Lecture 6-7Sedimentary and Metamorphic Rocks Part 1 Sedimentary Rocks Kyanite, Sillimanite, and Andalucite
What is a sedimentary rock? • Sedimentary rocks result from mechanical and chemical weathering • Comprise ~ 5% of Earth’s upper crust • About 75% of rocks at surface • Contain evidence of past environments • Record how sediment is transported • Often contain fossils
What is the economic importance of sedimentary rocks? • They are important for economic reasons because they contain • Coal • Petroleum and natural gas • Iron, aluminum, uranium and manganese • Geologists use them to read Earth’s history
Cementation • Precipitation of chemicals dissolved in water binds grains of a sediment together. • After the cements solidify, compaction drives out the excess water. • Important part of Lithification • Remember where cements come from?
Types of sedimentary rocks • Chemical rocks – sediment from ions that were once in solution • Detrital rocks –sediment transported as solid particles
Detrital sedimentary rocks • Constituents of detrital rocks can include • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks
Detrital sedimentary rocks • Mudrocks: less than .063 mm • 1. Mud: small particles easily kept in suspension • Settles in quiet water • Includes Shale: mud-sized particles <.004 mm deposited in thin bedding layers called laminae Most common sedimentary rock 2. Larger mudrock grains called silts silt-sized particles .004-.063 mm Gritty grains can be felt
Detrital sedimentary rocks • Sandstone • Made of sand-sized particles larger than .063 mm and less than 2mm • Forms in a variety of environments • Sorting, angularity and composition of grains can be used to interpret the rock’s history • Quartz is the predominant mineral (due to its durable nature)
Detrital sedimentary rocks • Conglomerate and breccia • Both composed of particles > 2mm in diameter • Conglomerateconsists largely of rounded clasts. Rounded pebbles in high velocity areas • Breccia is composed of large angular particles Breccia is made of shattered rock that accumulates at the base of a cliff
Energy • Coarse sediments are deposited in high energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams. • Fine sediments are deposited in low energy environments, e.g. the slow water of deep lagoons, the abyssal plain, etc.
Chemical sedimentary rocks • Precipitated material once in solution • Precipitation of material occurs two ways: • Inorganic processes: the minerals precipitate out of water • Organic processes: animals and plants precipitate the minerals to use as shells or skeletons http://www.ucmp.berkeley.edu/collections/micro.html
Common chemical sedimentary rocks • Limestone • Most abundant chemical rock • Made of the mineral calcite CaCO3 • Marine biochemical limestones form as coral reefs,coquina (broken shells), and chalk (microscopic organisms) • Inorganic limestones include travertine(caves) and oolitic limestone (Bahamas)
http://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpghttp://www.ndsu.nodak.edu/instruct/ashworth/coursework/g410/evaporites/saltbeds.jpg • Common chemical sedimentary rocks • Evaporites • Evaporation triggers deposition of chemical precipitates • Examples include rock salt and rock gypsum
Sedimentary environment determines roundness sorting, mineral diversity 6_5 Character of detrital sediments depends on time, distance, and energy. For example, in streams: Particles are large and irregular, and consist of a variety of lithologies, including the least resistant. Particles are mid-sized and of intermediate sphericity, and include resistant and nonresistant lithologies. Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz. HIGHLANDS LOWLANDS NEAR-COASTAL
Graded bedding Floods change the local conditions 6_6 Fine-grained sediment On floodplain Older sediment 1 Pre-flood Bounders on bottom, sands and muds suspended Flood water Erosion of uppermost fine-grained sediment 2 Flood stage Waning flow Fine-grained above Coarse-grained below Bedding plane 3 Post-flood
Sedimentary Facies • Different sediments accumulate next to each other at same time • Each unit (called a facies) possesses a distinctive characteristics reflecting the conditions in a particular environment • The merging of adjacent facies tends to be a gradual transition
Some Facies in an oversimplified drawing Abyssal Ooze Stillwater muds Nearshore sands
Cross Beds areripples in cross section • Irregularities lead to ripples, dunes, sand bars. • In cross section these look like lines at an angle to the horizontal – “cross beds” • Ripples can indicate direction of air or water flow if asymmetrical, a tidal environment if symmetrical. Size and shape indicate fluid velocity.
