Recitation 7 - Geologic Histories

Name: ______________________________ Recitation Section: ___________________________ Recitation 7: Geologic Histories Introduction: The first step in understanding the Earth is understanding how modern systems work. The second step is to use the rock record to move into the past. Once you are able to interpret how each rock type formed, you need to be able to put each geologic event that produced each rock unit into its proper sequence. Goals: 1. Learn to recognize geologic structures and erosional unconformities. 2. Understand how to use the Principles of Superposition; Lateral Continuity; Original Horizontality; and Cross- Cutting Relations, Inclusions, and Baked Contacts to reconstruct a history of geologic events. 3. Incorporate your understanding of the forces that produce geologic structures to further your understanding of the history reflected in geologic cross-sections. 4. Incorporate your knowledge of how igneous, sedimentary, and metamorphic rocks form to fully flesh out the geologic history reflected in cross-sections. Skills Developed: Geologic histories are like little puzzles. These problems help you to make and assimilate a series of interrelated observations and use them to construct a geologic narrative. Part A: Geologic Structures 1. Examine the block diagrams below. Determine whether each fault is a normal fault, reverse fault, or strike-slip fault and what type of geologic stress formed the fault. Draw arrows on either side of the fault to indicate the direction of motion; arrows have been added to the first diagram as an example. Fault type: Fault type: Fault type: ___________________________ ___________________________ ___________________________ Stress that formed feature: Stress that formed feature: Stress that formed feature: ___________________________ ___________________________ ___________________________

2. Examine the block diagrams below. Determine whether each fold is an anticline or syncline and what type of geologic stress formed the folds. Fold type: Fold type: ___________________________ ___________________________ Stress that formed feature: Stress that formed feature: ___________________________ ___________________________ Part B: Unconformities 1. Examine the diagrams below. Determine whether each unconformity is a disconformity, nonconformity, or angular unconformity. Unconformity type: (tilted rocks under erosional surface) Unconformity type: (igneous or metamorphic rocks under erosional surface) Unconformity type: (sedimentary beds under erosional surface) ___________________________ ___________________________ ___________________________ 2. What do unconformities represent in the geologic record?

2. The block diagram below shows another, more complex, series of geologic events. Starting with the oldest, number each geologic event represented by the diagram. Each rock unit, structure, and unconformity gets a number. a. How many different episodes of igneous intrusion were there? Note that each episode has a different pattern/coloring and is surrounded by a baked contact. For each intrusion, determine whether it is a pluton or dike. b. What type of fault is shown? Does it represent extension, compression, or shear (strike-slip motion)? c. What type of fold is shown? Could it have been produced by the same stresses as produced the fault? d. What type of unconformity is shown? What sort of geologic event(s) produced such an unconformity?

e. The Arroyo Granite has been radiometrically (absolute) dated at 126 ± 2 Myr and the Gulch Dike has been dated at 142 ± 3 Myr. i. What is the age of the period of erosion producing the unconformity? ii. How well constrained is the age of the fault? Part D: Absolute (Numerical) Ages The Amîtsoq gneisses of southern Greenland are some of the oldest rocks on Earth. The following datasets were collected by two independent labs. One set uses the decay of 87 Rb to 87 Sr (rubidium to strontium) whereas the other depends on the decay of 147 Sm to 143 Nd (samarium to neodymium). Whereas Rb has a chemistry similar to Na and Sr has a chemistry similar to Ca, Nm and Nd both have a chemistry vaguely similar to Fe. The contrast in the geochemical behavior of these elements means that there is no way that some geochemical process could skew both parent-daughter systems to produce ages that are both identical and wrong. 1. Four of the original nine Rb-Sr measurement pairs were selected to make graphing easier. Plot the four data points on the graph below. Connect the points with your best estimate of a straight line running through the y-axis. Use a straight edge/ruler. Graph is on second to last page. Sample 87 Rb/ 86 Sr 87 Sr/ 86 Sr 158529 0.310 0.7171 158530 0.996 0.7556 158526A 3.36 0.8807 158526 5.84 1.0133 a. Calculate the slope of this line. Slope = rise/run = (Y 2 - Y 1 ) / (X 2 - X 1 ). Pick two convenient spots on your line to get X and Y values. b. Now determine the age of the Amîtsoq gneisses by plugging your slope into the following formula: 𝑡𝑡 = ln( 𝑚𝑚 + 1) / 𝜆𝜆 where t is the time in million years, ln is the natural log function, m is the slope determined in part a, and λ (lambda) is the decay constant for 87 Rb: 1.42 * 10 -11 . Age in millions of years: _____________ = Age in billions of years: _____________

Graph for Question 1 in Part D Sample 87 Rb/ 86 Sr 87 Sr/ 86 Sr 158529 0.310 0.7171 158530 0.996 0.7556 158526A 3.36 0.8807 158526 5.84 1.0133 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 0 1 2 3 4 5 6 87 Sr/ 86 Sr 87 Rb/ 87 Sr

Graph for Question 2 in Part D Sample 147 Sm/ 144 Nd 143 Nd/ 144 Nd 158509 0.0996 0.510305 171757 0.1599 0.511783 171759 0.1858 0.512450 171756 0.2371 0.513762 0.508 0.509 0.51 0.511 0.512 0.513 0.514 0.515 0 0.05 0.1 0.15 0.2 0.25 0.3 143 Nd/ 144 Nd 147 Sm/ 144 Nd