7_sedimentary rocks_SP23
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Dec 6, 2023
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1 Lab 7: Sedimentary Rocks NAME_____________________________ Materials: Rock Study Kit, Mineral Study Kit Exercise 1. Sedimentary Rock Classes Sedimentary rocks form via weathering of preexisting rocks. The products of that weathering process can include clasts (smaller pieces of rocks and/or minerals), dissolved ions in solution, and/or new minerals. See Video 1: Introduction and
Video 4: Non-clastic Rock Types
for help. Below is a summary the sedimentary rock classes we will use in this lab. They are defined by how rocks are formed. Þ
Clastic
: made up of preexisting rock or mineral fragments that are “glued” together with cement Þ
Organic
: made up of fossilized cells (former living tissue) of organisms Þ
Biochemical
: made up of minerals that were precipitated by organisms (like shells or bones) Þ
Chemical
: made up of minerals that were precipitated via inorganic processes (ex: evaporation, cooling, or interactions between chemically different water) Classify the following sedimentary rocks according to the definitions above. Please note that some samples are in your rock kit and some are images posted to Canvas. a)
Sample 1a in the Lab 7_images file on Canvas – a 300 million year old rock from England. This rock was formed in a wetland where large amounts of plant matter was deposited in an anoxic environment. Note the tree bark imprint. b)
Sample 10 in your rock kit (limestone). This rock formed in a marine environment where shells of organisms were deposited and eventually get cemented together. c)
Sample 7 in your rock kit (calcareous tufa). This rock formed in a spring where groundwater continually seeps up at the bottom of a freshwater lake. As the groundwater interacts with lake water, minerals crystallize into tufa towers. d)
Sample 9 in your rock kit (conglomerate). This rock formed as pieces of a granite were weathered, eroded and transported down a mountain stream where they eventually were deposited. After deposition the pebble-size grains were buried where they were stuck together with a silica cement. Exercise 2. Clastic Rock Interpretation In this exercise we will focus on clastic sedimentary rocks only. As with igneous rocks, every sedimentary rock has a story to tell. You just need to know how to “read” it. In clastic rocks especially, characteristics like grain size, shape and sorting can tell you a lot about the environment in which sediments are deposited. See Video 2: Clastic Rock Texture
for a review. The source of most sediments that make up clastic rocks is geologic features that have high elevation, like mountain ranges. Here sediments are weathered and eroded from mountain sides and moved by a transportation agent to their depositional environment. Transportation agents include wind, water, glaciers and gravity. Here are some basic rules for how to interpret clastic rock texture: Þ
Larger grains or clasts tend to be deposited close to the source (mountains).
2 Þ
Grains tend to be more angular if they are deposited close to the source or carried by gravity or glaciers. Increased transportation time, especially by wind and water results in more rounded grains. Þ
Grains tend to be more poorly sorted when deposited close to their source, especially if carried by gravity or glaciers. Transportation by wind and water tend to increase sorting. Use the images in the Lab 7_images file on Canvas and the samples in your rock kit to fill out the table below. Note that more than one transportation mechanism is possible in the first column and more than one possible depositional environment is possible in the last column. See Figure 1 below for examples of depositional environments. REMEMBER you’re answering these questions for the sediment INSIDE the rock (e.g., what mechanism carries sand around before it is deposited to form a sandstone?). Likely Transportation Mechanism (wind, water, glaciers and/or gravity) Deposited close or far from source? (close, far, or intermediate) Possible Depositional Environment (several correct answers are possible) Sample 6 (Rock Study Kit) Sample 8 (Rock Study Kit) Sample 2c (Canvas Image) Sample 2d (Canvas Image) Figure 1. Sedimentary environments. From clockwise from left to right: sand dunes, braided river, mountains, glaciers, meandering river, delta and lake.
