Unit 5 Lab Sedimentary Rocks Fall 23 Rev11.16.22 sh 2

1 CHAPTER TEN: SEDIMENTARY ROCKS Bradley Deline, edited by Randa Harris INTRODUCTION We are particularly interested in the history and events that occur on the surface of the Earth both because it is easier to directly observe and test, and has direct relevance to our lives and our own history. Sedimentary rocks are the pages in which that history is written, since they contain powerful environmental indicators, traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes 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 a set of rocks, how they are layered, the fossils within, and patterns in the rocks a geologist can reconstruct an entire environment 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. In particular, fossil fuels as well as many other natural resources are, or are contained within, sedimentary rocks such as coal, natural gas, petroleum, 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. Learning Outcomes After completing this chapter, you should be able to: • Describe how erosion and weathering relate to the formation of sedimentary rocks.

2 • Identify sedimentary rocks and their features. • Describe the formation and history of different types of sedimentary rocks. • Describe the formation of different sedimentary structures. • Discuss the distribution of sedimentary rocks, sedimentary structures, and types of weathering in varying sedimentary environments. Key Terms (to be bolded throughout) Chemical Weathering Mechanical Weathering Erosion Clastic Sedimentary Rocks Maturity Chemical Sedimentary Rocks Biochemical Sedimentary Rocks Organic Sedimentary Rocks Sedimentary Structures Fluvial Depositional Environments Lacustrine Depositional Environments Eolian Depositional Environments Glacial Depositional Environments Transitional Depositional Environments Shallow Marine Depositional Environments Deep Marine Depositional Environments Weathering and Erosion Sedimentary rocks are formed by the weathering, erosion, deposition, and lithification of sediment. Basically, sedimentary rocks are composed of the broken pieces of other rocks. The obvious place to start this chapter 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),

4 then be transport with the water and then re-deposited as the concentration of ions increases, normally because of evaporation. Chemical weathering can also change the mineralogy and weaken the original material, which again is caused by water. A mineral can undergo hydrolysis, which occurs when a hydrogen atom from the water molecule replaces the cation in a mineral; this normally alters minerals like feldspar into a softer clay mineral. A mineral can also undergo oxidation, which is when oxygen atoms alter the valence state of a cation, this normally occurs on a metal and is commonly known as rusting. 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 surface area that is exposed to chemical weathering. Therefore, environments with multiple types of weathering can erode very quickly. 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. Identifying Sedimentary Rocks The classification of sedimentary rocks is largely based on differentiating the processes that lead to their formation. The biggest division in types of sedimentary rocks types is based on the primary type of weathering that leads to the material building the sedimentary rock. If the rock is largely made from broken pieces (called clasts) of rock that have been mechanically weathered the rocks are referred to as Detrital or Clastic Sedimentary Rocks . Simply put, these are rocks that are composed of the broken pieces of other rocks. In this case, the mineralogy of the clasts is not important, but instead we need to note the properties of the sediment itself.

5 Alternatively if the rock is largely the product of chemical weathering the classification is then based on the composition of the material as well as the processes involved in the materials precipitation from solution. Chemical Sedimentary Rocks form from the inorganic precipitation of minerals from a fluid. If the ions present within a fluid (water) become very concentrated either by the addition of more ions or the removal of water (by freezing or evaporation), then crystals begin to form. In this case the identification of the type of sedimentary rocks is based on the minerals present. If organisms facilitate the precipitation of these minerals from water we refer to the rocks as Biochemical Sedimentary Rocks. An example of biochemical precipitation is the formation of skeletal minerals in many organisms: from starfish and clams that grow calcite, to sponges that grow silica-based material, to humans that have bones made of hydroxyapatite. In many cases it is hard to differentiate whether a mineral was formed organically or inorganically, so in the current lab we will mostly group these two types of sedimentary rocks together. Rocks can also be formed from the carbon-based organic material produced by ancient life and are called Organic Sedimentary Rocks, which we will not have in this lab. Now we can discuss the identification and formation of particular sedimentary rocks. Clastic Sedimentary Rocks Weathering and erosion occur normally in areas that are at high elevation, such as mountains, while deposition occurs in lower areas such as valleys, lakes, or the ocean. The sediment is transported from the area of erosion to area of deposition by ice, water, or air. Not surprisingly, the sediment changes during its journey and we can recognize the amount of change and the distance the material has traveled, and the transport mechanism, by looking at its maturity (Figure 1). Maturity is defined as the texture and composition of a sedimentary rock

