Geologic Structures, Maps & Block Diagrams

Read Chapter 10 in Busch and Tasa. Read the introduction and carefully review structural geology and geologic mapping principles and symbols on Pages 259-282 plus the Figure 8.10, the Geologic time scale and
fossil succession chart on p 213. Read the rules for geological map interpretation on p 272 in Fig. 10.9.
Know and understand: original horizontality, continuity, correlations and stratigraphic sequences, superposition, faunal successions, inclusions, and cross cutting relations.
Learn to recognize the three different types of unconformities, various types of folds and 4 major types of faults.
Use stratigraphy as the key to structure. Knowing the succession allows you to tell what was deeper.
Unconformities: always label which type it is: , or !

  1. Disconformity – erosion or non-deposition generally lasting years to millions of years. Parallel beds above
    and below but missing time, fossils and facies between adjacent beds. Most channel cuts and bedding planes are weak disconformities but ignored if they are part of a uniform stratigraphic succession.
  2. Angular unconformity requires deformation and erosion, generally millions of years or more. Underlying beds are eroded at an angle before deposition resumes. As sea level rises, deposition may resume at one location before others thus the lowest stratum of the overlying sequence can vary laterally until sediments fill in.
  3. Nonconformity marks erosion clear down to crystalline basement rocks below the sedimentary basin. Often this is the basal unconformity for an entire sedimentary basin. The rocks below are deformed igneous and metamorphic that are hundreds of millions or billions of years older than the overlying sedimentary succession.
    These underlying rocks represent the roots of eroded mountain belts and they are often quite distant from a modern plate margin.

  4. Faults
    All faults show up as a break where the correlated strata or rock fabric is offset across a planar surface. They are
    the trace of past earthquake rupture and tend to form near or adjacent to plate boundaries where stresses build.
    Some faults moved only once and may have offsets of only a few centimeters. Major plate bounding fault zones may be several tens of kilometers wide with many fault strands which were active for tens of millions of years.
    Depending on the behavior and different strengths in a stack of rocks, and the level of the crust, faults may laterally fade away into the axes of folds, or join up as a master fault.
    Dip Slip Faults: Here the fault motion slid up or down the slope or dip of the fault plane
  5. Normal – This is the normal kind of fault to find in the top of the crust as erosion unloads the crust and it fails in tension. Typically these are high angle faults where the footwall falls, as in the collapse of valley walls, rift valleys, volcanic settings, or the basin and range. The dip on normal faults often decreases with depth in a“listric” fashion so that several normal faults merge into one basal shear plane of sideways slipping.
  6. Reverse – high angle fault (dip > 45°) where the footwall rises, typical of convergent settings. Typical in mountain fold and thrust belts like the Rockies or the Alps.
  7. Thrust – low angle fault (dip < 45° and often <10°) typical of convergent settings, fold and thrust mountain belts and overthrust ranges (Eastern Rockies, Wind River Mountains in Wyoming, Alps). This is also the fault
    type of subduction zones which typically dip at 10-15° where they intersect the seafloor. The symbol for this
    fault on a map view has teeth along the fault on the side of the upper plate. In cross section use arrows only.
    Strike-slip & Oblique Slip Faults: In these faults the motion is horizontal, along the strike of the fault and the
    fault plane is usually vertical or nearly vertical.
  • 2 –
  1. Strike-slip are high angle faults with purely lateral motion or offset, named either right lateral or left lateral
    for their sense of motion. This includes the major oceanic fracture zone transform faults that offset the mid
    ocean ridge system (Mendocino, Blanco, Sovanco, Clipperton) and major plate boundary transform faults like
    the San Andreas, Queen Charlotte-Fairweather, Alpine, Anatolian etc. Smaller scale strike-slip faults are found
    as tears between thrust panels in fold and thrust belts like those in the foothills of the Canadian Rockies. Strike
    slip transform faults take up differential rotation when blocks of crust slide or shear past each other.
  2. Oblique-slip Faults: In these faults the motion is at an angle to both the horizontal and to the steepest line of
    descent in the fault plane. The motion is oblique to the fault plane and is a combination of horizontal, along the
    strike and vertical along dip motions. The fault planes are often curved and this type of faulting occurs along
    curved contacts of mountain belts or where strike slip faults cut mountain ranges or are otherwise affected by
    rocks of different strengths on both sides of the fault. These occur in bends (strike slip) or at the ends of thrusts..
    Strike and Dip and other common map symbols:
    Please refer to the various map symbols and their definitions on p. 264-266 and especially Figure 10.5.
    Strike is the “long direction” on a geological map. This direction the trace of a horizontal line formed from the
    intersection of a plane of rock and the surface of the earth. It is measured in degrees clockwise from North at 0°
    in a circle with 360°. Strike is always parallel to the long direction of bedded formations on a geological map.
    The strike direction of a fault, dyke or vein is along that feature as it crosses the surface of the Earth.
    Dip is perpendicular to the strike line and points in the downhill direction. It is measured in degrees below the
    horizontal ranging from 0° to a maximum of 90°.
    Special cases: horizontal beds are denoted with a plus sign + or an X in a circle (also called quaquaversal
    dip). Vertical beds like intrusive dykes are denoted with a short cross bar perpendicular to the strike symbol
    With bedded rocks it is possible to tell the way up from the succession of fossils or local stratigraphy and also
    from the orientation of cross beds, fossils, pebble imbrications and other sedimentary structures such as mud
    cracks or graded beds. Because of this, it is possible to tell when beds are folded into upside down position. We
    use special symbols for overturned strike and dip, fold axes etc. that pay attention to the overturning direction
    (vergence) as well as angles. These symbols look like bent over croquet wickets or capital U’s or their upside
    down equivalent. The open end of the line always has arrown on it which point down dip.
    Fault traces are shown by a bold line with either U and D for up and down on respective sides for normal and reverse faults on a map or given by a laterally opposed pair of arrows pointing the direction of strike slip motion (to the right or to the left) on top of the map. For faults shown in cross section always use arrows to indicate the
    relative sense of motion across the fault.
    For Thrust faults, a series of triangular teeth is drawn on the map edge of the upper plate (subduction zones)
    When a fault has a compound motion that is neither purely dip slip nor purely strike slip, it is termed an oblique slip fault. Because of the compound motion, both the UD and opposed arrows are shown on the map.
    Fold axis symbols:
    Anticlines have outwards facing arrows, antithetic to the strike along the fold hinge. (outwards dips for Domes).
    Synclines have inwards facing arrows, synthetic to the strike, along the fold hinge. (inwards dips for basins)
    Recumbent folds lay down sideways & both limbs tend to be parallel or asymmetric, 1 overturned limb.
    Monoclines drape across basement faults, resembles half of a syncline/half of an anticline with 2 flats, 1 ramp
    Unconformites: Hiatus and unconformities are denoted by bold wavy lines between stratigraphic packages,
    whether these occur on a cross section or along a map surface. Frequently shows discordant strikes, formations
    pinch out along this type of contact. Label as , or .
  • 3 –
    Activity 10.1: Geological Structures Inquiry

