Lab%205---Geologic%20Structures%2c%20Maps%2c%20AND%20Block%20Diagrams

Lab%205---Geologic%20Structures%2c%20Maps%2c%20AND%20Block%20Diagrams

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Unformatted text preview: LABORATORY TEN Geologic Structures, Maps, and Block Diagrams -CONTR|BUTING AUTHORS- Michael J. Hozik - Stockton State College William R. Parrott, Jr. - Stockton State College Raymond W. Talkington - Stockton State College OBJECTIVES A. Be able to identify common kinds of geologic structures and know the symbols used to repre- sent them on geologic maps. B. Be able to read and interpret geologic maps. C. Be able to identify, describe, and interpret geo- logical structures in three dimensions using three—dimensional block diagrams. MATERIALS Pencil, eraser, laboratory notebook, ruler, set of col- ored pencils, scissors, Cardboard Models 1—6 (located at the back of this laboratory manual), and a geologic map (provided by your instructor, or obtained as noted by your instructor). INTRODUCTION Structural geology is the study of how geologic units (bodies of rock or sediment) are arranged when first formed and how they are deformed afterward. When a body of rock or sediment is subjected to severe stress (directed pressure), then it may eventually strain (un- dergo deformation, such as a change in shape). There- fore, deformed formations are geologic units that have adjusted to a severe stress. Much of the study of structural geology involves deciphering stress and strain relationships. Generally, geologists can see how bodies of rock or sediment are positioned where they crop out (stick “out of the ground as an outcrop) at Earth’s surface (Figure 10.1A). Geologists record this outcrop data on flat (two-dimensional) geologic maps using dif- ferent colors and symbols to represent the different units of rock or sediment and their positions (Figures 10.1B, 10.2, 10.3). They apply information from geologic maps to infer the three-dimensional arrangement of the units. The structural geology of an area can be described and interpreted from this three-dimensional arrangement, viewed as a con- ceptual model in your mind, or as a physical model. You will interpret as many as six different (physical) cardboard models of structural geology in this laboratory. PART 10A: STRUCTURAL GEOLOGY Three representations of Earth are commonly used by structural geologists. These are the geologic map, cross section, and block diagram: 0 Geologic map—shows the distribution of rocks at Earth’s surface. The rocks commonly are divided into mappable rock units that can be recognized and traced across the map area. This division is made on the basis of color, texture, or composition. 195 196 ° Laboratory Ten A. Outcrop Horizontal piano Dip direction FIGURE 10.1 Strike and dip of a rock layer as directly observed in nature (A) and as represented on a geolog— ic map (B). Strike is the direction of a line formed by the intersection of the surface of an inclined (tilted) rock layer and a horizontal plane. Dip is the maximum angle of inclination (tilting) of the rock layer, always measured perpendicular to the line of strike (looking straight down on it, in map view) and in the direction that the rock layer tilts down into the ground. Water poured onto a dipping rock layer drains along the angle of dip. The “T” and 45 together form the standard strike—and—dip symbol. The long top of the “T” is the line of strike, the short upright of the “T” shows the dip direction, and "45” is the dip angle in degrees. Such mappable rock units are called formations. They may be subdivided into members comprised of beds (individual layers of rock or sediment). The boundaries between geologic units are contacts, which form lines on geologic maps. A ge- ologic map also shows the topography of. the land surface with contour lines, so it is both a geologic and topographic map. 0 Geologic cross section—a drawing of a vertical slice through Earth, with the material in front of it FIGURE 10.2 Geologic maps with strike and dip symbols indicating the attitude of rock layers. Note that strike and dip can be expressed in quadrant or azimuth form. When expressing strike and dip directions as azimuth bearings, they should be expressed as three digits in order to distin— guish them from two-digit dip angles. Note also that a line of