8 Structural Geology Lab 2: Outcrop Patterns and Structure Contours I. Geologic Map Symbols A. Refer to Appendix F in back of lab manual for list of commonly used geologic map symbols 1. Emphasis: a. strike and dip of bedding (1) inclined (2) horizontal (3) vertical (4) overturned b. Fold axes (1) syncline (2) antincline (3) plunging folds c. strike and dip of joints d. strike and dip of foliation e. fault symbols (1) thrust (2) vertical II. Outcrop, structure and age relationships A. Folding of rocks 1. Inclined sequence of sedimentary beds a. layer cake relations (1) oldest on bottom, youngest on top b. Fold Types (1) Anticlines-upfolded forms, results in older rocks becoming enclosed within younger strata (2) synclines-downfolded forms, results in younger rocks becoming enclosed within older strata. (3) symmetrical folds - both limbs of the fold dipping at same angle away from fold axis (4) asymmetrical folds - both limbs of the fold not dipping at same angle away from fold axis (5) overturned folds - condition in which one limb of fold has been tilted beyond vertical (6) plunging folds- axis of fold is tilted (7) Domes- more or less circular equivalent of anticline, oldest rocks exposed in center of dome9 (8) Structural Basin- more or less circular equivalent of syncline, youngest rocks exposed in center of dome (not to be confused with depositional basin) c. Outcrops Patterns Associated with Folded Rocks (1) As rocks are folded, and subsequently subjected to erosion, regular patterns become evident in relation to type of rock that outcrops and age of the rock that outcrops in an area of folded strata. In essence, erosion exposes the interiors of the folds (2) Non-plunging Folds- axis of fold is horizontal, results in parallel bands of dipping strata about the fold axis (a) anticlines- oldest strata exposed along fold axis (b) synclines- youngest strata exposed along fold axis (3) Plunging Folds-axis of fold is tilted, results in alternating V-shaped bands of dipping strata oriented about the fold axis. (a) anticlines- oldest strata exposed in the center of the V, V points in direction of plunge of fold axis (b) syncline- youngest strata exposed in the center of the V, V points in opposite direction of plunge of fold axis. (4) Doubly Plunging Folds- fold axis is plunging in two opposite directions, results in a flattened oval pattern, or a double V-shaped pattern <<<>>>>. (a) anticlines- oldest strata exposed in center of flattened oval (b) synclines-youngest strata exposed in center of flattened oval. III. Using outcrop pattern to determine structural attitude A. Comparison of Outcrop Pattern with Topographic Contours 1. The Law of "V's" a. Contacts of horizontal beds appear parallel to contour lines (1) Contacts V upstream, when crossing valleys b. Contacts of vertical beds are not deflected at all when crossing valleys and ridges c. Inclined planes V updip when crossing ridges10 d. Planes that dip upstream, V upstream e. Planes that dip downstream (1) At angles the same as stream gradient: (a) bed contacts appear to parallel stream bed (2) At angles gentler than stream gradient, V upstream (3) At angles greater than stream gradient, V downstream Refer to Figures 2.2-2.7 in lab manual for visual summary B. Using geologic contacts and topographic contours to determine strike and dip of beds 1. Identify a single geologic contact between beds in stratigraphic sequence 2. Find a single contour line that crosses the geologic contact at two points a. A line connecting the points of equal elevation along the geologic contact defines STRIKE 3. To Determine Dip a. Take same geologic contact, and identify another (different) topographic contour line that crosses at two points, draw a line between them (1) You now have two strike lines on the same geologic bed (2) draw a line perpendicular to the strike lines and this will define DIP (a) remember that dip direction is in the direction of bed slope, or elevation decrease (3) Solve for dip using the following equation (a) Tan (Dip) = vertical /horizontal i) Inv Tan = angle of dip from horizontal IV. Structure Contouring A. Structure contour lines are similar to topographic contour lines 1. structure contours = lines connecting points of equal elevation on the surface of a bed of rock a. Structure contour lines = "continuous strike lines" 2. Data a. structure contour data is commonly derived from drilling of bore holes (1) The elevation of the top (or bottom) of a given unit is determined from drilling:11 b. Elevation of top of bed = (surface el. of boring - depth to rock contact) c. Data is plotted on a map base and contoured using the rules of drawing contour lines (1) This can also be accomplished using Golden Software Surfer program 3. Structure Contour Interpretation a. V-shape pattern with high elevations in center = plunging anticline b. V-shape pattern with low elevations in center = plunging syncline c. Bullseye patterns with high in middle = dome d. bullseye patterns with low in middle = basin e. Fault Interpretation (1) Abrupt gaps or terminations in contour pattern = normal fault (2) Overlaps in structure contour pattern = thrust or revers faults 4. Contour drawing techniques a. Inverse distance method (1) assume constant gradient between two data points and interpolate elevations b. Data interpolation (1) projecting from known control points to unknown areas c. Contour intervals: follow rules same as topographic contours V. Three-point Problems A. Method of determining strike and dip of bed 1. If the elevations of three points on a given planar surface are known, then the strike and dip of the plane can be determined B. Structure Contour/Graphical Approach 1. Determine elevation of three points known to lie on the surface of a plane (e.g. bed of rock, fault, etc.) a. Identify the high, middle, and low elevation points b. draw a line between the high and low elevation points ("line x-y")12 c. Assuming a constant grade between the two points, the middle elevation must also occur somewhere on line x-y. d. Determine total elevation difference between high and low points e. Determine total map distance between high and low point along line x-y (using engineers scale) f. Calculate a vertical-distance conversion factor: (1) map distance (inches) Vertical difference (ft