WOU ES 202 - Sedimentary Facies, and Stratigraphy

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1 ES202 Lab 4 - Introduction to Sedimentary Structures, Sedimentary Facies, and Stratigraphy (updated Jan, 2009, 8th edition AGI lab manual) Part 1 - Introduction to Sedimentary Structures Examine the samples that are located at the labeled stations in the lab. Answer the associated questions. For reference, use the attached "Sedimentology Tool Kit", and p. 126-127 in your lab manual. Station 1 - Sedimentation Patterns Fine "muddy" sediment was mixed with water in the jar, shaken, and left to sit for 1 week. Turn on the light / illuminator, and examine the results. Pay close attention to the subtle distribution of grain size from the bottom of the jar to the top. Answer the following questions: A. What are your initial observations of sediment size distribution? B. Is the deposit graded or ungraded? (refer to p. 126 of the lab manual for help with these terms). C. Where do you find the coarsest sediment? What is it's approximate grain size? (answer in both millimeters, and with the appropriate size term) D. Where do you find the finest sediment? What is it's approximate grain size? E. What is the implication for grain settling velocity vs. grain diameter? (i.e. which sediment sizes settle faster - or first? and which sizes settle slower - or last?). F. Consider a natural depositional condition in which pebbles, sand, silt and clay are deposited during flood discharge in a broad river valley. What would be a likely arrangement of grain sizes that you might find in such a deposit? (based on your above observations). Draw a diagram to illustrate your answer.2 G. Is the sediment sample in the jar "lithified" or "unconsolidated"? Station 2 - Cross-Bedding Examine the freshly broken surface of the red rock sample at Station 2A. Answer the following questions. A. What is the grain size and rock name of this sample? B. Is this sample well sorted or poorly sorted? C. Is this sample graded or ungraded? D. Is this sample best described as massive or cross-bedded? (massive is used where cross-bedding is not evident, see p. 126 of your lab manual for diagrams of cross-bedding) Examine the sample at Station 2B and answer the following questions. E. What is the grain size and rock name of this sample? F. Is it well sorted, moderately sorted, or poorly sorted? G. Is this sample massive or cross-bedded? H. Using p. 126-127 of your lab manual for comparison, is this sample right-side up, or upside down compared to it's original depositional position? I. Examine the north arrow taped to the top of the sample. Determine the general compass direction of paleocurrent that prevailed at the time of deposition. (north, east, northeast, or???; refer to p. 126-127 for help in your determination). Station 3 A. What is the sedimentary structure displayed in this sample? B. Are these structures symmetric or asymmetric?3 C. Can you determine the paleocurrent direction at the time of deposition? If so, what is it (note north arrow on sample). Station 4 - Sedimentary Processes and Determining "Up Orientation" Drop the loose shells into the tub of water, repeat 10 times and tabulate your results in the table below. Place a check on the appropriate line, determining whether the shells land convex-up, or concave-up. Trial No. Convex Concave Up Up 1 ____ ____ 2 ____ ____ 3 ____ ____ 4 ____ ____ 5 ____ ____ 6 ____ ____ 7 ____ ____ 8 ____ ____ 9 ____ ____ 10 ____ ____ A. What can you deduce about the position of a shell that is deposited on the sea floor, when the animal dies? B. Given your experimental results, do you think the rock sample at this station is currently in a right-side up or upside-down position, relative to it's original depositional environment? C. The correct answer to 1-4B above is that the rock is currently in a right-side up position. Assuming that this sample was deposited in a shallow-water, near-shore marine environment, suggest a process or processes that might provide an explanation for the discrepancy between your experimental results, and the fact that this sample is in a right-side up position.4 Station 5. Examine the glass dish of sediment at Station 5A, complete the following observations: A. What is the grain size? B. Is this sediment sample well, moderately or poorly sorted? C. Is this sample graded or ungraded? D. What is the sedimentary structure that is evident on the sediment surface? E. How did this sedimentary structure form (what are the variables and the process?)? Examine the display at Station 5B, answer the following: F. What is the primary sedimentary structure observable on this sample. G. Which of the following environments of deposition could this rock have formed in? Check all that apply, more than 1 possible. Explain your line of reasoning for each that you check. Environment Explanation ____ Deep Ocean Floor ____ Submarine Fan ____ Tidal Flat ____ River Floodplain ____ Shallow Lake ____ Gravel-dominated mountain stream5 Station 6. Examine the sample under the protective glass - fragile, do not touch!!! A. What is the grain size (millimeters and term)? B. Note the polygonal shape of the sample fragments, what sedimentary process might result in this pattern? C. What are your hypotheses as to the origin of the small circular patterns on the surface of the sample? D. Which of the following environments of deposition could this rock have formed in? Check all that apply, more than 1 possible. Explain your line of reasoning for each that you check. Environment Explanation ____ Deep Ocean Floor ____ Submarine Fan ____ Tidal Flat ____ River Floodplain ____ Shallow Lake ____ Gravel-dominated mountain stream Station 7. Refer to the catalog of sedimentary structures shown on p. 126-127 of your lab manual. A. What type of sedimentary structure is associated with this sample?6 B. Are these structures molds or casts (a mold is a form, a cast is a 3-D object made from the mold)? C. Is this sample right-side up or upside down relative to it's original depositional position? How do you know? D. Using the north arrow, what is the paleocurrent direction represented in this sample. Station 8. Note the finely layered interval between points A and B on this sample. Refer to the attached Sedimentology Tool Kit, and answer the following questions. A. Are these layers best described as bedding or


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