strike can be expressed as a bearing in either direction. For example (A), a line of strike with a quadrant bearing of North 45° West also has a bearing of South 45° East. A line of strike with an azimuth bearing of 335° also has a bearing of 155° (i.e., 180° less than 335°). removed: a cutaway view. It shows the arrange- ment of formations and their contacts. A good cross section also shows the topography of the land surface, like a topographic profile. Block diagram—a combination of the geologic map and cross section. It looks like a solid block, with a geologic map on top and a geologic cross section on each of its Visible sides (e.g., Figure 10.4). Each block diagram is a small three-dimen- sional model of a portion of Earth’s crust. Quadrant: North 45° West (or South 45° East), >\ 24° Southwest 24 Azimuth: Strike = 335° (or Strike = 155°), ' Dip = 24° @ 245° 43 Quadrant: North 90° East (or South 90° West), I 43° North Azimuth: Strike = 090° (or Strike = 270°). Dip = 43° @ 000° Geologic Structures, Maps, and Block Diagrams ° Strike and dip of strata X Vertical strata 7 Axis of an overturned anticline Axis of an overturned syncline Strike and dip of overturned strata Horizontal strata \\ R z ‘/ R (5% Lateral, or strike—slip fault; arrows indicate Strike and dip of foliation z// Silurian Permian Ordovician Pennsylvanian Cambrian Mississippian Precambrian Devonian FIGURE 10.3 Structural symbols and abbreviations used Figure 8.4.) Measuring the Attitude of Rock Units Attitude is the orientation of a rock unit or surface. Geologists have devised a system for measuring and describing attitude to understand three-dimensional relationships of formations and geologic structures. Strike and dip serve this purpose (see Figure 10.1): 0 Strike—the compass bearing (direction) of a line formed by the intersection of a horizontal plane (such as the surface of a lake) and an inclined layer (bed, stratum) of rock, fault, fracture, or other surface (Figure 10.1). If strike is expressed in Vertical foliation Axis of an antiform Axis of a synform Axis of a plunging antiform Trend and plunge of a line relative movement 197 High angle fault: U for up and D for down to indicate relative movement Reverse fault: Teeth are in the side of the hanging wall (upper block) Contact or other line solid where known, dashed where approximated, and dotted where only inferred Axis of a plunging synform Axis of a monocline. Arrow is on inclined beds of the monocline; opposite side is horizontal beds. W Unconformity Quaternary Tertiary Cretaceous Jurassic h Triassic on geologic maps. (For rock symbols, see degrees east or west of true north or true south, it is called a quadrant bearing. Strike can also be ex— pressed as a three-digit azimuth bearing in degrees between 000 and 360. In azimuth form, north is 000° (or 360°), east is 090°, south is 180°, and west is 270°. Dip—the angle between a horizontal plane and the inclined (tilted) stratum, fault, or fracture. As you can see in Figure 10.1, a thin stream of water poured onto an inclined surface always runs downhill along the dip direction, which is always perpendicular to the line (bearing) of strike. The 198 ° Laboratory Ten Disconformity Angular unconformity Nonconformity FIGURE 10.4 Unconformities. Arrows point to the unconformity surface (black line). A disconformity is an unconformity between relatively parallel strata. An angular unconformity is an unconformity between nonpara/lel strata. A nonconformity is an unconformity between sedimentary rock/sediment and igneous or metamorphic rock. inclination of the water line, down from the hori- zontal plane, is the dip angle. Dip is always expressed in terms of its dip angle and dip direction. The clip angle is always expressed in two digits (e.g., 45° in Figure 10.1). The clip direction can be expressed as a three-digit azimuth direction or as a quadrant direction (e.g., North, Northeast, East). Strike and clip are shown on maps by use of “T”- shaped symbols (see Figures 10.1, 10.2, and 10.3). The long line (top of the ”T”) shows strike direction, and the short line (upright of the “T”) shows dip direction. Note that clip is always perpendicular to the line of strike. The short line of the “T” points downdip. The accompanying numerals indicate the dip angle in de- grees. Refer to Figure 10.2 for examples of how to read and express strike and dip in quadrant or az- imuth form. Also note that special symbols are used for horizontal strata (rock layers) and vertical strata (Figure 10.3). Unconformities Structural geologists must locate, observe, and inter- pret many different structures. Fundamentally, these include unconformities, faults, and folds. There are three common types of anconformities (see Figure 10.4), which you may recall from Laboratory 8: 0 Disconformity—an unconformity between rela- tively parallel strata. 