Question 5 of 10: Question 4, Part 1: Geologic Map – Strike, Dip, Cross-Section, Thickness

Question 4, Part 1: Geologic Map – Strike, Dip, Cross-Section, Thickness (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Figure Q1 shows topographic contours from 140 to 200 m. There is a hill (closed 200 m contour) around point P, a valley along the straight N–S line through the centre of the map, and ground rising to about 185 m at Q on the east edge. The legend lists units A, B, C, D, E and a coal seam. The question states that C, D and E are parallel to the coal. Section X–X' runs due E–W through P and Q. Scale: 200 m = 89.7 pt on the page.

Find. (a) The strike and dip of the coal seam and of every bed; (b) the X–X' cross-section; (c) the true thickness of as many units as the map allows.

Approach. First decide which contacts follow the contours (horizontal) and which cut across them (dipping). Then build structure contours (strike lines) for the dipping contacts from the points where each one crosses a contour. Their spacing gives the dip, and the offset of the same contact across the straight central line gives the fault throw.

1. (a) Units A and B are horizontal. The A/B contact is the closed oval just inside the 200 m contour on hill P. The base of B runs just inside the 180 m contour right around hill P, again beside Q and in the northern wedge. A contact that follows a contour is level, so A and B are flat-lying (dip 0°, no strike). B lies between about 180 m and 200 m, and A caps the hilltop above 200 m.
2. (a) The coal, E, D and C form a tilted series. Their contacts cut straight across the contours, and all of them are truncated wherever the ground rises above about 180 m. B therefore rests on an angular unconformity cut across the tilted beds.
3. (a) Strike and dip of the tilted series. The coal crosses 160 m at the W foot of hill P and is cut off by the unconformity (≈180 m) S of P. Its V points north up the central valley and reaches the valley floor (≈136 m) at the fault. The C/D contact crosses 160 m and 180 m in the NW, and 140, 160 and 180 m east of the fault. Structure contours through these points run N64°W, and they fall about 26 m every 100 m towards the NNE: $\delta=\arctan(0.264)\approx\boxed{15^\circ}$. The coal seam and beds C, D and E strike ≈ N64°W (296°) and dip ≈ 15° towards N26°E (NNE). Beds get younger in the down-dip direction, so the order in the tilted series, oldest first, is E, coal, D, C.
4. (a) Fault. The straight line through the valley runs from the N edge to the S edge and trends ≈ N6°W. Its trace stays straight whatever the topography does, so the fault is close to vertical. East of it, the C/D contact and the coal V both reappear displaced up-dip (south). The fitted C/D plane is ≈ 30 m lower in the east block, so the east side is downthrown about 30 m. The base of B stays at 180 m on both sides (at P and at Q), so the fault offsets only the tilted series. It is older than the unconformity.

(b) Cross-section X–X'. The line is 767 m long from W to E. The ground rises from about 134 m at X over hill P (about 205 m), drops into the fault valley (about 136 m, 530 m from X), and climbs to about 187 m at X'. On an E–W section the NNE dip shows as an apparent dip of $\arctan(0.264\cos 64^\circ)\approx 6.5^\circ$ towards the east. In the west block the coal outcrops at 160 m, about 120 m from X, and descends to about 111 m at the fault. E lies below it and D above it; C/D projects above the ground everywhere west of the fault, so C has been eroded away there. Beneath hill P, B (180–200 m) and A (above 200 m) sit flat on the bevelled D. In the downthrown east block the coal is at about 82 m at the fault and 54 m at X'. The C/D contact (about 86 m of vertical separation above the coal) meets the ground about 50 m east of the fault. C then forms the slope up to 180 m, where flat B returns beneath Q.

(c) Thicknesses. Thickness can be measured only for a unit whose top and base both crop out. D: in the west block the C/D contact lies 86.3 m vertically above the coal (the difference between the two fitted structure-contour planes). True thickness $t=\Delta z\cos\delta=86.3\cos15^\circ\approx\boxed{83\text{ m}}$; the east block gives the same value, because the contacts there fit the same parallel planes. B: it is horizontal, so its thickness is simply the height between its base and top contours: $200-180\approx\boxed{20\text{ m}}$. A: its top has been eroded, so only a minimum can be given (≥ about 5 m, up to the summit). Coal: drawn only as a thin band, so it is a few metres at most and cannot be measured at this scale. E (base not exposed) and C (top removed at the unconformity) show only one contact each, so their thicknesses cannot be determined.