Question 6 of 9: Geologic Map – Strike, Dip and Stratigraphy

Question 6: Geologic Map – Strike, Dip and Stratigraphy (10 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. Scale bar: 500 m on the map; contact traces run close to N–S and are nearly straight while the flanking topographic contours swing widely (large "V"s in valleys/ridges); strike-and-dip tick marks on every exposed contact point consistently east.

Find. (a) strike and dip of the three beds; (b) the completed contact traces and the shale outcrop belt; (c) the depositional (oldest-to-youngest) sequence.

Approach. Use the Rule of V's (a contact's trace relative to the topographic contours it crosses reveals dip steepness and direction) together with the fact that a contact parallel/close to the local contour trend over a long run, while contours themselves swing through a large range of elevation, implies a steep dip; the tick-mark convention on the contact gives the dip direction directly.

1. Read strike from the trend of the contact traces. All four exposed contact segments run essentially north–south on the map (parallel to the north arrow), giving strike ≈ N–S (000°/180°).
2. Estimate dip magnitude from the map geometry. Measuring the upper contact's lateral "wander" against the local contour relief it spans (using the map scale bar): a lateral wander of $\approx 149$ m over a relief band of $\approx 300$ m gives $\delta = \arctan(300/149) \approx \boxed{71.6^\circ}$ – a steep dip, consistent with the contact staying nearly straight while the surrounding contours swing widely (a shallow-dipping contact would instead trace a much wider "V" through each valley/ridge).
3. Read dip direction from the tick marks. The short dip ticks attached to every exposed contact point toward the east, so the beds dip to the east, i.e. approximately 072°, dipping ~72° E (strike N–S, dip direction 090°).

(b) Completing the traces and shading the shale. With strike and dip fixed, each partial contact is extended across gaps in the outcrop map by projecting it along the N–S strike trend, honouring the Rule of V's through every valley and ridge it crosses at the ~72° dip (a steep dip produces only a small V, nearly following the strike line rather than ballooning far up-valley). Two rock types are directly labelled in the given segments (conglomerate and sandstone); the shale – the third named bed – must therefore occupy the belt(s) of ground between/adjacent to the mapped conglomerate and sandstone contacts that carry no legend pattern in the source figure. Shade that in-between belt as shale once the bounding contacts are extended per the strike found in step 1.

(c) Depositional sequence. For a homoclinal (uniformly dipping), non-overturned sequence, walking in the down-dip direction along the ground surface takes you up-section (into progressively younger beds), because each higher bed's outcrop belt is offset down-dip from the one beneath it. Since the beds dip east and the conglomerate lies on the up-dip (west) side of the mapped belts with shale on the down-dip (east) side, the deposition sequence from oldest (base) to youngest (top) is conglomerate → sandstone → shale – a fining-upward sequence, the classic signature of a marine transgression (progressively deeper, quieter water as the shoreline retreats landward, from coarse nearshore gravel through sandy shoreface to offshore mud).