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1 LAST NAME (ALL IN CAPS): FIRST NAME: 7. SEDIMENTARY ROCKS Instructions: Your work will be graded on the basis of its accuracy, completion, clarity, neatness, legibility, and correct spelling of scientific terms. If you want to test a mineral for effervescence and want to apply HCl, you must first ask permission from your instructor (remember HCl is an acid and is hazardous to health). Some rocks and minerals that you would be working with may have sharp edges and corners; be careful when working with them! When you are done with your lab work, please clean your area before you leave. INTRODUCTION The sedimentary rock cover of the continents of the Earth's crust is extensive (73% of the Earth's current land surface), but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding. The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other structures. Sedimentary rocks are also important sources of natural resources like coal, fossil fuels, drinking water or ores. The study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography, paleoclimatology and the history of life. Sedimentary Rock: form when sediments are compressed and cemented together. Sediments are loose grains and chemical residues of Earth materials, including rock fragments, mineral grains, and parts of plants or animals like seashells and twigs. DOMINANT SEDIMENTS TYPE IN ROCK Loose fragments of rocks or minerals broken off of bedrock Whole minerals (crystals) of the same kind (e.g. halite, calcite, gypsum, etc.) that precipitate directly out of water or due to seawater evaporation Shells (fossils) SEDIMENTARY ROCK CLASS Detrital (Clastic) Chemical Biochemical (Organic) DETRITAL SEDIMENTARY ROCKS Detrital sedimentary rocks are formed when sediment is deposited out of air, ice, wind, gravity, or water flows carrying the particles in suspension. This sediment is often formed when weathering and erosion break down a rock into loose material in a source area. The material is then transported from the source area to the deposition area. The type of sediment transported depends on the geology of the hinterland (the source area of the sediment). Diagenesis (all the chemical, physical, and biological changes, exclusive of surface weathering, undergone by a sediment after its initial deposition) causes the sediment to consolidate into a compact, solid substance from the originally loose material. Young sedimentary rocks, especially those of Quaternary age (the most recent period of the geologic time scale) are often still unconsolidated. As sediment deposition builds up, the overburden pressure rises, and a process known as lithification takes place. Sedimentary rocks are often saturated with seawater or groundwater, in which minerals can dissolve, or from which minerals can precipitate. Precipitating minerals reduce the pore space in a rock, a process called cementation. Due to the decrease in pore space, the original fluids (water and air) located between the sediments are expelled. The precipitated minerals form a cement and make the rock more compact and competent. In this way, loose clasts in a sedimentary rock can become "glued" together. Page 1 of 14

