Geological Structures Questions and Answers

Answer: C

Correct answer: evenly spaced straight lines, if the dip is uniform

A plane surface has straight, parallel and evenly spaced structural contours, exactly as a uniform slope does on a topographic map. Three points determine that plane, so the contours can be drawn by dividing the lines between the boreholes proportionally - which is the construction used to solve the three-point problem.

Answer: D

Correct answer: 4 m per kilometre

\[ \frac{60 - 20}{10} = 4\ \mathrm{m\ per\ km}. \] So an isopachyte drawn at every 4 m of thickness would fall a kilometre apart. A steady thickening of this kind usually indicates a basin subsiding faster in one direction while deposition kept pace.

Answer: A

Correct answer: 1-Q, 2-P, 3-R

The three are distinguished by what can be seen at the contact. An angular unconformity shows discordant dips, implying tilting between the two episodes. A nonconformity puts sediment straight onto crystalline basement, implying deep erosion. And a paraconformity shows nothing at all - parallel beds, no erosion surface - and is found only from the ages.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

The outcrop pattern is a direct expression of attitude. A horizontal bed follows the contours, since it crops out at one elevation. A vertical bed cuts straight across the map, since relief cannot deflect it. And a gently dipping bed gives a broad trace that swings into pronounced Vs where it crosses valleys, the Vs pointing down dip.

Answer: C

Deposition, tilting, erosion and renewed deposition is the sequence every angular unconformity records, and the interval unrepresented by rock may be very long. Reading that history from one surface is a classic exercise.

Answer: D

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: strike is by definition perpendicular to the true dip direction, and a horizontal bed crops out along the contours. Statement 2 has it backwards - the true dip is the steepest line in the plane, so any apparent dip is necessarily less.

Answer: A

Correct answer: an angular unconformity

Primary structures form as the rock is deposited or emplaced: cross-bedding and ripple marks record the currents that laid the sand down, and columnar joints form as a lava cools and contracts. An angular unconformity requires deposition, tilting, erosion and renewed deposition, so it is a secondary structure by any definition.

Answer: B

Correct answer: the compressive strength of the formations

A map records geometry - where boundaries run, how beds are inclined, where structures cut them - and a good deal can be inferred from it about the history. Strength is a material property that must be measured on samples or in situ, and no map can supply it.

Answer: C

Correct answer: is oblique to the strike, showing an apparent dip

A quarry face is cut where it is convenient rather than perpendicular to the strike, and any oblique face shows an apparent dip less than the true one. Boreholes give the true attitude, so a face reading gentler than the boreholes is the expected result rather than a contradiction - and the two can be reconciled by the apparent dip relation.

Answer: D

Correct answer: a paraconformity

Parallel beds, no erosion surface and a 40-million-year gap in the fossil ages is exactly the definition of a paraconformity. The other three are excluded: an angular unconformity would show discordant dips, a nonconformity would have crystalline rock beneath, and a conformable contact would show no gap in the ages.