EPS 50 Lab 4: Sedimentary Rocks

Transcription

1 Name: EPS 50 Lab 4: Sedimentary Rocks Grotzinger and Jordan, Chapter 5 Introduction In this lab we will classify sedimentary rocks and investigate the relationship between environmental conditions and sedimentary rock deposition. Remember that the diversity of rock in nature is generally continuous, and we are learning interpretive guidelines. The underlying principle for understanding sedimentary rocks and geological processes is that minerals and rocks are stable only under the conditions at which they form. If the temperature, pressure, water content, or other conditions change, the rocks will change to adapt to the new conditions. Objectives Learn to identify common sedimentary rocks (i.e. sandstone, shale, limestone and conglomerates) and the depositional environments they indicate. Learn to identify sedimentary structures and the processes that form them. Answers Please answer the numbered questions on the separate answer sheet handed out during lab. Only the answer sheet will be graded. Explanations should be concise a few sentences or fewer. All answers should be your own, but we encourage you to discuss and check your answers with a few other students as you work through the exercises. ===================================================================== Sedimentary Rocks Sedimentary rocks form by the accumulation of sediment (eroded bits of pre-existing rocks) or by the precipitation of minerals out of solution. Sediment is deposited in a number of environments by moving air and water. Sedimentary rock identification is primarily based on composition. Texture can be used, but texture of a sedimentary rock has a slightly different meaning than texture of an igneous rock. In this lab, texture of a sedimentary rock refers to the origin or type of sediment found in the rock. In this lab we will look at three different types of textures: clastic, chemical, and biological. The primary minerals found in sedimentary rocks include quartz, feldspar, clays (such as kaolinite), calcite, dolomite, gypsum and halite. These common minerals are generally either resistant to weathering and/or erosion processes (dissolution and transport) or are directly formed in sedimentary environments (e.g. by chemical precipitation, chemical weathering). Classifications (Tray 1) 1) Name and classify the minerals in Tray 1 (minerals may fall into more than one category) as silicate minerals, evaporite minerals and/or carbonate minerals. (9 pts) Clastic Rocks (Tray 2)

2 Name of Clastic Rock Breccia Conglomerate Sandstone Arkose Shale Siltstone Mudstone Fragment Type Coarse fragments of angular gravel and rocks Coarse fragments of rounded gravel and rocks Sand sized particles that are 90% quartz Sandstone composed of 25 % feldspar grains Clay particles Silt particles Mixture of clay and silt Siliciclastic rocks are composed of chunks or clasts of weathered rock material like gravel, sand, silt, and clay that become a solid mass through burial, compaction, and/or cementation in a process called lithification. Weather resistant quartz (SiO2) and other silicate minerals form the majority of clastic rocks, hence siliciclastic. As a pile of unconsolidated material grows, gravity compacts the sediment at its base, while squeezing out water and forcing grains together. Fluids circulate through the pile of sediment and cements the grains together, converting the sediment into a cohesive aggregate a rock. Chemical, physical or biological changes undergone by sediment following deposition are known as diagenesis, which occurs during and often following lithification. Clastic sedimentary rocks are divided into types based on clast size: clay, silt, sand and gravel. Clay particles have diameters less than 1/256 mm and silt particles have diameters from 1/256 mm to 1/16 mm; neither are visible to the unaided eye. Sand grains range from 1/16 mm to 2 mm, and gravel grains include clasts larger than 2 mm. In general, deposition concentrates sediments of common size together; some sedimentary rock names, such as siltstone and shale, depend on their grain size. However, in turbulent depositional conditions sediments may not be so well-sorted, and a greywacke, a rock that includes angular grains of quartz, feldspar and small rock fragments mixed with sand in a fine matrix, might be formed. A sedimentary rock with a very wide distribution of clast sizes is called a diamictite and is commonly of glacial origin. Common Components of Siliciclastic Rocks Quartz Feldspar Lithics Matrix Because quartz does not weather into any other mineral, it remains after everything else is weathered or sorted out. Feldspars are some of the most abundant minerals in the earth's crust. Most igneous rocks have large amounts of feldspar, e.g. Ca-plagioclase in gabbro, and Na-plagioclase and orthoclase in granite. If a siliciclastic particle is not quartz or feldspar it is classified as a lithic fragment. Lithic means "rock," and all mechanically weathered pieces of rock are included here. Lithics are usually small, dark in color, and their mineral composition is difficult to identify, except in conglomerates and breccias where clast size is commonly large. Matrix is the finer material in which larger particles are embedded. In a sandstone the matrix is commonly silt and clay or carbonate. In a gravel the matrix may be sand. However, since all minerals other than quartz will eventually weather into silt or clay-sized particles, silt or clay is very common. 2) Name two ways to distinguish between the quartz and the feldspar minerals you might find in

