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GEOL 101 Sedimentary Rocks

by: Victoria Williams

GEOL 101 Sedimentary Rocks GEOL 101

Marketplace > George Mason University > Geology > GEOL 101 > GEOL 101 Sedimentary Rocks
Victoria Williams
GPA 3.8

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About this Document

I cover all of the facts about sedimentary rocks in these notes including what they are, how they're formed, where they're formed, and terms used to identify them.
Introductory Geology
Mark Uhen
Class Notes
Geology, Earth Science, Sedimentary Rocks
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This 5 page Class Notes was uploaded by Victoria Williams on Wednesday February 24, 2016. The Class Notes belongs to GEOL 101 at George Mason University taught by Mark Uhen in Fall 2016. Since its upload, it has received 15 views. For similar materials see Introductory Geology in Geology at George Mason University.


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Date Created: 02/24/16
Sedimentary Rocks –   Most common sedimentary rock is shale  Sedimentary rocks are rocks that are rock formed from material derived from preexisting rocks.   Three classes of sedimentary rocks: o Detrital: Rocks that form from solid particles of preexisting rocks, formed from accumulating transported solid particles derived from mechanical weathering (ex. Sandstone) o Chemical: Rocks formed from soluble material produced by chemical weathering (ex. Evaporating liquid solution to reveal the rock, like Halite in water) o Organic: Rocks formed from organic material (ex. Coal)  Detrital Sedimentary Rocks o Texture: particle size. From largest to smallest, Gravel, sand, silt, clay. You can barely see silt with good eyes and a magnifying glass. You cannot see clay with the naked eye or with a magnifying glass. If you REALLY need to identify silt or clay, put it in your mouth. Silt will have a gritty texture, clay will turn to mush. But Prof. Uhen doesn’t recommend doing this, but he still told us anyways. o Detrital Sedimentary Rocks: Particle Size. Gravel size rocks, Conglomerate, Breccia. Sand size, sandstone. Silt/Clay/Mud stones are Mudstone, Shale, and Siltstone o Particle sorting and shape. Very poorly sorted rocks have a wide range of different grain sizes. The more uniform the particles are the better sorted it is. o Angularity and Sphericity. High sphericity (being round) is how circular the grain is.  Februrary 17  Lecture – Sedimentary Rocks Continued  Particle Composition of detrital sedimentary rocks o A rock magnified, the particles are sand­sized and the rock is made of quartz. It is called a quartz sandstone.   Detrital: Shale o Primary sediment: clay minerals (flat flakes of things) o Shale forms in layers and sheets. Often black, but not always. Can preserve fossils  Detrital: Siltstone o Primary sediment: silt o Can form in layers like shale, but they don’t do this all the time. These rocks are made of silt­size particles of any type.  Detrital: Sandstone o Primary sediment: sand o Particles are the same size as sand on the beach. Any composition is possible. Sand­size particles of garnet put together in a rock will be called garnet sandstone  Detrital: Conglomerate  o Primary sediment: Pebbles/Cobbles/Gravel o Generally very poorly sorted, particles of all sizes and types. Large chunks of other rocks  Detrital: Breccia o Primary sediment: angular fragments o Similar to conglomerate but the pieces of different rock are angular and not rounded like in conglomerate. Generally a mix of all size particles.  Chemical Sedimentary Rocks o Form from a solution, like in water. Ex. Limestone. Delicate crystals forming in a drop of water at the tip of a soda straw stalactite. The formation of crystals is triggered when some carbon dioxide escapes the water drop. o Primary component: Calcium carbonate CaCO 3  o Like the spikes at the top of a cave, the water dripping down has Calcium carbonate dissolved in it. The water can sit there for a while and eventually evaporate, leaving the calcium carbonate behind on the spike.  Chemical: Fossiliferous Limestone o Primary component: Calcium component shells o Made through biological processes, a lot of marine life makes their calcium carbonate. When they die, their skeletons/shells gather on the sea floor and compress into a stone. Easy to identify because you can see the shells.  Chemical: Oolitic Limestone o Small spherical grains called ooids are formed by chemical precipitation of calcium carbonate around a tiny nucleus and are raw material for oolitic limestone.  o These small spherical grains are mainly formed on shallow ocean beaches. The ocean here is saturated in calcium carbonate and as the particles on the beach gather the calcium carbonate and create spherical sand particles. When they are compressed together they make Oolitic limestone  Chemical: Chalk o The massive White Chalk Cliffs of Dover are made of a biochemical limestone made up almost entirely of the tiny hard parts of microscopic marine organisms.   Microcrystalline Quartz o Primary Component: Microcrystalline Quartz o Can be found in a lot of different forms like Agate, Flint, Jasper, Petrified Wood, and Arrowheads.  o Not very soluble in water but CAN dissolve in water, mainly hot water. o Agate quartz crystals are formed when this quartz infused hot water gets into a hollow stone and as the water evaporates, the quartz particles settle and create layers. Can be extremely colorful.   