7 pure white quartz beaches. Last we have shale (mudstone), which is composed of clay particles and has a finely layered or fissile appearance. This extremely mature sedimentary rock is made from the smallest particles that can be carried by wind or barely moving water and can be found thousands of miles away from the original source. Biochemical and Chemical Sedimentary Rocks As mentioned before, biochemical and chemical sedimentary rocks either precipitated directly from water or by organisms. The most recognizable chemical sedimentary rocks are evaporites. These are rocks that are formed by the precipitation of minerals from the evaporation of water. You have already examined multiple examples of these minerals/rocks in a previous lab, such as halite and gypsum. In this current lab, we will focus on siliceous and carbonate biochemical sedimentary rocks. Chert is a rock composed of microcrystalline varieties of quartz, and thus it has properties that are associated with quartz itself, such as conchoidal fracturing and hardness greater than glass. Chert is often formed deep in the ocean from siliceous material that Figure 1. Maturity in clastic sedimentary rocks showing how the sediments change as they are eroded further from their source.

8 is either inorganic (silica clay) or organic (skeletons of sponges and single-celled organisms). Rock gypsum in a chemical rock formed from evaporation. As water evaporates from a mineral- rich area, the mineral gypsum is formed. Both gypsum and rock gypsum are very soft – soft enough to be scratched with a fingernail. Carbonates are one of the most important groups of sedimentary rocks and can result in distinctive landscapes (karst) and human hazards (sinkholes). Limestone is a sedimentary rock composed of the carbonate mineral calcite and can vary greatly in its appearance depending on how it is formed, but can easily be identified by its chemical weathering. Limestone composed of the mineral calcite undergoes dissolution in acids, in other words it effervesces dramatically when we apply dilute acid. As with chert, limestone can be formed inorganically from a supersaturation of calcium and carbonate ions in water in varying environments from caves to tropical beaches. Limestone that consists of crystals of calcite or microcrystalline masses of calcite is called crystalline limestone. Alternatively, limestone can be formed biologically with the most striking example called skeletal packstone (or coquina), which are rocks made exclusively of fragmented carbonate (calcite or its polymorph aragonite) shells or coral. Organic Sedimentary Rocks Organic compounds are materials that contain a significant amount of the element carbon and are often associated with life. Organic sedimentary rocks are, therefore, rocks that consist mostly of carbon and are associated with significant biological activity. Other sedimentary rocks such as limestone and shale can contain carbon, but at much lower concentration (though shale can appear black from their carbon content). The most common organic sedimentary rock is coal, which is a very low density (light) black rock that has a dusty (sooty) or shiny appearance. It also produces a dark gray streak that can be seen both on a streak plate or a piece of paper. Coal is

10 Table 1. Classification of Sedimentary rocks.

11 LAB EXERCISE Materials Unpack your rock kit and get the following six samples, shown in Figure 2, samples G through L. In addition, you will need the following from your rock kit: 1) your glass plate, 2) your hand lens, and 3) your bottle of diluted acid. Using table one, start identifying the rocks by separating out the chemical and biological sedimentary rocks from the clastic sedimentary rocks. Make sure to use all of the tools available including the glass plate and the diluted acid to identify the chemical and biochemical sedimentary rocks (chert will be harder than glass and limestone will strongly react with dilute acid). Finally, use the hand lens to closely examine the size of the grains in the clastic Figure 2. Sedimentary rock samples.