0 Angular unconformity—an unconformity be- tween nonparallel strata. 0 Nonconformity—an unconformity between sedi- mentary rock/ sediment and non-sedimentary (ig- neous or metamorphic) rock. Any unconformity may be a very irregular surface, because it is usually a surface where erosion has oc- curred (before it was buried to form the unconformi- ty). For example, bedrock surfaces exposed on the slopes of hills and mountains in your region are part of a regional surface of erosion that could become an unconformity. If sea level were to rise and cover your region with a fresh layer of mud or sand, then the un- even regional surface of erosion would become a re- gional unconformity. Faults Faults in rock units are breaks along which movement has occurred. Faults form when brittle rocks experi- ence three kinds of severe stress: tension (pulling apart or lengthening), compression (pushing together, com- pacting, and shortening), and shear (smearing or tear- ing). The three kinds of stress force the rocks to fault in distinctive ways (Figure 10.5). Normal faults, reverse faults, and thrust faults all in- volve vertical motions of rocks. These faults are named by noting the sense of motion of the top surface of the fault (top block) relative to the bottom surface (bottom block), regardless of which one actually has moved. The top surface of the fault is called the hanging wall and is the base of the hanging wall (top) block of rock. The bottom surface of the fault is called the footwall and forms the top of the footwall block. The headwall block sits on top of the footwall block. STRESS TYPES FAULT TYPES and the developed when brittle rocks Strain (deformation) deform so much that they break they cause // ; I‘ Footwall / ___ I ' block . Hanglng _ I . ' wall ClTENSION] C> REVERSE FAULT (High angle) Hanging wall block THRUST FAULT _ -":;___ _li’ (Low angle) Footwall block STRIKE SLIP FAULTS Causes tearing and smearing Left Lateral Right Lateral FIGURE 10.5 Three types of stress and strain and the fault types they produce. 199 ‘ Laboratory Ten Normal faults are caused by tension (rock length- ening). As tensional stress pulls the rocks apart, gravi- ty pulls down the hanging wall block. Therefore, normal faulting gets its name because it is a normal response to gravity. You can recognize normal faults by recognizing the motion of the hanging wall block relative to the footwall block. If the hanging wall block has moved downward in relation to the foot- wall block, then the fault is a normal fault. Reverse faults are caused by compression (rock shortening). As compressional stress pushes the rocks together, one block of rock gets pushed atop another. You can recognize reverse faults by recog- nizing the motion of the hanging wall block relative to the footwall block. If the hanging wall block has moved upward in relation to the footwall block, then the fault is a reverse fault. Thrust faults are reverse faults that develop at a very low angle and may be very difficult to recognize (Figure 10.5). Reverse faults and thrust faults generally place older strata on top of younger strata. Strike slip faults (lateral faults) are caused by shear and involve horizontal motions of rocks (Figure 10.5). If you stand on one side of a strike slip fault and look across it, then the rocks on the opposite side of the fault will appear to have slipped to the right or left. Along a right-lateral (strike slip) fault, the rocks on the opposite side of the fault appear to have moved to the right. Along a left—lateral (strike slip) fault, the rocks on the opposite side of the fault appear to have moved to the left. Folded Structures Folds are upward or downward bends of rock layers (Figures 10.6, 10.7, 10.8, and 10.9). Antiforms are "upfolds" or "convex folds.” If the oldest rocks are in the middle, then