13. Sedimentary Rocks I (p )

Transcription

1 13. Sedimentary Rocks I (p ) Sediment Deposition Weathering results in rock being broken down into smaller fragments, called regolith. This regolith is then broken down to form soil. The regolith may also be eroded and transported away from where it first formed by the surface processes of water, wind, and ice. Eventually, eroded material must get dropped somewhere else- a process called. The loose material that is deposited is referred to as which eventually bonds together and hardens to form sedimentary rock. How much of the Earth's crust is comprised of sedimentary rocks? Nonetheless, sedimentary rocks cover most of the sea floor and about of the continents, so they are the type of rock that we see most often at the surface. Sedimentary rocks are usually banded into many layers, one on top of the next, like a layered cake. Example:. We call these layers or, and the overall appearance is called either: or. Each bed is separated from the next by a break called a. We can we distinguish one bed from another because they commonly show: Significance of Sediments The appearance of beds in sedimentary rocks and the type of sediment that makes up the rock can tell us many things about the history of the Earth. This is because sediments tell us something about the surface processes that created the sediment. We can understand this by looking at the types of sediments we find in different environments on Earth today. Example: How different are the sediments found in river channels, lake bottoms, and beaches? Fill in the type of sediment in each environment: RIVER CHANNELS: LAKES: BEACHES: So different environments and on Earth, and the processes that occur there, tend to be associated with different types of sediment. What do we call these different types of environments?. 1

2 Sedimentary rocks can tell us about the past distributions of rivers, lakes, oceans, deserts, and glaciers. We may even learn about how climate changed over time in certain places. Examples: (1) In the Great Lakes region, we see evidence of long-term climate change in the sediments that were: (2) In what type of ancient environment did the limestones in Florida form? Types of Sediment Sediment comes in all different shapes and sizes, as well as being formed through different processes in different depositional environments. We can divide sediment into three categories based on where it came from: sediment: broken down particles of rock produced by weathering and erosion. Also called sediment. sediment: precipitates out of water during evaporation or because of chemical reactions in water. sediment: mostly broken fragments of once living organisms, e.g. decayed plants; seashells. Detrital (Clastic) Sedimentary Rocks Also called clastic sedimentary rocks. Detrital sediment may be transported and deposited by rivers, lakes, glaciers, wind, waves, etc. (any surface process). The individual grains in detrital sedimentary rocks are called which are made up of bits of individual minerals and sometimes fragments of broken down rocks that we call. Fill in the following chart showing the different types of clast grain sizes, and examples of materials having a similar size: CLAST NAME SIZE SIMILAR SIZE STUFF OTHER INFO < 1/256 mm aka "mud" 1/256-1/16 mm aka "mud" 1/16-2 mm usually quartz > 2mm 2

3 The names of the detrital sedimentary rocks that form from the different types of clastic sediment are determined by the size of the clasts that make up the rock: ROCK NAMES clay -> or Contains thin layers of sediment called or We can tell the difference between these two rocks through these characteristics: Shale: if broken, it (i.e., the rock is ) Mudstone: this rock is more. ROCK NAME silt -> What are simple tests for the presence of clay or silt in a rock? Clay: Silt: ROCK NAMES sand -> or or How do these three rock types with sand-sized grains differ from each other? Sandstone: the clasts are mostly made of Arkose: some of the clasts must be made of Graywacke: some of the clasts must be ROCK NAMES pebbles + -> or We can tell these two apart because the clasts in conglomerate are whereas the clasts in breccia are. 3

4 Sorting and Rounding Some sedimentary rocks are made up of a range of sizes of clasts. The range of clast sizes is a characteristic called the sorting of the sediment. If there are a range of sizes, the sediment is but if the clasts are all about the same size, the sediment is. We can also characterize the clasts on the basis of their shapes, which we call the rounding of the sediment. The rounding can range from to. Rounding of sedimentary particles results from clasts being bashed together during transport. Small particles of the clasts chip off, causing grains to become more and more rounded over time. Examples: beach sand is: [well sorted or poorly sorted] [well rounded or angular]? Also, sand is: [well sorted] [well rounded] This type of sand will eventually form a type of rock called. Sediment deposited by glaciers is called and is dumped in a big heap. The type of rock that forms from it is called. glacial sediment is: [well sorted or poorly sorted] [well rounded or angular]? Chemical Sedimentary Rocks Chemical sediment forms when sediment particles form directly out of water through the process of. This can occur in one of two ways: (e.g., dripstone and ) or (i.e., involving living things) The most common inorganic mechanism by which chemical sediments form is by: : as water evaporates, minerals like or or precipitate out of the water. e.g. Great Salt Lake; Dead Sea. The type of rocks that results from evaporation are called. e.g., evaporation of the Sea in the Pliocene. 4

5 When organic processes result in the precipitation of chemical sediments, we say the rocks are: : plants and animals in water can alter the water chemistry. Organisms can change the amount of dissolved CO 2, which may cause (i.e., ) to precipitate out. This is how many are formed. What other mineral can precipitate out of water due to the presence of organic life, and what type of rocks get formed by it? MINERAL ROCK Biogenic Sedimentary Rocks Biogenic sediment is the broken down remnants of once living organisms and so is not the same as chemically produced biochemical sediment. The individual fragments of bones and teeth make a type of sediment particle called sediment. What two types of biogenic rocks contains lots of broken down fragments of coral, sea shells and the CaCO 3 skeletons of marine organisms? and A type of limestone containing large seashell fragments is called. Some organisms have siliceous shells, meaning they are made of silica. These may accumulate on the ocean floor to form a rock called, which is usually so fine-grained, you can't see the individual particles of quartz. (e.g., flint). Rocks that are made up of the remains of plants are called. The dead material accumulates in thick piles, first forming, which finally turns into coal because of high temperatures and pressures underground. Lithification Loose particles of sediment that have not yet turned to rock are called: The phenomenon by which this sediment is turned into rock is called. It literally means "turn to stone". We say the sediment lithifies. The chemical, physical, and biological processes that occur in the rock during lithification are called diagenetic processes. So the sediment is said to undergo as it lithifies. The three diagenetic processes are:. 5

6 Compaction is when sediments get buried deeper and deeper because of all the material being piled up on top. The sediments get compressed and any water present in the sediments starts to get squeezed out like squeezing a wet sponge. Cementation is when the water getting squeezed out of the sediments fills up gaps between the sediment grain, called pore spaces, and minerals precipitate out of the water. These minerals start gluing the grains together, so they are called cement. Recrystallization is when some minerals change into stronger types of minerals due to increasing pressure and temperature. The stronger minerals help bind the rock together. FINAL QUESTION: What minerals often form the cement that glues sediment particles together during diagenesis? and 6