Sedimentary Rocks and Their Environments

OBJECTIVES

• Review Igneous rocks: classify by texture/color level and name
• Understand the weathering processes and their products
• Know the processes that form sedimentary rocks
• Identify the common sedimentary rocks features and environments
• Be able to name the common sedimentary rocks

MATERIALS

• Igneous rocks from the previous lab
• Common Sedimentary rocks provided: Shale, Mudstone, Sandstone/Arkose Sandstone, Conglomerate, Breccia, Fossil Limestone, Chert/Flint, Coquina, Chalk, Lignite/bituminous coal, Rock Gypsum, Rock Salt
• Grain size chart
• Ruler

REVIEW IGNEOUS ROCKS

Your Instructor will provide you with a combined group of Igneous and Sedimentary rocks.  Physically separate the Igneous rocks that you worked with last week from the new sedimentary rocks.  Physically pick up and explore each sample.  

Once your instructor has checked your groups, use your knowledge and notes to review the Igneous samples.  Try to name them and distinguish the identifying textural characteristics (Figure 1). Recall the two-part chart with Texture and Composition from the previous lab. Fill-in the rock names. Can you identify any minerals?

Figure 1. Location relates to the cooling environment and therefore the grain size. Image: The Editor's Apprentice, CC0, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Igneous_rock_eng_modified.jpg

SEDIMENTARY ROCKS AND PROCESSES

INTRODUCTION

Sedimentary rocks are the pages in which Earth's history is written.  They contain powerful environmental indicators (glacial extent, shoreline locations), traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes (massive floods), overall climate, to the productivity of life.

The identification of sedimentary rocks is more than applying names, since each name is a loaded term that conveys information regarding its history, where it was formed, potentially when it was formed, and the processes that lead to its formation. Each sedimentary rock is a puzzle and by identifying the sediments within, how they are layered, the fossils within, and patterns in the rocks a geologist can reconstruct an entire paleoenvironment and ecosystem. Solving these puzzles is both an academic exercise to better understand the world around us as well as a tool for finding the resources that are important to our lives. Fossil fuels as well as many other natural resources are contained within sedimentary rocks such as coal, natural gas, salt, and the materials that go into wallboard or in the making of cement. Therefore, a better understanding of sedimentary rocks and how and where they are formed directly influences your everyday life.

LET’S RETURN TO THE ROCK CYCLE

Take note of the location on the rock cycle representing Sedimentary rocks and the processes that lead to them.  In order to create sedimentary rocks, sediment must first form (Figure 2).  

Figure 2. The Rock Cycle.  Sediments are produced by the weathering, erosion, transportation and deposition of sediment such as clay and pebbles.  Sedimentary rocks are formed when sediment is buries, compacted and cemented together. Image Steve Earle at opentextbc.ca.

MAKING SEDIMENT FIRST:  Weathering of Preexisting rocks

Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediments. Basically, sedimentary rocks are composed of the broken pieces of other rocks (sediment). The obvious place to start this lab is a discussion of how rocks are broken down, which is a process called weathering. There are two basic ways that weathering occurs in nature. First, rocks can be physically broken into smaller pieces (imagine hitting a rock with a hammer), which is called mechanical weathering (figure 3). Alternatively, rocks can be broken down and altered at the atomic level (imagine dissolving salt in a glass of water), which is called chemical weathering. There are multiple ways each type of weathering can occur. Therefore, both the rate that rocks breakdown and how, vary dramatically depending on the rock and environment.

Figure 3. Mechanical weathering turns larger pieces of rocks into smaller pieces of rock.  This increases the surface area and helps facilitate chemical weathering. Image CC BY S.A.4.0 International License by Johnson, Affolter, Inkenbrandt, Mosher at opengeology.org/textbook.

MECHANICAL WEATHERING

The most prevalent type of mechanical weathering is the collision, breaking, and grinding of rock by the movement of gravity, water, ice or air.  Several common methods of mechanical weathering are: Frost Wedging, Thermal Expansion, Unloading or Exfoliation and Biological mechanical weathering. When sediments produced mainly by mechanical weathering turn into solid rock, they produce a classification of sedimentary rocks called Detrital or Clastic. They are made up of small bits of sediment compacted and cemented together.

