LAB # 3 - GEOLOGIC MAPS AND STRUCTURES
Maps Used for Geological Studies
You are probably familiar with topographic maps; these depict the Earth's surface in three
dimensions. They accomplish this by means of contour lines, brown lines drawn on standard
topographic maps that connect points of equal elevation. In this lab, geologic maps will be
especially utilized for interpreting Earth history. These show the surface distribution of rocks,
often with with each geologic unit depicted by a unique color. Geologic cross sections show the
distribution of rocks in vertical section. They use data from outcrops, subsurface lithologic logs
or well logs to define the sedimentologic or structural trends across a given geographic region.
These may be constructed in two-dimensions, or three-dimensional cross-sections (termed Fence
Diagrams) may be drawn. The "layer-cake"-type cross sections may be combined with geologic
maps to form geologic block diagrams. These diagrams also yield three-dimensional figures,
and are thus very useful for interpreting geologic history.
There are also other maps that are useful for interpreting ancient facies and sedimentary
environments. Structure-Contour Maps depict the elevations of the tops of economic deposits,
key beds, formations, or prominent seismic reflectors. They are made by drawing contours that
connect points of equal elevation above or below some datum, usually modern sea level.
Structure-contour maps are used to map sedimentary basins, depositional facies, or subsurface
structural features. They are used quite frequently for mapping economic deposits, such as
hydrocarbon-bearing structural domes or anticlines. Isopach Maps show the thickness of a
stratigraphic unit or units. They are also often times used to show the distribution and thickness
of oil- and natural gas-bearing units, as thicker units potentially contain more economic deposits
versus thinner units of the same type. Isopach maps are also used to interpret the size and shape
of sedimentary basins, calculate ancient subsidence rates, and map centers of sediment
deposition. Lithofacies and Biofacies Maps reveal the composition or texture of rocks, or map
fossil distributions. They are often used to interpret depositional environments, for determining
the source area of sediments (provenance), and are often used to determine the paleoecology of
a region.
Structural Geology
Structural geology is the study of features formed due to rock deformation. In order to
understand the structural history of an area, the attitude of the rock layers must be determined.
The position and orientation of the rock layers is defined by means of their strike and dip. The
strike of a geologic layer is the direction of a horizontal line drawn on the plane of bedding.
This horizontal orientation is determined by means of a geologist's, or brunton compass. The
dip is the direction in which the rock layer is tilted down from the horizontal. Dip direction is
perpendicular to strike. The dip is not only defined as the direction of bed tilt, but also the
amount of tilt. The angle between a horizontal plane and the bedding plane defines the
magnitude of the dip. There are a couple of rules which may assist you in interpreting geologic
dip. Where a geologic contact (the boundary between geologic formations) crosses a stream, it
forms a V, the apex of which points in the direction of dip. The wider the V's, the steeper the
dip. Contacts between vertical beds form no V's whatsoever. Also, in a sequence of formations,
none of which has been overturned, the older beds dip toward the younger.
Folds are bends or flexures in rocks created by ductile deformation that takes place deep
within the Earth's crust. Folds are often associated with mountain-building events, termed
orogenies. In order to interpret geologic folds, we must define the parts of a fold. The opposite
sides of fold are termed the limbs, or flanks. The fold axis is a line that traces the maximum
curvature of a folded layer and separates the two limbs. The axial plane is an imaginary plane
which connects the fold axes of each rock layer and divides the folds symmetrically. Based on
these features, we may define several types of folds. Monoclines are folds which exhibit local
steepening of otherwise uniform dips. Anticlines are folds in which the limbs dip away from
one another and away from the fold axis. On geologic maps, anticlines often form elongate
bullseye-like features in which the oldest eroded bed is on the inside of the structure. Structural
domes are special types of anticlines that are roughly as wide as long. Synclines are folds in
which the limbs dip toward one another and toward the fold axis. They often have the same
general shape as anticlines on geologic maps, but in synclines the youngest eroded bed is on the
inside of the structure. Structural basins are synclines roughly as wide as long. The major
types of folds (anticlines, synclines, structural domes and basins) also vary according to the
orientation of their limbs, fold axes and axial planes. In an asymmetrical fold the strata of one
limb dips more steeply than those of the other. In an overturned fold both limbs dip in the same
direction, although perhaps not in the same amount. In a plunging fold the fold axis is inclined
at some angle to the horizontal, which often yields U- or S-shaped features on geologic maps.