13 c. contains sand-sized grains and scratches glass d. does not contain grains and scratches glass e. does not contain grains and can be scratched with a fingernail f. consists of grains too small to see, giving it a smooth appearance 6. The formation of Sample I includes: a. Chemical weathering, transport of ions, precipitation of minerals, lithification b. Mechanical weathering, transport of sediment, deposition of sediment, lithification c. Chemical weathering, transport of ions, precipitation of minerals as shells by organisms, deposition, lithification. d. Crystal precipitation during the evaporation of water, such as in a drying lake bed. 7. The formation of Sample J includes: a. Chemical weathering, transport of ions, precipitation of minerals, lithification b. Mechanical weathering, transport of sediment, deposition of sediment, lithification c. Chemical weathering, transport of ions, precipitation of minerals as shells by organisms, deposition, lithification. d. Crystal precipitation during the evaporation of water, such as in a drying lake bed. 8. Sample J is composed of ___________. a. clastic sediments the size of pebbles b. clastic sediments too small to pick out c. microcrystalline quartz d. calcite shells e. gypsum crystals 9. Sample K has the following characteristic: a. effervesces in dilute acid b. contains fossil shells and effervesces in dilute acid c. contains pebble-sized grains and scratches glass d. does not contain grains and scratches glass e. does not contain grains and can be scratched with a fingernail f. consists of grains too small to see, giving it a smooth appearance 10. The formation of Sample K includes: a. Chemical weathering, transport of ions, precipitation of minerals, lithification b. Mechanical weathering, transport of sediment, deposition of sediment, lithification c. Chemical weathering, transport of ions, precipitation of minerals as shells by organisms, deposition, lithification. d. Crystal precipitation during the evaporation of water, such as in a drying lake bed.

14 11. The formation of Sample L includes: a. Chemical weathering, transport of ions, precipitation of minerals, lithification b. Mechanical weathering, transport of sediment, deposition of sediment, lithification c. Chemical weathering, transport of ions, precipitation of minerals as shells by organisms, deposition, lithification. d. Crystal precipitation during the evaporation of water, such as in a drying lake bed. 12. Sample L is composed of ___________. a. clastic sediments the size of pebbles b. clastic sediments too small to pick out c. microcrystalline quartz d. calcite shells e. gypsum crystals 13. Fill out Table 3, the Sedimentary Rock ID Chart. PLEASE FILL OUT A ROW FULLY FOR EACH SAMPLE AND IDENTIFY WHAT KIND OF ROCK THE SAMPLE IS. In identifying each rock, think about some of the properties we learned about for minerals. Gypsum is soft, chert is hard. Carbonates fizz in acid, silicates don't. Use this to tell some of these samples apart and identify them!

16 Examples of sedimentary structures are given in table 2, but let us discuss them in more detail. The sedimentary structures that most students are familiar with are ripples and dunes. We are familiar with seeing dunes at the beach or in deserts or smaller ripples in mud puddles. In each case, these ripples are formed from either a wind or water current. Ripples form by the current pushing sediment into a pile; on the down-current side the sediment is shadowed and protected from the wind or water current. This means that there is erosion on the up-current side making a shallow slope and deposition on the steeper, down-current side. Therefore, if you cut a ripple in half and look at it in prospect you can see inclined layers of sediment building up on the steep down current side of the ripple. We can often see multiple layers of beds consisting of these inclined layers, which represent multiple generations of migrating ripples or dunes that are called cross-beds. Both ripples and cross-beds can indicate the presence and direction of the current in an environment. We can also see structures that indicate changes in the strength of a current. Imagine a fast moving current carrying a variety of sediment sizes; if the current slows it will no longer be able to carry the largest particles and they will be deposited first. Then as the current continues to slow progressively smaller particles are deposited on top of the bigger particles, forming a sedimentary deposit called a graded bed. This graded bed is a sedimentary layer with larger clasts on the bottom and smaller clasts on the top. These types of beds can be the result of floods in a river or storms in the ocean. We can also observe features that are pretty self-explanatory such as casts of mud cracks (covered and preserved cracks that are the result of the drying of wet Figure 3. Cross-section of a ripple showing the area of erosion and deposition.

