Detailed Lecture Notes on Alluvial Fans for GEOL 30040

Alluvial Fans Overview

  • Definition: An alluvial fan is a body of generally coarse-grained deposits such as conglomerates or ‘fanglomerates’ whose surface resembles a cone shape, radiating downslope from the point (apex) where a stream emerges from a mountainous catchment.

  • Common Locations: Found in the hanging wall (downthrown side) of normal faults and the footwall of thrust faults.

  • Example: Death Valley, California, located at -86 m below sea level.

Significance of Alluvial Fans

  • Sedimentary Cycle Origin: Alluvial fans are often the first depositional bodies formed as a result of weathering processes in uplifts, typically mountains.

  • Base Level Changes: Changes in base level are critical, as they allow for continuous deposition as the valley floor sinks along faults.

    • This ensures that the deposition area remains lower than the surrounding mountains.

  • Preservation Potential: Sediments deposited are more prone to weathering when they are located above base level, impacting the preservation potential of the stratigraphic record.

  • Morphodynamics: Alluvial fans play a crucial role not only in local morphodynamics of mountain areas and basinal sectors but also influence the long-term evolution of sediment-routing systems. These impacts extend to the propagation of stratigraphic signals of environmental change and the preservation potential of geological records on a larger scale.

Learning Outcomes for GEOL 30040

  • Ability to recognize alluvial fan deposits in geological records.

  • Distinguish alluvial fan deposits from gravel-bed river deposits.

  • Account for different morphological features of alluvial fans.

  • Explain controls on fan scale.

  • Reconstruct depositional processes and fan types from their deposits.

  • Interpret vertical grain size changes in fan deposits.

  • Use fan deposits in a broader basin-scale context.

  • Evaluate responses of alluvial fans to tectonic and climate controls.

  • Discuss the presence of fan-like features on Mars.

Fan Terminology and Scale

  • Critical Deposition Factors: Deposition on a fan is driven by:

    1. Spatial expansion of flows.

    2. Loss of transport capacity as flows leave hinterland valleys.

  • Cone Geometry: Alluvial fans are most commonly observed in desert environments.

Fan Gradients

  • Gradient Characteristics:

    • Alluvial fan gradients are typically steeper than those of rivers.

    • Two main categories:

    1. Steeper fans dominated by gravity transport (debris flow).

    2. Shallower fans characterized by water-lain processes (stream and sheet flood).

  • Examples: Blair & McPherson (1994) Badwater Fan in Death Valley exhibits gradients from 1-25 degrees, with higher gradients found in proximal deposits and less steep towards the fan toe.

    • Intersection points indicate a transition from channels to fan lobes.

Fan Clast Size Cycles

  • Maximum Clast Size (MCS): Average maximum diameter of the 10 largest clasts in a unit area.

  • Implications of Grain Size: Coarsening upward sequences may reveal sedimentary processes, while muddy deposits may signify periods of maximum displacement rates relative to controlling faults.

Clast Isopleths

  • Example: Triassic conglomerates in South Wales demonstrate systematic grain size variations, indicative of fan scale and palaeoflow direction.

  • Isopleth Definition: A line representing equal particle size.

  • Geometric Analysis: Grain size measurements aid in determining the geometry of alluvial fans.

Fan Scale Observations

  • Rule of Thumb: Average fan area approximates 50% of catchment area.

  • Factors Influencing Scale:

    • Subsidence rate and geological characteristics influence fan size.

    • Notable log-log relationships exist between hinterland drainage basin area and fan scale.

  • Case Study Analysis: Research on 116 modern arid region fans in SW USA shows variability often dictated by subsidence and lithological differences.

Tectonics vs. Climate Effects

  • Climate Impacts: Climate influences factors like available water for weathering and sediment transport, sediment generation, and vegetation cover.

  • Tectonic Controls: Tectonic activity impacts gradients, drainage basin scale, entry points, and the space available for sediment accumulation.

  • Alluvial Fan Adaptive Responses: Fans adapt through changes in the area-to-sediment (A:S) ratio, and deposits reflect cyclical environmental conditions, responding to both subsidence and climatic variations.

