Lab 2_ Maps and Structures F22 (Emma Born)

84 points total Name: Emma Born Lab Section: EPS 50: Fall 2022 LAB 2: MAPS AND STRUCTURES Due one week from today at the start of your lab section Introduction We often use simple maps to help us navigate roads and cities, but geologists are particularly interested in geologic maps and topographic maps. Geologic maps show the spatial distribution and orientation of different rock layers on Earth surface, while topographic maps use elevation contour lines to show the shape of the Earth's surface. Topographic maps are an important tool because they represent the 3-D landscape in only two dimensions, and detail the location of land features such as peaks, valleys , and ridges . Geologic maps help us infer the features and structures in Earth’s subsurface such as rock type, bedding planes, and faults and folds . Both geologic and topographic maps are not only important for interpreting the geologic history of an area, but also for resource geology, earthquake modeling, hazard mapping, identifying aquifers and groundwater networks, and even city planning. 1

1. Topographic Maps The main feature of topographic maps is topography : ridges, valleys, mountains, plains and other Earth surface features, i.e. elevation data. Changes in elevation are depicted on topographic maps using contour lines. Contour lines are lines representing equal elevation across a landscape, and thus connect points of equal elevation. They act as imaginary boundaries separating areas above a given elevation from areas below it to show the general shape of the terrain. The elevation difference between two contour lines is called the contour interval , which is typically specified at the bottom of the map or in the map legend. Contours that are closer to each other represent steeper slopes. Conversely, more widely spaced contours (or an absence of contours) represent shallower slopes. The slope direction is always perpendicular to a contour line. Map scale is a number on a map that relates the distances on the map to those in the real world. For instance, a map with 1 to 12,000 scale (1:12000), tells the user that one unit of distance on the map represents 12,000 units in the real world. Closed contours (circle) without other contour lines inside indicate topographic highs (e.g. mountain peaks, hilltops). A closed contour line with hash marks inside it indicates a topographic low (e.g. craters, lakes). peak depression Tips: ● The closer the contour lines, the steeper the slope. ● Contour lines never merge or cross, except at one feature: cliffs. 2

Question 1 (3 pts) Below is a detail showing the source area of Wildcat Creek. What is the slope (in % grade) from A to A’? Show your work. Answer 1 Elevation of point A: 460 Elevation of point A’: 370 Change in elevation between A and A’: 90 Delta x: 750 Slope = 90/750 * 100 = 12% 4

Question 2 (3 pts) Below is a detail showing an intermediate reach of Wildcat Creek. What is the slope (in % grade) from B to B’? Show your work. Answer 2 Elevation of Point B: 110 Elevation of Point B’: 40 Delta x: 2,200 Slope = 70/2200 *100 = 3.2% grade 5

Slope = 40/5250 * 100= 0.7% Question 4 (2 pts) Given your previous calculations of slope along Wildcat Creek, would the sediment grains deposited at C-C’ be relatively finer or coarser than those deposited at A-A’? Answer 4 Because the slope grade at A-A’ is larger, there will be coarser grains because there is a higher energy in the A-A’ system due to gravity. When the system starts to flatten, as in C-C’, we find coarser grains because there is less energy in that system to deposit larger grains Question 5 (10 pts) 7

The map below shows a stream and several points of elevation (ft) measured across a land surface. A) On the map below, draw topographic contour lines from 100’ to 800’ elevation, with a contour interval of 100 ft. (5 pts) B) Is the stream flowing in a topographic high or topographic low? (2 pts) C) Use your contour lines to sketch the topographic profile (change in elevation) across the transect Z to Z’. (3 pts) Answer 5 B) C) Z Z’ 900 ft _______________________________________________________________________________ 800 _______________________________________________________________________________ 700 _______________________________________________________________________________ 600 _______________________________________________________________________________ 500 _______________________________________________________________________________ 8

Question 7 (4 pts) Examine the common strike and dip symbols below. Assign the letter of each symbol to the appropriate rock block diagram. Strike and Dip Symbols a) strike and dip b) vertical strata c) overturned strata d) horizontal strata Answer 7 b d a c Strike is most commonly expressed as an azimuth , or degrees away from geographic north on a compass. Strike and dip measurements are often written as strike/dip : for example, 315°/60°. These measurements are written in accordance with the right hand rule (RHR), which determines the azimuthal value that will be recorded for strike. Using the RHR, one gives the strike direction such that the dipping bed is to the right when looking in the direction of the strike azimuth. Intuitively, 10

the RHR works only when using one’s right hand. Holding your right hand flat with your fingers pointed straight and thumb out to the side (so your thumb and fingers are ~90° apart): ● point your fingers in the direction of dip ● your thumb points in the direction of the appropriate strike azimuth Question 8 (1 pt) According to the RHR, which is the correct cardinal direction for the strike of these tilted beds? Answer 8 The correct cardinal direction for the strike of the tilted beds, according to the RHR, is West which corresponds to 370 degrees Question 9 (6 pts) Each of the map views below shows the outcrop pattern of a different rock bed. Using the map views, determine the strike of each rock bed using a protractor or by comparing to the compass on the right. Measure the dip of each bed with a protractor on each cross-section view. Write the strike and dip of each bed in the appropriate RHR format. 11