2 When sedimentation continues, an older rock layer becomes buried deeper as a result. The pressure in the rock increases due to the weight of the overlying sediment. This causes compaction, a process in which grains mechanically reorganize. Compaction is, for example, an important diagenetic process in clay, which can initially consist of 60% water. During compaction, this interstitial water is pressed out of pore spaces. Compaction can also be the result of dissolution of grains by pressure solution. The dissolved material precipitates again in open pore spaces, which means there is a net flow of material into the pores. However, in some cases, a certain mineral dissolves and does not precipitate again. This process, called leaching, increases pore space in the rock. Burial of rocks due to ongoing sedimentation leads to increased pressure and temperature, which stimulates certain chemical reactions. An example is the reactions by which organic material becomes lignite or coal. CHEMICAL SEDIMENTARY ROCKS Chemical sedimentary rocks are characterized by the presence of whole minerals such as halite, calcite, and gypsum. A chemical sedimentary rock generally contains one kind of mineral. Such minerals occur in their natural crystalline state, unbroken and unweathered. Most chemical sedimentary rocks contain either quartz (especially detrital rocks) or calcite (especially carbonate rocks). In contrast to igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals. However, the origin of the minerals in a sedimentary rock is often more complex than in an igneous rock. Minerals in a sedimentary rock can have formed by precipitation out of sea/ocean water or due to evaporation. Carbonate rocks dominantly consist of carbonate minerals such as calcite, aragonite or dolomite. Both the cement and the clasts (including fossils and ooids) of a carbonate sedimentary rock can consist of carbonate minerals. BIOCHEMICAL SEDIMENTARY ROCKS Biochemical (organic) sedimentary rocks are characterized by the presence of fossils. Fossils are most commonly found in sedimentary rock. Unlike most igneous and metamorphic rocks, sedimentary rocks form at temperatures and pressures that do not destroy fossil remnants. As a result, compared to igneous and metamorphic rocks, fossils are commonly found in sedimentary rocks. Dead organisms in nature are usually quickly removed by scavengers, bacteria, rotting and erosion, but sedimentation can contribute to exceptional circumstances where these natural processes are unable to work, causing fossilization. The chance of fossilization is higher when the sedimentation rate is high (so that a carcass is quickly buried), in oxygen-poor (or oxygen-free) environments where little bacterial activity occurs, or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Fossils can be both the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, and the woody tissue of plants. Soft tissue has a much smaller chance of being fossilized, and the preservation of soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while they were still alive are called trace fossils, examples of which are burrows, footprints, etc. WENTWORTH SIZE SCALE OF DETRITAL SEDIMENTS Gravel: > 2 mm Sand: 1/16 to 2 mm (feels gritty) Mud: < 1/16 mm {silt and clay} DESCRIPTION OF DETRITAL SEDIMENT SIZE TERMS Sand This is the size range of grains in a sandbox. The grains are visible and feel gritty when rubbed between your fingers. Silt: Grains too small to see but you can feel them as very tiny gritty grains when you rub them between your fingers or teeth. Clay: Grains too small to see, feel like chalk dust when rubbed between your fingers or teeth. Page 2 of 14

3 SORTING: Refers to the degree of similarity of sediment size. If sediments have nearly same size, they are said to be well sorted; if they are mixtures of different sizes, they are poorly sorted. Anything in between is moderately sorted. Winds and waves are most selective, they produce well sorted sediments; Glaciers and streams are unselective, they produce poorly sorted sediments. ROUNDING: Rounding, roundness or angularity are terms used to describe the shape of the corners on a particle (or clast) of sediment. Such a particle may be a grain of sand, a pebble, cobble or boulder. Well rounded sediments are produced by the action of water or wind when transported over long distances; angular sediments are produced by glacier or debris flow or short transport distance by river or wind. SEDIMENTARY ENVIRONMENTS The setting in which a sedimentary rock forms is called the sedimentary environment. Every environment has a characteristic combination of geologic processes and circumstances. The type of sediment that is deposited is not only dependent on the sediment that is transported to a place, but also on the environment itself. A marine environment means that the rock was formed in a sea or ocean. Often, a distinction is made between deep and shallow marine environments. Deep marine usually refers to environments more than 200 m below the water surface. Shallow marine environments exist adjacent to coastlines and can extend to the boundaries of the continental shelf. The water movements in such environments have a generally higher energy than that in deep environments, as wave activity diminishes with depth. This means that coarser sediment particles can be transported and the deposited sediment can be coarser than in deeper environments. When the sediment is transported from the continent, an alternation of sand, clay and silt is deposited. When the continent is far away, the amount of such sediment deposited may be small, and biochemical processes dominate the type of rock that forms. Especially in warm climates, shallow marine environments far offshore mainly see deposition of carbonate rocks. The shallow, warm water is an ideal habitat for many small organisms that build carbonate skeletons. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Warm Page 3 of 14

