LAB part 1 - Lithostratigraphy - Fall 2021 (1)

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GEO-102 – Fall 2021 Lab 3: Lithostratigraphy & Basic Principles of Relative Dating The principle of uniformitarianism states that the processes operating on Earth today are similar (if not identical) to those operating on Earth in the past. This idea is often summarized as: “the present is the key to the past.” Stratigraphy is a subfield of geology that studies layers of rocks. The goal of stratigraphy is often to correlate (match) rock layers from one area with rock layers from another area. Lithostratigraphy is a branch of stratigraphy that seeks to correlate layers based on the rock type, or lithology, of the layers. Geologists use several principles to help them determine which layer was deposited or formed first. These are the principles of relative dating : The principle of original horizontality : All sedimentary rocks were originally deposited in horizontal beds. If they occur in a different orientation (e.g. tilted, faulted, and/or folded), it is due to post-depositional processes. The principle of superposition : In an undisturbed sedimentary rock sequence, the older rocks are structurally and stratigraphically below younger rocks. The principle of lateral continuity : Sediments generally accumulate in continuous sheets within a given region. For example, two similar sedimentary layers on either side of a canyon can be assumed to have once been one continuous layer that was later eroded by the river that formed the canyon. The principle of inclusions : A rock containing an inclusion (a fragment of another rock) must be younger than the inclusion. For example, a sedimentary rock containing fragments (inclusions) of a metamorphic rock is younger than the metamorphic rock fragments. The principle of cross-cutting relationships : If a rock or structure (like a fault) cuts across another rock or structure, it is younger than everything it cross-cuts. Unconformities : Depositional time gaps in the rock sequence represented by an erosional surface. There are three basic types of unconformities: 1. Nonconformity : erosional surface that separates overlying (younger) sedimentary rocks from underlying (older) metamorphic and/or igneous rocks. The erosional surface separates two different rock types. 2. Disconformity : erosional surface that separates horizontal overlying (younger) sedimentary rocks from underlying (older) horizontal sedimentary rocks. 3. Angular unconformity : erosional surface that separates overlying horizontal (younger) sedimentary rocks from underlying tilted (inclined and older) sedimentary rocks. 1

GEO-102 – Fall 2021 Interpreting Stratigraphic Columns and Block Diagrams: Reconstructing Sequences of Geologic Events Geologists can use the principles of relative dating to correlate stratigraphic columns or interpret block diagrams. A stratigraphic column is a vertical sequence of sedimentary (and occasionally igneous) rock layers. Geologists often try to find similarities between two or more stratigraphic columns and attempt to correlate (match) the columns to interpret what may have happened over a geographical area (e.g. the two sides of the Grand Canyon). Block diagrams also represent a sequence of geologic layers and events but can often include a 3-D component to the representation and are not used in correlation. Block diagrams can be used to create a historical reconstruction of when layers were deposited and when other tectonic, igneous, or erosional events occurred. Tips for correlating stratigraphic columns: 1. Always begin correlations at the bottom of the stratigraphic columns. Connect the lowest layers first because those layers were the first to form. Work your way upwards. 2. Correlate layers by connecting the base of “Rock Type A” in one column to the base of “Rock Type A” in another column. 3. None of the lines you draw connecting the stratigraphic columns should cross each other. 4. If a rock type is present in one stratigraphic column, but is not present in the next column, connect the base and the top of that rock unit in a sideways triangle, showing that the rock unit becomes thinner in that direction. (See example below) 2

GEO-102 – Fall 2021 Part I – Correlation of Stratigraphic Columns 1. Correlate (connect) the rock layers in the stratigraphic columns below. Correlations should be based on similar lithologies (rock types). [7 points] 2. Correlate (connect) the rock layers in the stratigraphic columns below. Correlations should be based on similar lithologies (rock types). [7 points] 3

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GEO-102 – Fall 2021 3. Correlate (connect) the three stratigraphic columns below and answer the questions that follow. [10 points for correlation] a. Explain what is happening to the limestone unit at time 3 in the column sequence A-B-C. Can you suggest a reason for your observation? [2 points] b. If all the rocks were deposited in a marine environment, which column is the one most likely to have been deposited farthest from land? [1 point] c. Which column was deposited closest to land? [1 point] d. What was happening to sea level from time interval 1 to time interval 3? [1 point] e. What was happening to sea level from time interval 3 to time interval 5? [1 point] 4

GEO-102 – Fall 2021 f. What was happening to sea level from time interval 5 to time interval 6? [1 point] g. What was happening to sea level from time interval 6 to time interval 7? [1 point] Part II Block Diagram Exercises - Determining the order of geologic events For each of the following block diagrams, correctly identify the order of geologic events. Make sure to include all depositional, intrusive, erosional (unconformities), and deformational (folding, faulting, etc.) events. 4. Examine the block diagram below and answer the questions that follow. a. What happened first (i.e. which letter)? [1 point] b. The dark, squiggly lines (U1 and U2) represent unconformities. What happened during these events? [1 point] c. What happened first, the faulting or the folding? [1 point] d. If rock layer F is a sandstone and rock layer G is a limestone, what type of unconformity is U2? [1 point] 5

GEO-102 – Fall 2021 5. Examine the block diagram below and answer the questions that follow. a. What type of unconformity is X? [1 point] b. What type of unconformity is Z? [1 point] c. Are the inclusions in Rock T younger or older than Rock T? [1 point] d. List the order of events from oldest (1) to youngest (14) [7 points] 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 6

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GEO-102 – Fall 2021 6. List the order of events on Mars from oldest (1) to youngest (4) and explain your reasoning. [8 points] Uranius Tholus volcano, Mars (Viking Orbiter image 516A23) 1. 2. 3. 4. 7. Examine the block diagrams on the following two pages. For each block diagram, use the provided chart to list the order of events. Put the first (oldest) event at the bottom of the chart and work your way upwards. Each event (‘geological feature’) is either one of the named rock layers or one of the boundaries labeled with a letter. For the rock layers, place a checkmark indicating whether it is igneous, sedimentary, or metamorphic. For the boundaries, indicate whether the feature is a fault or one of the types of unconformities. [ 46 points ] 7