Draw cross section

Spencer Test.book Page 111 Thursday, September 28, 2017 12:07 PM 10 Folds on Geologic Maps Fold Geometry Depending on the conditions under which folding took place, fold geometry may range from simple curvilinear folds such as the Waterpocket Fold illustrated in Figure 10-1a to exceedingly complex shapes such as those most often seen in metamorphic rocks (Figure 10-1b). Less complex, geometrically simple folds of the types most often found on geological maps of sedimentary rocks are used in this discussion. The terms commonly used to describe such folds are illustrated in Figure 10-2. The crest line of a fold is a line drawn along the highest part of a folded layer (fold crest). The trough line similarly indicates the lowest part of the fold (fold trough). Portions of the folded layer that lie between crests and troughs are called fold limbs. The plane that divides a fold into two halves that are more or less mirror images of one another is called the axial plane or axial surface of the fold. The line defined by the intersection of this plane with the surface of the ground is the axial trace. Some geologic maps contain axial traces of large folds. The fold axis is an imaginary line formed where the axial plane intersects a folded layer. The axis may be horizontal, vertical, or inclined. If it is inclined, the angle measured in a vertical plane between the axis and horizontal projection of the axis is called the plunge (Figure 10-2c). Folds are described as upright, asymmetrical, or recumbent, depending on the orientation of the axial surface, also called the axial plane. Folds are described as symmetrical or asymmetrical, depending on the symmetry of the halves on either side of the axial surface, when viewed in cross section along the fold axis (Figure 10-3). Fold Patterns on Geologic Maps The pattern formed by the contacts of folded rock units on a geologic map depends on such factors as the size of the fold, the choice of map units, topographic effects, and the geometry of the folded rocks. 1. The size of the fold relative to the area covered by the map. Many maps show only portions of the limb of a large fold. The scale of the map and the thickness of map units affect the appearance of fold patterns. For example, the folds shown on state or national maps are generally so large that the topography does not have a great effect on the map pattern. Examine the folds shown 111 Spencer Test.book Page 112 Thursday, September 28, 2017 12:07 PM 112 Chapter Ten Figure 10-1 (a) Sketch of part of the Waterpocket Fold (a monocline) in Utah. This is an example of a geometrically simple structural feature. (After G. K. Gilbert.) (b) Geologic map of part of the Gwanda Greenstone Belt in the Zimbabwe craton. The rocks here have been highly deformed. Early formed folds were refolded producing a highly complex geometric pattern. (Republished with permission of the Geological Society of America. M. P. Coward, P. R. James, and L. Wright. 1976. “Northern margin of the Limpopo Mobile Belt, southern Africa.” Geological Society of America Bulletin, 87(4), 601–611.) Figure 10-2 Block diagrams showing parts of anticlines and synclines. (a) The axial surfaces of these folds are slightly tilted to the left. (b) This fold is asymmetric toward the left. (c) These folds are plunging to the top of the page. Note that the angle of plunge is measured in a vertical plane. Spencer Test.book Page 113 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps 113 Axial surface Symmetrical fold Overturned asymmetric fold Asymmetric fold Recumbent isoclinal fold on the Arkansas State map in Appendix B (p. 170). Map units must be very thick and the folds must be large to show on the map at this scale. 2. The selection of map units. Contacts of large folds may appear to be homoclinal if the map covers only a portion of the fold. Small folds within formations rarely show up on maps. Generally, folds are evident on geologic maps only if contacts of map units are affected by the folding. 3. Topographic effects. On maps at scales of 1:24,000 or less, even perfectly planar contacts may appear as complex patterns as a result of the way the contact between map units intersect the ground surface. Similar effects determine map patterns of folded rocks. 