Cross bedding in Sand Dune deposits Navaho Sandstone Sandstone deposited in ancient sand dunes Frosted Grains, well sorted
Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up
Terms for Marine (i.e. Ocean) Environments and some characteristic sediment facies 6_27 Continental slope Muds Continental shelf Sands Shallow marine Abyssal Plain Turbidite Graded Beds Ooze Deep marine Define Graded Beds Submarine volcanoes
Fossils are traces of prehistoric life generally preserved in sedimentary rock
Part 2 Metamorphism and Metamorphic Rocks
Metamorphism • … is the transformation of rock by high temperatures (heat) and pressure • Metamorphic rocks are produced by transformation of: • Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks • These are the source of many important minerals – Talc (lubricant, insulators, refractories), Corundum, Garnet (abrasives), Kyanite (ceramics), Micas (insulators), Chrysotile (“asbestos” for fireproofing), etc., etc.
Minerals do not melt during metamorphism Sedimentary rock 0km Metamorphic rock Igneous Sediment rock 10 km ~200ºC Sedimentary rock Metamorphismoccurs between about 10 and 50 km of depth Metamorphism Increasing depth and temperature 50km Melting ~800ºC
What causes metamorphism? 1. Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Increasing Heat with Depth
What causes metamorphism? • 2. Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially • All Directions = “Confining Pressure” • Differential = “Directed Pressure”
Origin of pressure in metamorphism (Burial) (Convergent Margin)
Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Source:Kenneth Murray/Photo Researchers Inc.
Main factors affecting metamorphism 3. Parent rock • Metamorphic rocks usually have the same chemical composition as the rock they were formed from. • Different minerals, but made of the same atoms. • Exception: water carries in new atoms and removes others. Important at MOR and in subduction zones.
Metamorphic Settings • Three types of metamorphic settings: • Contact metamorphism – due heat from adjacent rocks • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes: • Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated • Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated
Contact metamorphism Produced mostly by local heat source
2.Hydrothermal Metamorphism • Due circulation of water near Magma • Important at mid-ocean ridge
Metamorphism in a Subduction Zone Shallow Lithosphere Near trench Deep Lithosphere
Metamorphic Grade and Index Minerals • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form Notice Quartz and Feldspars are useless Note Quartz and Feldspar are not index minerals: Why? Some index minerals give us temperature info
Some Useful as Thermometers and Pressure Gauges Sillimanite Kyanite Polymorphs of Al2SiO5 Andalusite
7_21 CANADA New England Dynamothermal Metamorphism Mapped by index minerals MAINE Augusta CANADA U.S.A. Montpelier NEW HAMPSHIRE VERMONT Concord ATLANTIC OCEAN Boston Albany MASSACHUSETTS NEW YORK R.I. Hartford Providence Binghamton CONNECTICUT Unmetamorphosed y e Chlorite/muscovite zone Low grade l a l PENNSYLVANIA v Biotite zone Scranton Long Island Garnet zone Medium grade t f Staurolite zone i r NEW Newark High grade Sillimanite zone JERSEY Increasing pressure and temperature LOW GRADE DIAGENESIS HIGH GRADE MELTING INTERMEDIATE GRADE Chlorite and muscovite Biotite Garnet Staurolite Sillimanite
Common metamorphic rocks • 1. Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions
Common metamorphic rocks • Nonfoliated rocks (cont.) • Marble • Coarse, crystalline • Parent rock usually limestone • Composed of calcite crystals • Fabric can be random or oriented
Change in metamorphic grade with depth Metamorphism of a mudstone Increasing Directed Pressure and increasing Temps
A mica garnet schist Definition: Schist Garnets are abrasives, long lasting bearings, and jewels
Development of foliation due to directed pressure Granodiorite Gneiss