  • Geological structures are broadly divided into those which form under brittle (cold, shallow,
    dry, explosive, fast strain rates) conditions such as jointing, faulting, brecciation and those
    which form under ductile conditions (deep, warm, fluid-rich, recrystallization or plastic-flow)
    such as folding, diapirism, regional metamorphism. In some settings the rocks have experienced
    more than one episode of deformation or experienced changing environmental conditions. For
    example sedimentary salt beds can flow at 1-4 km depth then become uplifted and unloaded to
    break along joints. In the photo below by Dr. Callan Bentley of Northern Virginia Community
    College, there are dipping gravels and muds overlying a tilted angular unconformity, eroded
    down and deposited onto the normally faulted and intensely folded sandstones and mudstones
    of the Harpers Formation near Harper’s Ferry Virginia. It looks like the Appalachians
    Mountains were a happening place, more than once!
    A. Examine the geological photos 1-4 on p.273 in the AGI manual and indicate whether the rocks pictured are:
    a) undeformed (horizontally layered) b) uniformly dipping (planar but tilted) c) faulted (offset layers) d)
    jointed (fractured but not offset) e) folded (curved structures). Circle any answers that apply.
  1. Grand Canyon (Devonian-Permian): a) undeformed b) dipping c) faulted d) jointed e) folded (2)
  2. Cliff-South Central Alaska: (Permo-Trias) a) undeformed b) dipping c) faulted d) jointed e) folded (2)
  3. Quartzite, Maria Mtns, CA: (Proterozoic) a) undeformed b) dipping c) faulted d) jointed e) folded (2)
  4. Sandstone, Little Colorado Gorge: (Jurassic) a) undeformed b) dipping c) faulted d) jointed e) folded (2)
    B.1 Which of the images show ductile deformation? 1) 2) 3) 4) (2)
  5. Which of the images show evidence of brittle deformation? 1) 2) 3) 4) (2)
  • 4 –
    Activity 10.2: Visualizing How Stresses Deform Rocks
    A.Visualize all stresses or forces involved in the following deformations &complete the table below, checking
    any which apply.
    .
    Object/Action Confining Pressure Directed Pressure Compression Tension Shear
    1.Smashed Carton
  1. Sealed Soda Can
  2. Stretched elastic
  3. Rubbed hands
    B. Illustrate these block diagrams as per instructions in 1-3 below.
  4. a. _____________ b. ____________________ c. ______________
  5. On the Figure above trace on the stress directions as arrows on each of the 3 folded and faulted strata. (6)
  6. On each of the 2 sides of the fault blocks in the bottom row of block diagrams, put the appropriate symbols
    for the relative sense of fault motion across the fault. When you view a strike skip fault in cross section, the side
    that comes towards you is labelled with a bullseye (circle with a dot in the middle) the side that moves away is
    labelled with an x inside a circle. On the map view of the dip slip faults, put a U on the up-thrown side and a D
    on the down-thrown side. (6)
  7. (on p 2 fill in: 3a, b, & c) Below each of the 3 fault types, label the appropriate type of plate margin where
    these structures tend to occur: Transform/Strike-Slip, Convergent/Subduction, Divergent/Rift. (3)
    C.1 All of the above diagrams a, b & c, the stresses dominantly seem to act in the horizontal plane, w
  • 5 –
  1. For each of the 3 settings, explain how the vertical force manifests and describe what part of the deformation
    in the block diagrams it aids or opposes.
    a. ____________________________________________________________________________ (2) b. _________________________________________________________________________________ (2) c. _________________________________________________________________________________ (2) Activity 10.3: Map Contacts and Geological Formations To make geological maps or cross sections and to trace out structures, we need something to trace out or to correlate. The fundamental unit of stratigraphy and mapping is called the formation. A formation must be laterally extensive, thick enough to show up on a map and generally consists of a related stack of beds or lithologies which contain fossils of the same age and a related set of environmental indicators. Formations are organized into Groups and Supergroups. Formations usually are a several metres to hundreds of metres thick and represent a few million to many tens of millions of years of geological time. A. The photo below is near the leading edge of the Rocky Mountains in Western Montana. The Rockies formed in Late Cretaceous through Early Tertiary time because of rapid convergence between the Farallon Plate (former Eastern Pacific seafloor) and North America. These forces were so great that mountains were built from older rocks > 1000 km east of the subduction margin! Nearby, this same structure turns into the Lewis Thrust. 1 On the image below of Scapegoat Mountain trace in the dashed bedding plane contact for the cliff forming Lower Paleozoic limestones entirely across the width of the photo. __________________________ (2)
  2. What is the geological name for this kind of structure? (two words) _______ _______ (2)
  3. What kind of stresses formed this structure? _____________________________________ (2)
  4. Draw in arrows for the relative directions of those stresses above and below the photo. ___ (2)
  • 6 –
    B.1 Draw in the contacts for the base & top of: the buff coloured cliff forming Dolostone of the Late Cambrian
    Muav Limestone “M” near the base of the canyon and the base & top of the white coloured slope forming
    sandstone Pennsylvanian Watahomigi near the top of the plateau on the oblique air photo above. Note there are
    4 formations and 4 disconformites between these 2 formations. Transfer these 2 formations to the topographic
    map to the right _____________________________________________________________ (4)
  1. Colour the region between the 2 contacts on your map above red, and above the Watahomigi as Blue._ (2)
  2. Look at your geological map here and the related images and topographic map in the manual on p. 275 to
    decide whether the Watahomigi Formation is deformed into a geological structure to give it this shape.
    Choose one: a) Deformed b) Flat Lying (2)
  3. Explain your reasoning for the shape you found and why you interpreted its origin this way. __