4. The geometry of the folded strata, including the symmetry of the fold, the attitude of the limbs, the curvature of the fold, and the plunge of the fold axis. Fold shapes range from simple, symmetrical anticlines and synclines to complex refolded folds of intricate form. Because the appearance of a fold on a geologic map depends on the shape of the topography as well as on the geometry of the fold, maps of real folds may appear quite different from schematic illustrations, which do not show effects of topographic relief (Figure 10-4). Actual geologic map patterns may closely resemble the schematic patterns of illustrations if the mapped area has subdued relief, or if the size of the structural feature is large relative to the amount of relief. For this reason, large folds on state and national maps often resemble schematic maps more than do folds on 7.5minute quadrangle maps. Hints on Reading Maps of Folded Strata 1. When the folded strata are of different resistance to erosion, folds are likely to be reflected in the topography. 2. V-shaped patterns formed where streams cut contacts of folded layers reveal the direction of dip of those contacts just as they do for planar layers. Do not confuse the V-shaped patterns that form where streams cut across fold limbs with the V-shaped patterns formed where folds plunge (see Figure 10-4). 3. If a layer is uniform in thickness, and if the area has low relief, the relative width of the outcrop at different places along a fold or on different limbs of a fold is a good indication of dip. Thus, if the outcrop width of one limb is much greater than that of the other, the dips of the two limbs are different, and the fold is probably asymmetric in cross section. 4. Folds that have parallel limbs are called isoclinal folds. The limbs of isoclinal folds dip in the same direction. Both anticlines and synclines may be isoclinal. Figure 10-3 Examples of various types of fold symmetry as viewed in cross sections normal to the fold axis. Spencer Test.book Page 114 Thursday, September 28, 2017 12:07 PM 114 Chapter Ten If V-shaped patterns are present where streams cut across the limbs, the Vs on both limbs point in the same direction. Fold hinges of isoclinal folds are commonly sharp and narrow. 5. To obtain some idea of what the cross section of a structure should look like, it is useful to use a technique known as down-plunge viewing. If you know that a fold is plunging at an angle of 30 degrees, place the map on a flat surface and rotate the map until you are looking at it in the direction of the plunge of the Open, upright, symmetrical anticline N N Axial trace 4 4 3 2 3 4 0 1 2 3 3 4 2 3 1 Scale (a) N Asymmetric anticline N 4 3 2 3 4 0 1 2 Scale 3 2 3 3 1 4 (b) Plunging anticline (c) Figure 10-4 Block diagrams for (a) an open, symmetrical anticlinal fold and (b) an asymmetric anticlinal fold. Neither fold is plunging. (c) A plunging anticline. Compare Figure 10-4c with Figure 10-2c. Spencer Test.book Page 115 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps 115 fold. Then raise or lower your head until your line of sight is inclined at the same angle as the angle of plunge. In this position, you should see approximately what a cross section drawn perpendicular to the fold axis looks like. The same technique can be used to look down-dip to see what faulted beds look like in section. Constructing Cross Sections of Folded Rocks Freehand Cross Sections Where topographic relief is significant, preparing a profile of the land surface along the line of the section should always be the initial step in drawing cross sections. The second step is locating and indicating the dips of contacts along the profile. Any of several different methods may be used to complete the section. A method known as balanced cross section is now widely used. Most older sections were drawn freehand. In freehand drawing, smooth lines are drawn to represent the contacts between rock units. These lines must conform to the dip data available along the line of the profile, and they should also be consistent with the best available information about bed thickness and continuity. The cross section (Figure 10-5) across part of the Duffield Quadrangle, Virginia (p. 180) is a good example of the freehand technique. Use the following guidelines unless you have evidence that some other shapes are more accurate: 1. Draw the layers so they have uniform thickness. 2. Use uniformly curved lines rather than straight lines to represent the layers. 3. Use smooth lines rather than irregular lines. Figure 10-5 This cross section (line C–D) is drawn across the asymmetric syncline shown on the Duffield Quadrangle, Virginia (see the map in Appendix B). Balanced Cross Sections A balanced cross section is one in which the length of contacts and the area of beds shown in the cross section can be restored to their original condition—the shape they had before they were folded or faulted (see Figures 10-6 and 11-15). Perfectly balanced cross sections exhibit certain characteristics: 1. Bed lengths after deformation equal bed lengths before deformation. 