17 mud), and rain drop impressions (covered and preserved impacts of rain drops in soft mud). In both of these cases, the sedimentary structure tells us about the sediment, the water content, and its exposure at the surface above water level. Related to sedimentary structures are trace fossils, which are patterns in the rocks that are caused by the activity of organisms. These can occur in many different ways and can indicate many different aspects about the environment depending on the trace and the identity of the trace-maker. Traces can be terrestrial such as footprints, burrows or dens, or the traces of roots, which can inform us about the climate, ecosystem, and the development of soils. Traces can also be found in freshwater and marine environments, such as burrows, borings, footprints, or feeding traces, which tell us about the sediment, chemistry of the pore water, and the life that lived within it.

19 Depositional Environments A depositional environment is the accumulation of chemical, biological, and physical properties and processes associated with the deposition of sediments that lead to a distinctive suite of sedimentary rocks. Sedimentary environments are interpreted by geologists based on clues within such as rock types, sedimentary structures, trace fossils, and fossils. We can then compare these clues to modern environments to reconstruct ancient environments. We can break up the numerous depositional environments found on earth into common environments we find on land (continental environments) , in the ocean (marine environments), and at the interface between the two (transitional environments ). As each of the environments is briefly described, think about the types of sediments and sedimentary rocks most likely to occur in that environment, the maturity of those sediments if clastic, and the types of sedimentary structures that may be present. Continental Environments There are many different environments on the continents, but again we are limited to those that are dominated by the deposition rather than the erosion of sediments. Erosion occurs in high altitude areas and although continents are overall topographically elevated compared to the oceans, there are several different areas on the continent where we get distinctive depositional properties. Continental depositional environments are dominated by clastic sedimentary rocks, largely because of their proximity to the source of the sediments. Glacial depositional environments are controlled mostly by the weathering and erosion by glaciers and glacial meltwater. Glaciers most commonly occur in areas that are both high elevation and/or high latitudes. Glaciers are fairly slow moving (centimeters a day) and normally travel short distances from their source, but they can cause immense mechanical weathering.

20 Glaciers grind and bulldoze rock and create piles of poorly sorted sediment called moraines. Glaciers also produce a significant though fluctuating amount of melt water, which flows through the moraines building a system of braided rivers. These rivers carry the small sediments further from the end of the glaciers into an area called the outwash plain, which consists of poorly sorted sediment. We have spent a significant amount of time in chapter five discussing rivers and how they erode, transport, and deposit sediment. Sediments that are deposited through the action of rivers are referred to as fluvial depositional environments . The gradient and discharge of a river can greatly control the shape of the river, how it flows, and how it deposits sediment. Rivers alter sediment both chemically and physically as well as sort the sediment since there is a limit to the size or particles a river can carry. Within a meandering river we see several different types of sediments, from the pebbles and stones within the river channel to sandy point bars along the outer edge of the meander where the water slows. In addition, we see multiple types of sedimentary structures related to the flow of the river as well as those related to flood events. We also have sediments deposited within lakes, which are called lacustrine depositional environments . Unlike rivers, lakes do not have rapidly flowing water and thus there is significantly less movement of sediment. The sediment that accumulates in lakes can come from several sources including rainwater carrying sediment into the lake from the shores, rivers that flow into the lake, and sediment that is transported by the wind. Once the sediment reaches the lake it remains undisturbed so we see thin layers of fine sediment, with varying amounts of trace fossil. Lastly, there are eolian depositional environment , which are dominated by currents of wind rather than water. Since air is less dense than water, only smaller particles can be