2. The figure below shows a map of a tributary to the Amazon river. The river is 10 m deep, it carries sediment ranging in size from gravel down to clay, and the floodplain is heavily vegetated. Sketch what you would expect to see in a trench 15 m deep dug along the line A-A' shown in the map.
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3. Two sand layers are deposited at the same rate. They both have the same framework grain size, but one layer is an arenite and the other a wacke. Which unit would be more likely to show soft-sediment deformation, such as fluid-escape structures? ANSWER
4. Of the three sedimentary structures listed below, one is NOT indicative of conditions that would increase the chances of soft-sediment deformation. Which one is it, and why?
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5. We have talked in class about suspension feeding and deposit feeding as the two main types of sediment-related feeding behavior. Why do you suppose there is no such thing as "bedload feeding"? If there were such a feeding behavior, how might its trace fossils differ from those of suspension and deposit feeders? ANSWER
6. It is thought that Mars has had short-lived water floods when meteorite impacts melt frozen subsurface water. If such flows deposited sediments at rates comparable to those on Earth, would you expect Mars's reduced gravity to make it more or less likely that these sediments would show soft-sediment deformation relative to Earth? ANSWER
7. It has been suggested that in addition to carbon-based life as we know it, it might also be possible to develop silicon-based life forms. How would silicon-based life differ from carbon-based life in the formation of trace fossils if they had evolved on Earth? ANSWER
8. Given two sands with the same grain size and deposited at the same rate, how would reducing the density of the sand particles affect the probability of soft-sediment deformation in the sediment? ANSWER
9. In class we discussed the observation that the clay minerals in the world's oceans are zoned according to latitude, with kaolinite most abundant at low latitutudes, illite at mid-latitudes, and chlorite at high latitudes.
10. Consider a train of waves with a wavelength of 100 m propagating across the continental shelf from deep (100 m) to shallow water.
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A 1.1: Fine, cohesive sediment such as clay would be deposited only if the fluid were standing, or nearly so. The sand here is in the form of current ripples and so was clearly deposited under conditions of active bedload transport. So the clay was deposited under much quieter conditions than the sand. The deposit indicates variable flow conditions ranging from standing water to modest velocities capable of transporting fine sand. BACK NEXT
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A 1.2: Unlike noncohesive sediment, deposition of cohesive material such as clay is not reversible, i.e. it requires much higher stresses to erode it than those under which it is deposited. A clay deposited from standing water, once it has had a chance to consolidate, can be very difficult to re-erode. Hence it is not at all paradoxical that the clay is not eroded under the flows that transport and deposit the sand. BACK NEXT
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A 2: A reasonable quick sketch might look like this:
Key aspects to include: upward fining; upward decrease in scale of cross beds going up; lateral-accretion surfaces; bedform trough axes at right angles to dip of lateral accretion surfaces; evidence of plant roots near the top; correct vertical scale of deposit; correct orientation (i.e. dipping toward the right) of lateral accretion surfaces. BACK NEXT
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A 3: The difference between an arenite and a wacke is the presence of >15% fine matrix in the latter. The effect of a fine matrix on otherwise similar deposits is to reduce the permeability. This makes it more difficult to pass water out of the sediment and thus increases the water pressure in the sediment due to deposition. Hence the wacke would be more likely to show water-escape structures, given that both units were deposited at the same rate. BACK NEXT
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A 4: The answer is 4.3. Steeply climbing ripples indicate high rates of deposition, which favor SSD because it requires a high rate of fluid flow out of the sediment mass to accommodate the deposition. Flutes preserved as casts on the bottom of a sand bed imply deposition of sand on mud, which leads to an unstable density gradient and again favors SSD. If you were tempted by 4.3. because of the high transport rate implied, remember that high rates of transport do NOT necessarily imply high rates of deposition. BACK NEXT