3 siliciclastic sedimentary rocks. (7 pts) 3) Arrange the 9 clastic rocks in Tray 2 in order from smallest grains (label as 1) to largest grains (label as 9). Write the name of each clastic rock in the table next to its relative grain size. (9 pts) Chemical Rocks (Tray 3) Chemical sedimentary rocks are identified by their component minerals. Chemical sedimentary rocks form through the chemical precipitation or crystallization of elements and compounds from solution. Multiple precipitated minerals tend to be deposited together geographically and are not generally deposited in the presence of siliciclastic rocks. Chemical sedimentary rocks are so different from siliciclastic rocks that geologists often specialize in studying either one group or the other. Dissolved chemical species can precipitate from solution in several ways: - Water rich in dissolved species, like seawater or salty lake water in an enclosed basin, evaporate leaving behind sedimentary evaporites (sample in tray, not numbered). Rock salt or halite (NaCl) and gypsum (CaSO4*H2O) exist originally as dissolved constituents in the water, thus making the sea and enclosed lakes (such as Salt Lake) salty. When water evaporates in a closed area such as a lagoon, the salt concentration becomes supersaturated, and minerals precipitate. - Chert (sample 520A) is a siliceous (composed of SiO2) rock that forms from the recrystallized skeletons of single-celled marine creatures such as radiolaria and diatoms. Although the silica comes from skeletons, to become chert it must be chemically recrystallized, thus putting it in the chemical category. - The upper layers of oceans are near saturation with respect to dissolved calcium carbonate (CaCO3). Many marine organisms use calcium carbonate to construct shells and other hard body parts, and because these organisms live in solution, some of the calcium carbonate dissolves into the ocean waters. Under the right circumstances the dissolved calcium carbonate can precipitate, forming limestone (sample 589) deposits. The formation of the chemically similar mineral dolomite (CaMg(CO3)2) involves the chemical modification of limestone deposits by a magnesium rich solution. 4) List three methods for distinguishing the above chemical rock types from one another (think

4 back to your mineral identification skills and use Appendix 4 in the textbook). (6 pts) Biological Rocks (Tray 4) Mineral precipitation also occurs via plants and animals, which can extract the dissolved constituents from the seawater to make skeletons. These skeletons eventually become biochemical or biological sedimentary rocks. Biological sedimentary rocks form as the result of accumulation and lithification of organic material or biologic activity. Coal (sample 565) is the lithified remains of plants and can be identified by its dark brown to black color. Limestone (sample 62 and 173) deposits can form by lithification of coral reefs, marine organism shells, or marine organism skeletons (as opposed to the precipitation of dissolved calcium carbonate mentioned above). Limestone that contains fossils is referred to as fossiliferous limestone. Chalk (sample 64) is a particular variety of limestone that is composed of the skeletons of marine microorganisms like foraminifera. Diatomite (sample 47), which looks similar to chalk, is formed from silicate-shelled critters called diatoms. In both chalk and diatomite, fossils or fossil fragments may be microscopically visible in contrast to precipitated limestones or cherts where new crystal growth or recrystallization has occurred. Note that many carbonate rocks are formed by both chemical and biochemical processes; rocks observed in the field may be a combination of both. 4) Why are carbonates not formed in deep ocean environments? How, then, do we find them there? Read about the formation of carbonates in the textbook. (8 pts) Sedimentary Structures Sedimentary structures are features that tell us about depositional conditions. The images below demonstrate cross bedding, mud cracks and bioturbation. Cross-bedding (side view):

5 Mud cracks (top view): Bioturbation (side view): Bioturbation (top view):

6 6) Sketch each of the four structures next to the photo (be sure to include a scale bar). Briefly describe the processes that resulted in the formation of these sedimentary structures (refer to chapter 5 of the text). (24 pts) Sedimentary Wonderland The following is a map of Sedimentary Wonderland, a very special place where a variety of sedimentary rock-forming environments converge. 7) Using your knowledge of sediments and their formation processes, identify where on this map you expect the 15 sedimentary rock types listed below to form. Some rock types can fit into more than one environment and some environments can form more than one rock type. Write the location(s) next to each rock type. For each rock type, write down at least one observation you made that helped you decide which rock types formed in which environments. (37 pts) 1. sandstone (193) 6. volcanic breccia (278) 11. coal (565) 2. shale (15) 7. limestone (62) 12. chalk (64) 3. cross-bedded sandstone 8. chert (132) 13. greywacke 4. siltstone (419) 9. halite 14. diamictite 5. conglomerate (416) 10. gypsum 15. diatomite (47)

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