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Unformatted text preview: SEDIMENTARY ROCKS SEDIMENTARY ROCKS Sedimentary rocks are formed near or at the surface of the earth. They are derived from preexisting source rocks. They are composed by sediments, this is material that has been weathered, transported and deposited by processes such as running water (rivers) glaciation (glaciers), etc., or formed by chemical precipitation from solutions. They cover 75% of the Earth’s surface and are the sites of important resources such as COAL, OIL and GROUNDWATER. Sedimentary rocks frequently contain fossils that can be used to interpret earth’s history They are composed only by few mineral species. Their physical appearance is due to different factors SOURCE AREA (rock type present, relief, climate, degree of weathering) TRANSPORTING MEDIUM (running water, glacier, wind, gravity) DISTANCE OF TRANSPORTATION TYPE OF ENVIRONMENT OF DEPOSITION (water depth, climate, current strength) SEDIMENTARY ROCK FORMATION DEPOSITION COMPACTATION RECRYSTALLIZATION + COMPATATION LITHIFICATION SEDIMENTARY ROCKS 9/19/2003 1/6 SEDIMENTARY ROCKS Sedimentary rocks are classified based on their TEXTURE and COMPOSITION TEXTURE. Is the geometrical arrangement of the constituents of a rock (SIZE, SHAPE and MUTUAL RELATIONSHIPS OF PARTICLES). Based on their composition there are three main groups of sedimentary rocks: CLASTIC or DETRITAL BIOCHEMICAL Formed by fragments of mineral or rocks Formed by remains of organisms. Formed by chemical processes. Crystals that have precipitated from solutions CHEMICAL 9/19/2003 2/6 SEDIMENTARY ROCKS CLASTIC TEXTURES / ROCKS SEDIMENT GRAVEL(bouldercobblepebble-) SIZED SIZE > 2mm in ∅ DEGREE OF SORTING POORLY SORTED -TEXTURAL FEATURES / COMPOSITION ROUNDING ANGULAR Source dependent MINERALS ROCK NAME BRECCIA WELLROUNDED CONGLOMERATE SANDSIZED 1/16 - 2 mm ∅ (0.062 mm) WELL SORTED WELLROUNDED QUARTZ ORTHOCLASE QUARTZ SANDSTONE ARKOSE SILT-SIZED 0.005 1/16 mm ∅ Grains to small to be seen without the aid of a microscope Grains to small to be seen without the aid of a microscope Grains to small to be seen without the aid of a microscope SILTSTONE CLAYSIZED Feels gritty when rubbed <0.005 mm ∅ Feels smooth Grains to small to be seen without the aid of a microscope Grains to small to be seen without the aid of a microscope Grains to small to be seen without the aid of a microscope SHALE 9/19/2003 3/6 SEDIMENTARY ROCKS BIOCHEMICAL –CARBONATES Composition CaCO3 (calcium carbonate) CALCITE Fizz strongly with dilute HCl Fragment size Microscopic 1-5µ Name of the fragment MICRITE Origin Results from the lithification of lime mud, most of which originates from the breakdown of the hard "skeletons" secreted by calcareous algae which live in warm, shallow seas Textural Characteristics The color is variable, ranging from gray to tan, or other colors. Rock name MICRITIC LIMESTONE or MICRITE CARBONATES LIMESTONES are generally gray (but may be tan, pink, white, black, or other colors). Microscopic (larger than above) Probably formed by the accumulation of the remains of planktonic marine algae called coccolithophores or shells of Foraminifera and/or by chemical precipitation. White, but it may be stained with iron oxide or other impurities. It is a soft porous rock that crumbles easily. Less dense, and less compact than micrite. Whole fossils, broken shell fragments with a calcareous skeleton or body parts. CHALK Macroscopic ALLOCHEMS FOSSILS The remains of ancient plants or animals Many organisms have calcareous shells or skeletons, and their remains may accumulate in lime mud FOSSILIFEROUS LIMESTONE Composed almost entirely by broken shell fragments OOLITES are small (1/4 2mm; sand-sized)), concentrically layered, spherical grains, so named because they look like fish eggs. On a cut or broken surface they look circular, and internal concentric laminations may be seen with a hand lens or microscope. Commonly are formed by layers of material (usually calcite), that have been deposited around some tiny particle such as a sand grain or fossil fragment and are rolled back and forth in quiet waters COQUINA OOLITIC LIMESTONE 9/19/2003 4/6 SEDIMENTARY ROCKS BIOCHEMICAL -OTHER COMPOSITIONS Composition SiO2 SILICEOUS Fragment size Microscopic to very fine grained silica sediment Origin Chemical or biochemical origin. Some chert contains siliceous skeletons of microorganisms known as radiolarians ...

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