_______________________________________________________________________________ (2) Activity 10.4: Determine the Attitude of Rock Layers & a Formation Contact Strike is a map direction. It is the trace of a horizontal line (the map surface) where it intersects a dipping plane of rock. Think of this as the direction along the shoreline where the seabed falls away from the beach. When strike is expressed as a map quadrant, it can be something general like NNE, E, SE as per the Compass Rose on an old nautical chart. Azimuth is measured as degrees east of north so the same strikes might be given as: N 22.5° E, N 90° E & N 135° E. Review diagrams 10.2 on p.262 and map reading rules on p. 272 Fig. 10.9 first! Dip is always perpendicular to strike and points directly down the slope of a dipping plane of rock. A.1 This page is just as flat as the top of the rock wall pictured in A.1 on p,276. Put the appropriate structural symbol here to show its horizontal orientation and lack of dip. Symbol: ___________________ (1)

  • 7 –
  1. This plan view photo A.2 p.276 is a dipping (tilted) slab of rock much like a boat ramp at the edge of the sea.
    Assuming the top of this page is oriented North, Draw the Strike and Dip symbol here. After you draw it, give
    the Compass direction of the Strike (Quadrant strike) for your symbol and give its Azimuth Strike (compass
    bearing in degrees east of north).
  2. This oblique view photo A.3 p.276 is a dipping (tilted) slab of rock much like a boat ramp at the edge of the
    sea. You can see the dip direction by the wet line from the descending stream of water from the squeeze bottle.
    Assuming the top of this page is oriented North, Draw the Strike and Dip symbol here. After you draw it, give
    the Compass direction of the Strike (Quadrant strike) for your symbol and give its Azimuth Strike (compass
    bearing in degrees east of north).

