Stratigraphy MCQ Practice Paper – 002

Correct Answer – Principle of Superposition

This principle states that in an undeformed sequence of sedimentary rocks, each layer is younger than the one beneath it and older than the one above it. Because sediments accumulate over time, newer material is deposited on top of older material. When you look at a cliff or a core, you can usually read the history from bottom (oldest) to top (youngest) using this principle. It is one of the fundamental tools for relative dating in stratigraphy and is especially helpful before numerical ages are available. Geologists always check for signs of folding, faulting, or overturning, but when the sequence is undisturbed, superposition gives a straightforward age relationship between beds.

correct answer – A continuous shift from higher-energy to lower-energy depositional conditions.

This interpretation fits the observed gradual change in grain size and lithology, suggesting that the same general environment slowly evolved. For example, a shoreline might migrate landward due to sea-level rise, causing environments that once deposited sand to become deeper and quieter, laying down more mud and producing shale. The continuity of the transition supports environmental evolution rather than a time gap.

Correct Answer – A boundary representing a gap in the geologic record due to erosion or non-deposition

An unconformity is a surface in the rock record that represents missing time, usually caused by a period of erosion or non-deposition. Instead of continuous sediment accumulation, the area experienced either removal of existing sediments or simply stopped receiving new sediment for a significant period. When deposition resumes, younger layers are placed directly on older rocks, leaving a time gap between them. This surface is what geologists call an unconformity. Identifying unconformities helps reconstruct past environments, understand uplift or sea-level changes, and piece together regional geological histories by recognizing where parts of the record have been lost.

Correct Answer – Angular unconformity

An angular unconformity features older sedimentary layers that have been tilted or folded and then eroded, with younger sedimentary layers deposited on top in a different orientation, usually closer to horizontal. When viewed in outcrop or cross-section, you can see that the bedding planes below the unconformity surface meet the bedding planes above at an angle. This angular difference is the key diagnostic feature. It reveals a story in stages: first deposition, then deformation, then erosion, and finally renewed deposition. Because so many processes are involved, angular unconformities commonly represent large amounts of missing time and significant tectonic events.

Correct Answer – They represent organisms that lived for a short time and had wide geographic distribution

Index fossils are particularly valuable because they represent organisms that existed for a geologically short time but spread widely across different regions. A short duration means that whenever you find that fossil in a rock, you know the rock belongs to a relatively narrow time interval. Wide geographic distribution means the same species can appear in many different sedimentary basins. When both conditions are met, those fossils become powerful tools for correlating rock layers of similar age across long distances. This method allows geologists to build detailed relative time scales and connect local stratigraphic columns into regional and global frameworks.

Correct Answer – Deposition from moving currents such as rivers, tides, or wind

Cross-bedding is a sedimentary structure formed by the migration of ripples or dunes under the influence of moving currents, whether water or air. As these bedforms move, sand grains accumulate on their lee sides, building inclined layers that are preserved as cross-beds. The orientation and scale of cross-bedding can reveal the direction of flow, the strength of the currents, and even help distinguish between river, tidal, or aeolian (wind-blown) environments. Thus, seeing cross-bedding in a sandstone strongly indicates that the sediment was transported and deposited by flowing water or wind, not in a completely still setting.

The correct answer – They are different ages despite similar depositional environments.

The recurring nature of depositional environments means that similar sandstones can form multiple times in Earth’s history under comparable conditions. Fossils evolve and change through time, so different fossil assemblages signal that each sandstone was laid down during a distinct time interval. This combination of lithologic and paleontologic evidence is central to modern stratigraphic correlation and time assignment.

Correct Answer – To divide sedimentary successions into genetically related packages bounded by unconformities or their equivalents

Sequence stratigraphy is concerned with understanding and subdividing sedimentary deposits into packages that reflect the interplay of sea-level change, sediment supply, and subsidence. The primary aim is to recognize genetically related sets of strata bounded by unconformities or equivalent surfaces, such as maximum flooding surfaces or sequence boundaries. These boundaries represent shifts in accommodation and sedimentation that affect how sedimentary layers stack in time and space. By mapping these sequences, geologists can better predict where particular facies, such as reservoirs or seals, might be located and can correlate those units across basins in a way that is more process-based than simple lithologic matching.

