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Geology Ch 6
Terms in this set (51)
A very tough, abrasive, chemically-inert mineral. One of the most stable minerals in weathering and depositional environments at the Earth's surface. Commonly the last mineral to be destroyed during sedimentary transport and therefore, a major mineral constituent of most sandstones. At the bottom of Bowen's Reaction Series.
Adjective describing a limestone containing many small spheres of calcite (≈1 mm diameter) precipitated by inorganic processes in warm, shallow, marine water. The spheres are deposited where strong tidal or wave currents roll the spheres back and forth on a sea floor of calcareous mud. The spheres grow larger and rounder by the process of accretion as the mud adheres to their surfaces, similar to the way a snowball grows when rolled over wet, sticky snow.
General term for the binding mineral agent in a sediment that lithifies it into a sedimentary rock. It usually forms by the precipitation of minerals like calcite, quartz and hematite from groundwater moving through the pore spaces of a clastic sediment. This material binds the grains together and tends to decrease the rock's porosity.
The process in which angular clastic grains in a sediment are transformed by abrasion during sedimentary transport, causing the sharp edges and corners of the grains to be smoothed off. The degree to which this process has occurred, sometimes provides a very approximate measure of the sediment's distance of transport. This effect is most evident in the shape of sand grains deposited on sand dunes and beaches.
A major group of sedimentary rocks deposited by the direct precipitation of mineral grains from an aqueous solution like saline ocean or lake water. Examples of this group include limestone, gypsum and halite.
A clastic rock composed mainly of particles ranging in size between 0.06 and 2 mm. Quartz and feldspar grains are common (though not necessarily present) in many varieties of this rock. Coarser-grained than siltstone and finer-grained than conglomerate.
General term for the process by which transported sediments (clastic grains, precipitated mineral crystals and organic debris) come to rest and accumulate because the transporting medium can no longer hold them.
An ______ fan is a large, cone-shaped pile of sand, gravel and mud deposited at the base of a mountain range by a river as it enters a valley with more gentle slopes and loses it transport capacity.
A very fine-grained clastic sediment (grain size < 1 / 256 mm) which becomes shale or claystone after lithification. Also the mineral name for an important group of sheet-silicate minerals, including kaolinite and montmorillinite. Former last name of a world champion boxer.
Solitary or colonial animals that produce a hard shell of calcite and live attached to the sea floor. They grow upward in a shrub-like, tube-like or mound-like shape and, along with algae and other shell-forming animals, help form the cores of reefs.
A _________ current is a flowing layer of dirty, muddy, sediment-laden water that flows downslope along the floor of the ocean or a lake because it has a higher density than the surrounding clear water. Often triggered by submarine earthquakes, landslides or storms that cause mixing of seafloor muds with the overlying clear water. Many marine and lake-deposited graywackes are thought to originate from these currents. They sometimes deposit layers exhibiting graded bedding.
A region that undergoes erosion and provides the erosional debris for a sediment is called the sediment's source ____. By studying a sedimentary rock's textures, mineral composition, and sedimentary structures, this region can sometimes be characterized in terms of its rock types, climate, relief, location and distance from the site where the sedimentary rock was deposited.
Type of sedimentary rocks made of fragments of pre-existing rocks and minerals that were deposited mechanically under the influence of gravity. The individual grains are typically cemented or compacted together, sometimes giving the rock a granular framework with intervening pores. These rocks are classified primarily in terms of their grain size, which is also an indication of the amount of energy (current velocities and turbulence) in the depositional environment at the time of sedimentation. Examples include conglomerate, sandstone and shale. Because of their fragmental nature, these rocks usually preserve more information about the sediment's source area than chemical or organic rocks.
Small ridge, most commonly formed on the depositional surface of fine-grained sand and silt that is deposited by a moving current of water or wind. The ridges are generally perpendicular to the direction of current motion. Symmetric ones form beneath oscillating waves in shallow water. Asymmetric ones form in the unidirectional currents of wind-deposited sand dunes, as well as in rivers and deep bodies of standing water.
