Reading a geological cross-section

Problem statement

Question 1. Experimentation in Geology and Environmental Science (4 marks). Competency-based question. Answer only one of the following two parts, either 1.1 or 1.2.

Question 1.1. Given the following geological cross-section:

[Figure: north-to-south geological cross-section with a basement of gneiss and schist (Ordovician); above it, breccias (Permian), greywackes, quartz arenites and mudstones with $\textit{Calamites}$; on top, dolostones (Triassic) and anhydrites (Upper Triassic); then marls with $\textit{Ammonitina}$ and limestones with rudists; and, at the top, arkoses with $\textit{Dinotherium}$. A body of syenite cuts across the beds. Four contact surfaces, $a$, $b$, $c$ and $d$, and two faults, $e$ and $f$, are labelled. Source of the original figure: the examination board's own work. In the lesson it is redrawn schematically.]

a) (1 mark) Classify the materials in the section according to the type of rock they belong to. For the sedimentary materials, state which are detrital, chemical and mixed. State the type of metamorphism the materials have undergone and the metamorphic grade reached.

b) (1 mark) Arrange, on a relative time scale (from oldest to youngest), the materials that appear in the geological section.

c) (1 mark) Name the types of discontinuity represented by contact surfaces a, b, c and d. State which of them represents only an interruption in sedimentary processes, with no erosion or folding. State which of them contains the Jurassic as a hiatus.

d) (1 mark) Describe the tectonic stages that can be recognised in the sedimentary materials, stating in each case the structures that characterise them and the type of tectonic stress that produced them. Explain the time relationship (older and younger) between these tectonic processes.

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · Geology and Environmental Science · Question 1.1. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.