GEOL1100 Test 2
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This 12 page Study Guide was uploaded by Arnold on Monday May 2, 2016. The Study Guide belongs to Geol 1100 at Auburn University taught by Stephanie L. Shepherd in Fall 2016. Since its upload, it has received 10 views. For similar materials see Dynamic Earth in Geology at Auburn University.
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Date Created: 05/02/16
Sedimentary Rocks Classes of Sedimentary Rocks 4 classes Clastic o Cemented together clasts (fragments and grains of rock) Biochemical o Shells grown by organisms Organic o Formed from carbon-rich matter from dead organisms Chemical o Minerals dissolved in water precipitate and form Types of Sedimentary rock Siliceous rocks o Rocks made up of quartz Argillaceous rocks o Rocks made up of clay Carbonate rocks o Rocks made up of calcite and/or dolomite Clastic rock production Weathering o Physical and chemical elements create grains of pre-existing rock Erosion o Combination of processes (flowing water, wind, etc.) separates clasts from original rock Transportation o Clasts and dissolved ions carried away by wind, water or ice Deposition o Clasts fall out of transportation medium and accumulates Lithification o Transformation of clasts and dissolved ion into rock o Compaction is 1 part when pressure squeezes out water and air o Cementation is 2 nd part when minerals (usually quartz or calcite) precipitate from groundwater to fill space between clasts Classification of Sedimentary Rocks Clast size (most important when classifying) o Diameter of grains o Coarsest Finest Boulder, cobble, pebble, sand, silt, clay Mud- wet clay or fine silt Gravel- pebbles and cobbles Clast composition o Makeup of clasts in sedimentary rocks o Lithic clasts- chips of fine-grained rock mixed in with sand grains Angularity and sphericity o Angularity Smooth or angular corners and edges o Sphericity Degree the clasts resembles a sphere Sorting o The degree to which clasts are the same size or variety of sizes o Well-sorted Clasts all same size Matrix is the mass of small clasts if large clasts are mixed in Sedimentary maturity o Degree clasts evolved from crushed up rock to well-rounded group of clasts consisting only minerals that are most resistant to weathering Character of cement o Some cement consists of quartz, others with calcite, other minerals are rare Breccia o Lithification of an accumulation of angular clasts Conglomerate o Lithification of small river gravel with smooth edges Arkose o Granite breaks down into quartz, feldspar and clay in alluvial fan. Clay is washed away and the quartz and feldspar goes through lithification Sandstone o Layers of beach or dune sand lithify Shale o Layers of mud, exposed beneath marsh grass, goes through lithification 1. Rock tumbles off cliff and smashes into rock at the bottom creating sharp edges a. Breccia 2. Storm moves clasts into river where they are broken down and lose sharp edges a. Conglomerate 3. Granite is chemically weathered into quartz, feldspar and clay. Clay is washed away a. Arkose 4. Feldspar breaks down into clay and is washed away. The quartz sand washes on beach a. Quartz Sandstone 5. Silt accumulates in floodplains, deltas or mudflats a. Siltstone 6. Clay/mud accumulates a. Shale b. Mudstone Diamicton o Very poorly sorted sediment with clasts of all sizes o Diamicite Debris flows o Mud mixed with larger clasts Glacial till o Debris left behind after glacial melts Wacke o Poorly sorted sedimentary rock with sand and lithic fragments in matrix of mud Biochemical Sedimentary Rocks o Organisms extract dissolved ions in seawater to make solid shells o When organisms die, the solid material in their shell survives o When the shells are lithified it becomes a biochemical sedimentary rock Biochemical Limestone o Clams, oysters, snails and other organisms produce shells from calcium carbonate (CaCO3), either as calcite or aragonite o Rocks formed from this calcium carbonate material produces biochemical limestone and is a type of carbonate rock o Fossiliferous Limestone Contains visible fossil shells or shell fragments o Micrite Very fine carbonate mud o Chalk Consisting of plankton shells Biochemical Chert o Formed from plankton that produce shells made of silica (SiO2) o Cracks like glass, creating smooth fractures o Made up of quartz grains that are too small to be seen o Jasper Chert containing traces of iron oxide and tends to be red Organic Sedimentary Rocks o Dead organic matter from organisms buried, pressurized and lithified Coal o 40%-90% carbon, the remainder consists of clay and quartz o When compacted, hydrogen, water, carbon dioxide and ammonia break free and carbon atoms reorganize into macerals Oil shale o Clay and 15%-30% organic material in a form called kerogen o Kerogen is made from fats and proteins from plankton and algae Chemical Sedimentary Rocks o Rock formed by the precipitation of minerals directly out of water solutions Evaporites (products of saltwater evaporation) o Heat of the sun evaporates water, but the dissolved salt stays behind o Specific type of salt minerals depend on amount of evaporation 80% seawater evaporates gypsum 90% seawater evaporates halite 100% seawater evaporates 80% halite, 13% gypsum Travertine (chemical limestone) o Calcium carbonate (CaCO3) precipitates from groundwater from springs or caves o Groundwater “degasses” and loses carbon dioxide, decreasing ability to hold carbonate and increasing concentration of calcium carbonate o In caves, travertine builds up complex growth forms called speleothems Dolostone o Contains dolomite (CaMg[CO3]2) o Calcite and groundwater with magnesium Chemically Precipitated Chert o Limestone with nodules (lumps) of black chert o Small quartz crystals precipitate and replace calcite crystals within limestone o Replacement chert, nodular chert o Agate, a type of chert, precipitates inside open hollows of rocks Sedimentary Structures Bedding and Stratification Bed o Single layer of sediment with a recognizable top and bottom Strata o Several beds together Bedding o Overall arrangement of sediment into a sequence of beds Bioturbation o Worms, clams and other organisms leave behind burrows in layers Stratigraphic formation o Sequence of strata distinctive enough to be traced as package across large region Geologic map o Distribution of stratigraphic formations Consequences of Deposition in a Current Bedforms o Sediment structure developed between sediment and the fluid Ripple marks o Elongated ridge formed on a bed surface at right angles to the direction of flow Dune o Larger ripple marks Cross beds o Internal part of ripple marks and dunes Turbidity Currents and Graded Beds Turbidity current o Sediment flowing down in water Graded bed o Larger grains sink faster, small grains accumulate at top o Coarse at bottom, fine at top Turbidite o Deposit from turbidity current Bed-Surface Markings Mud cracks o Mud layer dries up after deposition and curl up at edges creating openings Scour marks o Currents flow over surface and erode small troughs parallel to flow Ripple marks and cross bedding layers deposited in current Mud cracks layer was exposed to air and dried out Graded beds deposition by turbidity currents Depositional Environments Conditions in which sediment was deposited Terrestrial (non-marine) Sedimentary Environments Glacial o Glaciers move all sediment of any size o When ice melts away, the sedimentary load is dropped in the glacial till o Diamicton Mountain Stream o Fast moving water has power to carry large clasts o Breccia and conglomerate depending on degree of rounding Alluvial-Fan o Fast moving water empties into broader area and cannot move large sediement o Sand from erosion of granite with feldspar breccia and arkose Desert o Dust is carried away and accumulates in large dunes sandstone River o Fine sediment settles on floodplains, coarse sediment collects in rippled layers o Sandstone, siltstone and shale o Fluvial sediments Lake o Fine clay forms mud shale o Deltas have gravel at top, gravel and sand middle and silt bottom Coastal and Marine Sedimentary Environments Marine Delta o Various environments such as swamps and floodplains produce variety of rocks Coastal Beach Sands o Sand washes back and forth on coast and becomes well sorted and rounded o Well-sorted, medium-grained sandstone with ripple marks Shallow-Marine Clastic Deposits o Waves doesn’t stir sea floor and finer sediment accumulates o Fine-grained, well-sorted, well-rounded silt Shallow-Water Carbonate Environments o Little sand and abundance of carbonate shells o Carbonate mud accumulates at lagoons and produce different limestone Deep-Marine Deposits o Fine clay and plankton produce mudstone, chalk or bedded chert Sedimentary Basins Sedimentary Basin o Result of lithosphere sinking and creating a sediment filled depression Subsidence o Lithosphere sinks Rift Basins o Lithosphere is stretched during rifting and earth becomes thinner o Faults create low areas bordered by mountains o Alluvial deposits form along base, salt flats or lakes develop between mountains o When rifting stops, the lithosphere cools, thickens, becomes denser and sinks o Thermal Subsidence Sinking due to cooling of lithosphere Passive-Margin Basins o Form along edges of continents that are not plate boundaries o Fill with sediment carried to sea by rivers and carbonate rocks from reefs Intracontinental Basins o Develop in the interiors of continents o Extensive swamps may form along shoreline, produce coal Foreland Basins o Form on continent side of mountain belt from collision o Some rock pushed up, some pushed down into lithosphere and filled with rock Transgression o Terrestrial sediment is buried by coastal sediment due to sea level rise Regression o Coastal sediment is buried by terrestrial sediment due to sea level fall Diagenesis o All physical, chemical and biological processes that transform sediment into rock Metamorphic Rocks Metamorphic