GEL 109 Midterm W01, Page 1 50 points total (1 point per minute is a good pace, but it is good to have time to recheck your answers!) 1. Where in a water flow is there usually a zone of laminar flow even if the bulk of the water is turbulent? (1 point) viscous/laminar sublayer at the base of the flow 2. Do high or low flow Reynolds numbers promote transport of more suspended load sediment? Why? (2 points) High Re because the turbulence keeps the sediment from settling out 3. What are two types of laminar flows that transport large volumes of sediment, and what characteristics of each flow makes it laminar? (4 points) Glaciers - high viscosity makes them laminar (not density - ice is less dense than water) Debris flows - high viscosity due to an abundance of mud compared to water (viscous sublayer doesn t count because it doesn t transport much sediment; it is too narrow) 4. Why is it possible to erode and transport pebble-sized chunks of mud when flow conditions are too low to erode individual clay grains? Include a property of clay-sized grains and relative flow speeds for transport in your answer. (2 points) Clay-sized grains are cohesive due to their surface charges. Thus, it requires a higher flow speed to erode individual clay-sized grains than pebbles. A pebble sized chunk of mud can be transported as a single grain, although abrasion with other sediment rounds them 5. In the following sketch of a subaqueous dune, sketch and label the features listed below: a) The areas where deposition occurs (1 point) b) The areas where erosion occurs (1 point) d) The internal cross stratification within the dune. Include both the cross beds and the erosions surfaces separating sets of cross stratification. (2 points) 6. On the following picture of some cross stratification, some of the laminae and erosional surfaces have been outlined. a) Label at least 3 erosional surfaces that developed on the backs of bedforms (dunes in this case). (1 point) b) Draw an arrow that shows the average direction the flow was moving. (1 point)
GEL 109 Midterm W01, Page 2 The wider lines are erosion surfaces. You can tell this by the fact that they truncate the more steeply dipping lines. The thinner lines are depositional surfaces along the slip face of the dunes (which are subaqueous dunes, by the way). This is a view of trough x-strat with flow going from left to right. To see a view of a perpendicular face, look at the photo on the web at http://wwwgeology.ucdavis.edu/~gel109/sedstructures /SedStructures.html, the third pair of photos. c) Briefly speculate on what caused the irregularities in the cross stratification. Give the reasoning behind your speculations. Think about how cross stratification geometry varies with the orientation of the rock surface compared to flow direction and how variations in dune shape, height, etc. affect the laminae deposited. (4 points) Variations in x-strat are commonly due to changes in flow direction. The view here is not ALWAYS parallel to the flow, so the laminae dip varies. This is particularly clear in the very thing layer in the middle of the photo. The key to identifying that is the very low angle the depositional laminae forms compared to the erosional surface, which suggests that you are looking more along the flow than perpendicular to it. Variations are also caused by non-linear dune crests (remember the movie...). The geometry of the layer that starts in the middle top of the photo and expands to the right is probably the result of a nonlinear dune crest. Change in flow strength can change dune height, but it is very difficult to tell how tall a dune actually was. For each layer, the MINIMUM dune height is the thickness of the layer. The dunes could have been any height greater than that thickness (although, with experience, you can make some good guesses as to what it might have been). Faster flows can transport more sediment, so if the flow speeds up, it could be that almost all of the sediment erodes away, leaving a thin layer, OR more sediment could be transported into the environment and remain there, leaving a thick layer. The critical factor is whether the flow came into this area with extra sediment (i.e. it slows down slightly and can t transport as much sediment as it used to be able to) or less sediment than it can transport (i.e. the flow is speeding up as it moves down the slope, or there wasn t very much sand of the correct grainsize upstream to erode). Changes in this balance are difficult to interpret from the rocks, although we know that the flow had at least a little extra sediment, because it was present to be turned into a rock! 7. Write in the name of the appropriate sedimentary structure for each area of the diagram. Also write in the area with no sediment transport. (5 points)
GEL 109 Midterm W01, Page 3 8. What produces the surface creep of grains up the up-wind side of wind ripples? (1 point) grain-grain collisions 9. What preservable sedimentary feature can surface creep produce that allows you to identify a ripple as a wind ripple rather than a subaqueous ripple? (1 points) reversely graded ripple laminations 10. Which of the following conditions are more likely to produce sand or clay-sized sediment? For Ca-feldspar and muscovite, say whether their susceptibility to alteration is high or low. Then mark which types of weathering are important in each type of climate. In one of the listed climates, both types of weathering can be important, so mark both. Put an S in the table if the conditions and mineral type are likely to produce sand (of any size) and a C if the conditions are likely to produce clay-sized sediment. You can put "S or C" in three boxes. (5 points)
