TITLE DETRITAL ROCKS CHEMICAL ROCKS BIOCHEMICAL ROCKS
INTRODUCTION TO SEDIMENTARY ROCKS
what is sediment? changing sediment to rock sedimentary rocks record Earth's history sedimentary rocks are an important resource three groups of sedimentary rocks
What is Sediment? Sediment is loose, hard material lying on top of solid rock. Sediment comes in many forms and sizes, and can originate from a variety of sources. Detrital sediment is produced by the weathering and erosion of rocks exposed at Earth's surface. In the first image, Bruce holds some detrital sediment scooped up from the desert surface. Chemical sediment forms as minerals crystallize and settle from water that contains lots of dissolved particles. The second image shows a cluster of salt crystals that have intergrown together. Biochemical sediment accumulates as plants and animals die and their hard parts, such as skeletons and shells, are deposited on the ocean floor. In the third image, Catherine stands on a beach composed mostly of shell fragments and urchin needles along the coast of Palos Verdes Peninsula, California.
detrital sediment chemical sediment biochemical sediment
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Changing Sediment to Rock Sedimentary rocks form from sediment grains or crystals that are deposited in a wide variety of natural environments at Earth's surface. As these grains and crystals are buried under more sediment, they become compacted together. This is an important step towards forming rock from sediment, but the final step occurs when elements dissolved in groundwater begin bonding to each other, cementing the sediment grains and crystals together. The microscope images below illustrate the two most common types of natural cement. Silica cement is very hard and durable, whereas calcite cement is less durable because it dissolves on contact with weak natural acids present within groundwater.
White to gray-colored silica cement in a
sandstone.
A
closer view of silica cement. (Image from Dr. Rick Behl.)
Colorful
calcite cement in a sandstone (left) and a limestone (right). 
You can
test for calcite cement by dropping a little hydrochloric acid on the rock's
surface. If there is a fizzing reaction, then the predominant cement
holding that rock together is calcite cement. In the image
above, calcite cement is reacting to HCl
acid.
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Sedimentary Rocks Record Earth's History Since sediment deposited in the ocean, for example, has different characteristics than sediment deposited in the desert, sedimentary rocks resulting from these different bodies of sediment will be noticeably different. Geologists use this type of environmental information to "read" about Earth's history contained in ancient sedimentary rocks. Furthermore, knowledge about the evolution of life on Earth comes directly from fossils contained within sedimentary rocks. Grand Canyon, pictured below, is a natural laboratory for studying some of Earth's history contained within the sedimentary rock record. Grand Canyon rocks include limestone, sandstone and shale, each of which formed in different sedimentary environments at different times in Earth's history. A stroll down a trail in Grand Canyon is an opportunity to read the information contained within its rocks.
Sedimentary rocks cover much of the older igneous and metamorphic rocks that comprise Earth's rigid outer shell, the crust. That makes them relatively easy to access and investigate, especially in a dry climate where there has been some crustal uplift and erosion such as in the desert areas of California, Utah, Nevada, and Arizona. Where exposures are limited, such as in Oklahoma and Texas, sedimentary rocks are studied by drilling wells for oil, natural gas or water, and bringing up drill cores to the surface for inspection on site or in a laboratory.
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Sedimentary Rocks are an Important Resource Sedimentary rocks such as limestone can be used for a variety of purposes including highway construction and building stones, and, in the classroom, as chalk. Rocks such as coal and oil shale can be processed for their valuable, energy-containing carbon compounds. Rock gypsum can be ground up and used in a number of ways, including plaster and wall board. The list for the uses of sedimentary rocks is almost endless.
row 1 The first image shows Diane standing in front of limestone blocks at the Getty Museum, Los Angeles, California. The second image finds MiAnn and Teresa relaxing on a limestone block on the grounds of the Museum. The third image is of a piece of chalk, derived mostly from chalk limestone.
row 2 The image on the left is of the Monterey Formation, an oil-rich sequence of sedimentary rocks that is well exposed along much of coastal southern California. This image shows oil, in the form of tar, naturally seeping from fractures in the rock. The second image shows a drill rig, used to probe rocks underground in search of oil and natural gas. (Both images were provided by Dr. Rick Behl.)
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Three Groups of Sedimentary Rocks Sedimentary rocks are usually classified into three groupings listed below:
DETRITAL SEDIMENTARY ROCKS These rocks consist of sediment grains derived from the weathering and erosion of other rocks, hence the name "detritus" (Latin for loose material derived from disintegration). They are by far the most abundant of the sedimentary rocks. Common detrital rocks are shale, sandstone and conglomerate. For more details about these rocks go to the Detrital Sedimentary Rocks page.
CHEMICAL SEDIMENTARY ROCKS These rocks are composed mostly of mineral crystals that form and settle (precipitate) from water, either lakes, the ocean, or groundwater. They are extremely valuable to industry. Common chemical rocks are rock salt, rock gypsum and chemical limestone. For more information about these rocks go to the Chemical Sedimentary Rocks page.
BIOCHEMICAL SEDIMENTARY ROCKS These rocks contain significant amounts of biologic material, usually in the form of fossil fragments that were originally deposited on the ocean floor. Examples of such rocks are coquina and chalk, which are types of limestones. Heating of fine-grained organic matter, trapped in between other grains or comprising most of a rock, can cause it to slowly alter into useful forms of carbon. Examples of such energy-rich rocks are oil shale and coal. For more information about these rocks go to the Biochemical Sedimentary Rocks page.