Question and Answer: Final Exam
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1: What is "uniformitarianism", and how is it applied in the
interpretation of sedimentary rocks? How representative are observations
made of processes on the modern Earth to the interpretation of ancient (1)
primary sedimentary structures (e.g. parallel lamination, cross-bedding) in
siliciclastic and carbonate sandstones, (2) formation of marine carbonates
in general including dolomite, (3) distribution of clay minerals in the
ocean, (4) continental shelf deposits, (5) trace fossils, and (6) fluvial
deposits.
ANSWER
2. Some sedimentary features and processes are useful indicators of
latitude, while others are unrelated to it. For each of the following tell
if it is latitude-dependent, and if so why, and in what sense. (1)
carbonate production, (2) clay mineralogy, (3) river type (braided vs.
meandering), (4) wave ripples, (5) lithic wacke
ANSWER
3. For each of the sedimentary features listed below, tell whether it can
be found in the each of the following environments: ff - fluvial
floodplain; fc - fluvial channel; dl - delta; cs - continental shelf; ap -
abyssal plain.
Features: kaolinite, trough cross-stratification, parting lineation, reef
carbonates, glauconite, normal grading, aragonite, quartz arenite,
coarsening-upward sequence
ANSWER
4. Consider a sedimentary basin with a granitic source terrain undergoing
constant subsidence. The climate changes slowly from cool and arid to warm
and humid. What vertical changes would you expect to see as a result of
this in the following: clay abundance and mineralogy, fluvial system, delta?
ANSWER
5. What vertical sequence of sedimentary environments would you expect to
find represented in rocks deposited in each of the following basin
scenarios? Assume sediment supply remains constant. What kinds of basins
might show these subsidence patterns?
(1) a basin that started out with a high subsidence that decreased through time
(2) a basin that started with a low subsidence rate that increased through time
(3) a basin in which subsidence remained constant and in balance with
sediment supply
ANSWER
6. At the beginning of the course we discussed the idea that the
sedimentary record is usually incomplete in the sense that deposition
almost never occurs continuously. Taking the fluvial and continental-shelf
environments as examples, discuss the main processes that cause deposition
to be discontinuous (episodic). Be sure to include some sense of the time
scale on which each process operates.
ANSWER
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A 1: Uniformitarianism is the idea that the processes that have formed the earth are essentially no different than those active on and in the earth
today. In its strictest (original) form, it held that the processes also
had to act at similar rates to those observed today.
Strict uniformitarianism is considered untenable today because we know that
the earth has evolved through time, changing the rates and in some cases
the nature of many processes. We also know that the earth's evolution has
been influenced disproportionately by rare extreme events ranging from
infrequent large floods and storms to magnetic reversals and meteorite
impacts. We also have a much greater appreciation for the variety of global
conditions the earth can exhibit, and the uniqueness of the one we have
evolved in. All of this has led to replacement of strict uniformitarianism
with the idea that the only thing that absolutely must be permanent through
time is the basic laws of nature. Indeed, it would be impossible to study
the history of the earth scientifically without this postulate. Hence we
attempt to base our interepretation of earth history on basic laws as
closely as possible, asking ourselves as we study the modern world whether
the relations we see are there because they must be, or because of some
quirk of our present world.
(1) Primary sedimentary structures. Although we still have a ways to go in
understanding the physics of formation of these, all the insight we have
says that they are a very basic feature of the interaction of sedimentary
particles and moving fluids, neither of which depends explicitly on time.
There is no reason to think fundamental observations of bedforms in the
modern world should not be applicable throughout geologic time.
(2) Marine carbonates. Although carbonate geochemistry is also fundamental
and applicable throughout geologic time, marine carbonates in the modern
world are "born, not made" as Clint Cowan said, and hence very much
influenced by, above all, the evolution of life. Early carbonates are all
algal or bacterial until Cambrian time, when shelled organisms appear.
Since then the nature of the dominant carbonate producers has changed
dramatically through time - for instance, the advent of rudist bivalves as
major reef formers in Mesozoic time and the dominance of corals today. It
would be a terrible mistake to analyze ancient carbonates based only on
what we can see today.
