Sediments and Sedimentary Rocks

Sediment Formation and Transport Dynamics

• Sediment Definition: A loose assemblage of broken rock pieces and other materials that accumulate over time through deposition and form sedimentary rocks via burial, compaction, and lithification.

• Erosion, Transport, and Deposition Model:

  • Weathering & Erosion: Bedrock at high elevations is broken down by climatic forces and removed.

  • Transport: Gravity, wind, and water move grains to lower elevations; transport capability depends directly on fluid energy, grain size, and grain density.

  • Deposition: Occurs when the transporting fluid loses sufficient energy to continue carrying sediment grains.

• Primary Control of Grain Size: The energy of transport—not erosion—determines the grain size deposited in an environment:

  • High-Energy Environments: River channels deposit coarser, larger grains.

  • Low-Energy Environments: Lakes deposit finer, smaller grains.

• Non-Transported Sediments: Precipitates and evaporites form in situ at the bottom of water bodies without undergoing erosion or transport; shell-derived sediments experience little to no transport.

Sediment Textural and Compositional Properties

• Grain Size Classification:

  • Gravel: Greater than $2.0\,\text{mm}$ ($> 2\,\text{mm}$); large visible grains.

  • Sand: $0.0625\,\text{mm}$ to $2.0\,\text{mm}$ ($1/16\,\text{mm}$ to $2\,\text{mm}$); visible grains with a gritty sandpaper feel.

  • Silt: $0.0039\,\text{mm}$ to $0.0625\,\text{mm}$ ($1/256\,\text{mm}$ to $1/16\,\text{mm}$); barely visible grains with a gritty feel.

  • Clay: Less than $0.0039\,\text{mm}$ ($< 1/256\,\text{mm}$); no visible grains, feels smooth.

• Grain Shape (Roundness):

  • Classified along a spectrum: Angular, Sub-Angular, Sub-Rounded, and Rounded.

  • Indicates the degree of grain abrasion; greater roundness correlates with longer transport distance or prolonged agitation.

• Grain Sorting:

  • Well-Sorted: Uniform grain size throughout, indicating efficient fluid separation.

  • Moderately Sorted: Intermediate variation in grain sizes.

  • Poorly-Sorted: Wide range of mixed grain sizes, notably coarse grains mixed with silt and clay fines.

• Mineralogy & Maturity:

  • Lithic Fragments: Rock fragments not assignable to a single specific mineral.

  • Textural Maturity: Sediments that are well-sorted, well-rounded, and composed primarily of stable, weathering-resistant minerals.

Clastic Sedimentary Rocks

• Conglomerate: Coarse gravel ($> 2\,\text{mm}$); rounded rock or mineral fragments; typically poorly sorted.

• Breccia: Coarse gravel ($> 2\,\text{mm}$); angular rock or mineral fragments; typically poorly sorted.

• Quartz Sandstone: Medium sand ($1/16\,\text{mm}$ to $2\,\text{mm}$); dominated by quartz grains; typically rounded and well sorted.

• Arkose: Medium sand ($1/16\,\text{mm}$ to $2\,\text{mm}$); contains at least $25\%$ feldspar; typically angular and poorly sorted.

• Graywacke: Medium sand ($1/16\,\text{mm}$ to $2\,\text{mm}$); sand mixed with mud matrix; typically poorly sorted.

• Siltstone: Fine silt/mud ($1/16\,\text{mm}$ to $1/256\,\text{mm}$); mostly silt-sized quartz and clay; blocky and gritty.

• Shale: Very fine clay/mud ($< 1/256\,\text{mm}$); mostly clay; splits into thin layers; may preserve fossils.

• Mudstone / Claystone: Very fine clay/mud ($< 1/256\,\text{mm}$); mostly clay; crumbles easily.

Chemical, Biochemical, and Organic Sedimentary Rocks

• Calcite-Based Rocks ($\text{CaCO}_3$, effervesce vigorously in $\text{HCl}$):

  • Crystalline Limestone: Fine to coarse crystalline texture.

  • Micrite: Microcrystalline with no visible grains; may show conchoidal fracture.

  • Coquina: Loosely cemented visible shell and shell fragments.

  • Fossiliferous Limestone: Well-cemented shells and shell fragments of various sizes.

  • Chalk: Soft, composed of microscopic shells and clay.

  • Travertine: Faintly layered calcite; often porous or cavernous.

• Dolomite-Based Rocks ($\text{CaMgCO}_3$, effervesces in $\text{HCl}$ when powdered):

  • Dolostone: Crystalline texture.

• Silica-Based Rocks ($\text{SiO}_2$, microcrystalline, conchoidal fracture, scratches glass):

  • Chert: Light-colored variety.

  • Flint: Dark-colored variety.

  • Agate: Banded variety.

• Evaporite Rocks:

  • Rock Gypsum: Composed of gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$); fine to coarse crystalline; soft.

  • Rock Salt: Composed of halite ($\text{NaCl}$); fine to coarse crystalline; distinct salty taste.

• Organic Sedimentary Rocks:

  • Bituminous Coal: Formed from altered plant fragments; black, brittle, organic, and often layered.