Exercises: Geological cross-sections

State the principle of original horizontality and explain what must have happened if today we find tilted or folded strata.

Four undisturbed horizontal strata appear in a cross-section: sandstone, mudstone, limestone and conglomerate, from bottom to top. Apply the principle of superposition to state which is the oldest and which is the youngest, and justify your answer.

In the geological cross-section shown in the diagram, five strata (A, B, C, D and E), an erosion surface (W) and a fault (X) are indicated. Order these events from oldest to youngest: formation of the five strata, erosion that caused W, and movement of fault X. State which principle you apply at each step.

A fault cuts through a series of strata and, in turn, is cut by an igneous dyke. Apply the principle of cross-cutting relationships to order these three events: formation of the strata, movement of the fault, and intrusion of the dyke.

Explain what an angular unconformity is and describe, step by step, the sequence of processes (sedimentation, deformation, erosion and new sedimentation) required to form it.

What is an index fossil? Explain what conditions a fossil must satisfy to be useful as an index fossil, and how it is used to correlate strata from different regions.

A student examines a geological cross-section showing three folded strata (1, 2, and 3), an erosion surface over them, two upper horizontal strata (4 and 5), and a fault that cuts across all strata from the base to the surface. In their report, they state:

"The fault is older than strata 4 and 5 because faults originate at great depth, and the folding occurred after stratum 5 was deposited, but only affected the lower ones because they are softer rocks."

1. a) Refute the student's two errors in a justified manner by applying the principles of superposition of strata, original horizontality, and cross-cutting relationships.
2. b) Deduce the actual geological history of the area by arranging all events in chronological order (stages of sedimentation, tectonic deformation, and erosion).

During a geological survey, a series of strata is found where a layer containing trilobite fossils is located geometrically above another layer containing ammonite fossils. An amateur concludes that trilobites outlived ammonites and evolved during the Mesozoic.

1. a) Refute the amateur's conclusion by explaining what the principle of faunal succession consists of and what role both groups play as index fossils.
2. b) Propose and justify two different tectonic hypotheses (specific geological structures) that explain how the positions of these strata could have become inverted.
3. c) What sedimentary structure or criterion would you look for in the strata to verify in the field whether the geological sequence is inverted or not?

In two stratigraphic columns separated by $50\ \text{km}$, completely different lithologies are observed: column A is composed of massive marine limestones and column B of dark shales. However, at one level in both columns the same ammonite species appears (Hildoceras bifrons).

1. a) Justify why correlating these columns based solely on rock type (lithology) would have led to an error regarding their contemporaneity.
2. b) Explain what two biological or taphonomic characteristics make Hildoceras bifrons a reliable index fossil to state that both strata are of the same age.
3. c) If a sandstone layer identical to another in column A is detected in column B, but located $20\ \text{m}$ above the Hildoceras level, can it be deduced that both sandstones formed simultaneously? Justify your answer using the principles of stratigraphic correlation.