ES120 Sedimentology/Stratigraphy

Transcription

1 Midterm Exam 5/05/08 NAME: 1. List or describe 3 physical processes that contribute to the weathering of rocks (3pts). exfoliation frost wedging many others. roots, thermal expansion/contraction also credit for erosive processes 2. List or briefly describe 3 factors that influence (control) rates of weathering (6 pts) Climate (precipitation, temperature) mineralogy, Rock resistance (hardness) Surface Exposure (erosion, renewing fresh surfaces) Surface area (area/volume ratio) Elevation (relief, tectonics 3. List or describe two properties of grain(s), other than sorting/size, in a rock that reflect upon transport history (2 pts). degree of rounding pitting frosting, mineralology, etc. 4. Flow Regimes & Reynolds #: In terms of velocity and water depth, explain the difference between laminar (critical) and turbulent flow (4 pts). Examples may help. Laminar flow occurs when the flow velocity are high relative to depth (high reynolds inertial forces >>viscous forces), whereas a decrease in inertial forces relative to depth allows flow to become turbulent (eddies) (low reynolds) 5. Gravity flows differ in part by how grains are supported during flow. Describe how grains/clasts are supported in a debris flow versus a turbidity current, and how this influences grain distribution (i.e., sorting) when the flow comes to rest and sediments are deposited (5 pts). In debris flows, flow is viscous (low reynolds) and the clasts are supported largely by grain to grain contact so that when the flow come to rests, it will be poorly sorted, or even reversely graded because of sieveing. In turbidity currrents, clasts are supported largely by fluid turbulence so as the flow slows, largest grains settle out first resulting in a graded deposit. 6. Viscosity is a measure of the resistance of a fluid to being deformed by shear stress. A. What is eddy viscosity (2 pts)? B. Under what kind of flow regime does a fluid experience eddy viscosity (2 pts). It is the viscosity in a flow that is created by eddies. Typically eddies form under low flow regime, low inertial forces (velocity) relative to 1

2 7. Draw and label a sand ripple that is being deposited in a stream in which the suspended load is very high relative to the bedload. Be sure to include foresets, bottomsets, topsets (4 pts). Foresets are curved and tangential to bottomsets, they may show climbining 8. You have a rock for which you have the following observations: 60% lithic clasts, 10% feldspar clasts, and 30% quartz clasts. Clast size on average is 0.5 mm (long axis), and there is approximately 17% matrix. Provide a name for the rock (as complete as possible). (2 pts) Lithic wacke 9. In terms of grain size, sorting, shape and mineralology, describe the characteristics of texturally immature siliciclastic rock (4 pts). Mixed grain size, poorly sorted, angular, high abundances of unstable mineral phases 10. In terms of grains sizes, shapes, sedimentary structures (bedforms), bedding, and other indicators, how would you distinguish a succession of sediments deposited in alluvial fan from those deposited in a submarine fan (6 pts)? List/describe 6 characteristics. Alluvial fans - overall larger grains over most of the fan, immature, angular and poorly sorted, withbraided river bedforms including bar deposits, imbricated clasts, extensive cross beds. Submarine fans finer, more mature grains (expection are the feeded channel deposits), clays/silts, bouma sequences, abundant soft sediment deformation structures, some marine microfossils. Debris flows are common to both 2

3 11. Given the bed forms in the photo below, what can you say about the direction of current flow and relative current velocity and water depth? (4 pts). Scale bar is inches slightly sinuous ripples with flow direction toward the top, probably water deeper than 50 cm, with low to moderate current velocities 12. Explain where the cherts of the Franciscan formation were originally deposited. In other words, describe the conditions required for the production and preservation of opal-rich sediments (4 pts). Would have formed in a low latitude pelagic setting with intense upwelling, probably equatorial, but coastal is possible as well (with low siliciclastic input) 13. The calcite compensation depth (CCD) determines the depth to which carbonate sediments are preserved in the deep sea. a. List or explain the factors that control the depth of the CCD (4 pts)? CO3 saturation decreases with increasing pressure (depth,) and decreasing temperature. Additionally, the input of CO2 (respiration) drives CO3 ion content lower. b. Why is the CCD shallower in the Pacific relative to the Atlantic (2 pts)? Circulations sweeps in older water which has collected respired CO2 14. Sediments deposited in lakes are often laminated. Why is that? In other words, what factors (~3) contribute to this (3 pts) Seasonal cycles in sediment fluxes, productivity, stratification, low oxygen 3

4 15. Using the diagrams below, explain how the current (direction/velocity) varies in a meandering river (both laterally and parallel to the direction of flow) and causes migration of the riverbanks and distribution/deposition of sediment (4 pts). x-section of a channel Plan view of a channel 16. Draw and describe the primary depositional facies of a meandering river system (8 pts). Label your diagrams. Channel lags (coarse grained, poorly sorted), point bar cross beds (sands/silts), levees (sands), floodplain deposits (fine grain muds & silts), oxbow lakes (fine grain, organic rich, fossil leaves), splay deposits, soils (organic rich, root traces) 4

5 17. For the stratigraphic sections described below, list the most probable environment of deposition (2 pts ea.). Be specific. A) 5 meters of interbedded mudstone with starved ripples and sandstone with bimodal current directions and clay drapes Tidal channel B) 5 meters of bedded limestones and cherts Pelagic C) 10 meters of planar and cross bedded poorly sorted sands, overlain by 40 meters of channel conglomerates and poorly sorted debris with angular clasts Alluvial fan D) 10 meters of well sorted, cross-bedded sands, with 3 to 4 meter thick, high-angled foresets overlying a lag gravel deposit Desert (sand dunes) 18. First sketch a profile/cross section (proximal to distal) for a prograding, riverdominated delta (i.e., similar to the Mississippi). Next sketch a vertical succession of the sediment facies at the proximal end of the delta. Label each unit, and provide information about sedimentary structures and relative grain size (8 pts). Vertical succession is a coarsening up (shallowing) and includes hemipelagic muds, overaliain by pro-delta silts, bar mouth sands, channel deposits, interdistributary muds/soils (organic rich) 5

6 ES120 Sedimentology/Stratigraphy 19. A. How would you describe the sedimentary structure in the photo below (3 pts)? Symmetric ripples (x-beds) bidirectional flow B. Where do you think this was deposited (2 pts)? Tidal channel, or shelf 20. List 3 oceanic processes that influence the transport and deposition of sediments on continental shelves (3 pts). Tides, ocean currents (geostrophic), storm waves 21. A. In terms of overall structure and depositional facies, how do ramped and rimmed carbonate platforms differ (2 pts)? Rimmed platforms have a distinct reef edge at the shelf edge. This typically protects a quiet water environment B. List or describe the key processes that control the buildup of carbonates platforms (4 pts)? Wave activity, nutrient and sediment fluxes, sealevel change 6