GEL 109 Midterm W05, Page points total (1 point per minute is a good pace, but it is good to have time to recheck your answers!

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1 GEL 109 Midterm W05, 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. Are the following flow types typically laminar or turbulent and why? (4 points) a) water - turbulent because of low viscosity or flow speeds tend to be too high or water depths tend to be too high for laminar flow b) air - turbulent because of very low viscosity or flow speeds tend to be too high or atmospheric length scales tend to be too high for laminar flow c) ice - laminar because of high viscosity or low flow speeds d) clay-water mixtures - laminar (if enough clay) because of high viscosity or low flow speeds 2. Where in a water flow is there usually a zone of laminar flow even if the bulk of the water is turbulent? Why is it laminar? (2 points) Water flow is laminar in the laminar/viscous sublayer of the boundary layer. It is because the flow speed is so slow that Re<500 for any flow depth. 3. What is the minimum flow speed necessary to transport 0.8 mm diameter coarse sand if it is already in motion? (1 points) 5 cm/sec 4. In the following sketch of a subaqueous dune, sketch and label the features listed below. (6 points) a) The separation and attachment points of the laminar/viscous sublayer b) The areas where deposition occurs c) The areas where erosion occurs d) The internal cross stratification within the dune. Include both the cross beds and the erosion surfaces separating sets of cross stratification. Make sure the geometry of the internal cross stratification corresponds to the other areas labeled.

2 GEL 109 Midterm W05, Page 2 5. On the following picture of cross stratification, some of the laminae and erosional surfaces have been outlined.

3 GEL 109 Midterm W05, Page 3 a) Label at least 2 erosional surfaces. (1 point) Marked with black and white arrows b) What direction(s) did the water flow? (1 point) to the left c) Does this cross stratification represent dune or ripple cross stratification? Why? Note the mechanical pencil at the top of the photograph. (2 points) Dune cross stratification because the thickness of the cross stratification sets is more than 3 cm, which is the maximum height for most ripples. 6. On the following picture of cross stratification, white arrows point out some of the erosional surfaces. a) The cross stratification are not even in thickness nor do the layers have constant dips. What do these features imply about the shape of the bedforms and the direction of flow relative to the orientation of the rock surface? (3 points) This view of cross stratification is almost parallel to the flow direction (e.g. the flow was into the rock from this view. The irregularities are due to non-linear dune crests which produce variable cross lamination sets. Variations in the dips of the laminae are due to variations in the slope at the front of the dune plus the angle of the rock exposure relative to the lamination. This style of cross stratification is very common and can be seen in the USGS movies of bedform migration that were part of the homework. b) Does this cross stratification represent dune or ripple cross stratification? Why? A 10 cm-long scale is present at the top of the photograph. (2 points) Dune cross stratification because the thickness of the cross stratification sets is more than 3 cm, which is the maximum height for most ripples. 7. What are type of weathering tends to most often produce grains of the following types? Choose one of these answers: Chemical Weathering, Physical Weathering, or Both (3 points) a) quartz sand Both b) Ca-feldspar or olivine sand Physical Weathering c) clay-sized clay minerals Chemical Weathering

4 GEL 109 Midterm W05, Page 4 8. For the following stratigraphic column, describe how the velocity of the flow changed through time from 0 to 80 cm. Include descriptions of at least 2 types of features that allow you to make this interpretation, e.g. grain size changes, stratification styles, a model for how the sediment was deposited, etc. (4 points) The flow velocity slowed down as demonstrated by the decrease in grain size that was deposited and the change in sedimentary bedform from upper planar lamination to ripples to no bedforms. 9. Which parts of the deposit in the above stratigraphic column above were deposited quickly and which part was deposited slowly, in relative terms? (Multiple Choice - Circle your answer. 3 points) 1-20 cm Slowly (1 cm/1000 years or less) Quickly (1 cm/hour or faster) cm Slowly (1 cm/1000 years or less) Quickly (1 cm/hour or faster) cm Slowly (1 cm/1000 years or less) Quickly (1 cm/hour or faster) 10. The following stratigraphic column represents a sequence of rocks deposited during a change in depositional environments. First there was erosion of igneous rocks; then rocks were deposited in a fining upward sequence. a) Next to the stratigraphic column, interpret the processes that deposited the sediment for the three sedimentary rock types (diamictite, sandstone, and shale). 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. by migration of a bedform, settling out, etc.). (4 points)

5 GEL 109 Midterm W05, Page 5 b) Draw a simple cross section through a landscape that includes the depositional environments in the stratigraphic column above. Remember that Walther s Law says that environments that grade into each other vertically in a stratigraphic section occur next to each other on a landscape. Label the environments on your sketch (erosion, #1, #2, and #3). Make sure that the processes that you described in part a are included in your sketch, e.g. water where there is water transport, etc.! (6 points) 11. What are three characteristics of sediment deposited in a playa lake environment? You can include associations with sediments from neighboring environments. (3 points) Fine grained (clay-silt-sized) sediment Evaporite/salt minerals Mud cracks Grade laterally (or vertically) into alluvial fan deposits Any fossils consist of arid plants or animals, possibly microbial mats May be associated with eolian deposits

6 GEL 109 Midterm W05, Page What are three characteristics of sediment deposited on an alluvial fan? You can include associations with sediments from neighboring environments. (3 points) Diamictites from debris flows Normally graded gravel to sand deposits from sheet floods Angular clasts common Fan-shaped deposits with the apex of the fan aligned with a canyon Sediments fine and thin from the apex of the fan to the foot of the fan May grade into playa lake deposits 13. What features might you look for to determine if an unsorted diamictite was deposited by a debris flow versus by glacial ice? Which feature would be present in which type of deposit? (2 points) Debris Flow Diamictites: unsorted, angular debris deposited across a moderately broad area, possibly at from the mouth of a canyon; relatively even thickness deposit over the area of deposition; clay-mineral matrix Glacial Diamictites: unsorted, angular debris deposited at the edges of the glacier, so usually localized with very uneven thicknesses if the glacier ends on land; associated with laminated sediment with drop stones if glacier ends in water; striated and faucetted clasts may be present; rock-flour matrix