Question

A geologist found a sequence of rocks in which 200 -million-year-old shales were on top of 100 -million-year-old sandstones. Hypothesize how this could happen.

Short answer

The 200-million-year-old shales are on top of 100-million-year-old sandstones likely due to geological processes like faulting or folding.

Step-by-step solution

Identify the Normal Sequence

In a typical geological sequence, older rocks are found beneath younger rocks due to the process of deposition over time. This is according to the principle of superposition, which 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. In this case, normally the 100-million-year-old sandstones would be expected to be beneath the 200-million-year-old shales.

Introduce Potential Hypotheses

Consider geological processes that could reverse the sequence of rock layers. These may include faulting, folding, overturning, or thrust faulting. Such processes can significantly alter the original stratification of rock layers.

Examine Faulting as a Hypothesis

One possibility is that a faulting event occurred, in which a section of the earth's crust slipped and shifted. This could result in older rock layers being moved over younger layers through processes like thrust faulting, which happens when compressional forces push an older section of earth's crust over a younger section.

Discuss Folding and Overturning

Another hypothesis could be that folding occurred, where the rock layers were compressed and folded over each other. If intense enough, this could possibly lead to an overturning of the layers, flipping the sequence upside down so that older layers are above younger ones.

Analyze Sedimentation Disruption

Examine if any unusual sedimentation conditions could have caused younger rocks to be deposited beneath older ones. Natural disasters, significant changes in sea levels, or other major environmental changes could lead to atypical deposition orders.

Key concepts

Principle of Superposition

The principle of superposition is a fundamental concept in geology and helps geologists understand the order and age of rock layers. It states that in any sequence of undeformed sedimentary rocks, each rock layer is younger than the one below it and older than the one above it. This principle assumes that deposition of sediment occurs in a relatively stable environment.

• Older layers lie at the bottom since they were deposited first.
• Younger layers are at the top, deposited gradually over time.

However, real-world geological scenarios can deviate from this simple sequence due to geological processes like folding or faulting. Understanding these exceptions is crucial for interpreting complex rock sequences.

Thrust Faulting

Thrust faulting is a geological process that can disrupt the normal sequence of rock layers as explained by the principle of superposition. In this process, compressional forces in the Earth's crust push older rock layers over younger layers.

• Occurs commonly in mountain ranges and areas of intense stress within the Earth's crust.
• Can result in older rocks appearing on top of younger ones, contrary to the typical order.

Thrust faults are significant because they can dramatically alter the landscape, leading to the formation of mountains and other elevated terrains, thus presenting a different sequence of geological strata.

Rock Folding

Rock folding involves the bending and buckling of rock layers due to compressional forces. This can occur over large areas and often results in wavy patterns of rock strata.

• Folds can be symmetrical (evenly shaped) or asymmetrical and range from small-scale folds to large mountain-sized formations.
• If the pressure is great enough, folds can cause an overturning effect, resulting in a reversal of the usual layered sequence.

Through folding, the older sedimentary layers may end up being positioned above the younger layers, offering another explanation for unusual rock sequences.

Sedimentary Rock Layers

Sedimentary rocks form from the accumulation and compaction of sediments, typically in horizontal layers. These layers tell the history of Earth's surface and environmental changes through time.

• Formed from materials like sand, mud, and organic matter.
• Play a crucial role in understanding Earth's geological history.

In normal conditions, sedimentary rock layers are deposited sequentially, with older layers at the bottom. Deviations from this sequence might suggest significant geological events or processes like faulting or folding, which require a deeper investigation into the area's geological past.