Joints and faults are created where rocks act in a brittle fashion, breaking rather than
folding. Although this often occurs at shallower depths versus folds, faults and folds often occur
together. Rock characteristics also influence whether a unit will fault or fold. Plastic units, such
as shales, are more easily folded than brittle units, such as hard limestone layers. A joint is a
break in a rock mass in which there is no relative movement of the rock on opposite sides of the
break. Joints are commonly created at and near the Earth's surface, especially in brittle rocks
such as limestones. A fault is a surface of rock rupture where there has been relative movement
on opposite sides of the break. There are a number of types of faults, which are defined based on
the form and relative movement of the blocks on either side of the fault. The hanging wall is
one of the blocks involved in fault movement; it is the block that would hang over the head of a
person standing in a tunnel along or across the fault. The footwall is the block involved in fault
movement that would be under the feet of a person standing in a tunnel along or across the fault.
If the displacement of these blocks is in the direction of the fault's dip, the type of fault is termed
a dip-slip fault.
There are three major types of dip-slip faults; normal faults, reverse faults and thrust faults.
In a normal fault, the hanging wall moves down relative to the footwall. That is, the hanging
wall has moved down, the footwall has moved up, or both blocks shifted relative to one another.
In a reverse fault the hanging wall moves up relative to the footwall. A thrust fault is a "low
angle reverse fault". That is, the hanging wall moves up relative to the footwall but the fault dip
is gentler than that seen in a reverse fault, typically at angles much less than 45°. Standing on
the surface, it is sometimes difficult to determine which side of the fault represents the upthrown
block. However, if you "straddle" a dip-slip fault trace, the foot standing on the oldest bed will
also be standing on the "upthrown" fault block. The second major grouping of faults are strikeslip faults, those in which block movement is parallel to strike. In a right lateral strike-slip
fault an observing looking across the fault would note apparent movement of the block on the
opposite side toward the right. In a left lateral strike-slip fault the opposite block has an
apparent offset to the left.
Another feature often observed in geologic sections are unconformities. An unconformity
is a substantial gap in the local rock record, indicating an interval of time during which rocks
either were not deposited or were removed later by erosion. There are several types of
unconformities. An angular unconformity is one in which older strata dip at an angle different
from that of the younger strata. For an angular unconformity to form, the earlier rocks had to be
deposited at the surface, buried, uplifted and eroded before the younger rocks were deposited
above them. In a disconformity beds on opposite sides of the unconformity are parallel to one
another. An unconformity between profoundly different rocks, typically sedimentary rocks
overlying plutonic igneous or metamorphic rocks, is termed a nonconformity.
NAME _________________________________________ Historical Geology Lab # 3
Geological Highway Map of Texas
1. To which "columnar section"/chart do you refer when studying the geologic map for the
Stephenville area?
2. Mineral Wells lies on the boundary between two series. Name the system and both series.
3. Name the system and series in which the city of Glen Rose lies.
4. Are there any unconformities between Cisco and Ft. Worth? Explain.
5. As you travel from Dallas to Houston, are the sequence of formations encountered older or
younger?
6. What kind of fossils should you expect to find within the Mineral Wells area?
7. Are Cretaceous fossils from the Glen Rose Formation younger or older than those from the
Goodland Limestone?
8. Some of the oldest fossils in Texas are algal colonies found in the Allamoore Limestone.
Give the location of these fossils and their eonothem.
9. What two areas in Texas are currently rising in elevation?
10. In the areas noted in question 9, should depositional or erosional processes dominate?
11. What is the strike direction of the formations in the Graham area? What is the dip direction?
Explain how you obtained the strike and dip directions.
12. Where are the nearest Precambrian rocks from Stephenville?
13. Referring to the area discussed in Question 12, is this a structural dome or basin? Why?
14. Referring to the area discussed in Questions 12 and 13, what do the long, dark lines
represent? What is the general compass trend of these structures?
15. Compare the Physiographic and Structural Maps of Texas. What geologic process created
the Balcones Escarpment?
16. Refer to the lowermost cross section at the bottom of the page. What type of unconformity is
present between the Precambrian and Cambrian rocks of the Llano Uplift?
17. Refer to the eastern portion of the middle cross section on the page. Did faulting occur
before the Tertiary beds were deposited in the East Texas Basin? Cite your evidence. Did
faulting occur after Tertiary beds were deposited? Cite your evidence.
18. What are the predominant rock types between Alpine and Marfa, Texas?