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 are frequent 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, favour fossilisation.

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 fining-upward and coarsening-upward sequences and explain what variation in the energy of the medium 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 indicates about the environment.

The bearings of cross-bedding and ripples are used as palaeocurrents. Explain how they are measured in the field and which palaeogeographic 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 a sedimentary basin with $2,5\ \text{Ma}$ of evolution, two boreholes that reached the basement were drilled:

• Borehole A: intersects $1\,000\ \text{m}$ of homogeneous quartz sandstones (initial sediment porosity: $40\,\%$; current rock porosity: $25\,\%$).

• Borehole B: intersects $600\ \text{m}$ of compacted shales (initial mud porosity: $70\,\%$; current shale porosity: $15\,\%$).

1. a) Considering that the volume of the solid mineral fraction is conserved during diagenesis, calculate the original thickness of uncompacted sediment deposited in each borehole.
2. b) Based on the numerical result, refute the claim in a technical report stating that "at point B the sediment supply rate was $40\,\%$ lower than at A".
3. c) Determine the real average initial sediment accumulation rate (uncompacted sediment) in $\text{mm/year}$ for both points in the basin.

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

• Core X: centimetre-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.

At the margin of a marine basin, a continuous stratigraphic column is recorded showing, from bottom to top: deep slope turbidites $\to$ outer shelf marls and siltstones $\to$ delta-front coastal sandstones $\to$ delta-plain coals. At the same time, the global geological record from that epoch shows evidence of a warm interglacial period with a global sea-level rise of more than $40\ \text{m}$.

1. a) Justify whether the observed vertical sequence corresponds to a marine transgression or a sedimentary regression.
2. b) Resolve the apparent contradiction between the global eustatic sea-level rise and the shallowing-upward trend recorded in the basin, formulating the conceptual relationship between the accommodation space generated (tectonic subsidence plus eustasy) and the fluvial sediment supply rate.
3. c) Reasonably predict what would happen to the coastline and the vertical succession of facies if, while the eustatic rise continues, a river capture on the continent drastically reduced the terrigenous supply to zero.

In a continuous borehole, a sharp and planar contact between two sedimentary formations is identified:

• At the base: laminar gypsum and red clays with desiccation cracks and anhydrite nodules, typical of an evaporitic tidal flat (supratidal sabkha).

• Directly above, in stratigraphic continuity without an apparent unconformity: turbidites with rhythmic alternations of graded sandstone and pelites containing bathyal planktonic foraminifera (deep-sea floor).

A novice geologist claims that, according to Walther's Law of facies correlation, at the time of deposition the arid tidal flat was directly laterally adjacent to the deep bathyal basin.

1. a) Refute the geologist's statement by analyzing the fundamental conditions and limitations for the application of Walther's Law.
2. b) Propose two different geodynamic hypotheses (one eustatic/oceanographic and one tectonic) capable of explaining the immediate vertical superposition of two sedimentary environments so far apart in depth.

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.

A geophysical cross-section across the Guadalquivir Basin shows extreme geometric asymmetry:

• Towards the northern margin (Sierra Morena), the Paleozoic basement is covered by barely $100\text{–}300\ \text{m}$ of shallow and continental horizontal sediments from the Upper Miocene and Pliocene.

• Towards the southern margin (Betic front), the sedimentary infill exceeds $4\,000\ \text{m}$; the deep Miocene sediments are folded, thrusted, and intermixed with chaotic submarine deposits (olistostromes).

1. a) Physically justify the asymmetric wedge geometry of the basin by applying the model of lithospheric flexure due to orogenic loading.
2. b) Explain why the age of the onset of marine sedimentation and the age of the subsequent compressive deformation become progressively younger as you move from the Subbetic front (south) toward Sierra Morena (north).
3. c) Deduce how flexural subsidence and the supply of eroded detritus from the Betics interacted to transform the basin from a deep marine trough open to the Atlantic into a shallow, infilled continental basin.

In a marine basin, a stratigraphic column of $3\,000\ \text{m}$ of compacted shales accumulated over $15\ \text{Ma}$ is measured.

1. a) Calculate the average net apparent sedimentation rate in $\text{mm/year}$ (or $\text{m/Ma}$).
2. b) A high-resolution microstratigraphic study in a section of that basin reveals that the sediment was deposited by storm surges: each $5\ \text{cm}$ layer was deposited in a few hours, but periods of non-deposition or erosion of about $250\ \text{years}$ elapsed between successive surges. Explain the "Sadler effect" (the dependence of accumulation rate on timescale) and deduce what represents most of the geological time recorded in the $15\ \text{Ma}$ of the column.
3. c) Knowing that the porosity of the clays decreased due to diagenetic compaction from an initial $60\,\%$ at the seabed to $20\,\%$ in the current consolidated rock, calculate the original uncompacted thickness of the series and determine the true gross sediment supply rate before diagenesis.