3 Exercise 3. Chemical, Biochemical and Organic Rock Interpretation Chemical, biochemical and organic sedimentary rocks are typically comprised of minerals or organic matter produced by either biotic or abiotic processes. Just like clastic sedimentary rocks their chemistry and texture can reveal a lot about their depositional environment. See Video 4: Non-Clastic Rocks
for help on this section. Chemical Sedimentary Rocks Typically produced by precipitation of minerals like halite, gypsum or calcite either as water evaporates or in groundwater that cools at the surface, drips slowly in cave systems or flows through rocks near the surface. I did not include oolitic limestone in the video. Oolitic limestone is made up of little balls of carbonate (primarily made of calcite) that form in hot, arid shallow marine environments. As the waves roll back and forth on the beach, tiny crystals of carbonate start to precipitate and form little round balls (ooids). Biochemical Sedimentary Rocks These are typically made up of the shells of organisms – big and small! But mostly small J
. The composition of the shells and the type of organisms can tell you a lot about the environment. Example: shells made of carbonate (like calcite) form in warm waters and shells made of silica form in cool water. Limestone is one of the most abundant types of biochemical sedimentary rocks. Organic Sedimentary Rocks This rock type is made up of organic matter (actual once-living tissue) that are preserved in rocks. The two main types are oil shale and coal. These rocks either require rapid burial of organic matter with some type of sediment (clay is the most common) OR deposition of organic matter in anoxic environments. Interesting note: these rocks are used as an energy source! They can be burned because they are made up of organically-bound carbon. Questions: a)
Sample 3(a) is made up of plant matter (no actual minerals!). Is this sample a chemical, biochemical or organic sedimentary rock? b)
What’s a likely depositional environment for Sample 3(a)? c)
Sample 3(c) is made up of gypsum (samples 9, 14 and 15 in your mineral study kit). Is this sample a chemical, biochemical or organic sedimentary rock? d)
What’s a likely depositional environment for Sample 3(c)? e)
Sample 3e (image of the rock comprised of shells from marine organisms) and Sample 10 in your Rock Study Kit are made up of the mineral (calcite). Compare these rocks to the sample of calcite (Sample 6) in your mineral study kit. Why do the sedimentary rocks made up of calcite look so
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4 much different that the mineral specimen in your mineral study kit? Think about how they’re formed. f)
Is Sample 3e a chemical, biochemical or organic sedimentary rock? g)
What is a possible depositional environment for Sample 3e? Exercise 4. Sedimentary Structures Sedimentary structures (large-scale patterns preserved in sed rocks) and fossils can also reveal quite a bit about depositional environment (see Video 5: Sedimentary Bedforms for help). Since we know what environments these features form in today, we can apply this knowledge to interpret the depositional environment of sedimentary structures preserved in ancient rocks. Here’s a summary of some common sedimentary structures: •
Ripples
: Small wave-like structures in sediments that are produced by water. Can be symmetric or asymmetric. •
Dunes
: Large wave-like structures in sediments that are produced by wind. Almost always asymmetric. Ripple/dune video: https://www.youtube.com/watch?v=ohRB_G47UcQ •
Crossbeds
: Inclined layering produced by migration of dunes and ripples. Crossbedding video: https://www.youtube.com/watch?v=gUIi2Pdqjgo •
Mudcracks
: Polygonal cracks formed where mud is deposited that is periodically flooded. Mudcrack video: https://www.youtube.com/watch?v=dJk0URePwos Use the Ex 4 images in the Lab 7_images file posted to Canvas to complete the table below. Name the sedimentary structure shown and list the possible depositional environment (sand dunes, river/stream, beach, floodplain). Note that the picture descriptions tell you the name of the rock. Sedimentary Structure Possible Depositional Environment Picture A Picture B Picture C Picture D Exercise 5. Sedimentary Rocks on Mars
Use the Ex 5_Mars Rock image in the Lab 7_images file posted to Canvas. This image was taken in Gale Crater on the Martian surface by Curiosity, the Mars Science Lab Rover. It provides important clues as to the past climate on Mars!
5 a)
What the name of the rock is pictured in this image? b)
What depositional environment does this rock form in on Earth? c)
Right now, Mars is extremely dry and cold. What does your answer for question b imply about how Mars’ climate was different in the past?