Stratigraphy & Sedimentology

Stratigraphy and sedimentology form the foundation for understanding Earth's history recorded in sedimentary rocks. These disciplines use the layer-cake nature of sedimentary sequences to reconstruct ancient environments, establish time relationships, and document changes in life and climate through geological time.

Fundamental Principles of Stratigraphy

Steno's Laws (1669)

Principle of Original Horizontality

Sedimentary layers are deposited horizontally or nearly so due to gravity. Exceptions (cross-bedding, etc.) occur within the larger horizontal framework.

Principle of Lateral Continuity

Sedimentary layers extend laterally in all directions until they thin to zero or terminate against a barrier.

Principle of Superposition

In an undisturbed succession, younger rocks lie above older rocks.

Additional Principles

Principle of Faunal Succession (William Smith, 1790s)

Fossil assemblages succeed each other in a definite and recognizable order. This allows for correlation of rocks of similar age across wide areas.

Principle of Cross-Cutting Relationships

Features that cut across other features are younger than the features they cut.

Principle of Inclusions

Fragments included in a rock are older than the rock that contains them.

Sedimentary Environments

Continental Environments

Fluvial (River) Systems

• Alluvial fans: Coarse deposits at mountain fronts
• Braided rivers: Multiple interconnected channels
• Meandering rivers: Single channel with point bars and cut banks

Lacustrine (Lake) Environments

• Proximal: Coarse-grained near-shore deposits
• Distal: Fine-grained deep-water deposits
• Seasonal: Laminated deposits (varves) showing annual cycles

Aeolian (Wind) Systems

• Dune deposits: Cross-bedded sandstones
• Loess: Fine-grained wind-blown silt
• Playa deposits: Evaporite sequences

Transition Environments

Deltaic Systems

• Prodelta: Offshore fine-grained deposits
• Delta front: Shoreface sands with marine influence
• Delta plain: River-dominated channel and overbank deposits

Tidal Flats

• Upper: Intertidal mudflats with desiccation features
• Lower: Subtidal carbonate or siliciclastic deposits

Marine Environments

Shallow Marine

• Shoreface: Wave-dominated sandstones
• Offshore: Storm-dominated deposits
• Reef: Carbonate framework structures

Deep Marine

• Turbidites: Submarine fan deposits from density flows
• Pelagic: Deep-sea oozes and abyssal deposits
• Hemipelagic: Slope and rise deposits

Sedimentary Structures

Primary Structures (Formed During Deposition)

Bedding and Lamination

• Horizontal bedding: Low-energy depositional conditions
• Cross-bedding: Current or wind direction indicators
• Graded bedding: Decreasing grain size upward

Load Casts and Flame Structures

• Load casts: Downward projections from overlying bed
• Flame structures: Upward flame-like projections

Deformation Structures

• Slump structures: Soft-sediment deformation
• Convolute bedding: Tightly folded soft-sediment layers

Secondary Structures (Formed After Deposition)

Diagenetic Features

• Concretions: Localized cementation
• Stylolites: Dissolution surfaces

Tectonic Features

• Sedimentary dikes: Soft-sediment injection
• Folding and faulting: Tectonic deformation

Depositional Sequences

Walther's Law

Facies that are adjacent to each other in a vertical sequence were also adjacent laterally during deposition.

Sequence Stratigraphy

Systems Tracts

• Lowstand systems tract: Deposition during sea-level fall
• Transgressive systems tract: Deposition as sea level rises
• Highstand systems tract: Deposition during sea-level highstand

Sequence Boundaries

Surfaces representing maximum regressive surfaces, often associated with subaerial exposure and erosion.

Biostratigraphy

Fossil Assemblages

• Index fossils: Species that are widespread, abundant, and short-lived
• Taxonomic range: First and last appearance of species
• Concurrent range: Overlap of multiple species ranges

Biostratigraphic Units

• Biozones: Intervals of rock characterized by specific fossils
• Range zones: Based on occurrence of taxa
• Assemblage zones: Based on groups of taxa
• Abundance zones: Based on relative abundance of taxa

Chronostratigraphy and Geochronology

Geological Time Scale

Eon-Era-Period-Epoch-Stage

• Phanerozoic: Visible life (last 541 Ma)
  • Paleozoic: Ancient life
  • Mesozoic: Middle life
  • Cenozoic: Recent life

Absolute vs. Relative Time

• Relative time: Order of events (older/younger)
• Absolute time: Numerical age (years before present)

Radiometric Dating

Where $t$ = age, $\lambda$ = decay constant, $D$ = daughter atoms, $P$ = parent atoms.

