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BIOCHEMICAL SEDIMENTARY ROCKS

biochemical limestone        diatomite        chert        coal

Biochemical sedimentary rocks form from sediment derived by biological processes.   This typically occurs in the ocean where a variety of atoms float among the water molecules.   Ions, such as calcium, magnesium, and potassium, along with trace elements like silicon, fluorine, iron and phosphorous, are used by marine organisms to form their hard and soft tissues.   Once the marine plant or animal dies, it may settle to the ocean floor as biochemical sediment, then become compacted and cemented together into solid rock.   Typically only the hard, skeletal parts of an organism are preserved as sediment.

sedCatherinebioclastic1.jpg (22350 bytes) Catherine Zekri holding shells washed ashore at Palos Verdes Peninsula, California.

On land, plants that die in a swampy environment may become deeply buried beneath other plants and detrital sediment.   Such plant matter, also biochemical sediment,  undergoes chemical reactions as it alters into the rock called coal.   Below are some of the many types of biochemical sedimentary rocks found on Earth.

 

biochemical limestone      Limestones are rich in calcium carbonate.   In a biochemical limestone the calcium carbonate is locked into the fossil shells and skeletons of marine organisms.   (In a chemical limestone, the calcium carbonate is in the form of calcite mineral crystals.)  As marine plants and animals die in the ocean, scavengers, waves or currents may break them apart, resulting in biochemical sediment comprised mainly of broken fragments of the original skeletons and shells.   Below are several images of so called skeletal limestones, the names of which are based on the sizes of the skeletal grains, or on the main type of fossil present within the skeletal limestone.

row A   Image 1 shows a skeletal limestone (technically a calcirudite based on the large sizes of a majority of the skeletal fragments).   The most common fragments are of trilobites and bryozoans.   Image 2 is a gastropod limestone (or gastropod calcirudite).   Image 3 is a view of an oyster limestone (or oyster calcirudite) exposed along the beach at Del Mar, California (image by Dr. Rick Behl).   Image 4 shows a brachiopod limestone (or brachiopod calcirudite).   Image 5 shows a much finer-grained limestone called micrite.   Most micrites are composed of microscopic, calcareous plant and animal fragments.   Click on an image to enlarge it.

   row A  sed_fossilimestclose_2.jpg (39182 bytes)    sed_fossilsgastrpods_1.jpg (39106 bytes)    oystercoquinaDelMarRick.jpg (48215 bytes)    sed_limestonebrach_close_1.jpg (37233 bytes)   Nov2062.jpg (34721 bytes)

                     image 1                   image 2                   image 3                    image 4                     image 5

 

row B      Images 1, 2 and 3 were taken using a petrographic microscope.   Each shows a thin-section view of a skeletal limestone where the rock sample was cut, ground and glued to a glass slide so that light would pass through showing excellent details of the fossils and surrounding matrix.   These thumbnail images are approximately 4X actual size, and all are from Ordovician-aged sedimentary rocks.   Image 1 is an ostracod valve.   Image 2 shows part of a brachiopod valve and a conodont (amber).   Image 3 shows another brachiopod fragment (long) and some echinoderm parts.   The last two images show limestone in natural settings.   Image 4 shows Jill, Bruce and Paul just prior to descending through the limestone layers that comprise the upper part of Grand Canyon.   Image 5 finds geology student Zach ripping into a limestone outcrop, searching for fossils.

   row B  sed1limestmicroostracod1.jpg (75228 bytes)    sedrocklimest1micro1.jpg (62244 bytes)   sed_fossillimestmicro_1.jpg (59072 bytes)   friends_JillGrandCanyon_1.jpg (28757 bytes)   sedlimestoneoutcrop1.jpg (42598 bytes)

                     image 1                image 2               image 3                image 4               image 5

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diatomite      Diatomite is a sedimentary rock composed mainly of the skeletons of a very common type of marine plankton - diatoms.   Diatoms are tiny plants that float near the ocean surface.   Their skeletons are composed of silica (silicon dioxide), a very durable substance.   Since diatom skeletons are highly porous, diatomite is extremely light in weight, and pure samples make excellent water filters.

Image 1 is a scanning electron microscope image of diatom skeletons clearly showing their porous nature (image by Dr. Rick Behl).   Images 2 and 3 show hand samples of relatively pure diatomite.   Note the white color and thin layering of this specimen, typical of diatomite.   Image 4 shows a sample of Monterey Formation diatomite.

                  diatomsspiculeSEMMontFmRick.jpg (27457 bytes)       sed_diatomitelonghand_1.jpg (22182 bytes)       sed_diatomiteclose_1.jpg (21536 bytes)       sedrockdiatomitehand1.jpg (37025 bytes)   

                       image 1                      image 2                      image 3                    image 4

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chert      The sedimentary rock chert can form in several ways.   From the biochemical aspect, it forms from deposition and compaction of silica-rich skeletons of diatoms, radiolarians (a common ocean planktonic animal), and tiny sponge fragments (called spicules) on the ocean floor.   Chemically derived chert can form as siliceous skeletons or silca-rich rocks dissolve, releasing silicon into the ocean water.   Silicon and oxygen will then bond together, precipitating onto the ocean floor.   Since chert is composed of silca, it is very hard and durable.   Ancient people often relied on chert as the raw material for the formation of tools such as hide scrapers, drills, and spear points.

Image 1 shows a collection of tiny radiolarian skeletons, common constituents in biochemical chert (image from Dr. Bert Conrey).   Images 2 provides a view of a typical chert hand sample.   Image 3 shows geology student Scott next to an outcrop of Monterey Formation shale (light) and chert (dark) along the California coast (image by Dr. Rick Behl).   Image 4 shows Bruce next to a chert outcrop in north-central Nevada..

    sedplanktonskeletons1.jpg (42454 bytes)         sed_chertclose_1.jpg (27080 bytes)         chertshaleMontFmRick.jpg (60372 bytes)         sed1Vininichert1.jpg (57706 bytes)   

           image 1                      image 2                       image 3                     image 4                  

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coal      The biochemical rock coal forms as concentrated plant matter becomes deeply buried over a long period of time.   Plants that die where there is an abundance of oxygen will begin to decay (oxidize) and become consumed by scavengers and bacteria.   But, plants that live and die in a swamp where there is little oxygen present within the water will not oxidize.   Instead, bacteria begin harvesting some of the plant matter, releasing oxygen and hydrogen into the water, resulting in a relatively increased level of carbon in the remaining sediment.  Temperature and pressure increase as more plants die and settle on top of the older plant matter.   This causes water and gases to be driven out of the organic matter, forming coal.

Images 1, 2 and 3 show outcrops of coal layers (also called seams) along different road cuts in the United States.   (Image 2 is by Dr. Stan Finney, and image 3 is by Dr. Rick Behl.)   Image 4, taken from a United States Geological Survey publication on coal, shows the distribution of coal in the conterminous United States.

               sed_coalroadcut_1.jpg (38751 bytes)       sed_coalStan_2.jpg (42652 bytes)       coalsandstoneSilveradoFmRick.jpg (34908 bytes)       coalbedsofUS.gif (68235 bytes)     

                       image 1                          image 2                          image 3                       image 4

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