Frost wedging occurs when water seeps into cracks in a rock and freezes (figure 4). Water has a         unique property in that it expands when frozen, which puts pressure on the rock and can potentially split boulders. The repeated freezing and thawing cycles can break a significant volume of rock over time. Gravity can move the broken sediment downward into talus piles forming at the base of mountains.

Figure 4. Frost Wedging. Liquid water seeps into small fractures in a rock then expands when the water freezes. This mechanically breaks apart rocks. Right image is from Southern Ireland. Left Image: https://opengeology.org/textbook/5-weathering-erosion-and-sedimentary-rocks/ Right Image:  https://nn.wikipedia.org/wiki/Frostforvitring https://www.wikiwand.com/simple/Weathering

        Thermal expansion weathering occurs when rocks are exposed to extreme heat and cold. This repeated         expansion and contraction as the rock heats and cools can cause a rock to break. An example of this is if         cold water is spilled on a hot light bulb it will shatter (figure 5). This mechanical weathering is common in         the extreme heat and cold found in desert environments. Sharp angled sediments form.

Figure 5. Parallel thermal fractures in rocks from Abisko, Sweden (left) Right image shows the thermal fracturing of rocks in Devils Marbles, Northern Territory Australia.  Image:  https://www.wikiwand.com/simple/Weathering https://upload.wikimedia.org/wikipedia/commons/7/73/Devils_Marbles%2C_Northern_Territory%2C_Australia%2C_2004_-_panoramio_%281%29.jpg by: ogwen

Unloading or exfoliation breakage can occur within rocks when they cool very quickly or experience extreme reduction in pressure (figure 6). Many igneous rocks cool/solidify at great depths within the Earth. When they are uplifted and exposed to lower pressure they expand slightly. This expansion creates joints.

Figure 6. Exfoliation fractures in granitic rocks in Yosemite NP, CA. Notice the layered appearance of the crystalline Igneous granite as it spalls due to the reduced overburden and pressure.  Image CC BY NPFlynn.

Biological mechanical weathering from plants, animals, and humans can cause significant amounts of weathering. The small roots grow and expand into fractures. Over time rocks can be split apart by the expanding power of tree roots (figure 7). Biological weathering from organics such as lichen and mosses also chemically dissolve minerals.

Figure 7. Plant roots grow into small fractures and eventually expand to break apart large rocks. Image: https://www.nps.gov/subjects/erosion/weathering.htm

Take a Look at the Sedimentary Rocks Provided.

Identify any that seem to be made of mechanically weathered sediments. Are they made of bits and pieces of other rocks? Put them into a pile.

CHEMICAL WEATHERING

Rocks can also be chemically weathered, most commonly by one of three processes: Dissolution, Hydrolysis and Oxidation. When chemically weathered materials form a solid rock, they form a group of sedimentary rocks called Chemical or Evaporites. If significant amounts of biological materials such as fossil shells or organic material is present a group of sedimentary rocks called Bio-Chemical rocks is formed.

Dissolution. A mineral or rock is broken apart by water into individual atoms or molecules. You are familiar with this process if you dissolve sugar in a hot beverage.         The individual ions, such as sodium (Na), Calcium (Ca), and others can then be transported with the water and then redeposited/precipitated as the concentration of ions increases, normally because of evaporation of the water. Rock salt and rock gypsum form this way (figure 8).  Chemical weathering can also change the mineralogy of rocks and weaken original material, sometimes producing caves and caverns (figure 9).

Figure 8. White gypsum dunes of the White Sands National Monument in New Mexico, USA (left). Right image is the formation of tufa towers of limestone forming from the highly alkaline soda lake in Mono lake Ca, Inyo Valley Caldera. Images: CC BY NPFlynn.

