EPS 50 Lab 6: Maps Topography, geologic structures and relative age determinations

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1 Name: EPS 50 Lab 6: Maps Topography, geologic structures and relative age determinations Introduction: Maps are some of the most interesting and informative printed documents available. We are familiar with simple maps that indicate places and the roads you are going to take from one place to another, but maps can reveal much more information. In this lab we will learn to interpret two very important types of maps used in geology and related disciplines: topographic maps and geologic maps. We will also learn to interpret geologic structures, and learn how they can be inferred from geologic maps. Objective: This laboratory will introduce you to topographic and geologic maps and also cover some geologic structures associated with folding and faulting. You will practice making relative age determinations of rocks and structures, and you will construct a geologic cross-section based on information you obtain from maps. Answers: All answers should be your own, but we encourage you to discuss and check your answers with other students. This lab will be graded out of 100 points. Part 1: Topographic maps (21 points) Topographic maps provide information about Earth s surface. The main feature of topographic maps is topography: ridges, valleys, mountains, plains and other Earth surface features. Maps usually contain a lot of information (i.e. roads, highways, buildings, water bodies, railroads, etc.), but for this exercise we will focus only on topography, (i.e. elevation data). Changes in elevation are depicted on topographic maps using contour lines. Contour lines are lines representing equal elevation across the landscape. Contours are drawn at regular intervals of elevation; these intervals vary between maps, but are typically specified at the bottom of the map or in the map legend. In addition, topographic maps provide a wealth of information regarding both natural and manmade features (see the topographic map symbol key). Map scale is another critical piece of information on all maps. 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 a 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. The scale, therefore, indicates the level of detail for the features on the map. The key feature of topographic maps is the contour line. Contour lines connect points with equal elevation. They serve 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. For most scientific maps, the units of the contour

2 lines, the distances and the elevation are all in metric units: meters and kilometers. However, older American maps frequently use units of feet and miles. In order to help the user determine the elevation at certain locations on a map, the index contours (usually every fourth or fifth contour line) are often marked by thicker lines with elevations labeled. Contour maps relay extensive information. Contours that are very close to each other represent steep slopes. Conversely, widely spaced contours (or an absence of contours) represent shallow slopes. The slope direction is always perpendicular to a contour line. Alternating convergent and divergent contour lines reflect a ridge and valley topography. A ridge line is an ideal line parallel to divergent contours passing through the point of maximum divergence (where the curvature of the contour is greatest). Similarly, a valley axis passes along contours of maximum convergence. Streams tend to follow the valley axes and contour lines intersecting them have a V (or U) shape with the bottom of the V (or U) pointing uphill. Closed contours without other contour lines inside are either high points (e.g., a peak) or low points (e.g., a depression). A saddle is an area bound by higher contours in two opposite directions and lower contours in the other two opposite directions. A watershed (or catchment) is the extent of land which can drain water to a common pour point. It can be identified on a topographic map by tracing two uphill lines in two directions running perpendicular to the contours until a ridge is found. The ridge line is then followed (possibly through peaks and saddles) until the other line perpendicular to the contours is met.

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4 On the previous page is a topographic map of the East Bay. The contour intervals (black) are 10 m, and the streams (light blue) are automatically generated. The Wildcat Creek (dark blue) runs from the Berkeley Hills northwestward into Richmond. 1) What kind of structural feature determines the linearity and flow path of Wildcat Creek? (1 pt) 2) Outline on the map or sketch below the alluvial fan that Wildcat Creek has built. (1 pt) We will now make some measurements from this map. In particular, we are interested in determining the slope of Wildcat Creek near its source, in an intermediate reach and near the mouth. The slope between two points is defined by the difference in elevation divided by the distance (rise over run). Elevation difference can be obtained by counting the contours between two points (multiplied by the contour interval), while distance can be measured using a ruler. As the units on your ruler are not the same as on the map, determine the appropriate conversion by measuring the scale bar at the bottom of each map. For example, if 1000 m on the map scale is 5 cm on your ruler, you would know that 1 cm on your ruler is 200 m on the map. Note that since we will work on some maps with detail views of the East Bay area in different scales, the scale bars will not be the same on different maps. There is no path of Wildcat Creek in the maps, so the first step of each exercise will be recognizing Wildcat Creek from the contour lines. When tracing the path of the creek between the indicated points, remember that contours intersect streams forming a V or a U with the apex of the V pointing upstream. Below is a detail showing the source area of Wildcat Creek.