Chemical: Evaporites o Evaporites are made when sea water evaporates. Primarily made from Halite and Gypsum.  o An example of these are the salt flats in Nevada.   Organic: Coal o Primarily made from plant matter. o Plant bodies that die and start to rot but not much because there’s not too much oxygen will create Peat, which is only partially decomposed plants o When you remove more and more water and compress Peat, you can get Lignite. Its soft, brown coal that is more compacted plant matter.  o Even more pressure, it makes bituminous, which is still soft but darker. o The hardest is Anthracite, which is the coal that is burned for the most energy  Lithification o Creating these sedimentary rocks.  o Compaction:  Loosely   packed  particles   with  water   filled   power  spaces   get compacted so the particles will line up and the water will leave because there is no space. More weight on top of it will make it compact more through gravity. o Cementation: Loosely packed sand or gravel size particles with mineral­bearing groundwater between them. Gradually the water evaporates and leaves behind the minerals, which cements together the particles and “glues” the grains together.  Classification: o Detrital Sedimentary Rocks, and Chemical and Organic Sedimentary Rocks tables are in your textbook  Sedimentary Environments: Where these rocks form o Two main places for these rocks to form: On land (terrestrial) and in the sea (marine) o Alluvial Fans – A stream going through a mountain, the water is infused with minerals. Then gravity pulls down the water a flat area. The water disperses and all the minerals settle in a ‘fan’ around the edge of the mountain.  o Sand Dunes – Sand particles moved by wind in various shapes, well sorted sediments o Stream – Tons of different types of steams. We will dedicate an entire lecture to streams o Lakes – Low spots in the earth, both water and sediment accumulate in that spot.  o Glacial Environment – frozen Rivers, they do the same things as rivers but since it’s solid it can carry gigantic chunks of rock (even house size!). While the glacier moves and melts at its end, it deposits the rocks it’s carrying at the end and that’s where rocks accumulate.  o Swamps and Bogs – Quiet water environments that are not well oxygenated, lots of mud and decayed plant material accumulates into rock o Caves – Develop in limestone that are sites where calcium carbonate is deposited as dripstone. The water is absorbed into the earth and goes into these caves that were created through water dissolving rocks through time. Also creates those spikes with more dissolving and depositing.  o Beaches – low­lying coasts and in sheltered coves are typically composed of well­ sorted sand or shell fragments. Lots of stuff can happen here because the particles can be affected by both waves and wind.  o Tidal Flats and Lagoons – Fine clay particles or carbonate­rich muds accumulate. Often changed by the tides. o Shallow Marine Environments – About 100 meters deep, sites where sand, clay, and carbonate­rich muds are often deposited. Ripple marks by wave activity may be present. In the photic zone where light from the sun can be used by plants and is also closer to the land where nutrients can be brought down. This makes an ecosystem that can help create rock/fossil flats. o Deep Marine Environments – Out of the photic zone, away from the continents. Adjacent to the continental slope often contain materials that was transported by dense underwater currents of suspended sediment. Each layer has coarser particles at the bottom and finer material on top. Not much sediment reaches these places. o Sedimentary Facies – Facies are suites of sedimentary deposits that are laterally adjacent, and are formed in but related depositional environments. From the near­ shore coast to the shallow ocean it goes Sandstone facies, shale facies, and limestone facies. This happens because as the water moves out, the water drops the particles it’s carrying. Water drops the sand­sized particles first, then the clay­ sized, and when it gets to the limestone facies there isn’t much sediment left.   Sedimentary Structures  o Strata are layers of rocks, different colors and widths. o Cross­Bedding is the presence of layers within a stratum or bed that are inclined to the horizontal, tend to show that the medium that deposited these things were moving like in a river or in a windy area. o Graded bedding describes a change in particle size from larger to smaller within a single bed. At the edge of the continental crust there is a continental shelf, which is the end of the continental crust changing to oceanic crust. Rivers bring sediment to the edge of the shelf and an event like an earthquake can send the sediment down, called a turbidity current. The sediment cascades down and creates something like an alluvial fan underwater. The larger pieces of sediment settle down first, so it creates a coarser under layer and the top are finder pieces of sediment. This is called grated bedding. o Ripple marks are waves of sediment on the surface of a layer formed by the moving action of water or air. o Mud cracks are polygonal shapes on the surface of a bed indicating that the bed formed in alternating wet and dry. This happens because the mud ‘shrinks’ when the water evaporates o Some structures are created by organisms. Some creatures can burrow through mud and then defecate the material out the back. Dinosaurs creating tracks in the sediment are also sedimentary structure


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