they are called anticlines. Synforms are "downfolds" or "concave folds.” If the youngest rocks are in the middle, then they are called synclines. In a fold, each stratum is bent around an imagi— nary axis, like the crease in a piece of folded paper. This is the fold axis (or hinge line). For all strata in a fold, the fold axes lie within the axial plane of the fold (Figure 10.6 and 10.7). The fold axis may not be horizontal, but rather it may plunge into the ground. This is called a plunging fold (Figures 1073). Plunge is the angle between the fold axis and horizontal. The trend of the plunge is A. Anticline (asymmetrical): oldest rocks (-6) occur in the center of the fold B. Syncline (symmetrical): youngest rocks (3') occur in the center of the fold FIGURE 10.6 Folds—the two common types. Letters on rock layers indicate their relative ages on the geologic time scale (Figures 10.3, 1.3). Note that solid lines (dashed where under- ground) are used to show the position of the axial planes of the folds. Note the symbols for axis of an anticline and axis of a syncline in Figure 10.3. Also note the orientation of symbols for strike and dip in relation to the attitude (orientation) of rock layers (strata). A. Horizontal fold \\ \ Limb of - ‘ fold Block diagram of a horizontal fold (anticline) Geologic Structures, Maps, and Block Diagrams ° 20 1 B. Plunging fold Block diagram of a plunging fold (plunging anticline} FIGURE 10.7 Fold terminology and block diagrams. A. Simple horizontal fold (anticline). B. More com- plex plunging fold (plunging anticline). Note that the fold axis plunges into Earth, but the trend is the com- pass direction (bearing) on the surface. Also note the orientation of rock layers, symbols for strike and dip, and symbols for the fold axes in the block diagrams. the bearing (compass direction), measured in the di- rection that the axis is inclined downward. You can also think of the trend of a plunging fold as the direc- tion a marble would roll if it were rolled down the plunging axis of the fold. Folds normally have two sides, or limbs, one on each side of the axial plane (see Figure 10.7). If a fold is tilted so that one limb is upside down, then the en- tire fold is called an overturned fold (Figure 10.8). Monoclines do not have two axial planes that sepa- rate two nearly horizontal limbs from a single, more steeply inclined limb (Figure 10.9). Domes and basins (Figure 10.10) are large, somewhat circular structures formed when strata are warped upward, like an upside-down bowl (dome) or downward, like a bowl (basin). Strata are oldest at the center of a dome, and youngest at the center of a basin. Geologic Maps and Block Diagrams Sample block diagrams and geologic maps are pro- vided so you can practice identifying, describing, and interpreting geologic structures. You will need to un- derstand and apply the symbols from Figure 10.3 and follow the set of simple rules for interpreting geologic maps (Figure 10.11). Refer back to Figures 10.4—10.10 as needed. Monocline Overturned anticline Geologic map of a monocline Block diagram FIGURE 10.9 Monocline. Not all folds have two limbs. of an overturned The monocline is a fold inclined in only one direction. A ant'd'ne monocline has two axial planes (dashed) that separate two FIGURE 10.8 Overturned fold Note that one “mb of the nearly horizontal limbs from a more steeply inclined limb. fold has been turned under the other, so it is overturned Note the Symbols used to indica’Fe “F’riz‘mta' Stratfa (rook (Upside down). Also note the Symbols used for strike and layers) and the axes of a monocline in the block diagram dip of strata (rock layers), strike and dip of overturned stra- and F'gure 10'3' ta, and axis of an overturned anticline in the block diagram and Figure 10.3. Youngest strata Oldest strata Oldest strata Youngest A. Burns FIGURE 10.10 Dome and basin. Both of these structures are bowl—shaped in three dimen- sions and appear as relatively circular “bull’s eye” patterns on maps. A. A dome is convex (bowed upward, like an upside-down bowl) and has the oldest strata in its center. Rocks dip away from the center of the dome (note strike—dip symbols). B. A basin is concave (bowed downward; bowl shaped) and has the youngest strata in its center. Rocks dip toward the cen- ter of the basin (note strike-dip symbols). 