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