2. Cross-sectional areas of beds after deformation equal cross-sectional areas of beds before deformation. Spencer Test.book Page 116 Thursday, September 28, 2017 12:07 PM 116 Chapter Ten (a) (b) (c) Figure 10-6 The concept of balance in cross section preparation is illustrated by the use of pins. In these drawings, pins are used to indicate the boundaries of the areas that have been involved in deformation. (a) A cross section of an area before deformation. (b) A balanced cross section showing a thrust fault. The fault is parallel to bedding at both ends of the cross section. The fault cuts up across layers at a “ramp.” The amount of shortening needed to produce this structure is indicated. (c) Folded beds. The upper two units are balanced for both length and area. The lowest bed must be highly deformed internally in order to accommodate the flat bottom and the folded upper surface. (After P. A. Geiser. 1988. “The role of kinematics in the construction and analysis of geological cross sections in deformed terranes.” In G. Mitra and S. Wojtal (Eds.), Geometries and Mechanisms of Trusting, with Special Reference to the Appalachians (pp. 47–76). Geological Society of America Special Paper 222.) 3. If faults are present, the angle between the fault and beds cut by the fault, called the cutoff angle, remains the same after displacement as it was immediately before displacement. If the area across which the cross section is being drawn has not been subsequently folded or faulted, balancing the section should be easy. If the area has been deformed, the shape of beds in the section will have changed from their original form, but neither volume nor bed lengths should have changed much during deformation. To test for balance, measure the length of each contact shown on the section. They should be approximately the same lengths. With a planimeter or digitizing tablet, measure the area of each bed. It should be possible to reconstruct an undeformed section from the preceding data. An example of a balanced section and its reconstructed original shape is shown in Figure 10.7. (Note: The preceding test is not completely valid unless the cross section extends far enough across the structure to reach a Spencer Test.book Page 117 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps 117 Figure 10-7 A balanced cross section across the Blue Ridge and part of the Valley and Ridge Provinces of Virginia. The top cross section has been restored below. BRT = Blue Ridge thrust; RVT = Rockfish Valley thrust; UNMR = Upper North Mountain ramp; LNMR = Lower North Mountain ramp. (Modified after M. A. Evans. 1989. Field Trip Guidebook T357 for the 28th International Geological Congress.) point at which the amount of slip between beds is minimal.) See Woodward, Boyer, and Suppe (1990) for more detailed information. Elementary discussions of balanced sections with exercises are also available in Marshak and Mitra (1988). Tracing Folds through the Topography The same technique used to trace the line of intersection of a plane bedding contact or a plane fault across the land surface (see Chapter 8) may be used to trace the contacts of folded layers through the topography (Figure 10-8). This technique is a useful way of learning how folded layers may appear on topographic maps, but application of this technique to real folds is limited. The method gives reliable results only if the fold axis is horizontal and if the shape of the fold in cross section remains uniform. Most real folds change in cross-sectional shape and plunge at one or both ends. Exercise 10-1 TRACING FOLDS THROUGH THE TOPOGRAPHY Refer to Figure 10-8. 1. The trace of the outcrop of the upper contact of a folded unit is already drawn on this map. The unit is approximately 200 feet thick, and is shown in the cross section. Locate the trace of the lower contact of this unit, and color the outcrop of the unit on the map. Structure Contour Maps of Folded Strata Structure contour maps provide one of the best methods of depicting the shape of folded strata. Structure contours are shown on the maps of Pine Mountain, Colorado, Atkinson Creek, Colorado, Davis Mesa, Colorado, and Salem, Kentucky Quadrangles in Appendix B. Unfortunately, geologists rarely have access to sufficient subsurface data to draw accurate structure contour maps. Structure contours are generally drawn on the top of a distinctive rock unit that is readily identified in well logs or on seismic sections. The surface on which the contours are drawn is referred to as a contact or horizon. If the horizon on which the structure contours are drawn crops out, elevation data are available all along the contact, but portions of the horizon above Spencer Test.book Page 118 Thursday, September 28, 2017 12:07 PM 118 Chapter Ten Figure 10-8 Tracing an anticlinal structure across the topography. The method used to trace the top or bottom of this layer through the topography