202 Geologic Structures, Maps, and Block Diagrams ° 203 A SET OF SIMPLE RULES FOR INTERPRETING GEOLOGIC MAPS 1. Contacts between horizontal beds “V” upstream and are parallel to topographic contour lines. 2. Anticlines have their oldest beds in the center. 5" Synclines have their youngest beds in the center. Anticlines plunge toward the nose (closed end) of the structure. Synclines plunge toward the open end of the structure. 99'? Contacts of horizontal beds, or of beds that have a dip lower than stream gradient, "V" upstream. 7. Contacts of beds that have a dip greater than stream gradient "V" downstream if they are dipping downstream. 8. Contacts of beds that have a dip greater than stream gradient will "V" upstream if they are dipping upstream. 9. Vertical beds do not “V” or migrate with erosion. 10. The upthrown blocks of faults tend to be eroded more than downthrown blocks. 11. Contacts migrate downdip upon erosion. FIGURE 10.11 Simple rules used by geologists to interpret geologic maps. 12. True dip angles can only be seen in cross section if the cross section is perpendicular to the fault or to the strike of the beds. Questions youngest at the top of the list. Indicate in your list, and on the map, where there are unconforrni- 1- For eaCh bIOCk diagram in Figure 10-12: ties (gaps or missing intervals of rock in the se- a. Note the ages of the rock units based on their symbols (Figures 10.3 and 1.3). b. Complete any blank sides of the diagram by drawing in the geologic units / contacts, then adding strike-dip and all other appropriate sym- bols from Figure 10.3 to the top (map) part of the block diagram. c. On the line provided, indicate what kind of ge- ologic structure is represented in the diagram. . Refer to the geologic maps in Figure 10.13. Do the following for each of these maps. a. Make a list of the ages of rocks present in the map, from oldest at the bottom of the list to quence), if any. b. Make a list of other geologic structures that you can identify on the map, then describe where each structure is located. c. Write a paragraph or outline of the general ge- ologic history of the region (i.e., describe when the rock layers formed and how they were de- formed or eroded). A. Add strike—dip and other appropriate symbols (Figure 10.3). B. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? G. Add strike-dip and other appropriate symbols (Figure 10.3). D. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? E. Add strike-dip and other appropriate symbols (Figure 10.3). F. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? FIGURE 10.12 Block diagrams to complete (Question 1). Continued on next page. 204 G. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? |. Add strike—dip and other appropriate symbols (Figure 10.3). J. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? K. Add strike—dip and other appropriate symbols (Figure 10.3). L. Add strike-dip and other appropriate symbols (Figure 10.3). What is the geologic structure in this block diagram? What is the geologic structure in this block diagram? FIGURE 10.12 (CONTINUED) Block diagrams to complete (Question 1). 205 NO DATA QUATERNARY RECENT AND PLEISTOCENE UPPER TERTIARY PLIOCENE AND MIOCENE in Western States includes Recent and Pleistocene LOWER TERTIARY OLIGOCENE, EOCENE. AND PALEOCENE In Alaska includes some Miocene CRETACEOUS lerarts of Rocky Mountains and Alaska includes Jurassic and Triassic JURASSIC AND TRIASSIC UPPER PALEOZOIC PERMIAN, PENNSYLVANIAN, AND MISSISSIPPIAN In parts of Rocky Mountains and Alaska includes middle and lower Paleozoic MIDDLE PALEOZOIC DEVONIAN AND SILURIAN LOWER PALEOZOIC ORDOVICIAN AND CAMBRIAN In parts of Missouri, Oklahoma, and Arkansas includes Devonian and Silurian YOUNGER PRECAMBRIAN In southeastern United States and Alaska includes metamorphosed Paleozoic YOUNGER PRECAMBRIAN Granite OLDER PRECAMBRIAN Metamorphic and igneous rocks FIGURE 10.13 Geologic maps for analysis and interpretation (Question 2). ...
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This note was uploaded on 04/16/2011 for the course GEOL 320 taught by Professor Mathewson during the Spring '11 term at Texas A&M.

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Lab%205---Geologic%20Structures%2c%20Maps%2c%20AND%20Block%20Diagrams

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