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Alluvial Fans

Alluvial Fans. Composition: siliciclastic , but all types of clasts are possible (usually compositionally immature). Texture : Immature (close to the sediment source), poorly sorted, large-sized clasts are common, clasts can range from angular to rounded.

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Alluvial Fans

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  1. Alluvial Fans

  2. Composition:siliciclastic, but all types of clasts are possible (usually compositionally immature)

  3. Texture: Immature (close to the sediment source), poorly sorted, large-sized clasts are common, clasts can range from angular to rounded

  4. Structures: beds with inverse grading, clast imbrication (with long axes parallel to the direction of flow or perpendicular to flow); subhorizontal beds (with normal grading or poorly-developed grading), rare cross stratification

  5. Geometry of the deposit: fan shaped (map view); wedge or lens (profile view)

  6. Lateral variations: rivers, lakes, playas

  7. Lateral variations: sometimes a series of alluvial fans form along the edge of a mountain range (bajada) Photos taken from satellites

  8. Vertical sequence: generally, sequences thicken and coarsen upward—prograding fans (retrogradational sequences are also possible, for example, when activity on a fan ceases)

  9. Biology: bones of terrestrial animals, pieces of terrestrial plants (but because of the high oxygen content and the porosity of the sediments, organic material disappears very rapidly) Chemical processes: sediments often have red or orange colors because of oxidation of iron minerals in a subaerial environment; soils often develop on inactive parts of alluvial fans

  10. Depositional processes?

  11. Depositional processes: debris flows, traction flows—sheet flow with rolling and saltating clasts; flow in channels that are shallow and broad. Alluvial fans may grade downstream into braided river environments. Ephemeral flow on fan surface

  12. Depositional setting: areas with high relief and steep slopes in tectonically-active regions, arid to semi-arid climates

  13. The most distinctive characteristics: coarse sediments and debris flows (poor sorting)

  14. Rocky Mtns, USA and Canadá Humid-region fans also found

  15. Fan deltas (coarse clasts, like alluvial fans, but adjacent to ocean or lake) Gulf of California (Baja México)

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