Geologic Maps and Structures Name ______Geology 100 – Harbor Section Read Ch

Geologic Maps and Structures Name ______________________________ Geology 100 – Harbor section Read Ch. 7 before you begin. The objectives of this lab are for you to learn the basic geologic structures in 3-D and to develop some facility in interpreting the nature of geologic structures from geologic maps and geologic cross sections. A big part of this task is developing a skill for thinking in 3-D. It won’t be easy for some. I can only ask you to keep trying! Check your answers as you finish each section. Strike and Dip Strike and dip define the orientation of rocks in space. Strike is the compass orientation of the line formed by the intersection of a horizontal plane with a planar contact between two units, a fault, or a sedimentary bed. Dip is the direction and degree of inclination measured down from horizontal. 33 Geologic maps give the strike and dip where it has been measured using the symbol at the right, which indicates a strike and dip of N45°E-33°SE. Assuming that the top of this piece of paper is North, give the strike and dip as shown by these symbols A B C 85 10 15 Now draw strike and dip symbols below each of the following bedding inclinations (Strike- dip). Both the direction (in the appropriate quadrant: NE, SE, SW, NW) and degree should be reported for dip. N30°E-45°SE S25°E-10°SW N85°W-80°NE Bright Angel Quadrangle and Structure sections Examine this classic geologic area and note how the geologic map portrays the layers of sedimentary rock over igneous rock and metamorphic rock. A geologic map shows the rocks that occur at the surface (or just below the soil) and is usually printed on top of a contour map so that the topography can be related to the geology. Note the different symbols used for different formations and other geologic data (faults, strike and dip, foliation, etc.). Memorize the names and dates on this abbreviated geologic time scale. 1. Using the geologic column to the right of the map, give the name of the oldest rock unit in the map. 2. Where does this unit crop out? (hill tops, slopes, river bottom, etc.) p. 2 3. In what geologic time period did the Redwall Limestone form? And the Muav Fm? 4. Explain what occurs between these two units. Be specific. 5. What type of unconformity occurs between the below the Tapeats Formation on the right side of cross section A-A´ (A-A´ are printed along the margins of the map and the line between the two indicates the location of the cross section or structure section). Devils Fence Quadrangle, Montana Examine the geologic map of this area in the Rocky Mountains of Montana. Again, note that different symbols and colors are used for different formations, including igneous rock bodies. Dark lines are used for faults, such as the Horse Gulch Fault in the southwestern corner of the map. Remembering the principles that one uses to define relative age in cross section, use the map pattern to determine if the relative age of the fault and the blue striped unit labeled “M1.” 1. Is the fault older or younger than M1. Which cuts across the other? Follow the fault to the northwest where it becomes a dotted line. Look up the meaning of this symbol in the key. 2. What is it? 3. Now determine if the unit labeled “Tfg” is older or younger than the fault. Study the legend at the right; it defines the system in which these formations are found. Use this and the geologic time scale in your book to determine the range of time in which the fault could have occurred (The “Paleogene” and “Neogene” formerly were called the “Tertiary.”) 4. After _______________ but before _________________________. Find the east-west line on the geologic map – marked B-B´ - on which the “structure section” was constructed. Compare the map to the structure section and answer the following questions. 5. On the far right of the section is an andesite porphyry labeled “ad.” Can you tell if it is older or younger than the surrounding Spokane shale (pCs)? 6. What information would you look for in the outcrop to tell you if this was a lava flow or a sill? (drawings may help) 7. If this andesite was intruded as a sill, was this before or after the folding? Why? 8. Is the Sagebrush Park granite older or younger than the surrounding sedimentary rocks? 