is similar to that used to trace a plane through the topography (see Figure 8-6). Spencer Test.book Page 119 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps Figure 10-9 A structure contour map of an area in Pennsylvania. Contours are drawn on the top of a Devonian sandstone, the Oriskany Sandstone. (From A. S. Cate et al. 1961. Subsurface Structure of Plateau Region North-Central and Western Pennsylvania on Top of Oriskany Formation. Pennsylvania Geological Survey, fourth series map.) ground level have been removed by erosion. If the horizon lies entirely in the subsurface, information used to construct the structure contours comes from well logs and/ or seismic data. For this reason, most structure contour maps depict areas where oil and gas are present. The Oriskany Sandstone is one of the principal gas-producing horizons in the Appalachian Basin. The structure contour map of an area on the edge of this basin (Figure 10-9) illustrates folds of the Valley and Ridge Province. Note that the spacing of contours on the fold limbs clearly shows the asymmetric shape of these folds, and the closed contour lines indicate that the folds plunge toward both the northeast and southwest. Exercise 10-2 FOLDS— NORTHERN AND WESTERN FLANKS OF THE BLACK HILLS, WYOMING Refer to Figures 10-10 and 10-11. Scale: 1:96,000. Small circles with a cross through them indicate gas wells. 1. The structure contours are drawn at a contour interval of 100 feet. The datum is sea level. Contours are drawn on top of the Fall River Sandstone, which does not outcrop in this map area. Using the structure contours, draw cross sections along the lines indicated by letters A, B, C, and D. Draw cross sections with 2× and 10× vertical exaggeration for each line. Note: At the scale of this map, 0.1 inch equals approximately 800 feet. 2. Using a transparent overlay, trace the contacts of the bedrock units where they are most likely to pass beneath the Quaternary alluvium. Draw the axial traces of the folds on the overlay. 119 Spencer Test.book Page 120 Thursday, September 28, 2017 12:07 PM 120 Chapter Ten Figure 10-10 Geologic sketch map of an area north of the Black Hills. Qal is Quaternary alluvium; all other units are Cretaceous in age. Km is the oldest unit; Kpf is the youngest. (After W. J. Mapel, G. S. Robinson, and P. K. Theobold. 1959. US Geological Survey Map OM-191.) a. Are these folds plunging? If yes, in what direction? b. Are the folds symmetrical or asymmetrical? c. If the folds are asymmetrical, which is the steep limb? 3. Why are the structure contour lines smoother than the mapped contacts? 4. Based on the shape of the contacts on this map, do you think the beds have low, moderate, or steep dips? The relief in this area is very low. Spencer Test.book Page 121 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps Figure 10-11 Structure contours drawn on the top of the Fall River Sandstone. For these cross sections, use graph paper with 10 divisions per inch. At this scale (1:96,000), 0.10 inch equals 800 feet. For 2× vertical exaggeration, 0.10 inch equals 400 feet. (After W. J. Mapel, G. S. Robinson, and P. K. Theobold. 1959. US Geological Survey Map OM-191.) Exercise 10-3 STRUCTURE ON THE ARKANSAS STATE MAP Refer to the map in Appendix B (p. 170). 1. Using an overlay, sketch the area in which Tertiary rock units outcrop, omitting all Quaternary deposits that are shallow alluvial materials. The total thickness of Tertiary rocks is no more than a few thousand feet thick. How would you describe the structure of the Tertiary rock units shown on this map? Are they folded, faulted, or homoclinal? 2. Examine the outcrop pattern of the Cretaceous rock units shown in the lower left corner of the map. Note the V-shaped patterns formed where streams cut across these units. What is the structure of the Cretaceous units? 121 Spencer Test.book Page 122 Thursday, September 28, 2017 12:07 PM 122 Chapter Ten 3. The red lines on this map are faults. Note that many rock units are repeated as a result of the faulting. Based on their map pattern and their relationship to the folds located west of Little Rock in the Ouachita Mountains, what types of faults are exposed in the Ouachita Mountains? Note: the Ouachita Mountains lie south of the Arkansas River and north of the post-Paleozoic rocks of the Coastal Plain. 4. Describe the changes in structure you would cross if you made a traverse due north from Arkadelphia to the northern border of the map. Can you determine if the folds are plunging? If yes, in what direction do they plunge? Are the folds symmetric? 