Dolomite formation is sensitive to oceanic chemistry. Much of carbonate
formation in the ocean today is conditioned by the fact that the ocean
contains too little Mg to form dolomite directly but too much to form
low-Mg calcite. There is no natural law that requires that this have been
true through geologic time. In particular, some researchers believe the
early ocean was much richer in Mg than today's, which might have caused
widespread formation of primary dolomite.
(3) Clay minerals. The fundamental geochemistry of clay-mineral formation
during weathering has not changed through geologic time. However, there are
important aspects of clay-mineral production and transport that are
dependent on life. First, the production of clay minerals is strongly
modulated by soils, which are in turn very dependent on terrestrial life.
Soils as we know them could not have formed before multicellular
terrestrial life evolved. One might expect that for the same overall
weathering regime, the balance of mechanical to chemical weathering was
shifted toward mechanical in the absence of strong soil formation. Second,
the transport of clay minerals to the sea floor is dependent on marine
organisms in ways that are still not well understood. There has certainly
been life in the seas for most of the Earth's history, but it is not clear
if its role in helping clay minerals reach the sea floor was as prominent
as it is now.
(4) Continental shelf. The modern continental shelf, as we discussed in
class, is generally a poor analog for what we find in the stratigraphic
record. We have evolved during an interval of large, high-frequency changes
in sea level, which is in itself not representative of most of earth
history. To make matters worse, the present sea level has been stable only
for the last 5,000 years or so, so the modern continental shelf is in most
of the world essentially a drowned fluvial system that is currently being
modified by submarine processes ("palimpsest" sediments). In contrast,
ancient shelf deposits often comprise great thicknesses of rock showing
evidence of nothing but shallow-marine conditions throughout. At this
point, it is still not entirely clear how to use observations of processes
on the modern shelf to analyze ancient shelf deposits.
(5) Trace fossils. Again, being closely linked to life, this is an area
where we expect that ancient forms could be quite different from modern
ones. We are helped in part by the fact that trace fossils are not the
remains of organisms but rather of behaviors. It seems reasonable to think,
for example,that the essential nature of deposit and suspension feeding, or
of the factors favoring each of them, have not changed with time, although
the specific implementations certainly have.
(6) Fluvial deposits. There is no reason to think that the basic physical
processes of rivers have changed through geologic time. But as we discussed
in class, most rivers are really a kind of biomechanical system in which
plants play an important role. In this sense rivers may well have undergone
an evolution. In particular, the advent of land plants in Silurian time
appears to have caused a shift favoring meandering. The role of plants in
producing soils has probably also contributed to higher production as well
as better retention of fines and hence better development of floodplains,
also favoring meandering.
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A 2: (1) carbonate production is inversely related to latitude (higher
production at lower latitudes) because it is temperature sensitive.
(2) clay mineralogy is quite sensitive to latitude because the degree of
alteration depends on the degree of chemical weathering, which is
controlled by temperature and availability of water. Highly weathered clays
like kaolinite are found at low latitudues, while unweathered ones like
chlorite are at high latitudes.
(3) river type is indirectly affected by latitude because of the role of
vegetation in stabilizing channel banks and promoting meandering. Braided
rivers are thus more common at high latitudes, though there are some very
important exceptions, like the Brahmaputra.
(4) wave ripples are unrelated to latitude.
(5) lithic wacke is a sandstone with a high proportion of lithic fragments
among the framework grains and more than 15% matrix. Although this sediment
type could occur at any latitude, production of unaltered lithic fragments
is favored by mechanical weathering and hence high latitudes. The
correlation is positive.
