Grain Shape in Detrital Sedimentary Rocks: Angularity, & Surface Textures / Sed Strat #3 | GEO GIRL

Summary

This lecture delves into sedimentary grain characteristics, focusing on grain sphericity and roundness, and their effects on transport, sorting, and deposition. The instructor explains how sphericity influences grain movement, while roundness results from transport abrasion. Differences between river and beach pebbles are discussed, emphasizing how shape and surface textures reveal the grain's history. The lecture also covers micro surface textures like polish, frost, and striations, which provide insights into a sedimentary rock's depositional environment and transport history. Key references are provided for deeper study.

Takeaways

• 🌍 The lecture discusses sedimentology and stratigraphy, focusing on the shape of grains in sedimentary rocks.
• ⚪ Sphericity of grains affects their transport and sorting, with spherical grains often separating from less spherical ones during deposition.
• 🪨 Roundness differs from sphericity, referring to the smoothness or angularity of a grain rather than its overall shape.
• 🏞️ Rounding occurs due to transport and abrasion, revealing the history of the grain's movement but not directly affecting transport itself.
• 📏 Surface textures, such as polish or frosting, provide clues about the last transport cycle of a grain, revealing environmental conditions.
• 🔍 Micro relief features, like striations and scratches, often form due to ice or tectonic activity and can help reconstruct depositional environments.
• ⚡ Frosted grains can result from both physical and chemical processes, with different patterns indicating varied environmental factors.
• ⏳ Surface textures, such as polish or frosting, are easily erased during transport, indicating the final phase of a grain's journey.
• 🧊 Striations can help identify glacial or tectonic origins, with different types of striations corresponding to different forces.
• 📚 The references for the lecture come from two major textbooks: 'Sedimentology and Stratigraphy' by Boggs and 'Sedimentary Rocks' by Pettyjohn.

Q & A

What is the significance of sphericity in sedimentary rocks?

Sphericity affects a grain's settling velocity and transport. Spherical grains tend to separate from less spherical grains during transport, resulting in sorting by shape rather than just size.

How does sphericity influence the sorting of sedimentary grains?

Sphericity influences sorting by allowing spherical grains to settle or move differently than less spherical grains, which results in sorting based on grain shape, not just size or density.

What is the difference between sphericity and roundness in grains?

Sphericity refers to how spherical or ball-like a grain is, while roundness refers to the smoothness or angularity of a grain's edges. They are distinct concepts, with roundness focusing more on edge texture and sphericity on overall shape.

How does transport affect the roundness of sedimentary grains?

Transport, through abrasion, smooths the edges of grains, increasing their roundness. Roundness is an effect of transport, rather than a factor that directly affects how a grain is transported.

Why is roundness important in understanding the history of sedimentary deposits?

Roundness can indicate the amount of transport a grain has undergone. Grains that are well-rounded have likely experienced longer or more intense transport, providing clues to the depositional environment and history.

What are some common terms used to describe the roundness of grains?

Grains are typically categorized as angular, subangular, subrounded, rounded, or well-rounded, depending on their level of smoothness and edge sharpness.

What are micro surface textures, and how do they form?

Micro surface textures are small-scale features on grains, such as polish, frosting, striations, or percussion marks. These textures can form from processes like abrasion, glacial scouring, or chemical weathering, and they provide insights into the last transport cycle of the grain.

What can polished versus frosted grains tell us about their transport history?

Polished grains result from fine abrasion, often in environments like deserts, while frosted grains are typically a result of coarser abrasion, such as sand blasting in aeolian environments. These textures can reveal whether a grain was transported by wind or water.

How do striations on grains help reconstruct past environments?

Striations, especially those formed by glaciers or tectonic activity, can indicate the direction and nature of past flows, such as glacial movement or fault slip, helping reconstruct past geological processes.

What is the significance of percussion marks on grains?

Percussion marks are small, concentric impact scars, often indicating high-velocity fluvial environments. They provide evidence of the forceful transport of grains, such as in rivers.