GEOL 101 1nd Edition Lecture 14 Outline of Last Lecture I Slope Failure II Land Failure Outline of Current Lecture I Sedimentary Rocks II Detrital Sedimentary Rocks III Chemical Sedimentary Rocks IV Environments V Energy Resources Current Lecture Sedimentary Rocks Sedimentary rocks products of mechanical and chemical weathering They account for about 5 percent by volume of Earth s upper crust Majority or rocks exposed at surface are sedimentary Sedimentary rocks make a thin veneer that coats the surface They are essential to geologists because they contain evidence of past environments Provide information about transport Contain fossil fuels Contain petroleum coal and economic resources Material that has been mechanically or chemically weathered may become a new rock through a process called digenesis Digenesis refers to all of the chemical physical and biological changes that occur to sediment after it has been deposited again The most common form of digenesis is Lithification Process by which loose unconsolidated sediment is transformed into solid rock This involves compaction and cementation o Compaction As sediment accumulates the weight of the overlying material compresses the sediment underneath Grains squeezed together and pore space between grains is reduced Most common in fine grained rocks silt and clay sized o Cementation Loose material is cemented together by materials that precipitate out of solution These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Cement forms in pore spaces and glues grains together Commonly the cement is calcite silica or iron oxide Most sedimentary rocks are lithified by a combination of these two processes Remember how there are two types of weathering Mechanical and chemical Since Sedimentary rocks form from weathered material there will be two types of sedimentary rocks Sedimentary rocks are based on the source of the material Detrital rocks sediment as solid particles These solid particles are typically the result of mechanical weathering but they may be from chemical weathering too Chemical rocks sediment that was once in solution These rocks form as material precipitates out of solution opposite of dissolving Detrital Rocks Detrital sedimentary rocks are made up of weathered solid particles Most of these particles are Clay minerals Quartz Feldspar Micas Detrital rocks are classified based on their grain sizes Detrital sedimentary rocks tell us a great deal about the environment where rocks formed Shale Most common sedimentary rocks Composed of mud sized particles clay and silt o Very fine grained o These fine particles are picked up by currents therefore in order to settle out of the water it must be calm o Common environments for shale to form include lakes lagoons and deep ocean basins As the silt and clay particles accumulated they can form layers or laminae Shale is often black which indicates high amount of organic matter o Black shales typically form in oxygen poor environments swamps o Leaf fossils are fairly common in shales o Because shale is composed of fine particles it is easily weathered o In an outcrop it is a slop former whereas more resistant rock layers are cliff formers Sandstone Composed of sand sized particles Forms in a variety of environments o Near shorelines beach sand o In deserts sand dunes o In deep water Characteristics that can help interpret the sedimentary environment include o Sorting o Shape o Composition Sorting how close in size the grains are to each other o Well sorted sandstone has grains that are all about the same size o Poorly sorted sandstone has grains that are variable in size within the sand size range Shape how angular or round the grains are o Angular grains are jagged o Rounded grains are smooth and spherical Composition mineral make up of the sand grains o Commonly Quartz is the dominant mineral in a sandstone quartzose sandstone Quartz is very resistant to weathering and so it lasts long enough to be incorporated into a rock Other minerals are more easily weathered and will dissolve oxidize or weather into clays o In some sandstones Feldspar is the dominant mineral arkose sandstone Feldspar will chemically weather into clay so if it gets preserved in a sandstone the sandstone must have formed very shortly after physical weathering o If a sandstone has mud sized combined with sand grains it is called a greywacke Greywackes generally form in ocean basins as submarine o Conglomerate consists of large rounded bits of gravel o Breccia is composed of mainly large angular particles Chemical Rocks Consist of material that has been precipitated out of solution Precipitation of material occurs in two ways o Inorganic processes o Organic processes biochemical origin Limestone Most abundant chemical rock Composed of mostly calcite Biochemical limestones form as coral reefs abundant invertebrate animal life fossils coquina broken shells chalk microscopic organisms Inorganic limestones include Travertine Oolitic limestone Dolostone formed secondarily from limestone Chert made of micro crystalline quartz Colorful varieties include o Agate banded form o Flint gray o Jasper red o Onyx black Evaporites saturated water evaporates the ions precipitate as minerals come out of solution and form rocks o Examples include rock salt and rock gypsum Coal remains of ancient plants o Different from other rocks because it is composed of organic material Often there are visible leaf and wood fragments o Stages in coal formation in order 1 Plant material 2 Peat 3 Lignite 4 Bituminous Sedimentary Environments Sedimentary rocks tell us a great deal about the environment in which they formed A sedimentary environment is any geographic setting where you have sediment accumulating o Streams beaches swamps Different environments produce different characteristics in the sediments Grain size grain shape composition sorting etc Types of sedimentary environments Continental Environments o Dominated by erosion and deposition associated with streams o Colder regions are glacier erosion o Wind takes on a great role in and out of coastal region Marine Environments o Shallow marine 700 feet Continental shelves 50 miles wide on average Large quantities of continental sediments that have been washed down Other areas have warm carbonate rich waters ideal for coral reef communities o Deep marine more than 700 feet Water is relatively