Question and Answer Set 2
1: Although there are many sand-bed braided rivers in the world, and a
number of gravel-bed meandering rivers, generally speaking gravel rivers
are more likely to be braided and sand-bed rivers to be meandering. Explain
this observation based on your understanding of the basic mechanisms that
favor meandering and braiding.
ANSWER
2. Consider a meandering river system with sand and mud undergoing slow
aggradation (vertical sediment accumulation) in a sedimentary basin.
Suddenly the supply of sand to the river system is increased by an order of
magnitude. Sketch the vertical succession of deposits that you would expect
from this event. Assume equal time intervals before and after the change.
ANSWER
3. The fluvial system shown in the sketch below comprises, starting at the
upstream end, a coarse-grained alluvial fan, a gravel-bed braided segment,
and a sandy meandering segment. Describe the vertical sedimentary sequence
that would be produced by progradation of this system. What would determine
the thickness of this succession?
ANSWER
4. Suppose that the rate of production of mud (silt + clay) relative to
sand has been constant throughout geologic time. If that were true, how
would the rate of delivery of mud to the sea have changed since the end of
Precambrian time?
ANSWER
5. According to the theory of avulsion and alluvial architecture that we
discussed in class (in which the avulsion frequency was assumed constant),
how would the rate of delivery of fine sediment to the shoreline by fluvial
systems depend on subsidence rate?
ANSWER
6. Consider a broad sandy depositional basin immediately west of a N-S
oriented mountain range. If the climate is monsoonal (the definition of a
monsoonal climate does not involve rain or storms; it means only that the
direction of the prevailing wind changes seasonally) so that the winds blow
W to E for half the year and E to W the other half, what sort of
sedimentary succession would you expect to find in the basin?
ANSWER
7. Describe the type of lamination you would expect to find for wind
ripples transporting a mixture of well sorted quartz sand and well sorted
magnetite of equal median size.
ANSWER
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 1: As you know, the basic requirement for meandering is some means of
stabilizing the overbank areas so that the flow is constrained to remain in
a single channel long enough to develop stable meanders. On the Earth today
the main mechanism of floodplain stabilization is vegetation, which works
both directly via root growth and indirectly by offering resistance to
overbank flow and by trapping fine sediments in suspension. Although this
is not in itself grain-size dependent, it is easier to plants to take root
and thrive in finer sediments, which are also generally richer in
nutrients. Also finer sediments are generally associated with lower slopes,
which tend to make overbank flows slower and thus make it easier for fine
sediments to settle out of them. A final factor is that gravel bed rivers
are generally found in more proximal (upstream) parts of the depositional
system and thus are subject to higher sediment loads than their finer
grained and more distal equivalents.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 2: The key thing to realize here is that the dramatic increase in sand load
would cause two things: an increase in the deposition rate, and a switch
from meandering to braiding. The base of the deposit would have classical
point-bar sequences with well developed fining upward and point-bar
(lateral accretion) sets. This meandering-dominated part would be much
thinner than the upper part, because as specified in the question both
parts represent equal time intervals. The upper part would be deposited
faster, would be much sandier, and would show the lack of stratigraphic
organization characteristic of braided-river deposits. There would be less
lateral-accretion depostion, and preserved channels would be wider,
shallower, and more symmetric.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 3: As shown in the sketch below, the vertical succession produced by
progradation of this fluvial system would coarsen upwards from the sand
associated with the meandering system to the coarse gravel of the alluvial
fan. The total thickness would be at least equal to the vertical relief of
the entire fluvial system. Of equal importance are the smaller-scale
sequences that would make up this overall CU sequence. The base of the
sequence would comprise deposits of the meandering segment and thus would
have well developed fining upward point-bar (lateral accretion) sets. The
thickness of these would be set by the flow depth and consequently would be
much smaller than that of the entire vertical succession. Going up the
deposits would coarsen and become less well organized. There would be less
lateral-accretion depostion, and preserved channels would be wider,
shallower, and more symmetric. The fraction of floodplain (ovebank)
deposition would decrease going upward as well. The coarse fan deposits at
the top of the succession would be the least organized of all, with little
or no channelization. Mass-flow deposits (matrix-supported, unstratified),
if present at all, would become more frequent going upward.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 4: It would have decreased because prior to the extensive colonization of
dry land by plants the only mechanism to trap mud on floodplains would have
been trapping in the pore spaces between larger grains. This is much less
efficient than trapping by plant stems and roots, so less mud would have
been retained in the world's fluvial systems.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 5: According to that theory (of which many of you were justifiably
suspicious), decreases in subsidence rate lead to less preservation of
floodplain fines in the deposit, i.e.a higher sand content. Thus the supply
of fines to the shoreline would vary inversely as the subsidence rate.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 6: The key to this question is that the basin would be very wet for half
the year (when the winds blow from the W and it is on the upwind side of
the mountains) and very dry the other half (when it is in the rain shadow
of the mountains). Hence the deposit would consist of a mix of fluvial and
eolian deposits. The latter would be recognizable by the criteria we
discussed in class (inversely graded wind-ripple lamination,
grain-fall/grain-flow deposits). The frequent drying would probably make
life difficult for vegetation in general, and the wind during the dry
season would help remove fines from the system, so one might expect the
fluvial phases of the system to be braided.
BACK NEXT
No Peeking!
No Peeking!
No Peeking!
No Peeking!
A 7: In class we explained the inverse grading of wind ripples in terms of
the fact that the coarsest material on wind ripples ends up on the crest
because the shear stress is highest there. For the case in the question,
the denser magnetite would end up on the crest of the wind ripples for the
same reason. Hence the laminae produced by the ripples would show grading
with increasing magnetite concentrations going upward.
BACK