4 shallow marine environments also are ideal environments for coral reefs, where the sediment consists mainly of the calcareous skeletons of larger organisms. In deep marine environments, the water current working the sea bottom is small. Only fine particles can be transported to such places. Typically sediments depositing on the ocean floor are fine clay or small skeletons of micro-organisms. At 4 km depth, the solubility of carbonates increases dramatically. Calcareous sediment that sinks below this depth dissolves; as a result, no limestone can be formed below this depth. Skeletons of micro-organisms formed of silica (such as radiolarians) are not as soluble and still deposit. An example of a rock formed of silica skeletons is radiolarite. When the bottom of the sea has a small inclination, for example at the continental slopes, the sedimentary cover can become unstable, causing turbidity currents. Turbidity currents are sudden disturbances of the normally quite deep marine environment and can cause the geologically speaking instantaneous deposition of large amounts of sediment, such as sand and silt. The rock sequence formed by a turbidity current is called a turbidite. The coast is an environment dominated by wave action. At a beach, dominantly denser sediment such as sand or gravel, often mingled with shell fragments, is deposited, while the silt and clay sized material is kept in mechanical suspension. Tidal flats and shoals are places that sometimes dry because of the tide. They are often cross-cut by gullies, where the current is strong and the grain size of the deposited sediment is larger. Where rivers enter the body of water, either on a sea or lake coast, deltas can form. These are large accumulations of sediment transported from the continent to places in front of the mouth of the river. Deltas are dominantly composed of detrital (clastic) sediment in contrast to chemical. A sedimentary rock formed on land has a continental sedimentary environment. Examples of continental environments are lagoons, lakes, swamps, floodplains and alluvial fans. In the quiet water of swamps, lakes and lagoons, fine sediment is deposited, mingled with organic material from dead plants and animals. In rivers, the energy of the water is much greater and can transport heavier clastic material. Besides transport by water, sediment can in continental environments also be transported by wind or glaciers. Sediment transported by wind is always very well sorted, while sediment transported by a glacier is called glacial till and is characterized by very poor sorting. SEDIMENTARY FACIES Sedimentary environments usually exist alongside each other in certain natural successions. A beach, where sand and gravel is deposited, is usually bounded by a deeper marine environment a little offshore, where finer sediments are deposited at the same time. Behind the beach, there can be dunes (where the dominant deposition is well sorted sand) or a lagoon (where fine clay and organic material is deposited). Every sedimentary environment has its own characteristic deposits. The typical rock formed in a certain environment is called its sedimentary facies. When sedimentary strata accumulate through time, the environment can shift, forming a change in facies in the subsurface at one location. On the other hand, when a rock layer with a certain age is followed laterally, the lithology (the type of rock) and facies eventually change. Facies can be distinguished in a number of ways: the most common are by the lithology (for example: limestone, siltstone or sandstone) or by fossil content. Coral, for example, only lives in warm and shallow marine environments and fossils of coral are thus typical for shallow marine facies. Sedimentary environments can shift their geographical positions through time. Coastlines can shift in the direction of the sea when the sea level drops, when the surface rises due to tectonic forces in the Earth's crust or when a river forms a large delta. In the subsurface, such geographic shifts of sedimentary environments of the past are recorded in shifts in sedimentary facies. This means that sedimentary facies can change either parallel or perpendicular to an imaginary layer of rock with a fixed age. Sediments are deposited in many different environments. Each environment has characteristic sediments, sedimentary structures, and organisms that can become fossils. The information gained from grain characteristics, sedimentary structures, and fossils can be used to infer ancient environments (paleo-environments). See Table 1 and Figure 1 for a list of representative paleo-environments and typical sedimentary depositional environments, respectively. Page 4 of 14