5 3) Trace the path of the creek between point A and point A. What is the distance (in map units) between A and A? The elevation change? The slope? Show your work. (3 pts) Below is a detail showing an intermediate reach of Wildcat Creek.

6 4) Trace the path of the creek between point B and point B. What is the distance (in map units) between B and B? The elevation change? The slope? Show your work. (3 pts) Below is a detail showing the mouth of Wildcat Creek. As no information is available between the contour lines between C and C, refer to the large map to estimate the path of the creek.

7 5) Trace the path of the creek between point C and point C. What is the distance (in map units) between C and C? The elevation change? The slope? Show your work. (3 pts) 6) Using the measurements you just made, estimate the type of particles (e.g. boulders, cobbles, pebbles, sand, silt) you may find on the bed of Wildcat Creek in reaches A, B and C. In what depositional environment would you expect to find conglomerate deposits? (3 pts)

8 The burst of light detection and ranging (LiDAR) technology gives the ability of generating high-resolution data to topographic maps. LiDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The map shown below is a detail of the Lone Tree watershed in Marin Co., California. The LiDAR data are in sub-meter spacing. Notice how details of the terrain are captured. Below is another detail of the same area. Notice the streams and ridges.

9 7) Outline the watershed above point B. In other words determine the region from which water could flow to point B. You will need to trace from B outward, and then follow the ridge all the way around. This region is the contributing area of point B. For extra credit, estimate the contributing area in square meters. (4 pts) The figure below shows a detail of the landslide scar. Notice the kinks in the contours above and below the landslide. At these kinks the contours become closer to each other, indicating an

10 increase in slope. Use this information to visualize the outer perimeter of the landslide and think about how deep it might be. The 5 contours closest to each other at the top of the scar (on the right in the map) could give you an indication of its maximum depth. In contrast, the opposite end is much shallower. 8) Estimate the volume of this landslide in cubic meters. Use an approximate perimeter and an average depth. Do not worry about being exact, but show your work. You can quickly obtain a ball-park figure by assuming a rectangular shape and vertical walls. (3 pts) Part 2: Geologic maps (13 points) The geologic map of Marble Canyon, Arizona is on your table. The purpose of a geologic map is

11 to show surface geologic features (e.g., rock types, ages) in terms of different colors and symbols. All geologic maps have several common features: [1] colored areas and letter symbols to represent the rock units on the surface in any given area, [2] lines to show the types and locations of contacts and faults and [3] strike and dip symbols to show the orientations of the formations. Rock units or geologic strata are normally shown by color or symbols to indicate where they are exposed at the surface. Because the geology of different regions can vary considerably, each map will have its own color labels. The creation of any geologic map often demands skilled interpretation by the authors as to the location of contacts between lithologies, the occurrence and character of faults and folds and identification of different rock units. Different colors on this map represent different geologic units. A geologic unit is a volume of a certain kind of rock in a given age range. For example, a sandstone in one age might be colored bright orange, while a sandstone of a different age might be colored pale brown. In addition to color, each geologic unit is assigned a set of letters to symbolize it on the map. Usually the symbol is the combination of an initial capital letter followed by one or more small letters. The capital letter represents the age of the geologic unit. 9) What is the scale and total area of this map (in meters squared)? (1 pt) 10) What major river occurs on this map? (1 pt) 11) What type of rock dominates this map? (2 pts) 12) What are the oldest exposed rock types in this map area, and what age are they? Where are they exposed on this map and what famous geologic structure are they from? (4 pts) 13) You want to study events from the Jurassic-Cretaceous boundary. Is this a good area to focus on your field work? Why or why not? (2 pts)

