Download Intro to Petrology: Sedimentary Rocks and Weathering - Prof. Helen Lang and more Study notes Geology in PDF only on Docsity! Dr. Helen Lang Dept. of Geology & Geography West Virginia University SPRING 2008 GEOLOGY 285: INTRO. PETROLOGY Sedimentary Rocks Rocks resulting from the consolidation f l dio oose se ment or chemical precipitation from solution h E h’ fat or near t e art s sur ace; or organic rocks consisting of the i i f l dsecret ons or rema ns o p ants an animals Two aspects to Weathering • Mechanical Weathering - physical breakup of rocks (esp frost wedging: water → ice causes . - 9% volume increase) Ch i l W th i h i l b kd f• em ca ea er ng - c em ca rea own o minerals in the presence of water δ+ δ+ δ- • Chemical Weathering is MUCH more important than mechanical weathering, due to the extraordinary dissolving power of H2O Goldich’s Weathering Series S ibili f i i l h iuscept ty o common gneous m nera s to weat er ng Olivine Calcic PlagioclaseMost Susceptible Pyroxene Amphibole Biotite Sodic Plagioclase Potassium Feldspar Muscovite Quartz Least Susceptible Weathering Reactions of Orthoclase Step 1: 3 KAlSi3O8 + 2 H+ + 12 H2O --> KAl3Si3O10(OH)2 + 6 H4SiO4 + 2 K+ orthoclase illite (~muscovite) soluble silica Step 2: 2 KAl3Si3O10(OH)2 + 2 H+ + 3 H2O --> illite 3 Al2Si2O5(OH)4 + 2 K+ kaolinite Iron in minerals weathers differently • Fe in most ferromagnesian minerals is red ced (Fe2+) beca se the ’re formed inu , u y reducing conditions (low oxygen) f i h i• Sur ace waters are very oxygen-r c , .e., oxidizing • Fe2+ released during weathering immediately oxidizes to Fe3+ • Fe3+ precipitates rapidly as EXTREMELY INSOLUBLE Fe(OH)3 and other hydroxides Weathering of pyroxene for example , C F Si O (F t f it ) + H O + H+a e 2 6 e par o aug e 2 = Calci m montmorillonite + H SiO + Ca2+ + Fe(OH)u 4 4 3 Fe(OH)3 and other Ferric oxyhydroxides precipitate (orange or rusty) and eventually dehydrate to Hematite, which gives subaerial soils and sediments (red beds) their red color The most common products of weathering are: • Quartz • Clay minerals-kaolinite illite montmorillonite , , • Cations in solution F i h d id d id (i l bl ) f• err c y rox es an ox es nso u e rom the weathering of mafic minerals Weathering of Granite • Where does alteration first occur? • Disaggregation of grains: forms “grus” • Surface (esp. of feldspars) gets soft and punky why?, • What are the mineral products of granite h i ?weat er ng Grus: disaggregated grains of quartz and feldspar Grus is a common first step in granite weathering Sedimentary Structures • Sedimentary structures can tell a lot about th d iti l i te epos ona env ronmen • Some sedimentary structures tell which way was UP during deposition • Some indicate current direction • You’ll hear about lots more in Strat-Sed Graded Bedding in Turbidite Flame Structures G d d UPra e - Scour surface Graded-UP Load casts or @ th a
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Cross-beds and laminar beds in b h keac roc Other Current direction indicators DOWNSTREAM A t i Ri l ksymme r c pp e-mar s Steeper slope is downstream DOWNSTREAM b i d lIm r cate c asts Elongate clasts dip upstream (pushed like falling dominoes) Flute Casts: Spoon-shaped sole markings on bottom of beds commonly , caused by turbidity currents Top view Side view deepest Flute casts are preserved on the bottom of overlying sand beds