Judging Dips from Geological Maps: The Earth’s surface to a first approximation is a
horizontal plane. A bed of rock, a dyke, a sill or a thick mineral vein is also a planar object.
Where 2 non-parallel planes intersect they make a single line. Take 2 sponges and do this. The
orientation of this linear geological contact on a map is called strike. Where land has a gentle
slope, streams erode and run downhill. A stream valley cuts down into the “horizontal” surface
and allows you to see a bit of the cross section. Streams tend to converge as tributaries come
together enabling us to see the flow direction. Rock beds which dip vertically show no offsets
as they cross streams. Dipping beds generally control the tilt of land and flow direction of
streams. For dipping beds having downstream dips, the strike of contacts is offset in the down-

  • 8 –
    stream direction, because the overlying bed is removed in the notch of the stream channel. Take
    a cut pot scrubber and lay it on top of a dipping sponge to visualize how this works when you
    look down on it from above. This trick that V’s point down dip is called “The Rule of V’s” . A
    corollary to this rule is that erosional removal of the top of the landscape causes strikes tomigrate in the down dip direction. Use a  stack of dipping sponges and view this from the side.
    B. Grab some coloured sponges or scouring pads from our collection on the carts and use them to make the
    maps and cross sections below. For each of the 2 maps below 1) Draw a possible cross section that agrees with the map in the open rectangle below. 2) Draw the strike and dip symbol on the map and 3) Give the numerical
    value of the strike azimuth with respect to North in the blank below the cross section. Hint use the Rule of V’s
    to determine the dip direction and sketch this in on the cross sections below. Here the dip angle is approximate as we do not have the topography nor the stream gradient to calculate a specific angle. (6)
    C. The following aerial photograph is 1 km square and taken from directly above the center of the photo in the Great Basin. Dipping strata have outcrop patterns that resemble chevrons on a Sergeant’s sleeve. As these beds differentially erode, the contacts between sandstones and shales obey the Rule of V’s. On the photo:
    Activity 10.5: Cardboard Model (Paper Copies) Analysis and Interpretation
    A.1 through F.1 Complete each of the geological maps placing all appropriate: faults, unconformities, strike
    and dip symbols, dip angle degrees., fold axes etc. Make sure you draw in all beds, unconformities, intrusions
    etc. on the ends or sides of the image if they are not drawn in already. Contacts need to match up where corners
    touch. Do not fold these up for handing in. Submit them complete and flat. (60 points)
  • Devonian strata were deposited.
    Deformation: Those strata were tilted, folded faulted etc. Erosion: this structure was eroded. And any new episodes of deposition etc.
    A.1 Explain the sequence of geological events that led to this geological map and block diagram. If
    sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each
    stratum. Do include any structural deformation, name the structures formed and include any erosional
    unconformities. S through P denote the Geological time Periods Silurian through Permian respectively. _Structure Name: _____________________________________________________________ (1)







______________________________________________________________________________ (3) B.2 Explain the sequence of geological events that led to this geological map and block diagram. If sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each stratum. Do include any structural deformation, name the structures formed and include any erosional unconformities. Bed B was deposited up the eroded surface of plutonic rock A. The block does not go far enough east to show beds G or younger strata. Structure Name: ___________________________ (1)











______________________________________________________________________________ (4) C.3 Explain the sequence of geological events that led to this geological map and block diagram. If sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each stratum. Do include any structural deformation, name the structures formed and include any erosional unconformities. Structure Name: ___________________________________________________ (1)








_________________________________________________________________________ (3)

  • 10 –
    D.4 Explain the sequence of geological events that led to this geological map and block diagram. If
    sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each
    stratum. Do include any structural deformation, name the structures formed and include any erosional
    unconformities. Bed B was deposited up the eroded surface of plutonic rock A. Ensure that both ends of your
    model show the same kind of fold! Structure Name: _______________________________ (1)










______________________________________________________________________________ (6) E.5 Explain the sequence of geological events that led to this geological map and block diagram. If sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each stratum. Do include any structural deformation, name the structures formed and include any erosional unconformities. My model has some extra steps compared to the one in the manual. Do mine! Structure Name: ________________________________________________________________ (1)












______________________________________________________________________________ (7) F.6 Explain the sequence of geological events that led to this geological map and block diagram. If sedimentation was in ordinary sequence without unconformities it is not necessary to list the deposition of each stratum. Do include any structural deformation, name the structures formed and include any erosional unconformities. Notice the different widths for the sandstone at the west edge and recall the rule of V’s. Structure Name: _________________________________________________________________ (1)





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