12 14) Faults are shown by heavy black lines. Note the location and general orientation of faults on this map. (2 pts) 15) Valuable ore deposits of gold, copper and silver are commonly associated with contacts between metamorphic and igneous rocks. Is Marble Canyon a good prospecting location? (1 pt) Part 3: Geologic structure (30 points) Geology exists in three dimensions, but when we display it on a map, we see only two dimensions. To convey accurately the nature of geologic objects, we must use a shorthand notation to specify orientation in three dimensions. For a planar feature, its three-dimensional orientation is described by its strike and dip. Strike is the line formed by the intersection of a horizontal plane with the plane of the feature. The strike of a rock layer fixes its orientation with respect to compass direction. The angle the layer dips into the ground is its dip. It is always perpendicular to strike. To visualize strike and dip, consider a tilted board placed in a tub of water. The board s strike corresponds to the line traced on the board by the surface of the water. The dip is the direction water poured on the board would flow down. The strike-and-dip symbol is T-shaped. The long line is parallel to the strike, and the tick mark indicates the direction of dip. These two lines (and strike and dip) are always at right angles to each other. A number next to the strike-and-dip symbol indicates the number of degrees the bed dips from the horizontal. The quantitative value of the strike is not shown explicitly with the strike-and-dip symbol. It is obtained by measuring the angle between the strike line of the

13 symbol and a line pointing north. Strike is expressed in terms of quadrant (e.g. S45ºE) or as an azimuth (e.g. 135º). In addition to the general T-shaped strike-and-dip symbol, three unique orientations of rock layers require special strike-and-dip symbols (see below). 16) Label the arrows below as either strike or dip. (1 pt) 17) Each of the six map views below show the outcrop pattern of a different rock layer. On each unit, draw the strike-and-dip symbol (you can choose either dip direction). Measure the strike of each layer and report it in azimuthal form (e.g. 122 ) in the provided blank space above each map. (6 pts) Strike: Strike: Strike:

14 (a) (b) (c) Strike: Strike: Strike: (d) (e) (f) 18) Below are five block diagrams illustrating different orientations of a planar rock unit. On the top of each block, draw the proper strike-and-dip symbol. Estimate the dip of the layer for (b), (d) and (e) and write it next to the dip symbol. (5 pts) (a) (b) (c)

15 (d) (e) 19) The following block diagrams will help you visualize in three dimensions. Each block provides incomplete information about some rock layer as it would show on cubes cut from the Earth s crust. The top of each block is a horizontal surface. The other two visible sides are crosssections formed when the cube was cut from the crust. Complete the subsurface view on the blank face(s) of each block. For (d) consider that the layers are dipping toward north with an angle of 45. (6 pts) N (a) (b) (c) (d) (e) (f) On flat surfaces, rock layers have fairly simple outcrop patterns. On a map, horizontal layers underlying flat topography will produce a map pattern showing only one pattern. However, a distinct pattern develops when beds cross stream valleys or gullies. If a valley cuts the same sequence, more layers will appear on the map, and those layers will define a particular pattern: a V shape. Interpreting these patterns leads to the rule of V s. From it, you can determine the dip direction of rock layers and something about the subsurface nature of an area.

16 20) Complete these 4 rules of V s: (6 pts) a. If a bed is vertical, contacts will (check one): point upstream point downstream run straight across a valley b. The contacts of horizontal beds will point (check one from each group): upstream downstream and parallel cross contour lines c. If a bed dips upstream, its contacts will point (check one from each group): upstream downstream and parallel cross contour lines d. Beds that dip downstream will have contacts that point (check one): upstream downstream 21) Below are two maps of rock units crossing a stream valley. Black bands on opposite sides of the maps identify the outcrops of the unit, and the numbers indicate the elevations of the contour lines. Complete the outcrop pattern for a rock body with a horizontal dip (A) and a vertical dip (B). (2 pts)