minerals o New minerals that grow within solid rock only in metamorphic conditions Texture o Arrangement of mineral grains Foliation o Parallel alignment of platy minerals (mica) and/or alternation dark and light Metamorphic Change Recrystallization o Changes shape and size without changing identity of mineral making up grains o Small, round, cemented together quartz grains irregular, large quartz grains Phase change o Transforms one mineral into another mineral with same composition but different structure. Rearrangement of atoms o Quartz (SiO2) Coesite (SiO2), denser than quartz Metamorphic reaction (neocrystallization) o Growth of new minerals different than protolith o Atoms diffuse through solid crystals or dissolve and precipitate at grain Pressure solution o Wet rock is squeezed more strongly in one direction than the other o Mineral grains dissolve where pressed and precipitate where not Plastic deformation o Rock is squeezed at high temperatures and pressures causing minerals to behave like soft plastic and change shape without breaking Types of Metamorphic Rocks Foliated Assemblage of parallel surfaces and/or layers in a metamorphic rock Striped or streaked appearance Ability to split into thin sheets Slate o Finest-grained foliated metamorphic rock o Shale or mudstone (rocks composed of clay) under low temperatures o Slaty cleavage- split into thin sheets Phyllite o Fine-grained, foliation caused by preferred orientation of very fine white mica o Fine-grained white mica and chlorite produced at high temperature from clay Metaconglomerate o Pressure solution and plastic deformation flatten pebbles into pancake Schist o Coarse-grained, larger mica grains than phyllite o Quartz, feldspar, garnet and amphibole can grow depending on protolith o Can form from shale or any other protolith that can produce mica Gneiss o Alternating dark and light layers showing a striped appearance. Very high temp o Light layers- quartz and feldspar, dark layers- amphibole, pyroxene, biotite Migmatite o Gneiss melts and mixes with igneous rock Nonfoliated Metamorphism occurred in absence of compression and shear Hornfels o Fine-grained Quartzite o Metamorphism of quartz sandstone o Quartz grains recrystallize creating larger grains Marble o Metamorphism of limestone o Calcite recrystallizes Granofels o Coarse-grained o Quartz and feldspar Amphibolite o Metamorphism of mafic rocks (basalt or gabbro) o Contains hornblende (type of amphibole) and plagioclase (type of feldspar) Metamorphic grade o Low-grade 250-400 degrees o High-grade 600+ degrees Contact metamorphism (heat) o Magma rises to rock at shallower depth o Heat and hydrothermal fluids causes wall rock to undergo metamorphism o High-grade rock forming at pluton, low-grade form farther away o Metamorphic aureole- metamorphic rock formed around igneous intrusion Burial metamorphism (pressure) o Metamorphism due to consequence of deep burial Dynamic metamorphism (shear) o Minerals in deep rock recrystallize at faults due to shear crust slides o Mylonite is produced Fine-grained Strong foliation parallel to fault Dynamothermal or Regional metamorphism (heat, pressure, shear) o Continental collision causes rock to go deep into earth surface o The rock heats up, pressure increases and the protolith is compressed o Protolith changes into foliated metamorphic rock Hydrothermal metamorphism o Cold seawater is heated by magma to create hydrothermal fluid o Hydrothermal fluid rises through crust to heat ocean-floor basalt Subduction Zone metamorphism o High pressure, low temperature Shock metamorphism o Large meteorites hit earth and release heat and compression o Quartz coesite Exhumation o Buried rocks end up back at surface o Continents squeezing together, mountains flatten as crust heats, erosion Geologists distinguish among metamorphic rocks according tothe type of foli ation and the mineral assemblage a rock contains.Hornfels is unfoliated and f orms as a result of contactmetamorphism. Mylonite develops when shearing creates afoliation but not necessarily a change in types of minerals. Slate,whi ch forms from shale, contains slaty cleavage; clay flakes aretypically aligned at an angle to bedding. Schist contains coarsegrains of mica (muscovite and/ or biotite) aligned parallel to eachother. Gneiss has compositional banding. (Migmatite forms whenpart of the rock melts, and thus it is a mixture of meta morphic andigneous rock.) Quartzite is composed predominantly of quartz (it ismetamorphosed sandstone), whereas marble is composedpredominantly of calcite or dolomite (it is metamorphosedlimestone or dolostone). Quartzite a nd marble are usuallyunfoliated. Compressive stress undergoes shortening Rock deformed quickly exhibits brittle behavior Central portion of high curvature on a fold is the hinge Faults are fractures with displacement, joins have no displacement Mineral assemblage in metamorphic rocks in dependent on pressure and heat
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