Figure 9. Large layers of limestone dissolved to form a karst topography in Minevre, France https://opengeology.org/textbook/5-weathering-erosion-and-sedimentary-rocks/

Hydrolysis occurs when a hydrogen atom from a water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into softer clay minerals such as kaolinite (figure 10). The clay minerals are often very dull in luster.  Minerals such as Quartz are very resistant to the chemical weathering of hydrolysis, therefore they may still appear vitreous and glassy.

Figure 10.  Unweathered granite (left) and weather by hydrolysis (right) Notice the chalky dull appearance of the feldspars as they become the clay mineral kaolinite. Image: https://opentextbc.ca/physicalgeology2ed/Steve Earle.

Oxidation is when oxygen atoms alter the valence state of a cation, this normally occurs in metal bearing         minerals such as magnetite and iron rich minerals. This is commonly known as rusting (figure 11).  The common yellow-red streaks or surface veneer is evidence of this chemical weathering process.

Figure 11. Iron bearing minerals such as the dark colored minerals Biotite Mica and Hornblende Amphibole oxidize (rust) to oxide minerals such as limonite (reddish orange form) (left). Right image shows the red rusty staining of a sandstone with iron bearing minerals.  Image: (left) https://opentextbc.ca/physicalgeology2ed/ Steve Earle. https://www.photos-public-domain.com/2018/09/19/sandstone-with-rust-spots-texture/

Chemical and mechanical weathering can work together to increase the overall rate of weathering. Chemical weathering weakens rocks making them more prone to breaking physically, while mechanical weathering increases the surface area of the sediment, which increases the area exposed to chemical weathering (figure 12). Therefore, environments with multiple types of weathering can erode very quickly.

 Figure 12. A combination of mechanical and chemical weathering produces these solution pockets in the soft sandstone at Capital reef National Park Utah. Image public domain https://www.nps.gov/media/photo/gallery-item.htm?pg=2189358&id=68DF6B38-155D-451F-674DACCB5A959BD2&gid=68D0E3D7-155D-451F-670C6155E53196CC

As you go through the following sections (on rocks and environments) think about the types of weathering required to make the sediment that will then make up different types of sedimentary rocks as well as what types of weathering you would expect to occur in different environments.

The main products of mechanical and chemical weathering

Lithic (rock) fragments: broken pieces of parent (pre-existing) rock. Produces a wide range of sediments such as silt, sand, pebble sized particles of varying materials (figure 13). They form a variety of sedimentary rocks such as Shale, Sandstones, Mudstones, Breccia and Conglomerates.

Figure 13. Rock fragments are formed by combinations of weathering and produce a wide variety of sediment sizes. Left image shows angular rocks fragments from The Grand Tetons National Park, Wyoming. Right image consists of rounded basalt pebbles from the beaches of Hellnar, Iceland near Arnarstapi. Images: CC BY NPFlynn.

Resistant mineral grains: some minerals are relatively stable at the Earth’s surface and are resistant to weathering processes. Quartz is the most common resistant mineral, and can vary greatly in their appearance (figure 14).  Quartz mineral grains are a main component of most Sandstones. Ions dissolved in groundwater can precipitate and cement particles together as seen in most Sandstones and Conglomerates.  

Figure 14. The wide variety of sand sized particles from glass sand, dune sand, quartz sand, volcanic sand, coral sand, garnet sand, and olivine sand. Samples are from Mongolia, Estonia, Hawaii, and mainland U.S. (1x1 cm each). Image: https://en.wikipedia.org/wiki/Sand#/media/File:00065_sand_collage.jpg Siim Sepp https://www.sandatlas.org/

Clay: clay is formed by the chemical weathering/breakdown, specifically hydrolysis, of feldspar minerals. Besides being a mineral, the term clay also refers to sediment that is smaller than 1/256 mm (figure 15).  The particles are so small that you can barely detect them as individual grains if you rub them between your fingers. Clay sized particles form the basis of Shale and Mud rocks. They can be found in a wide variety of colors.

