually to sideways forces that moved eastern belts toward the south. In
this way, thrust faults and reverse faults actually changed into right-lateral strike slip faults (like
today's San Andreas fault in California). One such strike-slip fault, the Nutbush Creek fault,
runs directly under the NCSU campus!
The Triassic basins were formed when the Atlantic Ocean began to form about 220
million years ago. At that time, the former supercontinent Pangea began to split apart. A
number of normal faults were formed as the crust lengthened in the region of the rift, in a
manner identical to the East African Rift Valley of today. One of these faults became the site of
the future Atlantic Ocean, and the other faults stopped moving when the ocean began to widen.
One of these would-be oceans basins, the Durham Triassic basin, is bounded on its east side
by the Jonesboro fault, which runs along the edge of RDU Airport and directly under the Angus
Barn restaurant on Route 70. This normal fault caused the basin to form, and rivers and
streams drained from the adjacent mountains (located where Raleigh is now) into the newly
created valley. The sediment these streams deposited became red sandstone, conglomerate
and shale. Plant and animal (early dinosaur) fossils are found in some of these rocks. When
basaltic magma began to rise from beneath the new ocean, some of it intruded cracks in the
continental crust as basaltic (diabase) dikes. Many of these basalt dikes can be found
throughout the North Carolina Piedmont, cutting across the older rocks.
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The Coastal Plain consists of late Mesozoic and younger sedimentary rocks that have
not been involved in any mountain-building activity, so they are flat-lying and undisturbed.
These soft rocks are derived from sediments that were mainly deposited along the shoreline of
the Atlantic Ocean. Fossils of marine animals, including ancient whales and sharks, are
plentiful in the Coastal Plain.
Study the accompanying maps and tables, the "Generalized Geologic Map of North
Carolina", and answer the questions indicated by your instructor.
1. How are the rocks in the Appalachian Plateau of east-central Tennessee similar to those in
the Valley and Ridge to the east? How are they different?
2. What major changes in the rocks take place in moving eastward from the Valley and Ridge
to the Blue Ridge province?
3. What are the major similarities between the rocks of the Blue Ridge and the rocks of nearly
all the Piedmont belts to the east?
4. Suppose the present-day spreading of the Atlantic Ocean stopped, and its crust began to be
subducted. What could eventually happen in North Carolina?
5. Which of the following faults represent compression, which extension, and which strike-slip
motion? Brevard fault ___________; Blue Ridge thrust ____________; Jonesboro fault
_____________; Nutbush Creek fault _____________
6. How do you suppose geologists discovered that the Carolina Slate belt was formed far away
from North America? What kind of evidence would suggest this?
7. At the beginning of the Paleozoic, there was an ocean where much of the Appalachian
orogen (belt of deformed and/or metamorphosed rocks; includes all the rocks in North Carolina
except the Coastal Plain and Triassic basins) now sits. What has happened to this ancient
ocean? What is the evidence?
8. Examine the nearly circular pink body of granite located in Richmond and Anson Counties
on the "Generalized Geologic Map of North Carolina." There are five different rock units that
are in contact with this granite body (called the Lilesville pluton). They are colored green,
brown, yellow-green, blue, and yellow with brown dots. Which of these five rock units should
show contact metamorphism from the intrusion of the Lilesville pluton? Why?
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GEOLOGIC PROVINCES OF THE SOUTHERN APPALACHIANS
FROM CENTRAL TENNESSEE EASTWARD THROUGH NORTH CAROLINA
APPALACHIAN PLATEAU
Flat-lying, undeformed sedimentary rocks of Paleozoic age
--separated physiographically by Cumberland escarpment from....
VALLEY AND RIDGE
Same Paleozoic sedimentary rocks, here folded and thrust-faulted
--separated by Blue Ridge thrust from....
BLUE RIDGE
Precambrian and early Paleozoic igneous and metamorphic rocks, complexly deformed
--separated by Brevard fault zone from....
INNER PIEDMONT
Metamorphic rocks of probable Paleozoic age, complexly deformed and with recumbent
folds
--separated by various fault zones from....
KINGS MOUNTAIN BELT, CHARLOTTE BELT, AND MILTON BELT
Mainly metamorphic rocks (Kings Mt. and Milton belts) and igneous rocks (Charlotte
belt) of Paleozoic age
-- separated by Gold Hill - Silver Hill and other fault zones from....
CAROLINA SLATE BELT
Late Precambrian and early Paleozoic volcanic and associated sedimentary rocks,
lightly metamorphosed
--unconformably overlain by rocks of the....
TRIASSIC BASINS
Younger sedimentary rocks deposited in normal-fault basins
--separated by Jonesboro fault from....
RALEIGH BELT
Paleozoic and Precambrian (?) metamorphic and igneous rocks
--separated by thrust faults from....
EASTERN SLATE BELT
Similar, and probably equivalent to Carolina Slate belt
--separated physiographically by the Fall Line, and unconformably overlain by rocks of....
COASTAL PLAIN
Young sedimentary rocks, mostly deposited along the continental margin, covering the
older rocks
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PLATE TECTONIC EVENTS IN NORTH CAROLINA'S GEOLOGICAL HISTORY
1,000 million years ago
Grenville Mountains; apparently no ocean to east
750-700 million years ago
Continent splits, forming Iapetus Ocean, and possibly a microcontinent (like
Madagascar)
650-550 million years ago
Volcanic island arc forms near eastern side of Iapetus
500-440 million years ago
Iapetus begins to close; volcanic arc collides with North America
320-280 million years ago
Iapetus closes completely; continents collide, but not straight on, to form supercontinent
Pangaea; thrust slices, including Blue Ridge, pushed westward onto continental margin
310-290 million years ago
Major strike-slip (lateral) faulting moves blocks southward on their east sides, due to
oblique convergence; deep granite magmas intrude
230-190 million years ago
Supercontinent begins to rift, forming Triassic basins; basalt (diabase) intrudes; Atlantic
Ocean begins to form east of earlier collision zone
150-0 million years ago
Region is eroded, Coastal Plain sediments are deposited on the new continental
margin; present physiography is established
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