Fan Entrenchment Topics

  • Abandoned Fan Segments: These can turn into soil formation sites or may undergo wind reworking, leaving distinctive deposits.

  • Causes of Entrapment: Factors include climate variability, tectonic impacts, or physical truncation of fan toes, leading to inactive or abandoned fan parts.

Types of Fans

  • Fan Delta: Forms when fans reach lakes or marine areas.

  • Megafans: Large rivers produce continuous fans that form broad, low-gradient depositional systems.

  • Terminal Fans: Occur when rivers lose water to evaporation/infiltration—channels decrease in size compared to tributive systems.

Alluvial Fan Analysis in Ireland

  • Devonian river deposits in SW Ireland formed large terminal fans as channels became progressively shallower.

Primary Depositional Processes

  1. Debris Flow: Poorly sorted and clay-rich, resulting in mounded deposits.

  2. Sheetflood: Exceeds water capacity, non-channelized, and leaves graded deposits.

  3. Stream Flow/Flood: Channelized, persistent flow marked by bedforms and gravel bars.

  4. Sieve Deposits: Features mounded gravels with open frameworks, indicative of uncollapsed interstitial areas.

  5. Inactive Sectors: Characterized by secondary soil and wind-blown deposits.

  • Transport Mechanisms in Arid Regions: Alluvial fan sediments are primarily transported through debris flow, stream flow, and mudflow.

Facies Succession

  • Coarsening Upward Profile: An alluvial fan's profile tends to be inversely graded with distal sections at the base, building outwards over time.

Spectra of Debris to Fluid Flow

  • Description of the transition between messy debris flows (lacking internal structure) to organized fluid flow scenarios is provided (reference study in Neogene, Tabernas, SE Spain, Nemec 1990).

Gravel-Bed Stream Deposits

  • Observations: Palaeosoil evidence indicates fan entrenchment, with noticeable grain size increases in vertical sections.

  • Pulses of Sediment Supply: Can indicate periods of fan abandonment and subsequent weathering—depicts the dynamics of braided river systems.

Sheetflood Deposits

  • Triassic Evidence: Examined in the Czech Republic revealing non-channelized deposits with a lack of cross-bedding, indicative of waning flow.

Sieve Deposits and Palaeosols

  • Characterized by calcite cementation, sieve lobe development gradually compacts multiple sublobes during deposition, reflecting the effects of intense precipitation.

Controls on Processes

  • Lithological Variations: Different rock types in catchments impact flow types and fan characteristics.

    • Case Study: Death Valley illustrates contrasting fan dominance (stream-flow vs debris-flow) based on local geology and erosion rates.

Death Valley Analysis

  • Differential impacts of tectonics on climate, fan activity, and the resultant asymmetry in fan distribution along the valley floor.

Vertical Clast Size Cycles

  1. Variation in grading is not merely inversely graded and reflects fan growth dynamics.

  2. Entrained finer sediments indicate deeper fans may show morphology differentiations.

  3. Changes in gradient due to fault activity impact sediment transport distance and material sorting.

Interactions Between Fans and Climate

  • Palaeosols Evidence: Inferred climatic conditions correlate with recorded periods of incision on fans and climate-driven precipitation shifts, notably the 8.2-kiloyear global cooling event.

Climate and Fans

  • Palaeoenvironmental Data: Recent research indicates associations between fan deposits and climate changes during the Holocene. Changes in sediment dynamics recorded by palaeosol development could signify broader climatic shifts, such as periods of increased aridity.

Alluvial Fans and Historical Weather Events

  • Case Study: Effects of Hurricane Hillary in Death Valley (August 2023), highlighting implications of rapid rainfall on fan dynamics.

Alluvial Fans in Viticulture

  • Vineyard Significance: Alluvial fans contribute to ideal vineyard locations due to well-drained soils, with notable examples in Alsace, France, and various locations in South America.

Summary of Alluvial Fans

  • Alluvial fans are prevalent near fault-controlled ranges and are characterized by a range of features, including variations in gradient and deposition processes.

  • The area of the fan typically correlates with drainage basin area under varying subsidence rates and lithology.

  • Tectonic processes drive fan development, with fans reflecting climactic shifts across geological timescales.