5 For ideal models for the formation of sedimentary rocks from an average granodiorite, see Figure 2; and for sedimentary rock analysis and classification chart, refer to Figure 3. Important: Most sedimentary rocks are cemented by the mineral calcite. Applying hydrochloric acid (HCl) on many sedimentary rocks can show fizzing due to the presence of some calcite in the rock. So, fizzing of a sedimentary rock doesn t necessarily make that rock a limestone. SEDIMENTATION RATES The rate at which sediment is deposited differs depending on the location. A channel in a tidal flat can see the deposition of a few metres of sediment in one day, while on the deep ocean floor each year only a few millimetres of sediment accumulate. A distinction can be made between normal sedimentation and sedimentation caused by catastrophic processes. The latter category includes all kinds of sudden exceptional processes like mass movements, rock slides or flooding. Catastrophic processes can see the sudden deposition of a large amount of sediment at once. In some sedimentary environments, most of the total column of sedimentary rock was formed by catastrophic processes, even though the environment is usually a quiet place. Other sedimentary environments are dominated by normal, ongoing sedimentation. In many cases, sedimentation occurs slowly. In a desert, for example, the wind deposits siliciclastic material (sand or silt) in some spots, or catastrophic flooding of a wadi may cause sudden deposits of large quantities of detrital material, but in most places eolian erosion dominates. The amount of sedimentary rock that forms is not only dependent on the amount of supplied material, but also on how well the material consolidates. Erosion removes most deposited sediment shortly after deposition. TABLE 1. PALEOENVIRONMENTS OF SOME SEDIMENTARY ROCKS SEDIMENTARY ROCK ENVIRONMENT OF FORMATION (PALEO-ENVIRONMENT) DETRITAL (CLASTIC) ROCKS Conglomerate Glacial, Stream Breccia Glacial, Stream Sandstone Stream, Sand dunes, Beach; Base of continental slope Mudstone Swamp (marsh), Floodplain CHEMICAL ROCKS Limestone Deep marine Chert Deep marine Travertine Evaporating (hot) springs Rock salt, gypsum, dolomite Evaporating playa lake/lagoon BIOCHEMICAL (ORGANIC)ROCKS Fossiliferous limestone Continental shelf Lignite, Coal Peat bog tropical climate Page 5 of 14

6 Figure 1. Typical Sedimentary Depositional Environments. Figure 2a. Detrital sediment characteristics along a river. Page 6 of 14

7 Figure 2b. Sedimentary rocks formed during a marine transgression. MARINE TRANSGRESSION/ REGRESSION AND CHARACTERISTIC SEDIMENTARY FACIES A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity). Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load. During the Cretaceous, seafloor spreading created a relatively shallow Atlantic basin at the expense of deeper Pacific basin. This reduced the world's ocean basin capacity and caused a rise in sea level worldwide. As a result of this sea level rise, the oceans transgressed completely across the central portion of North America and created the Western Interior Seaway from the Gulf of Mexico to the Arctic Ocean. The opposite of transgression is regression, in which the sea level falls relative to the land and exposes former sea bottom. During the Pleistocene Ice Ages, so much water was removed from the oceans and stored on land as yearround glaciers that the ocean regressed 120 m, exposing the Bering land bridge between Alaska and Asia. Sedimentary facies changes may indicate transgressions and regressions and are often easily identified, because of the unique conditions required to deposit each type of sediment. For instance, coarse-grained detrital (clastics) like sand are usually deposited in nearshore, high-energy environments; fine-grained sediments however, such as silt and carbonate muds, are deposited farther offshore, in deep, low-energy waters. Thus, a transgression reveals itself in the sedimentary column when there is a change from nearshore facies (such as sandstone) to offshore ones (shale and limestone), from the oldest to the youngest rocks. A regression will feature the opposite pattern, with offshore facies changing to nearshore ones. Transgression-Regression-Sedimentary facies (4 minutes) Page 7 of 14

8 Figure 3. Ideal Models for the Formation of Sedimentary Rocks from an average Granodiorite. Page 8 of 14

9 Figure 4. Sedimentary Rock Analysis and Classification Chart. Page 9 of 14

10 Q1. QUESTIONS i) What specific type of biochemical limestone is shown in Figure 5 below? Refer to Figure 4 for names and descriptions of sedimentary rocks. Type of biochemical limestone: ii) Explain how you determined this name. Scale: mm Figure 5. A sample of biochemical limestone. Actual size. iii) What is the roundness of the clasts in Figure 5 above? Choose and circle your answer. A) Rounded B) Sub-rounded C) Sub-angular D) Angular iv) Explain how and in what environment the shell clasts could have been deposited? Refer to Figures 1, 3, and 4 as needed to answer this question. Q2. Analyze the 3 sedimentary rocks given in the accompanying pictures (next page). For each picture, describe the rock s composition (rock of mineral fragments, unbroken or not-rounded minerals showing original crystal shape, fossils, etc.), texture (sediment size, roundness, and sorting), and paleo-environment by the side of the pictures. Page 10 of 14