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A 5: Bedload, almost by definition, tends to consist of particles that are relatively dense-otherwise they'd be in suspension. Since organic carbon occurs in water mostly as low-density aggregates, it is not generally found in high concentrations in bedload. Hence the bedload is not generally a likely food source. A possible exception to this would be silt and sand-size fecal pellets, which do move as bedload. In principle, it would not be impossible for some kind of organism to take advantage of this, but none have (other than bacteria, which don't count in this scheme), perhaps because substrates with active bedload are simply too difficult to live on. BACK NEXT
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A 6: Reducing the gravity would reduce the weight of the particles, hence the force resisting deformation. On the other hand, reducing gravity does not of itself reduce the pressure developed as water is forced out of the sediment (there is no gravity effect in Darcy's Law). Thus reduced gravity would make it easier to produce soft-sediment deformation. However, you might argue that the condition that rates of deposition be equal is not realistic, since the reduced gravity would also reduce settling rates and (in general) rates of deposition. BACK NEXT
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A 7: This of course is a very open-ended question with no right answer. However, there are a couple of key points. The most important is that organic carbon tends to travel with the fine sediment fraction because it occurs mainly as low-density aggregates (e.g. "marine snow"). This restricts sediment-feeding behavior to suspension feeding, mostly in regimes of moderate flow velocities, and deposit feeding, only in regimes where fines can settle out of the flow, which means low to no velocity. In contrast, silicon is everywhere in sediments, most importantly as silica. Hence there would be no restriction on deposit feeding, and the worms would be everywhere in sediments, with the possible exception of areas with extremely vigorous sediment transport. The whole sedimentary record would be ruined! With, ironically, the possible exception of the muds, which would probably be too full of foreign matter (e.g. organic carbon) for silicon-based life to bother with.
On the other hand, with so much to eat sitting in the bed, it's hard to see what motivation there would be for suspension feeding.
See if you can think of any other ways to play with this question! BACK NEXT
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A 8: All other things being equal, reducing the grain density reduces the submerged weight of the sediment. This reduces the force resisting sediment flow, and thus makes it easier to induce soft-sediment deformation. BACK NEXT
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A 9.1:At first thought, it doesn't seem paradoxical; all that seems to be happening is that the clays are being deposited in the latitudes in which they are produced. The paradox is that if you calculate the settling velocity of particles the size of individual clay flakes (of the order of 1 µm), they should not be able to settle out of the ocean before they are carried by ocean currents out of the latitude belt where they are produced. (The Stokes settling velocity for a sphere of diameter 1 µm and quartz density is about 10-6 m/s. The wind-driven surface circulation of the ocean extends to a depth of about 1000 m. Hence it should take of the order 109 s or 102 yr for an individual clay particle to settle out of the surface current system. Even a relatively slow current (0.01 m/s) would carry the particle a distance of 10,000 km in this time, more than enough to move the clay to a different latitude and destroy the climate signal.) BACK NEXT
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A 9.2: The implication of the calculation above is that the clays must be settling out of the water much faster than suggested by the Stokes Law estimate. The explanation, as we discussed in class, is that the clays are actually settling as much larger (though lower in density) particles produced by organisms. These include fecal pellets and ill-defined aggregates of decaying organic matter ("marine snow"). If there were no life in the oceans, this process could not take place, and the clay composition of marine sediments would not be zoned by latitude as it is now. BACK NEXT
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A 10.1: The main control on both the orbital diameter and near-bed velocity is the ratio of depth to wavelength (they are proportional to exp(-h/l) where h is depth and l is wavelength). Hence both would increase strongly as the water becomes shallower. BACK NEXT
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A 10.2: The first bedform to develop would be wave ripples ("vortex ripples"), which would appear in this case for water depths shallower than about 50 m (half the wavelength). As the bottom orbital diameter increased shoreward, so would the wavelength of the ripples. In the shallowest water, where the wave-induced bottom stress would be highest, experimental data would suggest that the bedform should be a plane bed. Also possible for relatively high bottom stresses would be hummocky cross-stratification, but its condition of formation is not well understood. BACK TOP
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