Mineralogy of Sedimentary Rocks: Weathering and Diagenesis

Summary

This script delves into the mineralogy of sedimentary rocks, focusing on terrigenous or clastic rocks formed from preexisting rock fragments. It discusses how minerals like quartz and clay, being the most stable, are commonly found in these rocks. The concept of sediment maturity is introduced, indicating the transformation of feldspar into clay over time. The script also covers the sorting and rounding of sediment grains during transport, the diagenesis process post-deposition, and the chemical changes leading to cementation or dissolution, ultimately affecting the rock's composition.

Takeaways

• 🌟 Sedimentary rocks are categorized into types based on their mineralogy, with terrigenous or clastic rocks being a primary focus.
• 🏔️ Terrigenous rocks are composed of preexisting rock fragments that have undergone weathering, erosion, and transportation.
• 💧 Acidic precipitation and groundwater affect the minerals at the surface, favoring the survival of relatively unreactive minerals like quartz and clay minerals.
• 📊 Bowen’s Reaction Series illustrates the reactivity of silicate minerals, with mafic minerals being highly reactive and felsic minerals, like quartz, being the most stable.
• 🕰️ Sediment maturity is indicated by the ratio of quartz and clay to feldspar, reflecting the time spent exposed at the surface and the degree of feldspar breakdown.
• 🧪 The most soluble minerals, such as evaporite minerals, are rarely found in terrigenous sedimentary rocks due to their susceptibility to chemical weathering.
• 🌊 Abrasion during turbulent flow contributes to the rounding of sediment grains, with longer transportation times resulting in more rounded grains.
• 🔍 Sorting processes, like those on a beach, separate grains based on size, with smaller particles like silt and mud being transported further offshore.
• 🏖️ Clastic rocks formed in beach environments, such as sandstones, are often rich in quartz and exhibit well-sorted and well-rounded characteristics.
• 🗻 Sediments deposited near their source tend to contain a variety of silicate minerals, including feldspar, and are angular and poorly sorted, as seen in breccia.
• 🔬 Diagenesis refers to the chemical and physical changes that occur in sediment after deposition, including compaction and cementation by minerals like quartz and calcite.

Q & A

What are terrigenous or clastic sedimentary rocks made of?

Terrigenous or clastic sedimentary rocks are made of pieces of preexisting rock that have been weathered, eroded, and transported.

Why are quartz and clay minerals commonly found in clastic sedimentary rocks?

Quartz and clay minerals are the most stable and least reactive, which allows them to survive the weathering and erosion processes and be commonly found in clastic sedimentary rocks.

What does a sediment's maturity indicate about its exposure time at the surface?

A sediment's maturity refers to the amount of time it spent exposed at the surface, which can be gauged based on the relative amounts of quartz plus clay to feldspar, indicating how long it has been subject to weathering.

Why are soluble minerals like salts rarely found as clastic sediments?

Soluble minerals like salts are rarely found as clastic sediments because they are easily chemically altered or weathered during transport, and thus do not survive to become part of the sedimentary rock.

How does the transport process affect the shape of sediment grains?

During transport, grains of sediment are abraded as they collide with each other in turbulent flow, which makes the grains more rounded over time.

What is the sorting process in sediment transport, and how does it occur on a sandy beach?

The sorting process in sediment transport involves the separation of grains based on their diameter. On a sandy beach, the constant wave action rinses out smaller silt and mud particles, carrying them offshore where they settle first, creating a silt deposit, while larger sand grains remain closer to the shore.

What characteristics define sediments that have been transported far from their source?

Sediments that have been transported far from their source are typically rich in quartz and clay, well-sorted, and well-rounded, as seen in sandstones formed in beach environments.

How does the composition of sediments deposited near their source differ from those transported far?

Sediments deposited near their source are composed of various silicate minerals, especially feldspar, and are relatively angular and poorly sorted, unlike those transported far which are more rounded and rich in quartz and clay.

What is diagenesis, and how does it affect sediment after it is deposited?

Diagenesis refers to the chemical and physical changes that occur during and after the burial of a body of sediment. It includes compaction, cementation, recrystallization, and dissolution, which transform the sediment into a rock.

What is the role of groundwater in the diagenesis process?

Groundwater, which fills the pore space in buried sediment, plays a crucial role in diagenesis by facilitating chemical changes such as cementation, where minerals precipitate into the pore space, and dissolution, where minerals like calcite dissolve or precipitate based on the chemistry of the pore water.

How does the depth of burial affect the compaction of sediment?

The deeper a sediment is buried, the more it compacts due to the weight of the overlying layers. For example, a 50% reduction in porosity can occur by a depth of about 2000 meters.