Alluvial Fans

Alluvial Fans

I. Continental Depositional Systems: 4 Main Types

1. Fluvial (rivers and streams)

2. Desert (eolian sand dunes)

3. Lacustrine (lakes)

4. Glacial

Of course, these are not mutually exclusive. Rivers in deserts, for example. Continental deposits are DOMINANTLY siliciclastic, fossils are rare and never marine. Tend to be reddish (redbeds) or yellowish or dirty brown in color. May find vertebrate fossils, and certain environments (swamps, some lake sediments) can be FULL of plant matter (coal, organic carbon for oil). Fresh water limestones and evaporates occur, but these are rare compared to good old sand and mud.

II. Fluvial deposits include all sediments laid down by rivers and streams. Three main types:

1. Alluvial Fan

2. Braided River

3. Meandering River

 Alluvial Fan: a broad fan-shaped deposit consisting of everything from boulders to mud that forms when a stream (especially in semi-arid settings) leaves a narrow mountain valley (canyon) and dumps onto an open plain.

1. In between major floods, physical and some chemical weathering causes mountain slopes to become littered with loose sediment.

2. A major storm washes sediment into the mountain gullies, where the water flow becomes so focused and deep that raging floods sweep sediment of all sizes down the canyon.

3. When the flood reaches the edge of the mountain range, it dumps out of the narrow mountain canyon onto the broad valley floor.

4. Instead of deep, channelized flow, you suddenly have broad shallow flow. Friction with the land affects most of the depth of the flow (draw vertical velocity profile) and thus dramatically slows the current velocities. This causes much of the sediment load to drops like rocks. Large sediment deposited immediately, finer stuff washed further down slope. A large fan-shaped pile of sediment accumulates.

5.Fan growth in cross-section. A big movement along a normal fault will create a cliff along a mountain front called a fault scarp. The first-deposited sediments form a small, steep fan of coarse sediments (boulders, gravel). As more sediments accumulate, the fan grows outward (progrades) and the slope is reduced. Also, erosion cuts into the floor of mountain canyon and thus also the first-deposited top of the fan.

6. Not whole fan surface is active at a given time. During normal floods, all water and sediment tends to wash down one particular area. Eventually, accumulation of sediment downstream makes it easier for a new flood to flow over another part of the fan. This switch from one side to another is called "avulsion".

Some Vocabulary and Features (Overhead):

1.Radial or Longitudinal Cross-section: follows the main stream flow Radial x-section is concave up, generally wedge-shaped in profile Cross-Fan Cross-section: cuts across flow lines Cross-fan section: lens-shaped profile.

2.Upper Fan (proximal fan or fanhead): single stream channel often entrenched as much as 20-30 m below surface of fan nearest the mountain front; meets surface at midfan. A new flood may cut new channel, and leave the old channel to get filled up with debris. Coarsest sediments.

3.Midfan has a kinder, gentler slope, gravelly/sandy braided stream systems (More on braided stream sediments in the next lecture topic!)

4.Distal fan (fan base) no well-defined channels; the gentlest slope and finest sediments (sands, silts, muds). The distal fan can grade into the silts, clays, and evaporites of playa lakes (desert lakes filled only during wet seasons). The distal edge of the fan may normally see only fine lake sediments. However, a large flood may carry a pulse of gravel and even boulders into the lake. Fault uplift followed by progradation: coarsening upwards sequence may migrate over lake sediments. A single flood event would bring just a pulse of sediments. Long-term evolution. Fault movement drops basin/raises highland. Old fan surface carried downwards, and tilted lake makes lake sediments migrate toward fault scarp. Soon, fan starts to build out again from fault face. As it grows, coarser and coarser sediments migrate out into the lake. This happens over and over again in an area being stretched apart. Thus, a core taken a certain distance from the fault scarp shows a whole series of coarsening upwards sequences: mixed boulders, gravels, and sand interfinger with fine lacustrine sediments (muds and evaporites).