Online Lab 8 Sedimentaray Rocks

Laboratory 4: Sedimentary Rocks 3

Laboratory 4: Sedimentary Rocks 6 Conglomerate Sandstone Siltstone Claystone

Laboratory 4: Sedimentary Rocks 9 Rock gypsum Anhydrite Peat Lignite Bituminous coal Anthracite coal

Laboratory 4: Sedimentary Rocks Prelab 1. What are the three categories of sedimentary rocks? 2. True or False? Mechanical weathering involves the dissolution or decomposition of sediments. 3. Clastic sedimentary rock textures are described in terms of: (Select all that apply.) a. Grain shape b. Grain source c. Grain size d. Grain sorting 4. Which of the following sedimentary rocks are clastic? (Select all that apply.) a. Mudstone b. Chert c. Sandstone d. Coal e. Rock Salt 5. Chemical sedimentary rocks form when crystals ___ from an aqueous solution. 6. Using the information in the table below, give an example of a possible environment where a fine-grained, well- rounded, and well-sorted quartz sandstone would form. Depositional Environments and Corresponding Rock Properties 12

Laboratory 4: Sedimentary Rocks Rock Sample 5. This rock reacts vigorously with acid and has the remains of fossil corals in it. Rock sample 6. This rock is very hard and very fine grained. It has a conchoidal fracture and does not react with acid. Rock sample 7. This rock has rounded pebbles that are cemented together with iron oxide. Some of the pebbles react with acid. 15

Laboratory 4: Sedimentary Rocks Rock Sample Rock Type (Clastic, Biogenic, or Chemical) Composition Texture (Grain size, etc.) Rock Name Other Observations 6 7 8 9 10 Part II Sedimentary Structures Sedimentary structures such as stratification (layering), ripple marks, cross-bedding, and mud cracks can be preserved in sedimentary rocks. These structures provide important information about depositional environments such as flow direction, climate (arid, semi-arid, or humid) and setting (e.g. fluvial, lacustrine, or marine). These structures also may indicate which direction was originally up within the rock. Tectonic forces can fold and overturn rocks, so establishing the original orientation is not always easy, but is often useful. Bedding Sedimentary rocks will often be deposited in discrete layers, which leads to a particularly important sedimentary structure called bedding. Bedding layers can range in thickness from millimeters to tens of meters. Typically, though not always, bedding is originally horizontal in orientation; tilted bedding indicates that the rock has been deformed in some way . The charts on pages 20 and 21 summarize the types of structures, descriptions and the environments they form in. Depositional Environments A sedimentary environment is a geographic location characterized by a particular combination of geologic processes and environmental conditions. Geologic processes include the currents that transport and deposit sediments (water, wind, or ice) and the plate tectonic settings that affect sedimentation. For example, the geologic processes of a beach environment include the dynamics of waves crashing against the shore, shoreline currents, and the distribution of sediments on the beach. Environmental conditions include the kind and amount of water (ocean, lake, river, arid land), the landscape (lowland, mountain, coastal plain, shallow ocean, deep ocean and type of biologic activity. 18

Laboratory 4: Sedimentary Rocks 21

Laboratory 4: Sedimentary Rocks Continental Sedimentary environments Depositional Environment Characteristics Rock Types Formed Lakes (Lacustrine) Lake deposits are generally low-energy environments where fine-grained sediments are deposited in thin layers. Lakes that freeze over seasonally may develop varves: alternating layers of light (coarser) and dark (finer) sediments. Large lakes can also have higher- energy, sandy beaches. shale, siltstone Glacial Glacial deposits form along the margins of and beneath glacial ice. Because ice can transport any size grain (unlike water or wind), deposits are typically very poorly-sorted Conglomerate and sometimes breccia If glacial origin is known, the rock is called a “tillite” Alluvial Fan Alluvial fans are fan-shaped wedges of sediment deposited along the margin of a steep slope. They often contain a lot of coarse-grained, moderately to poorly-sorted sediment. Conglomerate, sandstone, and sometimes breccia Rivers and Streams Rivers and streams typically deposit medium- to coarse- grained (sand to cobble-sized) sediment in their channels. River sediment is often moderately- to well-sorted. Sandstone, siltstone, some conglomerate Flood Plains Floodplains are relatively low-energy environments where finer (clay, silt, fine sand) sediments are deposited in well-defined layers. These plains are only periodically wet and when they dry out mud cracks often develop. Siltstone, shale Swamps, Marshes and Bogs Swamps are typically rich in organic material that is buried and compressed to form coal Coal Evaporite Shallow basins in arid regions and lagoons may become supersaturated and precipitate evaporite minerals Rock gypsum, Rock salt, crystalline limestone Deserts (Aeolian) Aeolian environments are arid and typically have winds that transport and sort medium- and fine grained sediment (sand to silt). Aeolian sediments are often well-sorted and show well-developed cross-bedding. Sandstone, siltstone Shoreline Environments Deltas Deltas form where rivers and streams enter larger bodies of water. They often contain fluvial-type deposits as well as swampy environments. Sandstone, siltstone, shale Beaches Beaches occur on the margins of large bodies of water. They generally contain deposits of well sorted, medium- grained sediment with planar bedding. Sandstone Marine Environments Shallow marine or continental shelf Shallow marine environments are formed on the margins of continents, on the continental shelf. Associated deposits are typically medium- to fine-grained and well sorted. In warm (subtropical to Sandstone, siltstone, shale, diatomite, oolitic limestone, chalk (Depends on supply of clastic 22

Laboratory 4: Sedimentary Rocks tropical) environments these sediments may be calcite-rich. and chemical sediments Deep marine Deep marine areas receive relatively little clastic sediment. Common deep marine sediments are either very fine-grained or microcrystalline (from recrystallization of microscopic silica-producing organisms). Chert, shale Reefs Organic structures composed of calcium carbonate-secreting organisms (i.e. coral) built up on continental shelves or oceanic volcanic islands Limestone (fossiliferous) Follow up Questions: 1. List the environments in which shale forms. What is the one thing that each has in common with the others? 2. What evidence indicates that sandstones form on the Earth's surface and not beneath the surface? 3.What is the general relationship between particle size and distance of transport for sediments from a source? (think about the distance and size of sediments transported by rivers to the continental shelf). 23

Laboratory 4: Sedimentary Rocks 4. Many limestones are dense, fine grained and black or dark in color. How would you distinguish them from basalt which is an igneous rock? 5. Many beaches on the volcanic islands of the pacific are white. The islands, however are composed of black basalt, which has no quartz and gives rise to black sand beaches. What do you suppose is the composition of the white beach sands? 6. This photo shows a microscopic view of sand collected from one of the canyons in the Huachuca Mountains. Would you consider this sand to be well sorted? What does the shape of the grains tell you about the history of the sand particles? 7. This photo shows a microscopic view of sand collected from a California beach. Would you consider this sand to be well sorted? What does the shape of the grains tell you about the history of the sand? 24

Laboratory 4: Sedimentary Rocks 8. South Point on the Big Island of Hawai’i is the most southerly point of land in the United States and the source of this sand. At South Point on Mauna Loa’s southwest rift, the pounding surf erodes a forty-nine-thousand-year- old volcanic cinder cone. Based on your knowledge of the type of volcanic rock produced in Hawaii, what minerals would these sand grains be composed of? How would you classify this sand in terms of its texture? 9. Sand grains from the beaches of tropical islands like Bermuda are filled with fascinating remnants. What do you suppose these sands are composed of? How would you classify this sand in terms of its texture? 25