Sedimentary Geology Earth and Planetary Sciences Final Exam December Duration 3 Hours

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1 Sedimentary Geology Earth and Planetary Sciences Final Exam December 2005 Duration 3 Hours General instructions: 1) All answers must be written in answer books provided and turned in at the end of the exam. 2) Notes/textbooks or other memory aids are not permitted. 3) Use sketches whenever appropriate. Part 1 Answer only two of the following four questions (25%) 1. Define and explain the three forces acting on sediment grains as they are put into motion by unidirectional currents, and the mechanisms of sediment transport once a grain is put into motion. Lift force (Bernoulli effect) and drag force combine to exert fluid force. Gravity, and possibly cohesion and friction, acts as a force to resist movement. Once set in motion particles travel as bedload (rolling, sliding, saltation in contact with bed), suspension (held in suspension by upward component of turbulence; intermittent suspension possible), wash load (fine-grained suspended sediment e.g., clays brought in from up current areas). 2. Describe the Modified Dunham classification scheme and explain its basis for classifying limestones. Sedimentary Geology Fall 2005, p.1

2 3. List the four main diagenetic processes that affect siliciclastic and carbonate sedimentary rocks, and give an example of how each might be recognized in thin section. Compaction caused by weight of overburden chemical and/or physical effects. concavo-convex boundaries, stylolites, sutured boundaries, broken grains, bent grains (e.g., mica), squashed soft grains. Dissolution chemical dissolution of minerals. secondary porosity Precipitation cements. Overgrowths, pore-filling. Not just quartz or calcite. Recrystallization replacement of one mineral and simultaneous replacement by another no volume change. E.g., carbonate replacement by quartz. pseudomorphs, ghost fabrics 4. Define all of the following terms: i) Petroleum liquid hydrocarbon accumulations ii) Flute mark erosional scour marks at the base of beds, used to give paleocurrent info iii) Floodplain flat area within a valley, flanking channel that is inundated during high discharges iv) Stylolite interlocking surface in a rock caused by pressure solution v) Till glacial deposit deposited directly by ice vi) Chert cryptocrystalline silica Sedimentary Geology Fall 2005, p.2

3 vii) Coarsening-upward succession upward increase in grain size, often related to progradation viii) Soil unconsolidated deposits that support plant life ix) phi scale grain size scale -log base 2 of grain dia in mm x) trough cross-bedding sed structure produced by migration of 3-d dunes xi) Wave fetch area over which wind blows to form waves xii) Turbidite deposit of a turbidity current Part 3 Answer four of the following five questions (50%) 5. Define lithostratigraphy and sequence stratigraphy. Why is sequence stratigraphy preferred for reconstructing earth history? What are the primary surfaces used by sequence stratigraphers, how do they form, and how are they recognized? Lithostratigraphy characterization and correlation of rocks based on lithology (composition, color, etc.). Divides rocks into formations, members, groups, etc. Sequence strat analysis of stratigraphic successions in terms of genetically related packages of strata, bounded by discontinuities. Primary surfaces: 1) unconformities form when sea level falls. Recognized by erosional truncation and/or exposure, 2) flooding surfaces form during rsl rise/transgression - Recognized by abrupt deepening of strata., 3) maximum flooding surfaces form at turnaround from transgression to regression recognized by hot shales (gamma ray log) or downlap (e.g., log cross-sections, seismic). Main problem with lithostratigraphy: lithostratigraphic boundaries can cross timelines. 6. Use Walther s Law, your knowledge of carbonate facies distributions, and the Modified Dunham classification to predict what the vertical succession of facies would be for a prograding unrimmed carbonate platform. Walther s Law states that in a section without significant hiatuses, the vertical succession of facies (at least partially) reproduce lateral succession of facies found in modern settings. Unrimmed platform no protective rim, so wave energy is dissipated over entire platform. Vertical succession similar to prograding clastic shoreface gradational boundaries. Outer shelf mudstones (micrites), bioturbation -> innershelf/shoreface high energy grainstones. Probable storm deposits. Pacakages capped by flooding surfaces. 7. Describe 1 st, 2 nd, 3 rd, 4 th and 5 th order relative sea level cycles in terms of the time periods over which they operate and the possible causative mechanisms. Sedimentary Geology Fall 2005, p.3

