Assignment - Lesson 3.4 - depositional environments_individual

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Clemson University *

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1010

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Geography

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Oct 30, 2023

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Assignment 3.4 - Interpreting Environmental Change from a Stratigraphic Section Depositional Environments Background For this lesson you will be investigating the geologic history of an unknown location by interpreting a hypothetical, regional stratigraphic section. A stratigraphic section is a vertical sequence of rock units (lithologies) with oldest units on bottom and youngest on top (Principle of Superposition) . Accordingly, you can make interpretations about how the environment changed through time at this location by looking at the sequence of lithologies and their relationships to one another, starting by interpreting the bottom layers and then moving to the top. What you need to do For this exercise, you will answer the following questions about the provided stratigraphic section (1) What is the depositional environment represented by each unit/lithology (2) Determine if the changing depositional environment through time indicates rising or falling sea level Use the abundant evidence preserved in the stratigraphic record to support your interpretations of depositional environment for each unit. Interpret whether sea level rose or fell over this time period by thinking about whether deeper water depositional environments are at the bottom of the section and increasingly shallow environments are at the top of the section (this would indicate water level fell through time) or whether shallow water environments are at the bottom and the environments get progressively deeper towards the top of the section (this would indicate rising sea level through time). Words of wisdom There is not necessarily a unique depositional environment for each lithology. However, a benefit of having a continuous stratigraphic record is that you are provided with context. What this means is that you have additional information about possible depositional environments by considering the sequence as a whole. Specifically, neighboring units in the stratigraphic sequence can aid the interpretation of a logical progression of environments through time. A good way to approach stratigraphic interpretations is to focus first on observations for each sedimentary unit or lithology then to use those observations to make informed interpretations of the likely depositional environment(s) in which the unit formed. From there, think about each individual identification in the broader context of the complete section.

Ask yourself the following questions, the answers to which will help you interpret the depo. environment: (1) What does grain size/composition/texture, fossil content, etc. in each unit tell me about the depositional environment’s energy, location, etc.? How does this change through time? (2) At what levels do I see evidence for marine deposition and when do units reflect deposition in continental environments? (3) What does a marine-to-continental or continental-to-marine change tell me? The following questions are in a Canvas Quiz in Lesson 3.4 – answer them there and submit the online . Don’t forget you also have a regular Lesson 3.4 lesson quiz in addition to this assignment! 1) First, list the units in order from oldest to youngest a. 1, 2, 3, 4 b. 4, 3, 2, 1 c. 4, 1, 3, 2 d. 2, 3, 4, 1 2) Which is the depositional environment of Unit 1 a. Stream (channel and floodplain) b. Shallow marine (continental shelf) c. Deep ocean d. Beach e. Coastal delta with swamps f. Playa lake Unit 3 – This unit contains medium to dark gray siltstone and shale that has a lot of organic matter and fossils of clams, snails and other marine organisms. Fossil rich limestone beds are present at the top of the unit with mostly whole shell fossils Unit 2 – This unit is mostly yellowish well-rounded quartz sandstone with small pieces of broken marine shells. It also contains ripples. Unit 4 – This unit includes sandstone, mudstone and layers of coal. The upper part of the unit contains sandstone beds with small cross beds. The mudstone has mudcracks and plant fossils. The lowest part of the unit contains tan sandstone with broken marine shells. Unit 1 – The lowest unit includes a conglomerate with rounded pebbles and coarse sand containing pieces of fossilized wood and scour marks. The conglomerate is overlain by reddish, maroon, and gray shale and mudstone with fossilized plant fossils. These muddy layers contain dinosaur footprints where a large herbivorous dino sank into the then-soft sediment.

g. Mountain glacier h. Desert dune field 3) Which is the depositional environment of Unit 2 a. Stream (channel and floodplain) b. Shallow marine (continental shelf) c. Deep ocean d. Beach e. Coastal delta with swamps f. Playa lake g. Mountain glacier h. Desert dune field 4) Which is the depositional environment of Unit 3 a. Stream (channel and floodplain) b. Shallow marine (continental shelf) c. Deep ocean d. Beach e. Coastal delta with swamps f. Playa lake g. Mountain glacier h. Desert dune field 5) Which is the depositional environment of Unit 4 a. Stream (channel and floodplain) b. Shallow marine (continental shelf) c. Deep ocean d. Beach e. Coastal delta with swamps f. Playa lake g. Mountain glacier h. Desert dune field 6) Does the change of environment from the base of the section (unit 1) up to the gray siltstone (unit 3) indicate rise or fall in relative sea level? a. Rise b. Fall 7) Does the change of environment from the gray siltstone (unit 3) to the overlying sandstone (unit 4) indicate a rise or fall of sea level? a. Rise b. Fall 8) Which of the following phrases summarizes the history of the entire sequence? a. Rise of sea level b. Fall of sea level c. A sea level rise followed by a fall d. A sea level fall followed by a rise

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