GEL 109 Midterm W03, Page 1 Name:

50 points total(1 point per minute is a good pace, but it is good to have time to recheck your answers!)

1. a) Where in a water flow is there usually a zone of laminar flow even if the bulk of thewater is turbulent? (1 point)laminar/viscous sublayer

b) Why is it laminar? (1 point)because the flow speed is so slow that Re<500 no matter what the flow depth is

2. List one thing that each of the following characteristics could tell you if they were presentin a sandstone. (4 points total; 1 point each)

a) grains are well sorted1. consistent flow speed that washes all smaller grains downstream and does nottransport larger grains2. only a narrow range of grain sizes is present in the sediment that is eroding upthe flow

b) grains are poorly sorted1. sediment was deposited very rapidly from a flow that slowed down quickly2. so much sediment was present in the flow that grain-grain collisions were moreimportant than turbulent flow grain sorting3. sediment was deposited from a laminar flow such as ice or debris flow

c) grains are coarse sandflow was rapid enough to transport coarse sand

d) grains are silt sizedflow was slow enough to allow settling of silt-sized grains or so slow that silt sizedgrains were being transported as bedload rather than in suspension

3. What property of clay minerals makes consolidated mud more difficult to erode thansand? (1 point)they are cohesive and stick together

4. What mineral do you predict makes up the most sand in almost every environment?Why? (2 points)quartz because it is most resistant to chemical weathering and is hard enough thatit resists physical breaking and abrasion

5. Two rivers in Costa Rica have different concentrations of suspended sediment; one has aconcentration that is high enough that the water is brown, whereas the other river has a verylow concentration, and the water is clear. The two rivers merge in a mountainous area that isheavily forested (see projected photo). What are two reasons that the concentration ofsuspended sediment might be different in the two rivers? Be sure to include whether eachprocess would promote high or low suspended sediment concentrations. (4 points)

1. Different rock types weathered in the watershed of each river. The one withhigh concentrations of suspended sediment might have bedrock that is rich inminerals that alter to clay minerals (mafic rocks). This would provide thesuspended sediment. The one with low concentrations of suspended sedimentmight have bedrock that has few minerals that produce clays such as sandstones,

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granitic rocks, etc. Thus, there would be much less clay sediment to erode andbecome suspended sediment.

2. The river with high suspended sediments has a source of clay-sized grains toerode somewhere in its watershed, whereas the other river does not. A sourcemight include older floodplain deposits.

3. The river with high suspended sediment concentrations might have substantialdeforestation in its watershed, whereas the other river might have intact forest.Deforestation allows much more dramatic erosion of soil that is rich in clay-sizedclay minerals. Without the roots holding the soil in place and the plants to protectthe soil from strong rains, the clay-sized sediment can be eroded more easily.

4. The watershed providing abundant suspended sediment might have higherrainfall and more extensive chemical weathering than the watershed for the riverwith low suspended sediment concentrations. The rivers are close enough to eachother that the temperatures are probably about the same in the watersheds, butrainfall could vary depending on rain shadows from nearby mountains.

6. What are two conditions that results in laminar flows that transport large volumes ofsediment, and what characteristics of each flow make it laminar? (4 points)

Glacial flow is laminar and transports large volumes of sediment. Ice is laminarbecause it has a very high viscosity.

Debris flows are laminar and transport large volumes of sediment. They are alsolaminar because of high viscosity. Their viscosity comes from the cohesivebehavior of the clay-sized grains that are abundant in debris flows.

7. In the following sketch of a subaqueous dune, sketch and label the features listed below.a) The separation and attachment points of the laminar/viscous sublayer (2 points)b) The areas where deposition occurs (1 point)c) The areas where erosion occurs (1 point)d) The internal cross stratification within the dune. Include both the cross beds andthe erosion surfaces separating sets of cross stratification. Make sure the geometryof the internal cross stratification corresponds to the other areas labeled. (2 points)

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8. On the following picture of cross stratification, some of the laminae and erosionalsurfaces have been outlined.

a) Label at least 2 erosional surfaces in the part labeled B. (1 point)