Lithostratigraphy

Formal Units

• Formation: Mappable rock unit with consistent lithology
• Member: Subdivision of a formation
• Group: Collection of related formations
• Supergroup: Collection of related groups

Criteria for Formation Recognition

• Lithologic distinctiveness: Different from adjacent units
• Mappability: Laterally traceable
• Stratigraphic continuity: Consistent position in sequence
• Thickness: Sufficiently thick for mapping

Correlation Techniques

Lateral Correlation

• Physical correlation: Similar lithology and structures
• Biostratigraphic correlation: Similar fossil assemblages
• Chemical correlation: Similar geochemical signatures
• Magnetic correlation: Similar paleomagnetic signatures

Cyclostratigraphy

Recognition of climate cycles in sedimentary sequences:

• Milankovitch cycles: Orbital parameters affecting climate
  • Eccentricity: ~100,000 and ~405,000 year cycles
  • Obliquity: ~41,000 year cycle
  • Precession: ~23,000 year cycle

Sequence Analysis

Transgression and Regression

• Transgression: Landward shift of facies (sea level rise or subsidence)
• Regression: Seaward shift of facies (sea level fall or uplift)

Marine Regression/Transgression Identification

• Transgression: Onlap geometry, landward facies shift
• Regression: Offlap geometry, seaward facies shift

Specialized Applications

Sequence Stratigraphy

Unconformity-Bounded Sequences

• Sequence boundary: Surface of subaerial exposure
• Maximum flooding surface: Time of maximum marine transgression
• Systems tract architecture: Predictable facies arrangements

Sequence Hierarchy

• First-order: ~200 Myr (supercontinent cycles)
• Second-order: ~10-80 Myr (tectonic cycles)
• Third-order: ~1-10 Myr (eustatic cycles)

Practical Considerations

Core Correlation

Systematic correlation of subsurface data for hydrocarbon exploration.

Outcrop Analysis

Ground-truth for subsurface interpretations and sequence identification.

Basin Analysis

Understanding subsidence, sediment supply, and accommodation controls on stratigraphy.

Real-World Application: Basin Evolution Analysis

Understanding how sedimentary basins evolve through time helps predict hydrocarbon and groundwater resources.

Basin Fill Analysis

# Sedimentary basin fill analysis
basin_data = {
    'initial_depth': 100,    # meters (water depth at beginning)
    'sediment_supply_rate': 0.2,  # mm/year (sediment accumulation rate)
    'subsidence_rate': 0.15,      # mm/year (basin deepening rate)
    'time_period': 100,      # Myr (duration of deposition)
    'sea_level_change': 50   # meters (net sea level change over time)
}

# Calculate net accommodation space
subsidence_mm = basin_data['subsidence_rate'] * basin_data['time_period'] * 1e6  # mm
sediment_fill_mm = basin_data['sediment_supply_rate'] * basin_data['time_period'] * 1e6  # mm

# Convert to meters
subsidence_m = subsidence_mm / 1000
sediment_fill_m = sediment_fill_mm / 1000

print(f"Initial water depth: {basin_data['initial_depth']} m")
print(f"Total subsidence: {subsidence_m/1000:.1f} km")
print(f"Total sediment accumulation: {sediment_fill_m/1000:.1f} km")
print(f"Net accommodation space: {(subsidence_m - sediment_fill_m)/1000:.1f} km")

# Determine basin type based on sediment fill
fill_ratio = sediment_fill_m / subsidence_m
if fill_ratio < 0.3:
    basin_type = "Starved basin - slow sedimentation"
elif fill_ratio < 0.7:
    basin_type = "Moderately filled basin"
else:
    basin_type = "Rapidly filled basin - sediment-starved"

print(f"Sediment fill ratio: {fill_ratio:.2f}")
print(f"Basin type: {basin_type}")

# Estimate potential for organic-rich facies (source rocks)
# Organic-rich facies typically form under specific conditions
if basin_data['initial_depth'] > 100 and fill_ratio < 0.5:
    source_rock_potential = "High - deep water, slow fill"
elif basin_data['initial_depth'] > 200:
    source_rock_potential = "Moderate - deep water"
else:
    source_rock_potential = "Low - shallow water, oxidizing conditions"

print(f"Organic-rich facies potential: {source_rock_potential}")

Sequence Stratigraphic Interpretation

Analyzing the vertical arrangement of sedimentary facies to understand basin evolution.

Your Challenge: Stratigraphic Column Analysis

Analyze a stratigraphic column to interpret paleoenvironments and establish time relationships.

Goal: Interpret depositional environments and correlate stratigraphic units across multiple sections.

