Geology 101 with Willsey, Episode #15: Sedimentary Rocks

Summary

This video covers key concepts in sedimentary rock formation, focusing on the characteristics of grain shape, sorting, and bedding. It explains how angular or rounded grains reveal the transport distance, and how well or poorly sorted sediment indicates energy levels in the depositional environment. The video also discusses the importance of bedding, where variations in energy lead to layered formations, and highlights different depositional environments, such as glaciers, which move large and small particles. The video concludes with a quiz to test knowledge on sedimentary rock identification and depositional processes.

Takeaways

• 😀 Sedimentary rocks form in various depositional environments, including marine, transitional, and terrestrial settings like deserts and glaciers.
• 😀 Grain shape provides clues about the distance sediment has traveled; angular grains suggest short transport, while rounded grains indicate long-distance movement or reworking.
• 😀 Sorting of grains in sedimentary rocks is affected by the distance the sediment has traveled; well-sorted rocks usually travel further, while poorly sorted rocks remain close to the source.
• 😀 Bedding refers to the layering of sedimentary rocks, which can vary in thickness and does not directly correlate with time but rather with fluctuations in energy during deposition.
• 😀 High-energy environments like rivers and glaciers can transport large, angular, and poorly sorted sediment, whereas low-energy environments like lakes and swamps are more likely to have fine, well-sorted sediments.
• 😀 A diversity of grain sizes and shapes within a rock suggests it was deposited in a high-energy environment capable of transporting large particles.
• 😀 Beaches tend to have well-sorted, rounded particles due to continuous wave action, whereas environments like glaciers can transport and deposit a wide range of particle sizes and shapes.
• 😀 The size and sorting of sedimentary rocks give insight into their formation and transport history, helping to identify the environment of deposition.
• 😀 Sorting is a general characteristic of natural sediment deposition; longer transport distances tend to result in better sorting due to the selective action of water or wind.
• 😀 The appearance of angular grains and poorly sorted rocks in a deposit suggests a relatively short transport history, such as in a landslide or in areas near the source of the sediment.

Q & A

What does the shape of grains in sedimentary rocks tell us about their transportation?

The shape of grains helps determine the distance they have traveled. Angular grains suggest short transport distances, typically from a local source like a landslide. Rounded grains indicate longer transportation, often from a more distant source, or repeated reworking, such as in beach environments.

Why are rounded grains often found in beach environments?

In beach environments, waves continuously rework sediment. This constant movement causes particles to collide with each other, leading to rounding. Even though the sediment doesn’t travel far, the consistent wave action causes this physical change.

What is the significance of sorting in sedimentary rocks?

Sorting refers to the uniformity of grain sizes within a sedimentary rock. Well-sorted sediment indicates long-distance transport, where energy (like water or wind) has been consistent enough to sort the grains. Poorly sorted sediment suggests short transport or rapid deposition, often from high-energy events like floods.

How can energy levels in a depositional environment affect the characteristics of sedimentary rocks?

Energy levels in a depositional environment can impact both the size and sorting of sediment. High-energy environments, like floods or glaciers, can carry large, poorly sorted grains. Low-energy environments, like lakes or swamps, typically lead to the deposition of smaller, well-sorted particles.

What is bedding in sedimentary rocks, and how does it form?

Bedding refers to the layers of sediment that accumulate over time in sedimentary rocks. Variations in energy levels, such as changes in water speed or storm surges, cause different types of sediment to be deposited in distinct layers, forming beds. These layers are often a result of fluctuating depositional conditions.

Does the thickness of bedding in sedimentary rocks correlate with the time it took for the sediment to accumulate?

No, the thickness of bedding does not necessarily correlate with the time it took for the sediment to accumulate. A thick layer could form in a short period during a high-energy event, like a flood, while a thinner layer could accumulate over a much longer time in a low-energy environment.

What types of environments are most likely to have poorly sorted sediment?

Poorly sorted sediment is typically found in high-energy environments like landslides, glaciers, or floods, where the sediment is transported over short distances or deposited rapidly. These environments cannot efficiently separate sediment by size, resulting in a mixture of particle sizes.

What role do glaciers play in the transport of sediment?

Glaciers can transport sediment of all sizes, from fine particles to large boulders, without significant sorting or rounding. As glaciers move, they pick up and carry debris from the land, depositing it at their terminus when they melt or retreat.

What is the main difference between clastic and non-clastic sedimentary rocks?

Clastic sedimentary rocks are made from fragments of other rocks or minerals that have been transported and deposited, whereas non-clastic sedimentary rocks, like chemical or organic rocks, form through precipitation of minerals or accumulation of organic material.

How does the energy of a river system affect sediment deposition?

In a river system, varying energy levels lead to the sorting of sediment. High-energy events like floods can transport larger, poorly sorted particles, while low-energy periods result in finer, well-sorted sediments. The river's energy also influences the types of sedimentary structures, such as bedding.