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They are the thicker white lines as well as many surfaces that are close tohorizontal

b) What direction(s) did the water flow for part B? (1 point)from left to right

c) Was part B deposited by ripple or dune migration? Why? (2 points)ripple migration because the cross lamination is only 1 or 2 cm high. Dunesusually produce thicker cross stratification and the laminae are coarser

d) Assume that the lamination in part A is upper planar lamination. How did flow speedchange through time? Why? (2 points)It slowed down through time because upper planar lamination forms from fasterslow speeds than ripple cross-lamination forms. Because the upper planarlamination is below the ripple lamination and older rocks are below youngerrocks, the flow slowed down through time.

e) The cross lamination sets in part B are not even in thickness nor do the laminae haveconstant dips. What do these features imply about the shape of the bedforms and thedirection of flow relative to the orientation of the rock surface? (3 points)Irregular cross lamination is often due to non-linear ripple crests which producevariable cross lamination sets. Also, when flow is not exactly parallel to the fieldof view (rock face), variations in the geometry are common. If the direction of flowchanges direction by a few 10’s degrees, the apparent geometry of the crosslamination will change. Finally, the shapes of the ripples can be changing in spaceand time like the flume movie for homework #2. In rapidly flowing currents,turbulence is high and flow speeds are irregular producing bursts in flow speed.These cause ripples to migrate irregularly and produces irregular crosslamination.

9. Answer the following questions about this stratigraphic column.

a) How did the velocity of the flow change through time from 0 to 80 cm? What are atleast 2 features that allow you to make this interpretation? (3 points)

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The flow velocity slowed down as demonstrated by the decrease in grain size thatwas deposited and the change in sedimentary bedform from upper planarlamination to ripples to no bedforms.

b) Which part of the deposit was deposited quickly and which part was deposited slowly, inrelative terms? (2 points)The sands were deposited quickly and the shale settled from suspension veryslowly.

c) Describe the flow characteristics that generate this type of deposit, including the earlystages of flow before deposition. (3 points)About 4 of these ideas are sufficient for full credit: Turbidity currents areinitiated with slope failure. They are commonly erosive initially (producingbedding plane scours of various types and ripping up cohesive clasts of underlyingshale). They flow down a slope as a very turbulent grain flow with high sedimentconcentrations. As they slow down due to a decrease in slope and at the tail of theturbidity current, they deposit the coarser grained sediment that is poorly sorteddue to the high concentration of sediment. As the turbidity current slows moreand sediment concentrations decrease due to deposition, upper planar laminationand then ripples develop. They demonstrate that the flow is more typical of aturbulent water flow rather than a grain flow. As the current slows even more,very little sediment is transported as bedload and most of the sediment that isdeposited is settling from suspension. Once the flow has stopped, settling fromsuspension is the main depositional process.

10. The following stratigraphic column represents a sequence of rocks deposited during achange in depositional environments. First there was erosion of igneous rocks; then rockswere deposited in environment #1, which gradually changed to environment #2.

a) Next to the stratigraphic column, interpret the processes that deposited the sediment forenvironments #1 and #2. Please include the way sediment was transported (i.e. bedload,suspension, in a laminar/turbulent flow, by wind/water/ice) and how it was deposited (i.e. bymigration of a bedform, settling out, etc.). (4 points)

(Section not scanned.)

#2 (top): Clay-sized grains were transported in suspension in water. They weredeposited by settling from suspension in very still water. The larger cobbles weretransported in icebergs that melted allowing the cobbles to “settle” or fall to thebottom. You can tell they were not transported by a current because they are largeenough that the flows required would probably erode at least some of the clay.Also, they are isolated from each other, which suggests that there was little sortingof grains. Finally, there is evidence for ice transport in part #1, so melting oficebergs is reasonable.

#1 (bottom): Some of this sediment was derived from ice transport asdemonstrated by the striated clasts. The diamictite could have been deposited bymeling of ice above this section. In this case the sediment would have beentransported by ice and deposited by the sediment falling to the bottom of alake/ocean as the ice melted. Alternatively, it could have been deposited by a seriesof debris flows that incorporated ice-transported sediment. The lack of grainsorting requires deposition from a laminar flow.

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b) Draw a simple sketch that illustrates the processes that deposited the sediment in bothenvironments and shows the spatial relationship among the two environments and theerosional area, i.e. where they were located relative to each other. (Remember thatWalther’s Law says that environments that grade into each other vertically in a stratigraphicsection occur next to each other.) Label the environments on your sketch (erosion, #1 and#2). Make sure that the processes that you described in part a are included in your sketch!(6 points)