A black, soft, low-density organic sedimentary rock formed by the compaction and carbonization of plant matter in an oxygen-poor swamp or peat bog environment. Commonly contains the carbonized remains of leaves, stems, branches and trunks of trees and other vegetation that did not decay completely because the decay process used up oxygen in the system, thereby inhibiting further destruction of the plant matter.
Coarse-grained, clastic sediment containing rounded mineral grains or rock fragments that are greater than 2 mm in diameter. Forms conglomerate after lithification.
environment of deposition
The location and conditions under which sedimentation occurs. It is described in terms of the physical, chemical and biological processes and conditions that exist at that location. Examples include rivers, desert valleys, alluvial fans, beaches and the sea floor.
Type of terrestrial depositional system that produces elongate lenses of crossbedded sandstone and conglomerate in lensoid channels that are embedded in broad floodplain deposits of shale and mudstone.
A bioclastic limestone containing many coarse shells and shell fragments that are cemented together by calcite. It usually accumulates in shallow, warm, ocean water near the shoreline, where waves sometimes abrade and redistribute the shell material before cementation.
A coarse-grained sedimentary rock with many angular fragments of rocks and minerals. If the debris were transported far enough for the grains to be rounded, it would be called a conglomerate.
Type of sedimentary structure in conglomerate, sandstone and siltstone in which rapidly-moving currents of wind or water deposit inclined sets of layers. The slope of the inclined layers sometimes indicates the direction the currents were flowing.
Organic material that when deeply buried and compacted, forms the parent material for coal. It is a brown, soft, loose, spongy deposit of plant debris that accumulates in swamps and bogs.
A soft, white, porous type of bioclastic limestone formed by the accumulation of calcareous shells of microscopic marine organisms. Reacts with HCL and writes on blackboards.
A fine-grained clastic sedimentary rock composed of clay and silt. Often fissile.
Conglomerate is a coarse-______ clastic rock.
A clastic sedimentary rock in which more than 25% of the grains are made of feldspar. The presence of abundant feldspar (and commonly other more unstable minerals like mica) imply a relatively short distance of transport, only minor chemical weathering, and a source area containing rocks rich in feldspar like granite. Commonly deposited in desert alluvial fans and river-dominated environments at relatively short distances from their source areas.
The formation and growth of mineral crystals in a rock. In sedimentary rocks, this process generally occurs by precipitation of mineral crystals from an aqueous solution like saline marine or lake water, and produces an interlocking crystal texture in the rock. The process is most pronounced in chemical sedimentary rocks (e.g. limestone, gypsum, halite) and is one of the processes by which these rocks become lithified. The process is also involved in cementing clastic rocks - another process of lithification.
Fine-grained, water-saturated, clastic sediment composed mainly of clay and silt-sized particles. Generally forms shale and mudstone after compaction and lithification.
A hard, brittle, very fine-grained to microcrystalline, chemical sedimentary rock made of silica. Commonly occurs in limestones as nodules. Also occurs as laterally-continuous beds, some of which apparently form by the recrystallization of silica-bearing shells of microscopic marine organisms that accumulated on the ocean floor.
The bounding surface between two adjacent rocks of different lithology or age. In sedimentary rocks they are often represented by bedding planes.
A large wedge or lens-shaped pile of coarse-grained, clastic sediment (sand and gravel) that accumulates where a river exits a mountain canyon and enters a less-confined, more gently-sloped valley and drops its sedimentary load. Alluvial ____.
A massive ridge or mound of limestone that accumulates in shallow, warm seawater by the growth and accumulation of densely-populated communities of coral and/or algae. The organisms cement their shells together to form a wave-resistant ridge or mound of calcite skeletons. Waves pounding on the windward side on this feature break off shell debris which accumulates in steep slopes on the windward side in deeper water. On the landward (often leeward) side of the structure, horizontal beds of finer-grained carbonate sand and mud typically accumulate in shallow-water lagoons.