When rock strata intersect Earth’s surface, their apparent thickness will change depending on the local topography and the dip of the strata. Since we cannot directly measure true thickness (unless the bed dips 90°), we must use other measurements of exposed rocks in the field to acquire the true thickness of rock beds. The relationship between true (t) and apparent (a) thickness is expressed below, where (θ) is the dip angle: ? 𝑎 = ?𝑖?θ Question 10 (3 pts) Using the true thickness equation given above, calculate the true thickness t (meters) of the purple rock bed below, where a is apparent thickness (meters) and theta is the dip angle (degrees). Show your work. Answer 10 A = 130m Theta = 20 ° T = asin(theta) T = 130*sin(20°) = 118m 13

2.2 Faults Sometimes, when rocks are pulled or squeezed, they stretch and fold. Other times, when they are deformed too quickly or too much, they crack, or fracture . If there is differential movement on either side of this fracture surface, this is known as a fault . There are three main types of faults ( normal, reverse, strike-slip ), each typically associated with a certain type of deformation: Question 11 (5 pts) A) On block diagrams 1 and 2 below, draw arrows on the fault plane indicating the relative motion of each fault block. ← B) Identify the type of faults in diagrams 1 and 2. If strike-slip, also indicate whether dextral or sinistral. 14

Question 13 (1 pt) Which rock type (igneous, metamorphic, sedimentary) dominates this mapped area of Arizona? Answer 13 Sedimentary. Because the mapped area is a canyon, there are deposits of sediment from natural erosion. *BONUS* Question 14 (4 pts) A) What is the name and symbol of the oldest geologic unit on this map? B) From what geologic time period is this unit? C) This unit forms the basement rock of which famous geologic feature? Answer 14 A) X v or Vishnu Schist B) Precambrian C) Grand Canyon Types of Geologic Unconformities Unconformities are erosional surfaces that represent a large gap in time in an otherwise relatively continuous rock record. Unconformities are usually caused by a period of erosion, or sometimes pause in deposition, followed by deposition of flat overlying rock layers. The rocks that got eroded away are now “missing time” in the rock record. We call this time gap an unconformity, of which there are three main types depending on the rock type and angle of rock beds above and below the unconformity: 16

A disconformity represents a period of erosion or nondeposition between parallel beds of sedimentary rocks. An angular unconformity arises when horizontal rock strata are deposited on previously tilted and eroded sedimentary rock beds, such that they meet at an angle. A nonconformity exists where sedimentary rock beds are deposited on pre-existing and eroded metamorphic or igneous rocks. Question 15 (4 pts) Using the Correlation of Map Units on the Marble Canyon map, and the unconformity definition above, to determine the type of unconformity (disconformity or nonconformity) present between each set of rock units. Answer 15 Yc / Yn: disconformity because Yc and Yn are both sedimentary (sandstone) Ybh / Xv: nonconformity because Y bh is limestone (sedimentary) and X v is metamorphic (schist) 4. Geologic Cross Sections We can construct these cross sections , vertical planes that show geologic structures beneath the surface, by using information from geologic maps. Geologic information such as rock contacts and strike and dip are used to project the geometry of these exposed units into the subsurface. We can more easily visualize geologic structures like folds with geologic cross sections, demonstrated below in Question 16. Geologic cross sections are more generally useful for visualizing spatial and temporal relationships between geologic units and features of a given area. Usually, the steps for making a cross section include: 1. Choose a line of section (e.g. A-A’) that is roughly parallel to the dips of the beds and geologic features of the area. 2. Construct a topographic profile along that line of section using information from the contour lines. 17

Question 17 (1 pt) What type of unconformity exists in your cross section? Draw this contact as a squiggly line in your cross section. Answer 17 Disconformity between limestone and sandstone. Nonconformity between shale and limestone Question 18 (1 pt) What is the youngest rock formation in this map area? Answer 18 The youngest rock formation is basalt, the only igneous rock in the cross section. Clearly, after 19

the deposition of the sediments that formed sandstone and limestone the extrusive igneous rock, basalt was formed Question 19 (1 pt) Look at Steno’s Laws of Stratigraphy below. Which stratigraphic principle helps verify your answer to Question 18? Answer 19 Steno’s Law’s of Stratigraphy say that the layer closest to the surface is the youngest Types of Folds Folding can happen when rocks are squeezed or compressed. In response to this compressional stress, the rocks will “shorten” and form wave-like structures called folds . The limbs of the fold are the rock layers on either side of a fold’s axial plane. Notice how the anticline fold and syncline fold share a limb. Synclines have limbs pointing into the fold axis, while anticlines have limbs that point away from the fold axis. You can see in the diagram below that the rocks in the middle of a syncline are younger, and the rocks in the middle of an anticline are older. 20