Grain Sizes and Arrangements Flashcards

Grain Sizes and Shapes

• Wentworth Size Scale categorizes grain sizes in sediments.
• Argillaceous sediments contain significant amounts of mud.
• Argillite is a low-grade metamorphic rock formed from mudrock.
• The term "clay" has dual meanings in sedimentology: grain sizes less than 4 µm and a group of layered silicate minerals.

Measurement Techniques

• Grain size can be measured through fluid displacement (volume), weight, linear measurement, visual estimation, and sieving.
• Hydraulic settling is effective for fine-grained sediment analysis.
• Electrical or optical properties assess suspended fine particles in fluids.

Grain Transport Dynamics

• Two types of sediment transport: bedload (traction and saltation) and suspended load (particles transported within the fluid).

Maturity of Sediments

• Immature sediments have >5% mud, poorly sorted, and angular grains.
• Submature sediments contain less dark minerals and less than 5% mud, with angular to subangular grains.
• Mature sediments are quartz-dominant with <5% mud, sorted, and subangular to subrounded grains.
• Supermature sediments consist almost entirely of quartz, <5% mud, well sorted, and rounded grains.
• Sediment maturity relates to weathering, transport energy, and duration, not geological age.

Sediment Characteristics

• Textural inversion involves mixing sediments of different maturities.
• Grain shape is defined by roundness (smoothness of corners) and sphericity (shape similarity to a sphere).

Surface Features and Support Types

• Striations are linear scratches indicating glacial transport.
• Chatter marks are crescent-shaped grooves from rock impacts.
• Frosting on quartz sand grains results from wind transport creating tiny pits.
• Grain-supported sediments have framework grains in contact, while matrix-supported sediments are surrounded by finer matrix particles.

Packing Arrangements

• Cubic packing has a loose arrangement with 48% pore space.
• Rhombohedral packing is more space-efficient, with only 26% pore space.
• Tabular grains can be randomly oriented, parallel to bedding, or imbricated (angled overlapping).

Biogenic Fabrics

• Sediments can display biogenic fabrics formed through organism activity.
• Examples include algal or bacterial mats and coral reefs.

Porosity and Permeability

• Porosity measures void space as a percentage of total volume, affecting fluid capacity in rock (oil, gas, water).
• Permeability indicates how easily a material transmits fluids under pressure.