Correct Answer – Coarse conglomerate at the bottom, grading upward into sandstone, then shale, and finally limestone

This pattern—coarse at the bottom, becoming finer and often more carbonate-rich upward—is characteristic of a marine transgression over a broad continental area. As sea level rises, shoreline environments move landward, and deeper-water facies shift over previously shallower settings.

In a typical transgressive sequence, you might first deposit coarse nearshore sediments like conglomerates or sandstones when the sea is relatively low. As water deepens and the shoreline retreats, finer sediments such as shales are deposited farther inland, eventually followed by more offshore or carbonate-rich facies like limestones. This creates a vertical succession from coarser to finer, and sometimes more carbonate-rich, units upward, reflecting the overall deepening of water through time.

Correct Answer – The layers must have been deposited horizontally and later deformed

The Law of Original Horizontality states that layers of sediment are initially deposited in a nearly horizontal position because gravity causes particles to settle on flat surfaces. Therefore, when a stratigrapher sees sedimentary layers that are now steeply tilted or folded, the logical conclusion is that these layers were altered after they formed. Tectonic processes such as folding, faulting, and tilting must have occurred, deforming the once-horizontal beds into their present orientation. Recognizing this allows geologists to distinguish between features formed during deposition and those created later by crustal movements.

The correct answer – lithostratigraphic units are defined by rock type and observable physical characteristics.

They provide a way to describe and map rocks based on what can be seen and measured directly, including lithology, structures, and sometimes thickness patterns. These units can cross time boundaries because similar depositional environments can exist at different times, producing similar rock types. Thus, lithostratigraphic classifications emphasize the rocks themselves rather than the time they represent.

Correct Answer – Walther’s Law of Facies

The best choice among the options is Walther’s Law of Facies. This concept states that, in a conformable sequence, the vertical succession of facies reflects the lateral changes in depositional environments. Fossil assemblages are one of the key clues that define these facies. If a particular fossil community is characteristic of shallow marine conditions today, finding a similar assemblage in ancient rocks supports the interpretation that those rocks record a shallow marine environment in the past, fitting into the framework described by Walther’s Law.

Correct Answer – A transition from coastal swamp to a marine transgression

Coal commonly forms in peat-forming swamps, often near coasts, where abundant vegetation accumulates and is preserved under waterlogged, low-oxygen conditions. Marine shale typically indicates offshore, deeper-water deposition of fine mud in a marine setting. When a marine shale overlies a coal seam, it suggests that the shoreline moved inland over time, drowning the swamp and allowing marine mud to settle on top. This pattern is a classic indicator of rising sea level and landward movement of the sea, known as a transgression.

Correct Answer – Principle of Faunal Succession

The Principle of Faunal Succession states that fossil assemblages succeed one another in a recognizable and predictable order through the stratigraphic record because organisms evolve, appear, and go extinct over time. This principle allows geologists to match rock units of similar age across different regions, even across continents, by comparing their fossil content. If two separated rock sequences contain the same characteristic fossil assemblage, they are interpreted as having formed during the same time interval, enabling long-distance correlation in biostratigraphy.

Correct Answer – A maximum flooding surface associated with peak transgression

A maximum flooding surface marks the time when marine waters have transgressed furthest landward over a basin, and accommodation for sediment is greatest. Below such a surface, you might see more terrestrial or coastal deposits like fluvial sandstones; above it, more marine facies such as limestones can dominate. The stratigraphic shift from river sands to shallow marine limestone suggests that the area transitioned from a land-dominated to a marine-dominated setting due to rising relative sea level. In sequence stratigraphy, this is commonly interpreted as part of a transgressive systems tract and near the maximum flooding surface, where marine influence is at its peak over that location.