9. Did this stock cool recently (tens of thousands) or many millions of years ago? 10. How do you know? p. 3 11. Locate the yellow unit labeled “Ttg” in the south-central portion of the map (“T” is for the old time name “Tertiary”). Based on what you know from the cross section and from what you can see on the map, sketch a hypothetical east-west cross section showing the relationship of the unit Ttg to those surrounding it. 12. What is the name of the contact between these units? Topography and Outcrop Pattern The outcrop pattern (where the rock “crops out” at the surface) or geologic units will change depending on topography. For horizontal strata, like at the Grand Canyon, the outcrop pattern is parallel to the contour lines. Thickness of given formation can be determined by reading the contour lines. 1. Using the Bright Angel Geologic quadrangle, determine the thickness of the Muav Formation (contour interval = 50 ft.) _______________________________ 2. In what period was this rock formed? 3. Describe the orientation of the strata portrayed on this map. Strata that are dipping create the appearance of greater thickness when viewed at the surface of the Earth. To demonstrate this, mark the width of the outcrop along the hillside for the four cross sections below (These are slices through a hillside). 4. Given what you observe above, state a relationship between hillslope gradient (or steepness), outcrop width, dip, and true bed thickness. Using the red and white wooden blocks out in the lab, determine the dip (E or W) from the outcrop pattern on the following maps (dotted lines are contours). Below, the stippled unit is a planar bed (like the red one in the models) with an orientation that is either dipping west, dipping east, vertical, or horizontal (flat-lying). 5. Ans. _______________ _______________ _______________ _______________ 6. Can you state a general rule that describes the relationship of dip direction to the “v” of the outcrop pattern where strata cross as stream (there is one exception)? p. 4 Geologic Structures - - Folds Look at the block models of u-shaped bends (folds) in rock strata in the lab. The following questions will help you to visualize the three-dimensional pattern of folded and faulted rocks. For the following questions, refer to the block models and online images for help. The ages of the units in the diagrams are Paleozoic, so use your time scale to get relative age. Assume that the tops of the blocks shown below are flat unless otherwise stated. The following are MAP VIEWS with north oriented to the top. 1. Name this structure. How did you determine this? Draw a simple E-W cross section through it. O C C O S 2. What is this structure? Assuming that the thickness of C the beds are uniform, is this fold symmetrical or asymmetrical O (using you new law of bed thickness and dip)? Draw a cross S D S O section. M M D S D 3. What is this structure? How did you determine this? Draw a cross section M D S D M 4. Here’s a tough one. The determination of structure gets more difficult with the addition of topography. From what you’ve stream learned about dipping contacts and stream valleys, the “Geologic Rule of V’s, draw in the contacts on this same structure as they cross a stream. p. 5 Somerset Co., PA (Map #2) 1. Study the southern part of the geologic map from this part of the gently folded Appalachian Plateau. From the map and geologic column, determine whether this map shows an anticline or a syncline. Justify your choice. South of the line labeled C-D are two stream valleys flowing west called Cove Run and Glade Run. Use the Geologic Rule of V’s in these valleys to determine the direction of dip (generally East or West) for the contact between units Mm and Pp. 2. Dip? Justify your answer. 3. Does this agree with what you stated as to the type of structure? 4. What other evidence is printed on the map to help you with this question? Plunging Folds Find a definition of “limbs,” “axial plane”, “hinge”, and “axis” for geologic folds. Not all folds have an axis that is horizontal. When the axis dips into the Earth, it is said to “plunge.” 1. What type of fold is shown in the fold diagram to the north left, an anticline or syncline? • Which direction (north or south) does it plunge? • Does the fold “open” (increasing distance between outcrops of a single unit) or “close” in the direction of the plunge? north 2. Following the example above, draw in the block diagram at left, a syncline plunging east. Do plunging syncline limbs open or close in the direction of plunge? Find good illustrations folds online. For this Wyoming structure, note the asymmetry and direction of plunge. For geologic map below showing folded Paleozoic formations (north to the top), draw the trace of each folds axial plane, with the direction of plunge (an arrowhead, like the Wyo example) and adorn the line with symbols P IP for anticline ( ) or syncline ( ).

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