GEL 109 Midterm W01, Page 4 Susceptibility to Alteration Warm Humid Climate Arid Climate (warm or cold) Cold Climate (wet or dry) Weathering Style xxx Physical Chemical Physical Chemical Physical Chemical Olivine Very High c c or s c or s (depends on how wet) Ca-feldspar high c s c or s (depends on how wet) Muscovite low S or C s s Quartz Very Low S s s The type of weathering depends to a large extent on the presence of water. In a warm, humid climate, chemical weathering is very abundant, but physical weathering can also occur. In an arid environment, i.e. lacking water, chemical weathering is very slow, and physical weathering processes are most abundant. For cold climates, chemical process are slower, and physical weathering is most abundant. Some chemical weathering can occur in cold and wet environments, but it is much slower. The sediment type that is produced depends on the type of weathering and the mineralogy of the original rock. Minerals with a very high susceptibility to alteration almost always form clay minerals, although in some environments, especially dry and cold ones, they can form sand. Minerals that are less susceptible to alteration almost always form sand. 11. What mineral do you predict makes up the most sand in almost every environment? (1 point) quartz 12. There are two slopes in northern California with the same climate, including rainfall. One is extensively forested. The other is not, because it faces south and the hot summer sun dries out the soil. a) Which slope is likely to be more deeply weathered chemically? Why? (2 points) The forested slope is more likely to be deeply weathered because the roots release organic acids which increase weathering. They also hold moisture in the soil making it wetter, and thus, increasing weathering. Vegitation doesn t keep water from getting to the soil, it just slows down the water, reducing erosion. b) Which is more susceptible to erosion and why? (2 points) The unforested slope is more susceptible to erosion because it doesn t have the tree roots to help hold the soil in place, and the forest canopy to reduce the effects of rain impact, etc. 13. The following stratigraphic column represents a sequence of rocks deposited during a change in depositional environments. For each sediment type, list the flow type, depositional processes and describe the probable depositional environment. For the flow type, include whether the flow was laminar or turbulent and the medium that deposited the sediment (ice, water or wind). (7 points)
GEL 109 Midterm W01, Page 5 (Column not scanned) Depositional Environments Diamictites: Diamictites are unsorted deposits, thus the flow must be laminar. Thus, deposition is probably either from debris flows or glaciers. The surrounding sediments (see below) consist of turbulently deposited stream and eolian deposits, which also could be either in glacial or arid environments, although eolian deposits are most common in arid environments, because glacial environments commonly have a lot of water. The diamictites are a couple of meters thick and have a mud matrix, which suggests a debris flow origin, which is what they were intended to be. Trough cross-bedded sandstone: Trough cross-bedding form from irregularly crested dune migration, and stratification requires turbulent flow. Water was the transport medium; air can t transport gravel-sized grains under normal conditions. They are coarse deposits that fine upward in some cases, which could be braided river/stream deposits or sheet flows. The trough cross stratification suggests more braided-river type deposits, which could be due to deposition during floods that have lower flows than those required for sheet floods. Fine sandstone with meter-high cross bed sets: The fine grain sizes and m-high cross bedding suggest eolian deposition (i.e. wind) and turbulent flows. Wind is always turbulent due to the low viscosity of air. Another clue to eolian deposition is the reversely graded ripple laminae, which form from collision impacts in air, but not water. Overall Environment: Walther s Law states that neighboring depositional environments are respresented vertically in a stratigraphic column (if there are no intervening unconformities). Thus, the overall depositional environment in this case is one where diamictites, trough cross-bedded sandstones, and eolian dunes can all be deposited. I intended the sequence to represent a small alluvial fan with some debris flows, water deposition of sand during storms, and development of a dune on the surface of the fan. The dunes are one of the key points that suggests an arid environment rather than a glacial environment. 14. These three stratigraphic columns represent contemporaneous deposits from different and changing depositional environments. Draw cross sections of the land surface during deposition at times 1 and 2, showing the sediment types deposited in each environment. Include any bodies of water, like a lake or the ocean. Label each environment with the processes dominating deposition (i.e. stream deposition, etc.). (7 points) (Strat columns not scanned) Assume they were labeled A, B, and C from left to right. Column A, from base to top: The unconformity on top of the granite represents the erosion under a glacier. As the glacier retreats, diamictite is deposited as a morraine or a till sheet. The till sheet is more likely based on the fact that the diamictites are also present in column B, so the diamictite is widespread. Morrains are commonly localized into more isolated mounds (but not always). Above time 2, shale with dropstones was deposited. Silt and clay sized grains need VERY slow flow speeds to settle out of suspension. Thus, these deposits represent settling out from standing water, like the ocean or a lake. Overall, this section goes from erosion beneigth a glacier to a large sediment influx from melting of the
GEL 109 Midterm W01, Page 6 glacier, to settling of sediment from suspension with some ice rafted debris from melting ice bergs. Column B from base to top: The diamictites were deposited as either till from a melting glacier or debris flows. The matrix composition, i.e. lithic fragments, suggests a glacial origin for the sediment, whether it was deposited as a debris flow or directly from melting ice. The probably interpretation is that it was deposited directly from melting ice because there are no associated turbidites. If there was enough slope to produce debris flows, turbidites could also be expected. Above the diamictites, shales settle out with drop stones from melting icebergs, again indicating a glacial environment. The glacier was receeding, and eventually little ice rafted debris was deposited, either because the glacier melted or it was too far from site B. Column C from base to top: The gravel sized clasts in silt to clay-sized sediment, suggest ice rafted debris, because silt and shale require very low flow speeds to settle from suspension, whereas gravel requires high flow speeds to be transported, unless it is dropped into place, i.e. transported by floating ice. The laminated character of the shale suggests that it did settle from suspension rather than being deposited as a mud flow or debris flow, both of which lack good lamination. An ideal answer to the question: Time 1 A B C erosion Granite Glacier Ice berg ice rafted debris and suspension settling Debris from melting ice Time 2 A Ice berg B C ice rafted debris and suspension settling suspension settling Granite