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A 3: kaolinite: ff, dl,cs, ap. Clay is not usually found in river channels,
but in the unlikely event that it were, it could certainly be kaolinite.
trough x-stratification: fc, ff, dl, cs certainly. Large-scale
cross-stratification is unusual (though not impossible) in abyssal plain
deposits (turbidites) for reasons that are still not entirely clear but
probably have to do with the short-lived nature of the flows.
parting lineation: ff, fc, dl, cs, ap (all the environments)
reef carbonates: cs only, at least if they are in place. Conceivably reef
carbonates from the shelf could be transported to abyssal depths by
slumping, but if you want to include this in your answer you should be sure
to explain why.
glauconite: cs only, subject to the same caveat as reef carbonates
normal grading: ff, cs, ap are the best answers: crevasse splays on
floodplains show normal grading, as do tempestites and turbidites (found in
cs and ap respectively). If you include fining-upward sequences (which is
not strictly correct since normal grading is something that occurs within a
single bed), then you could add fc (from the classic point-bar sequence)
and by extension dl since deltas have river channels at their tops. Again,
this would be a place where it would be important to explain your reasoning
on an exam.
aragonite: cs is the obvious answer here. You could argue for ff, since
carbonates do sometimes form in soils (as you have seen in class), though
we didn't talk about what kind of carbonate. You could also argue for ap as
long as the abyssal plain was above the CCD - not likely, but not
impossible either. Explain, explain, explain.
quartz arenite: ff, fc, dl, cs, ap (all the environments) This is a
compositional rock name, and so could be found anywhere you could get a
well sorted sand.
coarsening-upward sequence: dl. This is the obvious conventional (and the
best) answer. You could argue for other environments if you invoke
progradation (e.g. a fluvial environment in which more proximal parts of
the system prograde over more distal parts). If you include inverse
grading, which is again not really correct, you could also argue for any
environment with mass flow deposits. But none of these are listed in the
choices I gave for this particular question.
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A 4: The total amount of clay delivered to the basin would increase up
section, and the mineralogy would change from illite-dominated to
kaolinite-dominated. The increase in clay supply would drive the fluvial
system toward meandering, and would increase the amount of floodplain fines
in the section. The deltaic system would be driven towards a bird's foot
morphology and the increase in ratio of suspended load to bed load would be
reflected in longer and more tangential-style deltaic foresets.
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A 5:
(1) this basin would start off undersupplied, which usually means it would
fill with water. Sedimentation would initially be in deep water, probably
as turbidites. The upward succession would be towards shallower-water
environments as the diminishing subsidence rate allowed sedimentation to
catch up with subsidence and fill the basin in. The uppermost deposits
would be subaerial if the process continued to completion. This subsidence
pattern is typical of extensional basins (though in reality sedimentation
often keeps up with subsidence throughout the life of these basins).
(2) this basin would start out with supply keeping up with subsidence and
so would be filled with sediment. The initial (lowest stratigraphically)
deposits would be subaerial or perhaps shallow-marine, depending on
sea-level variations. The accelerating subsidence would eventually outstrip
supply, which would be seen in the deposits as progressive deepening of the
water. The topmost deposits would be turbidites if the process went to
completion. This subsidence pattern is typical of foreland (thrust-related)
basins.
(3) the ongoing balance between subsidence and sediment supply in this
scenario would be reflected in a continuous vertical succession of deposits
showing neither deepening nor shallowing trends overall, since the basin
surface would remain at its initial level (usually at or above sea level).
There is no basin type for which constant subsidence rate is the rule, but
some strike-slip basins show significant periods of constant (albeit very
rapid) subsidence.
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A 6:
Fluvial: floods (years to tens of years), channel migration, e.g.
meander migration and cutoff (tens to hundreds of years), avulsion
(hundreds to thousands of years)
Continental shelf: storms (years to tens of years), sea-level change
(thousands of years to tens of millions of years), and, though indirect,
floods in the river system supplying the shelf (years to tens of years).
These sources of variability are intrinsic to the environments in question.
Other possible sources of variation in sedimentation rate, which are
external to the depositional environment (hence applicable to both rivers
and shelves) include: climate fluctuations (time scales not well known:
decades to hundreds of thousands of years) and tectonic fluctuations in
sediment supply and subsidence (time scales again not well known: thousands
to millions of years, at least).
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