Part 1 Sedimentary Rocks

Transcription

1 Lecture 6-7 Sedimentary and Metamorphic Rocks Part 1 Sedimentary Rocks Kyanite, Sillimanite, and Andalucite

2 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

3 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

4 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?

5 Types of sedimentary rocks Chemical rocks sediment from ions that were once in solution Detrital rocks sediment transported as solid particles

6 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

7 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 mm Gritty grains can be felt

8 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)

9 Detrital sedimentary rocks Conglomerate and breccia Both composed of particles > 2mm in diameter Conglomerate consists 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

10 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.

11 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

12 Common chemical sedimentary rocks Limestone Most abundant chemical rock Made of the mineral calcite CaCO 3 Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) Inorganic limestones include travertine (caves) and oolitic limestone (Bahamas)

13 Common chemical sedimentary rocks Evaporites Evaporation triggers deposition of chemical precipitates Examples include rock salt and rock gypsum

14 Chemical Sediments: Coal

15 Sedimentary environment determines roundness sorting, mineral diversity Character of detrital sediments depends on time, 6_5 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

16 Graded bedding Floods change the local conditions 1 Pre-flood 6_6 Fine-grained sediment On floodplain Older sediment Bounders on bottom, sands and muds suspended Flood water Erosion of uppermost fine-grained sediment 2 Flood stage Waning flow 3 Post-flood Fine-grained above Coarse-grained below Bedding plane

17 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

18 Some Facies in an oversimplified drawing Abyssal Ooze Stillwater muds Nearshore sands

19 Strata- Bedding Planes

20 Slabs of eroding sandstone with ripple marks

21 Cross Beds are ripples 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.

22 Cross bedding in Sand Dune deposits Navaho Sandstone Sandstone deposited in ancient sand dunes Frosted Grains, well sorted

23 Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up

24 Submarine volcanoes Terms for Marine (i.e. Ocean) Environments and some characteristic 6_27 sediment facies Continental shelf Continental slope Abyssal Plain Define Graded Beds

25 Fossils are traces of prehistoric life generally preserved in sedimentary rock

26 Dinosaur footprint in mudstone

27 End of Sedimentary Rocks

28 Part 2 Metamorphism and Metamorphic Rocks

29 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.

30 Increasing depth and temperature 0 km 10 km ~200ºC Minerals do not melt during metamorphism Sediment Sedimentary rock Sedimentary rock Metamorphic rock Igneous rock Metamorphism Metamorphism occurs between about 10 and 50 km of depth 50 km ~800ºC Melting

31 What causes metamorphism? 1. Heat Most important agent Heat drives recrystallization - creates new, stable minerals Increasing Heat with Depth

32 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

33 Origin of pressure in metamorphism (Burial) (Convergent Margin)

34 Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: foliation Source: Kenneth Murray/Photo Researchers Inc.

35 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.

36 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

37 Contact metamorphism Produced mostly by local heat source

38 2. Hydrothermal Metamorphism Due circulation of water near Magma Important at mid-ocean ridge

39 Hydrothermal Metamorphism

40 Metamorphism in a Subduction Zone Shallow Lithosphere Near trench Deep Lithosphere

41 Metamorphic Grade and Index Minerals Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

42 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?

43 Some Useful as Thermometers and Pressure Gauges Sillimanite Kyanite Polymorphs of Al 2 SiO 5 Andalusite

44 CANADA New England Dynamothermal Metamorphism Mapped by index minerals Albany NEW YORK Binghamton PENNSYLVANIA Scranton NEW JERSEY Montpelier VERMONT r i f t 7_21 NEW HAMPSHIRE MAINE Augusta Concord MASSACHUSETTS Hartford CONNECTICUT v a l l e y R.I. Low grade Boston Long Medium Island grade Newark High grade ATLANTIC OCEAN Providence Unmetamorphosed Chlorite/muscovite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone Increasing pressure and temperature DIAGENESIS LOW GRADE INTERMEDIATE GRADE HIGH GRADE MELTING Chlorite and muscovite Biotite Garnet Staurolite Sillimanite

45 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

46 Common metamorphic rocks Nonfoliated rocks (cont.) Marble Coarse, crystalline Parent rock usually limestone Composed of calcite crystals Fabric can be random or oriented

47 Change in metamorphic grade with depth Metamorphism of a mudstone Increasing Directed Pressure and increasing Temps

48 A mica garnet schist Definition: Schist Garnets are abrasives, long lasting bearings, and jewels

49 Gneiss displays bands of light and dark minerals

50 Development of foliation due to directed pressure Granodiorite Gneiss

51 Migmatites- When Partial Melting Starts Heat the rock, when the minerals with the lowest melting points (Quartz, Feldspar) at that pressure melt, then recrystallize. We get separate regions of Metamorphic (dark, mafic) and Igneous (light, felsic) rock Part igneous, part metamorphic