5. How closely can you determine the age of the folding and faulting in the Ouachita Mountains? Exercise 10-4 FOLDS—PITTSFIELD EAST QUADRANGLE, MASSACHUSETTS Refer to the map in Appendix B (p. 200). Note: Most of the rocks exposed in this map area are marbles. They were deeply buried and became highly ductile during deformation. The black line that passes between the letters “L” and “D” in the label “Pittsfield” is the axial trace of a fold. 1. What type of fold is the fold for which the axial trace is shown? 2. The red line that trends north–south across the map shows the axial trace of another fold. What type of fold is this? 3. Using a piece of tracing paper, make an overlay for this map and draw axial traces for the other folds shown on the map. 4. How can you explain the two sets of fold axes? 5. Which fold set is older? Exercise 10-5 FOLDS—WILLIAMSPORT QUADRANGLE, PENNSYLVANIA Refer to the map in Appendix B (p. 214). Scale 1:24,000. The topographic contour interval is 20 feet. In Figure 10-12a, a cross section has been drawn across the map along the line X–Y. Some of the rock units shown on the map have been lumped together in order to simplify the cross section. In this section Sr and Sm, all units between Sb and Dmt, and Dh and Dtr are combined. Figure 10-12 Cross section across the Bald Eagle Mountain from the Williamsport Quadrangle, Pennsylvania. (Based on O. B. Lloyd, Jr. and L. D. Carswell. 1981. Groundwater Resources of the Williamsport Region, Lycoming County, Pennsylvania. Commonwealth of Pennsylvania Department of Environmental Resources, Water Resources Report 51.) Spencer Test.book Page 123 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps 123 Spencer Test.book Page 124 Thursday, September 28, 2017 12:07 PM 124 Chapter Ten 1. Draw a cross section across the map along a line from point 349 (near the top of the map) to a position between points 87 and 90 (located near the bottom of the map). Use the blank profile shown in Figure 10-12b. Draw the following contacts: Dm/Don, Sb/Sm, Sr/St, and St/Oj. 2. Explain the map pattern at point A. Why does the rock unit exposed here occur as an isolated, elongate outcrop? 3. Why does the outcrop of Dh on the north and south limbs of this fold almost join at point B? 4. Explain the map pattern of Sm (the long, narrow connected strips) along an east– west line through point C. 5. Explain why the width of the outcrop belt of St varies so much. Exercise 10-6 FOLDS—SALEM QUADRANGLE, KENTUCKY Refer to the map in Appendix B (p. 208). The structure contours on this map are drawn on the base of the Cypress Sandstone (Mcb). Negative values indicate elevations below sea level. 1. Is the area where *ca is exposed best described as a dome, basin, plunging anticline, or plunging syncline? 2. Are the rock units that are exposed outside the area bounded by faults folded? 3. What are the approximate strike and dip of the rock units located outside the faultbounded area? Exercise 10-7 FOLDS—MAVERICK SPRINGS ANTICLINE, WYOMING Refer to Figure 10-13. The structure contours show elevations above sea level on the base of the Permain age Phosphoria Formation and on the top of the Upper Cretaceous Frontier Formation. The stratigraphic interval between these two formations includes Spencer Test.book Page 125 Thursday, September 28, 2017 12:07 PM Folds on Geologic Maps 125 Figure 10-13 Structure contours drawn on top of the Frontier Formation except in the Maverick Springs and Circle Ridge anticlines, where the contours are drawn on the base of the Phosphoria Formation. The structure contour interval is 500 feet. (After D. A. Andrews. 1944. Geologic and Structure Contour Map of the Maverick Springs Area, Fremont County, Wyoming. Oil and Gas Investigations Map 13, US Geological Survey.) Spencer Test.book Page 126 Thursday, September 28, 2017 12:07 PM 126 Chapter Ten the entire Mesozoic section. A dashed line separates the portions of the map drawn on these two surfaces. The thickness of the stratigraphic section between the base of the Phosphoria Formation and the top of the Frontier Formation is approximately 3,800 feet. 1. Draw a cross section across the Maverick Springs anticline along the line A–B showing the shape of the Frontier and Phosphoria Formations. Suggestion: draw the cross section using contours on the top of the Frontier Formation at the two ends of the cross section. Then use the thickness of the Phosphoria to estimate where the top of the Phosphoria will be below the top of the Frontier. Use dotted or dashed lines to show where you infer the position of the contacts to be across portions of the cross section where no control data are shown. 2. Draw cross sections across Circle Ridge and Little Dome anticlines along the lines C–D and E–F. 3. Compare the shape of Little Dome with that of the Maverick Springs and Circle Ridge anticlines. 10000 5000 0 10000 5000 0 10000 5000 0

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