Figure 15. Large clay deposits (left). Right image shows the rock shale formed from the finest size clay grains which are typically 1/256th of a mm in size.  Image (left): By Siim Sepp - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=328890. Right image:https://ohiodnr.gov/wps/portal/gov/odnr/discover-and-learn/rock-minerals-fossils/common-rocks/shale

Ions in solution: chemical weathering of non-resistant minerals releases ions such as Si, Ca, Na, Fe, Mg. These ions are present in lakes, rivers, groundwater, and the ocean where chemical sedimentary rocks form by precipitation or evaporation. The Bonneville salt flats are produced by the evaporation of super saturated saline sea waters evaporating, as well as the White sands (gypsum) found at the White sands National Monument in New Mexico (figure 16). This process produces such rocks as Rock Salt and Rock Gypsum.  

Figure 16. White Sands NM in New Mexico is actually Gypsum sand, not quartz sand (left). Right image shows gypsum as a rock. Image: https://www.sandatlas.org/wp-content/uploads/White-Sands-gypsum-dunes.jpg davebluedevil/Wikimedia Commons.Right: http://skywalker.cochise.edu/wellerr/rocks/sdrx/gypsum1.htm

Biological and Chemical Material:

Many weathered sediments occur in marine environments, such as the ocean or terrestrial environments such as lakes or swamps. As a result, biological organisms are often a small or large part of some sedimentary rocks. If you can identify biological remains such as shells this is a strong indicator of a sedimentary rock since living organisms could not survive in igneous/magma conditions such as a volcano. The list below describes a few of the common biological materials that combine to create the classification of Biochemical sedimentary rocks.

Fossils and shell fragments when mixed with limestone material can produce fossil Limestones. Limestone can also form from non-biological marine conditions and may not contain fossils. Limestones are the most common sedimentary rock and have a wide variety of colors and overall appearances. When shell fragments dominate the rock, they can produce a rock called Coquina (figure 17).

Figure 17. Images of fossiliferous limestone and brachiopod fossils on top. Bottom images show a coquina with cemented shell fragments. Images Roger Weller Cochise college. http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Powdered carbonate coccoliths are microscopic marine organisms composed of calcium carbonate microfossils which dominated some marine environments in the late Mesozoic era (figure 18). Their outer layers build up and form massive chalk layers such as the white chalk cliffs of Dover England (figures 19 & 20).  This material is used to make the common chalk used for drawing on sidewalks.

 Figure 18. Coccolithopore while still allive. The microplates shed as the organism grows. There are mostly a low magnesium calcium carbonate. Image:

https://en.wikipedia.org/wiki/Coccolith#/media/File:Gephyrocapsa_oceanica_color.jpg

Photo by NEON ja, colored by Richard Bartz, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

Figure 19. White Chalk Cliffs of Dover England from the cretaceous period. Large scale deposits of coccolithophores. Image R Weller http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Figure 20. Sedimentary rock formed from coccoliths.  The Bio-chemical sedimentary rock Chalk is formed. Image http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Silica tests of radiolarians and diatoms form in deep marine waters when they build-up they produce nodules or layers of silica-based chert (figures 21 & 22). The organic material can cause the rock to appear dark grey and black. The high silica content can produce concoidal fracture similar to the Igneous volcanic rock Obsidian and the mineral Quartz. As all three are primarily composed of Silica this should help you recognize their connection.

Figure 21. Silica tests of micro fossils of Radiolaria (a) and Diatoms (b) Marine zooplankton from the oceans. Image: By Hannes Grobe/AWI - Grobe, H., Diekmann, B., Hillenbrand, C.-D.(2009). The memory of the Polar Oceans, In: Hempel, G. (ed) Biology of Polar Oceans, hdl:10013/epic.33599.d001, pdf 0.4 MB., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=8831059 https://simple.wikipedia.org/wiki/Radiolaria#/media/File:Marine-microfossils-major_hg.jpg

Figure 22. Silica based Bio-Chemical sedimentary rock called Chert.  The silica and organic material devitrifies into a powdery surface layer. Image: http://skywalker.cochise.edu/wellerr/GEO/sedimentary/sedimentary-list.htm

Terrestrial organic material from swamps produce a common material such as peat (figure 23). When this material compacts in an oxygen poor environment it can produce a low-quality type of coal called Lignite (figure 24).  Coal primarily comprised of hydrogen and carbon and is otherwise known as a fossil fuel.