A small, empty void space between the grains in a rock. These empty spaces are normally filled with groundwater which migrates slowly through them and may eventually fill them up with precipitated mineral cement. Oil reservoirs are produced if these voids are filled with oil.
A category of sedimentary rocks composed mainly of the remains of animals and plants. Coal is an example.
Type of sedimentary rock that forms the bedrock throughout much of northeast Texas, including the Collin County region. A chemical sedimentary rock composed mainly of the mineral with the chemical formula CaCO3. Fizzes with HCL. Usually deposited in warm, shallow marine water and commonly contains marine fossils. The CaCO3 is precipitated by either the growth of CaCO3-secreting organisms or by direct precipitation from ocean water by inorganic processes.
The remains or evidence of a plant or animal preserved by natural means in a rock. In the case of animals, it usually represents the animal's hard parts (shells, teeth, bones) that don't break down rapidly from bacterial decay and are thereby buried before destruction is complete. Hard parts are commonly replaced by later minerals or completely dissolved, producing molds and casts. Can also include traces of an organism like tracks, trails and burrows. Since many animals and plants are sensitive to specific environmental conditions, these features contain clues about the depositional environment of the rocks they are preserved in.
A textural term for the shape of a clastic grain exhibiting sharp corners and edges. This shape usually indicates that the sediment was not transported far before deposition. The opposite of rounded. Sedimentary breccia exhibits this texture.
A clastic sediment composed of particles ranging in size between 0.06 and 2 mm. Often composed of quartz and feldspar, though any mineral can be present. Coarser-grained than silt and finer-grained than gravel. A common sediment on beaches and in wind-blown dunes.
A clastic sediment composed of particles ranging in size between 0.004 and 0.06 mm. Coarser-grained than clay but finer-grained than sand.
Coarse-grained, clastic rock containing mineral grains or rock fragments that are more rounded than those in a sedimentary breccia, implying a greater distance of transport. Formed by the lithification of gravel.
Type of layering in which a single layer contains relatively coarse grains at the bottom that gradually decrease in grain size toward the top. Commonly deposited by turbidity currents as they slow down and gradually loose their ability to keep successively smaller and smaller clastic grains in suspension.
The removal of rocks and minerals on the Earth's surface by running water, wind, glacial ice or gravity. Provides the material for the deposition of sediments.
One of the processes of lithification in which a rock is squeezed by the weight of overlying rock and loses some of its volume and pore space. Individual grains are brought closer together and the rock becomes more cohesive. This process also entails the expulsion of large volumes of water that occupied the pores in the rock, especially in clay-rich muds.
Type of ripple marks formed by oscillating currents (e.g. waves).
A type of chemical sedimentary rock that forms by the precipitation and growth of mineral crystals in highly saline water bodies as a result of evaporation of the water. Examples include rock salt (halite) and rock gypsum.
A chemical sedimentary rock made up primarily of a mineral with the chemical formula CaMg(CO3)2. This rock is formed by the alteration of limestone in which some of the calcium in the calcite is replaced with magnesium that is brought into the rock by percolating groundwater.
A small, polygonal fracture that forms on the surface of mud when it dries in contact with the atmosphere. They form because clay in the mud shrinks when it dries, causing tension cracks to form in the uppermost layer of the mud. They look like an arrangement of patio tiles on a bedding plane. They imply very shallow-water conditions that became dry when the mud was exposed to the atmosphere. They typically form in mud puddles, on tidal flats, or on the floodplain of a river after the flood waters evaporate. Not found in deep marine sediments.
A shale that can split into thin layers is said to be ________. Rhymes with "missile".
The fine-grained clay and silt-sized particles filling the pore spaces between larger sedimentary grains. It causes a rock to have a wide range in grain sizes and therefore a poorly-sorted texture.
Shale is a __-grained sedimentary rock, deposited in a low-energy depositional environment.
If a sedimentary rock contains a wide range in grain sizes, its sorting is said to be ____. Turbidites and the deposits of glacial ice exhibit this kind of sorting.
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