11 Scale: mm Scale: mm Page 11 of 14

12 Q3. Using Figures 1, 2a, 2b, and 3 in this worksheet and relevant sections from your textbook and the video from Professor Harris ( answer the following questions. Journey to the beach: i. You are walking to a beach. You are walking along a river. What sediment do you see? What other features do you see? ii. You are on the beach, what sediment do you see? What other features do you see? iii. You keep walking and you are under the ocean. What sediment do you see? What other features do you see? Sea levels rising and falling: iv. You are standing on a river delta. What sediment do you see? v. Ocean levels rise. What are you now standing on? vi. Ocean levels continue to rise. What are you now standing on? vii. Ocean levels fall. What are you now standing on? Q4. Answer the following questions. Refer to Table 1, Figures 1 through 4, and your notes as needed. i) A great portion of Florida, Texas, and California is made up of ca. 150 million years old limestone bedrock. Based on the prevalence of limestone, what would you say about the paleogeography/paleo-environment (distribution of land and sea/ocean) of those states 150 million years ago? Explain. ii) Consider conglomerate and breccia. These two detrital sedimentary rocks ae made up of gravel sized rock fragments. While conglomerate is made up of rounded gravel, the gravel in breccia are angular. What do these textural differences tell you about the conditions under which these two sedimentary rocks formed? Explain. iii) Regarding the rock fragments shown in the side figure, how would you describe the rounding and sorting of the sediments? For each category, choose and circle your answer. Hint: If the sediments in the picture all fall within one category of Wentworth size scale, for example, gravel, or sand, or mud, the sediments are well sorted; if it is made of more than one class, e.g. sand and gravel, or mud and sand, etc., it is poorly sorted. Rounding: Sorting: A) Rounded A) Well sorted B) Sub-rounded B) Poorly sorted C) Sub-angular D) Angular Scale: mm Page 12 of 14

13 Q5. Refer to images of volcanic rocks and sediments photographed from around a volcano similar to Mt. Rainier in Washington State. Note how the sediments changes from A to D. Image A was taken from near the top of the volcano; Image B was taken near the middle of the volcano s slope; Image C was taken in a River that drains away from the base of the volcano; Image D was taken 20 miles (approx. 30 km) downstream, at a delta A C D Scale: mm B (i) What is the Wentworth size class of each grain of the sediment at each location? A. B. C. D. (ii) What is the grain roundness at each of the following locations? A. B. C. D. (iii) In general, would you classify the sediments in each image as detrital, biochemical, or chemical? A. B. C. D. (iv) Name the kind of sedimentary rock the sediment would turn into if it were to be lithified (compacted and cemented). For a list of rock names, refer to Figure 4, step 3. A. B. C. D. (v) Based on your answer for (i) and (ii) above, describe what happens to detrital sediment with distance from its source. Page 13 of 14

14 Q6. Obtain six sedimentary rock samples from your instructor. Analyze them and for each sample complete a line on the given Sedimentary Rocks Worksheet using the steps to classify a sedimentary rock given in Figure 4. SEDIMENTARY ROCKS WORKSHEET SAMPLE # COMPOSITION (DETRITAL, BIOCHEMICAL, CHEMICAL) TEXTURAL & OTHER DISTINCTIVE PROPERTIES (ANGULAR/ROUNDED; GRAVEL/SAND/MUD; LAYERED; EFFERVESCES*, ETC.) ROCK NAME S-1 S-7 S-9 (Example) S-11 S-12 S-14 *Most sedimentary rocks are cemented by the mineral calcite. Applying hydrochloric acid (HCl) on many sedimentary rocks can show fizzing due to the presence of some calcite in the rock. So, fizzing of a sedimentary rock doesn t necessarily make that rock a limestone! Page 14 of 14