Alluvial Fans

as fan advances into a valley what happens to slope?

-slope decreases as the channel cuts deeper and the fan advances outwards

what is goal of stream

-smooth longitudinal profile that flattens out at the local base level defined by the valley floor -to erase the effect of the fault offset

if more time between big fault movements,

-the more the fan can prograde -the less time, the smaller the fans

how clast-supported conglomerates are formed

-water-lain fan boulders that are clast-supported -next flood will wash finer clasts between the boulders

why do thickest sediments accumulate near thrust?

-wetter facies (coal swamp, evaporites) tend to be closer to mountain belt -also bc of greater subsidence there

2D alluvial fan model step 1

1. movement along a normal fault leads to a fault scarp and a sudden drop to a nearly flat valley floor -fault = big break in profile, and valley is flat -mountain stream ends up plunging down the fault scarp and hitting the valley floor

are alluvial deposits the same as alluvial fan deposits?

NO

what type of fault in foreland basins form alluvial fans

active thrust faults

examples of alluvial deposits

alluvial fan, braided stream, meandering river deposits

flash floods in deserts...

are especially dramatic sediment movers

why don't alluvial fans form where there are classic smooth longitudinal stream profiles?

because the gradual decrease in stream energy means that boulders, cobbles, etc., tend to be left upstream in the head waters

at distal fan, many fans grade into..

broad sandy gravel deposits

river channels are ____ by foreland basin

captured

sites of deposition on fan

change over time -seen with desert varnish

below about 15km depth, the rocks become more

ductile, which leads the normal fault to flatten out

when do mountain streams transport most of their sediment load?

during relatively rare major flood events

overbank (flood) deposits tend to be

finer-grained

what do alluvial fans need

flat valley floors

in mid- to distal fan we find...

gravel conglomerates

active faults (normal or thrust) are....

great way to juxtapose topographic relief and flat valleys

crust cannot maintain open overhanging gap 15km deep, so

hanging wall slides down across the footwall -footwall also rises

distal fans can also grade into

lake deposits

over time, the smoother profile allows....

larger sediments to advance across the valley floor

thinner CUS cycles reflect periods of

more frequent fault-driven earthquakes (less time for fans to prograde)

extension makes ______ faults

normal

basin and range consists of

normal faults

both sides of a rift basin may have coarse-grained clastics because..

often a minor antithetic normal fault creates some movement on opposite side of the valley

debris flow lobes accumulate where?

on the proximal fan (fanhead)

alluvial fans along thrust faults tend to get

overrun by the hanging wall

where would largest grains settle

proximal fan; closest to canyon mouth

thinner coarsening upward sequences cycles reflect...

pulses of uplift along a boundary fault responsible for the alluvial fan deposition in the first place

Q(al) =

quaternary alluvium

imbrication

refers to a primary depositional fabric consisting of a preferred orientation of clasts such that they overlap one another in a consistent fashion, rather like a run of toppled dominoes.

alluvial

refers to any sediment deposited by rivers and streams

alluvium

relatively recent (unconsolidated) sediment deposited by rivers and streams

with thrust faults, alluvial fans over time migrate in ___

same direction as the thrusting

in distal fan/lake deposits, clastics grade from

sands along shore to muds

thicker CUS sequences reflect periods of

slower fault movement

inactive faults allow...

smooth profile to become re-established

Death Valley

stretching and normal faulting have produced a series of basins (valleys) and ranges (mountains)

alluvial fans form in

tectonically active areas

some large floods entrain so much sediment that...

they become debris flows

plate-shaped clasts always end up imbricated if...

they were deposited by running water

thickest sediments accumulate near

thrust

______ sometimes folds the overridden fan deposits

thrusting

extension causes valley floors to

tilt down toward the normal faults as the block between the normal faults rotates

broad sandy gravel deposits mapped as...

-quaternary alluvium -mostly mixture of distal alluvial fan and braided stream deposits

what triggers the sudden deposition of sediment

-SUDDEN decrease in slope -alluvial fans form best where mountain canyons suddenly dump onto relatively flat valley floors

walther's law with alluvial fan

-as fan progrades, a single spot first sees valley floor sediments, then distal fan sediments, then medial fan sediments, and finally proximal fan sediments

fines are deposited when?

