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This 41 page Class Notes was uploaded by Shelby Green on Friday February 5, 2016. The Class Notes belongs to Geol101 at Clemson University taught by Dr. Coulson in Spring 2016. Since its upload, it has received 46 views. For similar materials see Physical Geology in Geology at Clemson University.
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Date Created: 02/05/16
Announcements • Exam 1 next time! Lecture 5- Sedimentary Rocks & Processes • PT 1- Forming Sedimentary Rocks • PT 2- Classifying Sediment & Sedimentary Rocks • PT 3- Mass Wasting Sedimentary Processes • Why do we care? • Most common rock type at Earth’s surface Landslide PT 1- Forming Sedimentary Rock • Parent Rock: pre-existing rocks • Step 1- Weathering – Break down of rock into particles – All the debree is called sediment • Physical vs chemical See file 5b Physical Weathering • Plant roots: split rock as the plant grows – Ex: the tree to the right • Frost wedging: water gets into cracks freezes ice expands splits the rock – Happens again and again, expanding a little more each time – Ex: pot holes in roads can be result of frost wedging Chemical Weathering • More common in many environments reaction series turned upside down Chem Weathering w/ Water • Feldspar + H 2 + H C2 3aolinite + dissolved ions Chem Weathering w/ Water • Saprolite formation: substance that has undergone extensive chemical weather – Translates to “rotten rock” – Falls apart easily Step 2- Erosion • Requires energy: • Transport process from point A to Point B – Water – Wind – Gravity (like landslides) – Glaciers Step 3- Deposition • Deposition • Basin: any place that can be filled with sediment • Accommodation space: measurement of how much sediment can fit in a basin Deposition continued • Subsidence: lowering of ground level in a basin and sediment sinks downward – Creating extra accommodation room • Adding sediment into a sediment adds a lot of weight – Ccauses sinking, repeat…re room up top, adds more sediment, Deposition • Layers (strata or beds): sediment piles up in layers Step 4- Lithification • Compaction-sediment gets compact as layers are added to basin – fills the gaps between the particles as shown below – Ex: Time 1 stratum of sediment Compaction – added layers help compact the sediment Lithification continued • Cementation: when the constant compaction pushes absorbed water out of the sediment layers and leaves behind little crystals that act as natural cement PT 2- Classification • #1- Detrital (aka *clastic) sediment • Created by physical weathering Traits of Detrital Material • Sorting: how uniform is the grain size – Do they vary or are they close in size? – How long did that erosion continue? • The further it goes, the better sorted it becomes – Long distance = well sorted – Short distance = poorly sorted Moder Well sorted ately Poorly sorted sorted Traits of Detrital Material • Rounding: how long did the erosion go on/how far are they from the parent rock? – Long erosion = well rounded – Short erosion = poorly rounded Poorly rounded Well rounded Class Exercise • Describe the sorting and rounding of each sample Well rounded/sorted Poorly rounded/sorted ID Clastic Sedimentary Rocks • Grain size is key – Pick out the dominate grain size • Ex: sand = sandstone • Ex: silt = siltstone Types of Sediments • #2- Chemical Sediments • Form via chemical reactions – dissolution & re-precipitation – saltwater evaporation Chemical Sedimentary Rocks • More crystal look will be due to chemical processes • Usually comprised of 1 major mineral type – Ex: halite = rock salt – Ex: quartz = chert • Economically viable: high concentration of that 1 mineral so it’s easy to find if you’re looking for the particular mineral without the processing costs Types of Sediments • #3 Biogenic Sediments: • Sediment particles come from living organisms • Ex: shells, partially decayed plants, microscopic organisms Biogenic Sedimentary Rocks • Recognize that it contains shells or other bio material • Chalk: poorly cemented, tiny shell fragments • Limestone: well cemented, tiny shell fragments • Coal: compressed plant material PT 3- Mass Wasting (Landslides) • Why do we care? Slope Destabilization • Angle of Repose-max number of steepness – An average • how steep the slope can get before whatever is added to its top just falls down ~ 35° Slope Destabilization • Lack of moisture: low water content/dry Slope Destabilization • Excessive moisture: too much moisture turns sediment into mud which landslides easily Slope Destabilization • Lack of vegetation: plant roots stabilize slopes and hold sediment in place Slope Destabilization • Excessive vegetation: too much vegetation cause several issues – The roots can make more channels for the water to travel down – Too much mass on a steep slope – Plants absorb the water and make the soil dry See file 5c Types of Mass Wasting/Landslides • Categories based on: – Material: • sediment or mud or rock or snow or ice – type of movement • Rolling/sliding/flowing/falli ng downhilll – Speed • Rapidly or gradually Types of Mass Wasting • Rockslides – Pretty quick but not as fast as other materials bc of the friction between the ground and the rock Slow Mass Wasting • Creep – Slow moving downhill – Fence hints that the sediment is moving Causes of Mass Wasting • Unstable slopes do NOT automatically have a landslide occur Thunderstorms/h eavy rains Earth quakes Clear cutting Human Landscaping Risk Assessment • Risk Assessment Maps – Have to be updated frequently Prevention • Drainage control Prevention • Decrease slope grades Prevention • Building codes – Ex: cant build big houses on steep hill sides • Retaining walls – Concrete barriers to block falling rock from reaching road • Rock bolts – Drive bolt through rock to rock behind it to keep front rocks from flaking/falling off Prevention: Cost • Expensive to build/implement changes, but damage is more costly • Estestimated return is $10-$2,000 per $1 spent on prevention Case Study: Thistle, UT • 1983 slide caused $200 million in damage • Deemed preventable if $0.5 million had been spent on drainage systems Review- Things to Know • How sedimentary rocks form • Types of sediments & sedimentary rks • Things that make slopes unstable • Things that trigger mass wasting • Ways to prevent mass wasting
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