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326-97 Lab Final S.D Geol 326-97 Name: KEY 5/6/97 Class Ave = 101 / 150 Geol 326-97 Lab Final s.d. = 24 This lab final exam is worth 150 points of your total grade. Each lettered question is worth 15 points. Read through it all first to find out what you need to do. List all of your answers on these pages and attach any constructions, tracing paper overlays, etc. Put your name on all pages. 1. One way to analyze brittle faults is to calculate and plot the infinitesimal shortening and extension directions on a lower hemisphere, stereographic projection. These principal axes lie in the “movement plane”, which contains the pole to the fault plane and the slickensides, and are at 45° to the pole. The following questions apply to a single fault described in part (a), below: (a) A fault has a strike and dip of 250, 57 N and the slickensides have a rake of 63°, measured from the given strike azimuth. Plot the orientations of the fault plane and the slickensides on an equal area projection. (b) Bedding in the vicinity of the fault is oriented 37, 42 E. Assuming that the fault formed when the bedding was horizontal, determine and plot the original geometry of the fault and the slickensides. (c) Determine the original (pre-rotation)orientation of the infinitesimal shortening and extension axes for fault. 2. All of the following questions apply to the map shown on the next page. In all of the rocks with cleavage, you may assume that both cleavage and bedding strike 024°. Hint:[ This problem will be easier to do if you recall the relationships between cleavage and bedding in asymmetric folds]. (a) Determine the correct stratigraphic sequence and note where there are unconformities in the section (no need to determine thicknesses). (b) Determine the strike and dip of the fault which runs approximately north-south through the middle of the map (“fault A”). Is there any feature on the map which might allow you to determine the true slip on the fault? (c) Construct a pi-diagram for the cleaved rocks and use it to determine the general geometric style of the folds. [Note: Because several of the points overlap completely, you will have to “offset” them slightly from one another to see the pi-diagram pattern.] What is the vergence of the folds? (d) Is the boomerang shape of the outcrop of siltstone in the western part of the map due to multiple folding or topography? How can you tell? (e) Is the fault that runs approximately east-west through the southern part of the map (“fault C”) a strike slip or a dip slip fault? How can you tell? Show the sense of offset on the map using appropriate symbols. (f) Construct a cross-section along A-A'. Be sure to use arrows that show the relative sense of slip on the faults crossed by the section. (g) List the geologic events, in order from oldest to youngest, which produced the present geometries observed in the map. page 1 of 5 GEOL 326-97 Lab Final Name: ______________________________ 660 2502 2815 45 1500 1721 20 062 05 2993 3500 3426 3121 1000 38 1800 05 066 2496 18 064 04 1450 932 2000 22 Lake 1500 2000 1002 32 1500 900 21 500 49 50 2150. 22 2000 2500 1267 21 36 1504 37 19 1194 21 2000 61 900 35 20 52 53 1516 06 2602 066 61 1907 706 A 14 A' 3200 1516 52 25 450 49 2731 60 22 fault B 1091 10 40 1000 fault A 63 1502 1900 36 15 3000 1395 49 2000 50 1515 1197 fault C N 1114 65 36 15 2504 52 38 24 21 1190 1467 21 22 Key (units are not shown in stratigraphic sequence) 02 conglomerate siltstone gray slate km sandstone limestone black slate drainage 50 fault bedding dip38 cleavage dip1467 spot elevation (m) A A' 4000 2000 0 m ? note that the displacement of the limestone is larger ? –2000 than that of the folded rocks. This suggests that fault A may have moved first as a thrust (pre limestone) and later as a normal fault (post limestone) Geol 326-97 Lab Final Name: KEY ANSWER PAGE QUESTION #1 — ANSWERS Equal Area Slickensides 297, 48 bedding pole to fault Equal Area pole to fault infinitesimal axis original orientation of fault original orientation 064,86 S of slickensides 210, 83 infinitesimal axis movement plane page 3 of 5 Geol 326-97 Lab Final Name: KEY ANSWER PAGE QUESTION #2 — ANSWERS limestone (a) sandstone unconformities conglomerate gray slate siltstone black slate (b) three point problem calculated position of the 3500 elevation 3200 m along the line between 2000 and 3500 m vertical section on AA' A A' strike = 356° 3200 m 1200 m 2000 A' 2000 m A 1620 m map distance tan (dip) = 1200 m / 1620 m = 0.7407 = 1620 m 3200 dip = 36.5° map trace of fault A The truncation of the base of the sandstone beneath the limestone defines lines on either side of the fault which can be used to get the necessary piercing points. page 4 of 5 Geol 326-97 Lab Final Name: KEY ANSWER PAGE (c) Equal Area fold axis: 024, 00 axial surface The tight clustering of the points in two groups means that the folds are: asymmetric, cylindrical kink (or chevron) folds with vergence to the pole to axial surface bisects the angle between the point concentrations (because one flank is overturned) N = 26 C.I. = 2.0%/1% (d) The boomerang shape is due to topography. The π-diagram from part (c) shows a single set of perfect cylindrical folds. The might be possible with type 0 superposed folds but boomerangs are typical of type 2 superposed folds and would show up. Likewise, there is only one generation of cleavage (which is axial planar to the one generation of folds). Additionally, the elevations within the boomerang are all lower than surrounding elevations. (e) Fault C is mostly astrike-slip fault. The trick is to find piercing points on either side of the fault so that one can define the slip (offset planar markers just give you the separation). For fault C the best bet is to use the fold axis on a particular stratigraphic horizon. The axis at the siltstone/slate contact in the eastern side of the map has been offset vertically by ~100 m but horizontally by ~1500 m so the fault is primarily a strike-slip fault. Note that fault C is nearly vertical (because it has a straight trace across topography). (f) See map and cross-section page. (g) Geologic events from oldest to youngest: 1. Deposition of mudstone to conglomerate 2. Folding with ESE vergence and cleavage formation; metamorphism to slate 3. Left Lateral movement of fault C 4. Thrust or oblique slip motion on fault A 5. Uplift and erosion 6. Deposition of sandstone 7. Tilting, uplift, and erosion 8. Deposition of limestone 9. Normal movement on fault A; probably movement of fault B. page 5 of 5.
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