3.3 Sedimentary processes and depositional environments

Sedimentary processes shape our planet's surface, from mountains to ocean floors. Erosion breaks down rocks, while transportation moves particles via wind, water, ice, and gravity. Deposition occurs when energy decreases, creating diverse landforms and rock layers.

Marine environments host unique depositional systems. Coastal areas like beaches and deltas contrast with deep ocean abyssal plains. Deltas form where rivers meet water bodies, while turbidites result from underwater avalanches. Understanding these processes helps reconstruct Earth's past and locate resources.

Sedimentary Processes

Describe the major processes of sediment transport

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• Erosion
  • Weathering of rocks and minerals breaks down larger particles into smaller ones
  • Physical weathering fragments rocks through mechanical processes (freeze-thaw cycles)
  • Chemical weathering alters mineral composition (dissolution of limestone by acid rain)
  • Biological weathering caused by organisms (plant roots, burrowing animals)
• Transportation
  • Wind carries sand and dust particles (sand dunes, loess deposits)
  • Water moves sediment in rivers and ocean currents (river deltas, turbidity currents)
  • Ice transports debris in glaciers (moraines, erratics)
  • Gravity drives mass wasting events (landslides, debris flows)
• Deposition
  • Settling of particles occurs when transport energy decreases
  • Influenced by particle size and fluid energy determines settling rates
  • Coarse particles settle first, finer particles remain suspended longer

Explain the concept of sediment sorting and its importance in sedimentary environments

• Sediment sorting
  • Process separates particles by size, shape, and density during transport
  • Influenced by transport medium energy and duration of transport
• Importance
  • Indicates depositional environment helps reconstruct past conditions
  • Affects porosity and permeability of sedimentary rocks impacts fluid flow
• Sorting scales
  • Well-sorted uniform particle size suggests consistent energy environment
  • Poorly-sorted mixed particle sizes indicates variable energy conditions

Discuss the factors that influence sediment deposition rates

• Particle characteristics
  • Size larger particles settle faster than smaller ones
  • Shape affects drag and settling velocity
  • Density denser particles settle more quickly
• Environmental factors
  • Water or wind velocity higher speeds keep particles suspended longer
  • Turbulence can keep particles in suspension
  • Depth of water affects settling time and particle distribution
• Stokes' Law
  • Describes settling velocity of particles in fluid
  • $V = \frac{2}{9} \frac{(ρ_s - ρ_f)}{μ} g r^2$
    • V settling velocity
    • ρ_s density of particle
    • ρ_f density of fluid
    • μ fluid viscosity
    • g gravitational acceleration
    • r particle radius

Depositional Environments

Identify and describe the main types of marine depositional environments

• Coastal environments
  • Beaches dynamic zones of sand and gravel deposition
  • Estuaries mixing zones of fresh and salt water with fine sediment accumulation
  • Deltas where rivers deposit sediment into standing water bodies
• Shallow marine environments
  • Continental shelf gently sloping submerged extension of continents
  • Coral reefs biogenic structures in warm, clear waters
• Deep marine environments
  • Continental slope steeper descent from shelf to deep ocean
  • Abyssal plain flat, deep ocean floor with fine sediment accumulation
  • Mid-ocean ridges underwater mountain ranges with hydrothermal deposits
• Polar environments
  • Ice shelves floating extensions of land ice
  • Glacial marine sediments deposited by melting icebergs and sea ice

Explain the characteristics of deltaic depositional systems

• Delta formation
  • River meets slower-moving or stationary body of water causing sediment deposition
  • Sediment accumulation builds land into water body
• Delta types
  • River-dominated (elongate) form when river flow dominates over waves and tides
  • Wave-dominated (cuspate) shaped by strong wave action redistributing sediment
  • Tide-dominated (irregular) influenced by strong tidal currents creating complex channels
• Delta components
  • Topset beds horizontal layers at delta surface
  • Foreset beds steeply dipping layers at delta front
  • Bottomset beds gently sloping layers beyond delta front
• Sediment distribution
  • Coarse sediments near river mouth due to rapid deposition
  • Finer sediments farther offshore carried by currents and waves

Discuss the formation and characteristics of turbidites

• Turbidity currents
  • Dense, sediment-laden underwater flows move down submarine slopes
  • Triggered by earthquakes, storms, or overloading of shelf sediments
• Turbidite sequence (Bouma sequence)
  • Graded bedding coarsest at bottom, fining upward
  • Parallel lamination horizontal layers from suspended load
  • Ripple cross-lamination formed by decreasing current velocity
  • Upper parallel lamination fine-grained sediment settling
  • Pelagic mud slowest settling particles from water column
• Importance
  • Indicator of past submarine landslides helps reconstruct geologic history
  • Potential hydrocarbon reservoirs significant for oil and gas exploration

Compare and contrast the sedimentary processes in shallow and deep marine environments

• Shallow marine environments
  • Higher energy conditions due to waves and currents
  • Wave and tidal influences shape coastlines and nearshore deposits
  • Coarser sediments predominate (sand, gravel)
  • Biological activity more prevalent (shell fragments, bioturbation)
• Deep marine environments
  • Lower energy conditions below wave base
  • Gravity-driven processes dominant (turbidity currents, slumps)
  • Finer sediments accumulate (clay, silt)
  • Less biological influence except for pelagic organisms
• Sediment sources
  • Shallow terrestrial and biogenic inputs (river sediment, coral fragments)
  • Deep pelagic and hemipelagic sources (planktonic remains, fine particles)
• Depositional rates
  • Shallow generally higher due to proximity to sediment sources
  • Deep generally lower except in turbidite settings where rapid deposition occurs