Lecture Outline Wednesday - Friday February 14-16, 2018

Transcription

1 Lecture Outline Wednesday - Friday February 14-16, 2018 Quiz 2 scheduled for Friday Feb 23 (Interlude B, Chapters 6,7) Questions? Chapter 6 Pages of the Past: Sedimentary Rocks Key Points for today Be able to identify the 3 categories of sedimentary rocks using texture and composition Be able to recognize the different types of sedimentary structures Understand how sediments are converted to sedimentary rocks Sedimentary Rocks Clastic (Detrital) Biochemical Chemical Where are the different types of sedimentary rock types found? Abundance by Sedimentary rock types Siltstone, mudstone, and shale 75% Sandstone and conglomerate 11% Carbonate rocks 14% What about chemical precipitants? ( ) Clastic/Detrital Sedimentary Rocks Composed of pieces of (clastic sediment). Classified by texture (, and arrangement of sediments.) Mudstone Sandstone Conglomerate or Breccia Texture is affected by transport process (Terms may be used to describe sediment or sedimentary rocks) Particle size, silt,, gravel (pebble, cobble, boulder) Shape Angular to Well Rounded

2 Sorting ( ) Well Sorted to Poorly Sorted Sorting are the particles about the same size? If yes well sorted. If no poorly sorted. How do grain size, shape and sorting change with increase in transport distance? The more the the transport. The more the the transport. With more transport the particles become and (similar in size). The size of the particles reflects the energy it takes to move them. Example: for a stream to move large particles like cobbles and boulders the moving water must be very fast high energy. However if the stream is only capable of moving fine particles like silt and clay then the water in the stream must be very slow low energy. Biochemical Sedimentary Rocks Comprised of the remains of organisms (plants and animals). Classified by. Ex: Limestone Chemical Sedimentary Rocks Composed of (like salt from evaporating seawater). Looks crystalline (but crystals may be very small. Classified by composition (minerals). Ex: Rock Salt, Rock Gypsum Sedimentary Structures (be able to recognize or identify from slide or description) Parallel layers of sediments. Each layer is called a bed. Sets of bedded sediments at an angle to horizontal. Deposited by currents (wind or water). Ex: Dunes Cross-bedding indicates the direction of current. Beds progress from coarse grains at bottom to fine grains at top of bed. Indicates waning of current. Wavy lines formed on top of a bed of sand or silt by waves or currents. Smaller scale than larger cross-beds. Can be preserved in rocks. Common on shorelines. Ripples can be symmetrical or asymmetrical Polygonal pattern of cracks that develop in mud as it dries.

3 Ex: Mudflats (or other areas exposed to wet/dry cycles). Burrow marks left in sediments by animals. Footprints, etc. left in sediment by animals. Why are sedimentary structures so important? (earth history). The present is the key to the past Principal of Uniformiantarianism Sediments are transported and deposited by one of the agents of erosion - wind, water, ice, or gravity The depositional environment is the geographical area in which the sediment is deposited. Depositional Environments are characterized by a combination of: sedimentary rock types sedimentary textures sedimentary structures present Geologists observe these features in sedimentary rock to determine the ancient environment where the sediment was deposited. Diagenesis - the chemical and physical changes that transform sediment into rock As sediments accumulate, older layers become buried. Sediments become compacted and heated. Sedimentary rocks generally form up to about 10km deep (around 300 degrees C). Lithification - hardening of soft sediments into rock: pore space volume is reduced due to weight of overlying sediment, and/or chemically precipitated minerals in pore spaces (acts as a glue by binding the materials together) common cements -,, hematite Lecture Outline Friday February 16, 2018 Key Points for today Understand how water, wind, ice, and gravity affect the size of the sediment being moved Be able to recognize and name some common depositional environments Common Sedimentary Environments Continental (Clastic dominates) Streams (fluvial) Desert (minor chemical) Lake Glacial Shoreline (Clastic dominates) Deltaic (where rivers enter the ocean)

4 Tidal Flat (exposed at low tide) Beach Marine (Mostly Clastic with some Chemical and Biochemical) Continental Shelf Continental Margin Reefs Deep Sea Clastic Sedimentary Environment Agents of Transport Wind Most selective agent Results in sediments Sand, Silt or Dust (sand grains may be frosted) Example description: Very Well Sorted Quartz Sandstone Cross Bedding What environment is it? Water Selection and sorting vary. Depends of. Ex: Faster moving waves on a beach can move gravels, deeper, grain size decreases. Glacial ice Least selective Results in sediments Note: Lack of bedding or other structures. Gravity Results in poorly sorted sediments angular Examples of Clastic Sedimentary Wind transport be able to recognize from a slide) Dunes can form in or Water transport ( be able to recognize from slide) Fan Sandstone can form in all these locations. The particles that make-up the sandstone will vary depending on where they originated and were they were deposited. Ice transport Glacier Biochemical Sedimentary Environments Carbonate deposition Marine Setting

5 and dolostone Composed of carbonate minerals (calcite, aragonite). Property?? Precipitated by organisms or inorganically Reefs are mound shaped organic structures composed of carbonate skeletons like coral. How Atolls are formed reefs formed around extinct volcanoes. The volcanoes erode and sink as the ocean plate subsides, but the reef continues to grow at a faster pace than subsidence. Floating organisms also secrete calcium and silica skeletons. When they die they settle to the ocean floor and creating limey and silica rich sediment which can be lithified into sedimentary rocks like limestone and chert. Where does coal come from? Where does the oil and gas that we use in cars and trucks come from? Chemical Sedimentary Environment (deserts and marine) Salts form as water from a shallow basin. Forms minerals like Halite and Gypsum. Modern Ex: Great Salt Lake, Ancient Ex: