GEOL 4334 Lab 03: Outcrop patterns v. 2015 LAB 03: Geologic Maps I: Topo Maps & Outcrop Patterns Objectives: This lab will develop & exercise the skills of 3D visualization and description of geologic structures using correct terminology and will acquaint you with: a) symbols used to describe different structures, b) the relationships between inclined layers, their true thickness vs. apparent thickness, topography, and how to predict where planar layers may occur in the subsurface, c) the outcrop pattern of folds in map view, d) the ‘rule of V’s’ to discern the orientation of planar features on geologic maps, and e) the use of geologic cross sections to discern 3-D perspectives on structures. Background: You should be familiar with reading topographic maps. Please refer to your Physical Geology laboratory exercises on topographic maps for review. Materials: tracing paper, pencils, ruler, protractor, color pencils, scissors Introduction To properly assess the shape and orientation of a subsurface structural reservoir in an oil field, or the extent of a seismically active fault, it is necessary to learn how to visualize such features in 3-D based on their 2-D representation on geologic maps. This set of laboratory exercises will focus on a critical element of geologic map analysis: How to recognize the orientation of structures based on outcrop patterns. 0 2000 ft. D Map Legend S Tertiary dike K O 77 18 N 15 K Cretaceous D Devonian S Silurian O Ordovician K S 27 K S 25 O contact; unconformity (circles on younger rocks) D O 24° 15 inclined vertical horizontal bedding bedding bedding plunging anticline syncline S Figure 1 shows a geologic map and oblique relief map to help visualize the three important features: faults, folds, and unconformities. Also shown are the common map symbols used to illustrate a geologic map. All maps should contain: • a legend • scale & geographic coordinates • appropriate caption. In this map, note how the structure symbols correspond to the orientation of the underlying rock unit and also indicate attributes such as the dip of faults, etc. Fig. 1. Geologic map and oblique perspective diagram. O Page 1 of 7 GEOL 4334 Lab 03: Outcrop patterns v. 2015 Topography & Outcrop Pattern Recognition Outcrop patterns of different geologic units provide a qualitative means of understanding the 3D orientation of units along the uneven surface of the Earth. Topographic maps display relief, or changes in elevation, using lines of equal elevation called topographic contour lines (Fig. 2). ’ 600 400’ 400’ 0’ 20 600’ 200’ Fig. 2. Shaded relief and resulting topographic map. Note how contour lines on the oblique shaded relief map project upwards onto the 2D topographic map. 800’ 800’ 200’ A If an exposed geological unit (e.g., a fault, stratigraphic bed, unconformity, etc.) is planar over some distance, the resulting outcrop pattern over uneven topography may be used to predict the unit’s orientation. On a geologic map, horizontal surfaces, such as the contact between sandstone and shale beds, would appear as a line parallel to the topographic contour lines (Fig. 3A) Inclined planar units would make an angular pattern on a non-flat topographic surface (Fig. 3B). Note how the inclined layer dips “towards” the mountain and makes a “V” that points “up” the drainage. In contrast, a vertical surface such as a strike-slip fault would appear as a line on the geologic map that “cuts across” the topographic contour lines (Fig. 3C). In map view, such surfaces would be linear over some spatial distance. Fig. 3. Oblique perspective shaded-relief perspectives and equivalent 2D topographic maps depicting the trace of planar units intersecting topography. A) horizontal bed; B) inclined bed, dipping 25° N; C) vertical unit, striking EW/90° dip. Note structure symbols that depict horizontal, inclined, and vertical contacts, respectively. Oblique View Map View N structure symbols B 25° N C N Page 2 of 7 GEOL 4334 Oblique perspective view Lab 03: Outcrop patterns v. 2015 Map View “Rules of V’s” A) 300 m m 200 300 m 200 m 100 m N B) N 30° m 300 m N 35° m 300 N m N 200 300 m E) m N While one can memorize these rules, it is best to work on visualizing the geometrical relations until your understanding becomes second nature. Figure 4. A-F various planar beds in oblique view (left) and the resulting outcrop pattern on a 2D topo map (right). Soccer man in A is climbing up the drainage wall. Arrows point down the drainage. Martini glass is on a horizontal surface. 200 m 12° m 300 E) Vertical surface; no “V” in map view. Outcrop pattern is linear in map view and “cuts across” topographic contour lines. F) Inclined surface dips upstream, “V” points upstream. 200 300 m 60° F) C) Surfaces that dip downstream at the same gradient as the stream appear parallel to the stream. D) Inclined surfaces with dips downstream, “V” points down stream. m 200 D) A) On a geologic map, horizontal planes appear parallel to contour lines (Fig. 4A). B) Planes that dip downstream at a shallower gradient than the stream “V”, or point, upstream. The outcrop pattern has a “narrow” profile in map view. 200 C) It is important to be able to visualize the intersection of a planar unit and topography. The “rule of V’s” provides a general means of interpreting outcrop patterns and determining the orientation of planar geologic features as they intersect topography. Page 3 of 7 GEOL 4334 Lab 03: Outcrop patterns v. 2015 Outcrop patterns and folds Folded, planar beds can make an infinite number of interesting and variably complex patterns. Figure 5 depicts two examples: On the left a horizontal, upright fold, on the right a plunging, upright fold. A B X’ X slice through fold D horizontal (map view with no topography) horizontal slice through fold (map view with no topography) trace of axial plane X X’ X’ X N N 0 500 ft. C map view (with topography) map view (with topography) 800 E 0 0 60 00 4 0 60 80 40 0 0 20 0 20 Figure 5. Two views of antiforms (“A”-shaped folds). A) Cross section X-X’ depicts the profile shape of the fold. B) horizontal slice through a horizontal, upright antiform. at the elevation of soccer man (i.e., a map view with no topography). Note how the map pattern of the limbs of the fold are parallel in the horizontal slice. C) Map depicting the outcrop pattern of the same fold as it intersects an uneven topographic surface. Can you visualize and draw a profile of this fold? D) Map view of a plunging, upright antiform. E) Map depicting the outcrop pattern of the same fold as in D) as it intersects an uneven topographic surface. Visualize the profile view of the fold. NOTES: Page 4 of 7 GEOL 4334 Lab 03: Outcrop patterns v. 2015 Predicting Outcrop Patterns If you know the orientation of a unit at one locality on a topo map, you can predict where the unit will crop out within the map area, assuming the unit is planar and continuous. Figure 6A depicts a topo map with some structural data (strike/dip symbols) noted at three localities. At station 5 (St. 5; elevation 240’) the attitude of a planar bed is 090°/20°. Complete the outcrop pattern of this bed across the map area. Step 1: draw a profile plane with a vertical scale that includes the lowest to highest elevations in the map area (Fig. 6B). Project the point at St. 5 from the map onto the profile plane at 240’ (black arrow). Step 2: draw a line through this point with an inclination of 20° toward the south. You have now drawn the cross section of the planar bed (Fig. 6B). A) ’ 240 ’ 250 0’ 3 2 20° N 0’ ’ 210 ’ 260’ 240’ 220’ 100’ 0 N drainage contact 24 St. 5 220 20° Step 3: At each elevation on the profile, project a line across the topo map. Place a point where the line of a given elevation crosses the equivalent topographic elevation. For example, in Fig. 6B note how the open circle at profile elevation of 210’ is projected onto the topo map and two open circles are drawn where the line crosses the 210’ topographic contour interval. B) ’ N 23 0’ 24 0’ 220 ’ 210 20° ’ St. 5 20° δ=20° 240 ’ 250 Step 4: Once all points have been drawn on the map, sketch the outcrop pattern by “free-handing” a line that connects the points. DO NOT USE A RULER when connecting points of topography. Figure 6. A) incomplete geologic map displaying bedding symbols and field station, St. 5. B) Method to complete outcrop trace of planar, inclined contact. 100’ S 0 fold line NOTES: Page 5 of 7 GEOL 4334 Lab 03: Outcrop patterns Common Map symbols v. 2015 Wells strike/dip of foliation, trend/plunge of lineation in foliation plane Strike/dip of bedding, open circle=upright based on geopetal structure oil well gas well strike/dip of joint/fracture oil and gas well strike/dip of cleavage strike/dip of overturned bedding dry hole strike/dip of igneous modal layer strike/dip of dike or schileren strike/dip of magmatic foliation, trend and plunge of lineation in foliation plane horizontal bedding vertical bedding Contacts Contacts - solid = exposed & well located, dashed where approximate, dotted where inferred; U = relative displacement “up”, D = relative displacement “down” unconformity; circles on younger rocks contacts U D thrust/reverse fault; “teeth” on hanging wall, tick = dip of fault where measured, arrow = plunge and trend of striations on fault surface. Dash/dotted as above. Note that dip value (40°) is greater than plunge of striae (30°). U normal fault; ticks on hanging wall, tick = dip of fault where measured, arrow = plunge and trend of striations on fault surface. Dash/dotted as above. D alternate symbols for low-angle (< 30°) normal fault, called a detachment fault D U strike-slip fault trace of fold axis Folds plunge direction and amount upright anticline small dome overturned anticline, axial surface dips to N trace of fold axis syncline overturned syncline, axial surface dips to S 72 small basin N trend and plunge of asymmetric “z” (left) and “s” (right) fold axes plunge direction and amount Antiformal syncline, dip of axial plane 72 trend and plunge of small-scale fold axis Synformal anticline, dip of axial plane Common structure symbols used on geologic maps. Be able to visualize the structures that these symbols depict. Page 6 of 7 100 m 200 m 300 m 200 m 300 m m 100 m light grey lines = topographic contours in meters above sea level cut along border of polygon after creasing the paper along the fold lines. 30° fold lines m 300 300 m 300 m 200 m 100 m Page 7 of 7 200 GEOL 4334 Lab 03: Outcrop patterns v. 2015 100 m Cut-out PRIOR lab session to help visualize outcrop patterns. 200 m