Stratigraphic Data

import math

# Stratigraphic column data for two sections
section_a = [
    {'depth': 0, 'thickness': 15, 'lithology': 'sandstone', 'structures': ['cross-bedding'], 'fossils': None, 'environment': 'fluvial'},
    {'depth': 15, 'thickness': 8, 'lithology': 'shale', 'structures': ['ripple marks'], 'fossils': 'brachiopods', 'environment': 'delta front'},
    {'depth': 23, 'thickness': 20, 'lithology': 'limestone', 'structures': ['bioturbation'], 'fossils': 'corals', 'environment': 'shallow marine'},
    {'depth': 43, 'thickness': 12, 'lithology': 'shale', 'structures': ['horizontal bedding'], 'fossils': 'ammonites', 'environment': 'deep marine'},
    {'depth': 55, 'thickness': 25, 'lithology': 'sandstone', 'structures': ['graded bedding'], 'fossils': None, 'environment': 'turbidite'}
]

section_b = [
    {'depth': 0, 'thickness': 10, 'lithology': 'limestone', 'structures': ['bioturbation'], 'fossils': 'corals', 'environment': 'shallow marine'},
    {'depth': 10, 'thickness': 22, 'lithology': 'shale', 'structures': ['horizontal bedding'], 'fossils': 'ammonites', 'environment': 'deep marine'},
    {'depth': 32, 'thickness': 28, 'lithology': 'sandstone', 'structures': ['graded bedding'], 'fossils': None, 'environment': 'turbidite'},
    {'depth': 60, 'thickness': 6, 'lithology': 'shale', 'structures': ['ripple marks'], 'fossils': 'brachiopods', 'environment': 'delta front'},
    {'depth': 66, 'thickness': 18, 'lithology': 'sandstone', 'structures': ['cross-bedding'], 'fossils': None, 'environment': 'fluvial'}
]

# Calculate total section thickness
total_thickness_a = sum(unit['thickness'] for unit in section_a)
total_thickness_b = sum(unit['thickness'] for unit in section_b)

# Identify key marker beds for correlation
marker_beds = []
for unit_a in section_a:
    for unit_b in section_b:
        if unit_a['lithology'] == unit_b['lithology'] and unit_a['fossils'] == unit_b['fossils']:
            marker_beds.append({
                'lithology': unit_a['lithology'],
                'environment': unit_a['environment'],
                'section_a_top': sum(u['thickness'] for u in section_a[:section_a.index(unit_a)]),
                'section_b_top': sum(u['thickness'] for u in section_b[:section_b.index(unit_b)])
            })

# Calculate paleowater depth based on environments
env_depths = {
    'fluvial': 0,          # Near surface
    'delta front': 10,     # Shallow marine
    'shallow marine': 50,  # Restricted circulation
    'deep marine': 200,    # Normal marine circulation
    'turbidite': 1500      # Deep water
}

# Estimate depth for each unit
section_a_depths = [env_depths[unit['environment']] for unit in section_a]
section_b_depths = [env_depths[unit['environment']] for unit in section_b]

Analyze the stratigraphic columns to interpret sea-level changes and correlate between sections.

Hint:

• Use Walther's Law to interpret lateral facies changes
• Look for key marker beds that can be correlated between sections
• Consider the vertical sequence of environments
• Estimate paleowater depths from sedimentary environments

# TODO: Interpret sea-level changes and correlation
sea_level_trend_a = ""  # Transgression or regression for section A
sea_level_trend_b = ""  # Transgression or regression for section B
correlation_points = [] # List of correlated marker beds
relative_age_relationship = ""  # Which section is older/younger or equivalent
net_sea_level_change_a = 0  # meters (estimated change in section A)
net_sea_level_change_b = 0  # meters (estimated change in section B)

# Calculate correlation between sections
# Identify equivalent stratigraphic levels
equivalent_units = []
for i, unit_a in enumerate(section_a):
    for j, unit_b in enumerate(section_b):
        if unit_a['lithology'] == unit_b['lithology'] and unit_a['fossils'] == unit_b['fossils']:
            equivalent_units.append((i, j, unit_a['lithology']))

# Print results
print(f"Section A total thickness: {total_thickness_a} m")
print(f"Section B total thickness: {total_thickness_b} m")
print(f"Sea-level trend in Section A: {sea_level_trend_a}")
print(f"Sea-level trend in Section B: {sea_level_trend_b}")
print(f"Net sea-level change in A: {net_sea_level_change_a} m")
print(f"Net sea-level change in B: {net_sea_level_change_b} m")
print(f"Correlation points: {equivalent_units}")

# Assess paleoenvironmental changes
if 'corals' in [unit['fossils'] for unit in section_a] and 'ammonites' in [unit['fossils'] for unit in section_a]:
    paleo_environment = "Transition from shallow to deep marine"
else:
    paleo_environment = "Mixed depositional environments"
    
print(f"Paleoenvironmental interpretation: {paleo_environment}")

What does the facies succession tell you about the tectonic setting and eustatic sea-level changes during this time period?