-at end of big flow events -or when flood flows out of channel and soaks between grains

chaotic jumble on debris flows means

-clasts sitting in ways that seem unlikely with a water-lain traction deposit -large oblong clasts sitting on end -oblong clasts tilted in various ways (showing no imbrication)

alluvial fan general sequence

-coarsening upwards sequence

zig-zag facies shifts

-each CUS followed by a sudden fining -prograding fan was reset by sudden fault movement that sank fan, so valley sediments shifted back toward fault

What happens when fault starts slipping again?

-each big slip makes a big earthquake and can create a meter or 2 vertical offset on the fault -mountains rise, valley sinks

debris flow lobes

-extremely poorly sorted chaotic jumbles of everything from boulders to clay -still clast-supported

what are alluvial fans

-fan-shaped deposits consisting of everything from boulders to clay that forms when a stream leaves a mountain and suddenly dumps everything into a relatively flat valley

what dominates center of rift basin with major fault and antithetic fault

-fluvial and lacustrine (lake) sediments dominate the center -depo-centers closer to the main normal fault

playa lakes

-form in deserts -water evaporates between rare storm events and leaves white salt deposits behind

bajada

-geomorphic surface that forms when fans from adjacent valleys merge and coalesce -depositional part of a piedmont

rapidly slowing flow when flood hits valley floor means

-instant deposition of the bedload -rapid deposition of the suspended load

debris flows lack.... but commonly show...

-internal bedding -inverse grading at the base

how to recognize debris flows on fans

-lack of sorting -chaotic jumble

surficial deposits

-look big on surface but don't extend below valley floor

when a raging flood hits a flat valley floor, the flow rapidly slows bc

-lost its slope - has turned into a shallow flow spread out over the valley bottom -> shallow = more slowed by frictional shear forces

Desert varnish on alluvial fans

-makes rust colors -dark hard film of oxides formed on exposed rock surfaces in arid regions -darkness of rust proportional to age of the exposed surface

coarsest clasts deposition in fan 1 vs fans 2 and 3

-mountain stream is very fast -fan 1 sees a very abrupt decrease in slope, so stuff deposited instantly -fans 2 and 3 see more gradual changes; coarsest sediments can advance farther across valley floor

what happens to alluvial fans when fault moves again?

-movement along fault drops the fan -distal fan then gets buried by lake and/or river sediments -then fan can start prograding again

coarsest sediments accumulate ______, finest sediments _______

-near the fault -farthest away

how to build a fan

-need abundant clasts with a large size range -mountains often feature lots of physical and some chemical weathering that produces many loose blocks of rock -combination of mass wasting plus a really big rainstorm can flush quite a lot of coarse sediment down a canyon

in basin and range, where is the major fault?

-on side with smaller alluvial fans -major fault causes dip in valley, so sediments get covered, and not flat surface to extend over

overall thinning near top may reflect

-overall slower offsets resulting in less topographic relief

3 parts of alluvial fans

-proximal fan: top -mid-fan: middle -distal fan: bottom

2D alluvial fan model step 2

2. stream carries sediment down canyon, creates a steep fan along the fault surface -as time passes, fan builds out away from the fault -at first fan is very steep, as it advances slope becomes gradually less steep -streams goal is a smooth longitudinal profile that flattens out at the local base level defined by the valley floor -goal is to erase the effect of the fault offset

to develop an alluvial fan facies model, divide into how many parts?

3

2D alluvial fan model step 3

3. over time, the stream cuts a canyon deeper into the mountains, starting at the fault scarp and working backwards (headwards). Fan progrades into the valley -overall smoother longitudinal profile

2D alluvial fan model step 4

4. canyon cuts deeper into mountain and the top of the fan. Alluvial fan continues to prograde into the valley

normal faults form at an angle of

60 degrees below the horizontal

Walther's Law

Vertical change in facies result form lateral changes in environments