Exercises: Sedimentary rocks

Which processes make up diagenesis? Briefly describe compaction and cementation.

Order these clastic rocks from largest to smallest grain size: mudstone, conglomerate, siltstone, and sandstone. What information does grain size provide about the energy of the depositional environment?

What is clastic sediment and how does it differ from sediment of chemical or biogenic origin? Give an example of a rock of each type.

1. a) Define what a sediment is.
2. b) Name three examples of modern sediments and indicate the sedimentary environment in which each is deposited.

What are sedimentary structures and what type of information do they provide about the depositional environment? Name three examples.

What is a sedimentary basin and which mechanisms, such as thermal subsidence or tectonic loading, make its floor sink and accumulate sediments?

Ripple marks and desiccation cracks (also called mud cracks) are common sedimentary structures. Explain what each one indicates about the environment and the water level at the time of deposition.

What is a fossil? Explain which conditions, such as rapid burial, hard parts or a reducing environment, favor fossilization.

Observe the sedimentary bed in the diagram, where the grain size decreases from base to top (normal grading or graded bedding):

1. a) Explain how this structure forms during the sediment deposition process.
2. b) State what characteristic submarine process produces it.

Sandstones with cross-bedding and ripple marks appear above marls containing marine fauna remains. Interpret the change in depositional environment recorded by these layers.

Evaporites (gypsum, halite) form in restricted marine basins in arid climates. Explain the process and reason what their presence in the Spanish Mesozoic subsurface indicates about the climate of that era.

Cross-bedding forms inclined layers within a stratum. Explain how it is generated at the front of a dune or a ripple and what it allows us to deduce about the direction of the current that produced it.

Distinguish between fining-upward and coarsening-upward sequences and explain what change in the energy of the depositional environment each one records. Which process can produce reverse grading in an alluvial fan?

Tempestites are layers with an erosive base and grading deposited by storms on the marine platform. Explain how they form and how they differ from a turbidite (process and position on the platform).

A fossil coral reef forms a massive limestone body surrounded by inclined layers of fragments. Explain how a reef grows, what its fore-reef slopes are, and what the whole structure indicates about the environment.

The bearings of cross-bedding and ripples are used as paleocurrents. Explain how they are measured in the field and which paleogeographic maps (river directions, position of the coastline) they help to reconstruct.

In an intermontane basin, a continuous $800\ \text{m}$ succession is recorded showing, from base to top: massive angular proximal alluvial fan breccias and conglomerates, channelized braided river sandstones, floodplain siltstones with paleosols, and, finally, lacustrine micritic limestones with charophytes.

1. a) Reasonably interpret the morphotectonic evolution of the adjacent relief and the dynamics of the basin's base level over time.
2. b) A preliminary report states that this "fining-upward" sequence demonstrates a progressive increase in precipitation in the basin that facilitated the transport and eventual deposition of limestones. Rebut this statement by demonstrating why the variation in grain size is a response to the erosive dismantling of relief and the loss of topographic gradient.
3. c) Predict what lithological and structural changes you would observe vertically in the series if a sudden tectonic reactivation of the basin-bounding fault occurred at the end of the lacustrine period.

In two drill cores (X and Y) from the same marine sedimentary basin, the following facies and fossil associations have been recorded:

• Core X: centimeter-scale sandstone beds with normal grading (coarse to fine grain), erosive base with current marks (flute casts), rhythmically alternating with dark shales. It contains planktonic foraminifera and fossil trails of the Zoophycos ichnofacies, without benthic macrofauna.

• Core Y: well-sorted quartzitic sandstones with hummocky cross-stratification and oscillation ripples, alternating with bioclastic limestones rich in fragmented, thick-shelled bivalves, calcareous algae, and Cruziana fossil trails.

1. a) Deduce and identify the exact sedimentary environment to which each core belongs (storm-dominated shallow marine shelf vs. deep submarine slope or lobe).
2. b) Reasonably explain the physical mechanism of transport and deposition that generated the sandstone beds in each core, resolving the apparent paradox of how coarse-grained sands can reach the deepest sedimentary environment.

In a mountainous region, three lithostratigraphic units have been distinguished:

• Unit A (lower): Cretaceous marine limestones affected by tight folds with limbs dipping $60^\circ$.

• Unit B (intermediate): Paleogene continental conglomerates and sandstones resting on an erosional surface that truncates the folds of A; the beds of B dip uniformly $25^\circ$ to the south.

• Unit C (upper): perfectly horizontal Miocene lacustrine clays and gypsum resting on a flat surface that bevels the beds of B.

1. a) Reasonably classify the type of stratigraphic unconformity present between Unit A and B, and between Unit B and C.
2. b) Reconstruct the geodynamic chronology of the region, deducing the number of sedimentary cycles, the number of independent compressive tectonic phases, and the erosional periods involved.
3. c) Predict what internal geometric arrangement the beds of Unit B would have if the tectonic deformation that tilted them had acted simultaneously with their sedimentation (syntectonic) rather than subsequently.