Figure 23. The terrestrial formation of organic peat and then sedimentary coal into bituminous coal.  Image: https://openpress.usask.ca/app/uploads/sites/29/2017/05/Formation-of-coal.png Steve Earle CC BY 4.0

Figure 24. Dull black lignite coal. It is a low-quality coal. Image: https://www.usgs.gov/media/images/lignite-coal-0 lignite coal donna pizzarelli USGS Public Domain

Transportation and Deposition of Sediment

Weathering products that are eroded from their source can be transported by moving water, ice, or wind to a new location. The size of the carried sediment depends on the speed and type of material carrying the sediment such as ice, water or wind. A fast fluid/high energy (like a rapidly flowing river) can carry large particles and cause immense amounts of weathering while a slow fluid (like a calm stream) would hardly cause any weathering. The density of the fluid also controls the size of the particles that can be transported, for instance denser fluids (like water or glaciers) can carry larger particles than less dense fluid (like air). The shape, round or angular also indicates if the sediment has been transported to a nearby deposition or transported a great distance. The sorting of the grains can also tell us about the transportation and deposition of the sediments. If the grains are all a similar size (all very small such as clay or sand) or of a wide variety of sizes such as pebbles mixed with mud. The three main sedimentary textures that tell us about the transport history of sediments are listed below:

Grain size of a sedimentary rock can be interpreted to indicate several things. The energy of the         environment at the time of deposition. The higher the energy (e.g. the swifter the water), the larger the grain size that can be moved.  Lower energy environments encourage deposition of transported grains.         

Clay and silt sized particles will settle out of transportation only in the lowest energy environments such         as lakes or deep marine settings, producing Shales. A rock made of mostly sand sized particles produces         Sandstones. The grain size of sediment generally decreases as it gets farther from the source area due to         breakage, abrasion, or chemical weathering (Figure 25). The Wentworth scale is a chart of grain sizes of         sediment.        

Figure 25. Wentworth scale of grain size. Image usgs.gov   http://pubs.usgs.gov/of/2006/1195/htmldocs/images/chart.pdf

Rounding is the removal of sharp edges of rock fragments and resistant mineral grains as they grind         against one another or the ground surface. Angular grains have not experienced as much abrasion as         well-rounded grains. Therefore, the presence of rounded sediments indicates that the materials have been transporting for a greater amount of time than angular sediments (Figures 26 & 27).  The rounding of the individual, and usually larger grains can be used to distinguish between the Detrital/Clastic Sedimentary rocks called Conglomerate (rounded) and Breccia (angular).

Figure 26.  The 4 stages of angular sediments to rounded sediments. More rounding indicates greater weathering. Image: http://publications.iodp.org/proceedings/352/102/figures/02_F05.png

Figure 27. Angular grains, both large (top left) and small (bottom left) and more rounded large (top right) and small (bottom right). Energy from tumbling rounds the sharp edges for the sediment. Image: CC BY Steven Earle. https://opentextbc.ca/physicalgeology2ed/

Sorting is a process through which sediment grains are selected and separated according to grain size, and in some cases grain shape or density (Figure 28). Well-sorted sediments indicate constant energy over time. Poorly-sorted sediments may indicate inadequate time to winnow and sort grains. Silt/Mudstone and Sandstones are generally well sorted while Arkose Sandstone, Conglomerate, and Breccia are generally poorly sorted.

Figure 28. Sorting of grains: Poorly sorted grains have very mixed grain sizes including large mixed with small. while well sorted sediments are of more equal size. Image: Geology Commons oer.galileo.usg.edu

Let’s Look at the Samples and Identify the Size, Shape, and Sorting of the Sediments

Indicators of Energy and Deposition Environment

Fast, turbulent waters are high energy environments, and calm waters are low energy environments. Higher energy environments produce large, poorly-sorted sediments, especially if the energy/movement changes suddenly.  Whereas low energy environments produce small, rounded, well-sorted sediments. Well sorted sediments, mostly one size range indicates a stable level of energy, not a varying energy level.

