N30 E-45 SE S25 E-10 SW N85 W-80 NE

Transcription

1 Geologic aps and tructures Name Geology 100 Harbor section The objectives of this lab are for you to learn the basic geologic structures in 3- 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-. heck your answers as you finish each section. You will have to look up terms as you go along. trike and ip trike and dip define the orientation of rocks in space. trike 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. ip 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 E. Assuming that the top of this piece of paper is North, give the strike and dip as shown by these symbols A B Now draw strike and dip symbols below each of the following bedding inclinations (trikedip). Both the direction (in the appropriate quadrant: NE, E, W, NW) and degree should be reported for dip. N30 E-45 E 25 E-10 W N85 W-80 NE Bright Angel Quadrangle and tructure 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.). Have you learned the time scale? emorize the names and dates. 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.) 3. In what geologic time period did the Redwall Limestone form? And the uav Fm?

2 4. Explain what occurs between these two units. Be specific. p 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). evils Fence Quadrangle, ontana Examine the geologic map of this area in the Rocky ountains of ontana. Again, note that different symbols and colors are used for different formations, including igneous rock bodies. ark 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 Is the fault older or younger than 1. 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. tudy 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. 4. After but before. Find the east-west line on the geologic map marked B-B - on which the structure section was constructed. ompare the map to the structure section and answer the following questions. 5. n the far right of the section is an andesite porphyry labeled ad. an you tell if it is older or younger than the surrounding pokane shale (ps)? 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 agebrush Park granite older or younger than the surrounding sedimentary rocks? 9. id this stock cool recently (tens of thousands) or many millions of years ago? 10. How do you know?

3 p Locate the yellow unit labeled Ttg in the south-central portion of the map. 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 utcrop 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 anyon, 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 uav Formation (contour interval = 50 ft.) 2. In what period was this rock formed? 3. escribe the orientation of the strata portrayed on this map. trata 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). The stippled unit is a planar bed with an orientation that is either dipping west, dipping east, vertical, or horizontal (flat-lying). 5. Ans. 6. an 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)?

4 p. 4 Geologic tructures - - 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 AP VIEW with north oriented to the top. N 1. Name this structure. How did you determine this? raw a simple E-W cross section through it. 2. What is this structure? Assuming that the thickness of the beds are uniform, is this fold symmetrical or asymmetrical (using you new law of bed thickness and dip)? raw a cross section. 3. What is this structure? How did you determine this? raw a cross section stream 4. Here s a tough one. The determination of structure gets more difficult with the addition of topography. From what you ve 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.

5 p. 5 omerset o., PA (ap #2) 1. tudy 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. outh of the line labeled - are two stream valleys flowing west called ove 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 m and Pp. 2. ip? Justify your answer. 3. oes 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. north 1. What type of fold is shown in the fold diagram to the left, an anticline or syncline? Which direction (north or south) does it plunge? oes 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. o plunging syncline limbs open or close in the direction of plunge? Find good illustrations folds online. For this figure (link), in which direction do the folds 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) and adorn the line with P symbols for anticline ( ) IP or syncline ( ). Lastly, put strike and dip symbols on the evonian unit on the limbs of the fold.

6 p. 6 Geologic tructures Faults 1. Look up the definitions of fault types given below. For the 2 cross sections illustrated below, draw in the blank boxes the position of the black unit following dip-slip faulting. location of future fault reverse fault normal fault 2. Here are map views of two strike-slip faults. raw in the displacement you should expect to see in a road or fenceline for each case. left lateral choose the fault right lateral ap and ross section Patterns of ip-slip Faults For situations the second two pairs of map (above) and cross sections(below), complete the cross section and give the fault type shown on the map (normal or reverse). Explain your answer as if you were walking across this map from left to right observing the geology as you go. Unfaulted Fault type? Fault type? fault fault Answers trike and ip A= 50 E-10 W B=N5 E-15 E =N90 W-85 N

7 Bright Angel Quadrangle and tructure sections 1. Vishnu chist 2. River Bottom, east end 3. ississippian, uav=ambrian 4. a GIANT unconformity, a isconformity 5. Angular unconformity p. 7 evils Fence Quadrangle, ontana 1. Younger cuts 2. concealed or cover by another unit 3. younger 4. after ississippian (320a) and before Tertiary (67 ma), that s as precise as it gets 5. Younger, it cross-cuts, ps 6. For a lava flow, the bottom margin only would be bake/chill whereas both sides would chilled for an intrusive dike 7. Before, it was folded too 8. Younger, it cuts these units 9. any millions 10. Granite cools slowly and therefore deep. Uplift and erosion must have exposed it. Tta Angular Unconformity Topography and utcrop Pattern ft. 2. ambrian 3. Nearly horizontal or flat-lying 4. utcrop width increases as the angle and direction of dip approach the hillslope angle. The true bed thickness is only shown in map view when the unit is vertical. 5. vertical; horizontal; dips West; dips East 6. Beds (or contacts) dip in the direction of v s formed in stream valleys, EXEPT horizontal beds (v follows contours), vertical beds (no v) and (rare!) those that dip downstream at a slope less than that of the stream (v in opposite direction to dip). Geologic tructures Folds 1. yncline, youngest in the middle 2. syncline, asymmetrical ( verges left or west) 3. Anticline, oldest in the middle stream wide outcrop uniform thickness narrow outcrop 4.

8 omerset o., PA (ap #2) 1. Anticline, ldest in the middle 2. ips West, ontact between units V to the west (the point of the v is to the west) 3. Yes, the beds dip away from the fold axis 4. The trace of the axial plane shows the word anticline, golly Plunging Folds 1. Plunging anticline; Plunges North, anticline limbs close in the direction of plunge p Here the syncline limbs open in the direction of plunge. Figure 7.4 in the book shows that the folds plunge toward ENE (syncline limbs opens in that direction) nline tructure: away from you into the page IP P Geologic tructures Faults location of future fault reverse fault normal fault left lateral choose the fault right lateral ap and utcrop Patterns of dip-slip faults Unfaulted Fault type = normal Fault type = reverse fault fault p Walking west to east on the map you would encounter repeated units crossing the normal fault and missing or truncated units crossing reverse fault. You cannot distinguish these faults unless you know the direction of dip.