4 1 st order sea level fluctuations over 100s of millions of years. makeup and breakup of supercontinents (eustatic) 2 nd order 10s of millions of years tectonic (spreading rates - eustatic) 3 rd order millions of years problematic in much of Phanerozoic. Regional uplift/subsidence? 4 th order 100s of 1000s of yrs milankovitch cycles, autocyclic processes, 5 th order 10s of 1000s of yrs climate, tectonics, autocyclic 8. What are the controls on whether a shoreline will experience transgression or regression? Describe what they are and how they interact. Transgression landward movement of the shoreline Regression basinward movement of the shoreline Global sea level change eustacy Regional subsidence/uplift These two combine to cause relative sea level changes fix the amount of accommodation space Sediment supply (ss) Add sed supply to rsl changes to get changes in water depth If ss>rsl rise progradation, if ss<rsl rise transgression, if ss=rsl rise aggradation 9. Describe the differences between wave-dominated and river-dominated deltas, in terms of their morphology, sedimentary facies and sandbody geometry. How would you distinguish between a wave-dominated delta and a strandplain? Delta: shoreline protruberance formed where sediment is supplied by a river faster than it can be redistributed by basinal processes. Wave dominated waves redistribute most of sediment supplied by the river. Regular/smooth shoreline. Progradation of entire delta front. Shoreparallel, sheet-like delta front sand. Coarsening-upward succession, facies show strong wave influence (e.g., hummocky cross-stratification) Sedimentary Geology Fall 2005, p.4

5 River dominated supply > redistribution/reworking, birdfoot geometry, progradation only around river mouths coarsening upward successions, complex facies distributions in interdistributary marshes, levees, crevasse splays, etc. Wave-dominated delta will resemble strandplain need to map out to see whether there is a protuberance Part 4 You must answer the following question (25%) 10. Congratulations! You have just landed a summer job with Terra Incognito, a prestigious international environmental consulting firm. They already have your first assignment: tell them what they need to find out about the Salsa Verde Formation, a possible source of potable water (i.e., an aquifer) for the town of Las Tamales. A map view and simplified geologic cross-section of the area are shown on the following page. Unfortunately the Salsa Verde Fm is poorly studied, and not much is known about the depositional environment, diagenesis, etc. Outcrops of the formation about 100 km west of town have been described as fluvial, but the style of fluvial deposit (e.g., meandering, braided) is unknown. Porosity is present in the sandstone units exposed at the surface there, but the amount (if any) of porosity in the formation below the town is unknown. Your company knows that you have taken EPSC-455 and so has assigned you the following specific tasks: A) Porosity characterization. Define the types of porosity present in the sandstones, and how/when the different types of porosity originated. Determine the amount of porosity present, and how effectively the rocks will transmit fluids. (This isn t a hydrogeology course, so forget pump tests or anything else like that) Porosity - % of void space in rock that may contain fluids. need to look at thin sections to characterize. Primary (intergranular) vs secondary (dissolution, fracture, moldic, etc.). Use textural evidence. Impregnate with blue expoxy. Need to get samples from subsurface and outcrop outcrop may have porosity that formed during teleodiagenesis. Cementation may have destroyed porosity in subsurface. Test porosity and permeability using plugs porosity Boyle s law, permeability Darcy s law. Collect wireline logs. B) Aquifer geometry. Define, or predict, the lateral continuity of the sandstones and the sand/shale ratio. These variables affect the size of the Sedimentary Geology Fall 2005, p.5

6 (possible) aquifer and the ease with which water could flow from one part of the system to another. Work with/collect outcrop, core, wireline log, and maybe seismic data. Thin section analysis of samples to characterize type of porosity in outcrop AND core types of porosity in subsurface may be different from what is present in outcrop. Impregnate samples with blue epoxy before cutting thin sections. Measure porosity and permeability on core or outcrop samples using methods described in handout to Lab 7. Lab 7 also introduced people to porosity logs, but nobody got this and so I didn t dock marks for not mentioning it. Note that sorting and grain size affect primary porosity, but cementation can completely fill in primary porosity therefore not a very reliable predictor of porosity in consolidated sedimentary rocks. Use facies and facies successions observed in outcrop, core, logs and/or seismic data to define (interpretation) whether fluvial system was meandering or braided. Meandering systems moderate - low sand/shale ratio, lenticular channels, shales - floodplain, fining upward successions less continuous poorer aquifer. Braided sheet sandstones, high sand/shale ratio more continuous, therefore probably better aquifer. Note that type of fluvial system could change from braided to fluvial (or vice versa) over distance from outcrop to area below town, and that it may be that fluvial system originally flowed toward the mountains depends on tectonic history (nobody got either of these didn t dock any marks). Also, structure (dips) on formation are probably not the original depositional slope. Note that sand/shale ratio is defined over the thickness of the formation, not from thin sections. Write a short essay that addresses all of the issues identified in Tasks A and B above (there are no tricks just make sure you address all the points). In your answer, make sure that you state what data you would want to collect and/or use, why you would collect/use those data (i.e., what do you hope to learn from them), and generally state how you would resolve the problems being posed. Give examples. Use drawings as necessary. Integrate what you have learned in this course. Assume that you will be able to collect any outcrop or subsurface data you like (not necessarily a realistic assumption, but ). Sedimentary Geology Fall 2005, p.6

7 Sedimentary Geology Fall 2005, p.7

Depositional Sequences Sequences

Depositional Sequences Sequences Depositional Sequences Transgressive and Regressive packages can be bound by unconformities Because sediment can only be preserved during net aggradation and progradation All other times there is either