17 (A) Bed follows contour interval (B) Bed only visible downstream In general, most rock layers will have dips that are between horizontal and vertical. When a valley cuts such beds, the map outcrop pattern will be deflected along the valley. The direction of the deflection is determined by the bed s dip relative to the slope of the valley. The diagram below shows two dipping units across a small stream valley in block diagrams (left) and map views (right). Both types of diagrams should illustrate how contour lines deflect in the valley, but the outcrop pattern in the map view is only shown up to the valley rim. 22) Complete the outcrop pattern on each map. (4 pts) A B Part 4: Geologic cross-sections (36 points)

18 Geologic cross-sections are useful for visualizing spatial and temporal relationships of geologic units that occur in an area. Steep river canyons, sea cliffs, mountain sides and road cuts yield natural cross-sections, but most cross-sections are made by projecting surface features into the Earth or by interpreting data from seismic waves. From geologic maps, we can construct geologic cross sections (vertical planes of the subsurface) that show geologic structures beneath the surface. The top of a cross-section shows the topography of the land and below that the geometric relations of the different rock units that crop out at the surface. Geologic information such as strike and dip is used to project the units that crop out at the surface into the subsurface. Cross-sections are extremely useful for visualizing the spatial and temporal relationships of the geologic units that occur in an area. With a little bit of practice, it is a fairly straightforward procedure to create a cross-section from a map. Definitions Disconformity: A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a

19 period of erosion or non-deposition. [1] Paraconformity is a type of disconformity in which the separation is a simple bedding plane (i.e. there is no obvious buried erosional surface). [2] Blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols or beds of pebbles derived from the underlying rock. Nonconformity: A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding by metamorphism, the plane of juncture is a nonconformity. Angular unconformity: An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers. The whole sequence may be deformed and tilted by orogenic activity afterward. Below is a map of a region with generally flat topography and the corresponding legends. A blank cross-section template is shown below this map. The best orientation for your cross section is a horizontal line which cuts all the rock units exposed. This traverse line is drawn on the map with the right end marked A and the left marked A. Your cross-section (currently blank) is also labeled this way. Complete the following steps: 23) Assign a number to each unique geologic unit, starting with 1 for the youngest. (3 pts)

20 24) According to the time period (the right column of the geologic time scale to the right), write the numbers of the units that were formed during which time (i.e., from 1 to 13). Indicate each unconformity (identifiable by a lack of rocks from that time period) in the section by drawing a wavy line over a horizontal line to the right of the time scale. Label each line with a Roman numeral starting with I for the youngest. (Hint: you should find three.) On the map, mark the location of each unconformity with a wavy line and label it with its proper Roman numeral. (4 pts) 25) Using the steps described earlier in this lab, complete the cross-section in the box in the previous figure. Be sure to mark the unconformities with a wavy line. (9 pts) 26) Three more unconformities of limited time spans cannot be identified using the previous procedure. Use your cross-section to locate these additional unconformities. Identify each with a capital letter (A for youngest) at the top of the map. Mark their positions on the map with wavy lines. (3 pts) 27) In the first column of the table below, list the letters and Roman numerals of the unconformities (in chronological order). In the second column, match the letters with the type of unconformity (angular, disconformity or nonconformity). Report the geologic processes (uplift, deposition and/or erosion) that are indicated by each unconformity in the third column. (Hint: it usually requires more than one process.) (6 pts) Letter Type of unconformity Geologic event

21 28) Review the geologic map of Marble Canyon, Arizona and try to imagine the structures in three dimensions. How many unconformities are present? (1 pt) 29) The map below presents a slightly more complicated configuration in an area of different rock types, but still similar to those you've just seen in the previous example. Construct a crosssection across line A-A' and then answer the questions. (3 pts)

22 30) What is the oldest rock unit here? (1 pt) 31) What is the youngest rock unit here? (1 pt) 32) What two types of folds can you identify in the cross-section? (2 pts) 33) Label each fold on the cross-section with the corresponding fold type and label the fold axes. (1 pt) 34) What is the plunge of a fold? Assuming north is at the top of the map, in which direction do these folds plunge? (2 pts)