DEPOSITION: Sediment is deposited when transporting agents, such as running water, glacial ice, or wind, lose energy and can no longer transport the sediment load. Deposition also refers to the accumulation of chemical or organic sediment, such as calcium carbonate (CaCO3), clamshells on the seafloor, or plant material in a swamp.

Sediments are deposited in layers on top of one another, which packs loose sediment grains tightly together (compaction). Compacted sediment can be hardened even further by the precipitation of cement (ions dissolved in circulating groundwater) in the pore space between the grains (figure 29). Common cements are calcite (CaCO3), silica (SiO2), and iron oxides.

Figure 29. Deposition of sediment followed by the compaction and cementation of the grains into a hardened lithified sedimentary rock. Image: CC BY 4.0 Karla Panchuk. https://opentextbc.ca/physicalgeology2ed/

Depositional Environments

Sediments accumulate in depositional environments such as alluvial fans, river channels, flood plains, deltas, lakes, desert valleys, beaches, shallow marine, and the deep-sea floor.  An important task of a geologist who studies sedimentary rocks is to interpret the ancient environment in which the rock formed (Figures 30-33). By making detailed observations, a geologist can read the many clues that tell the depositional story of a rock.

Figure 30. Properties you can use to interpret the depositional environment of sedimentary rocks.

Take a Look at Your Sedimentary Samples. Can You Identify Any Possible Environments?

Figure 31. Common depositional environments and their corresponding sedimentary rocks.

Figure 32. This chart is another way to look at some of the information listed in Fig. 30.

Figure 33. A visual representation of many terrestrial and marine depositional environments. Image CC BY S. Earle opentextbc.ca/geology.

Let’s Practice the Classification of Detrital or Clastic Sedimentary Rocks

Sedimentary rocks made from sediments such as bits and pieces of weathered rocks are classified as Detrital or Clastic.  They are classified based on the size of the grains, the sorting of the grains, and the rounding of the grains.  Each of these components indicate the weathering, transportation and depositional environments.  Try to identify the Shales, Silt/Mudstone, Sandstones, Conglomerate and Breccia.

Next to the rock name write a few characteristic features and how they formed:

Shale

Silt/mudstone

Sandstone

Conglomerate

Breccia

Let’s Practice the Classification of Chemical, Biochemical, and Organic Sedimentary Rocks

This classification of sedimentary rocks form from chemical weathering processes.  Many are formed from the evaporation of marine waters to produce rocks such as Rock salt and Rock gypsum.  Biochemical sedimentary rocks form from marine organisms that extract chemical components from water to build shells and other body parts.  Rocks such as Fossil limestone (which can also be inorganic), Chalk, Coquina, and Chert/Flint. Organic matter can build up in layers of meters and meters of organic debris to form terrestrial coal seams.

Next to the rock name write a few characteristics that will help you identify each sample:

Rock Salt

Rock Gypsum

Fossil Limestone

Chalk

Coquina

Chert/Flint

Lignite Coal

Using the Charts try to distinguish between Clastic/Detrital and Biological/Chemical Sedimentary Rocks.  

Then try to name them. Look carefully at the sediment size, shape & sorting.  Can you connect a depositional environment to each of the samples?

TRY A FEW QUESTIONS:

1. How are Breccia and Conglomerate the same but different?

2. How would you identify Coquina from Chalk?

3. What environmental conditions lead to the formation of Shale vs Sandstone?

4. Rock salt can form in what type of geologic setting?

5. If you find coal measures/seams what can this tell you about the paleoenvironment of that area?

6. Why are Sedimentary rocks important?

7. Explain how you would distinguish an Igneous from a Sedimentary rock. Name a few.

8. Identify the following images as type of rock, details of their environment, and components.  Write a brief statement of what this rock can tell you about its formational environment. (Images first 2- CC BY S. Earle, 4th image openpress.org)

https://www.wikiwand.com/en/Coquina

https://opengeology.org/textbook/wp-content/uploads/2017/02/XBedsZion.jpg

CC BY NC SA The following 4 images Skywalker.cochise.edu R Weller