3120lect 5

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Geology 3120 - Sedimentary Structures Outline • Review the geologic history exercise from last time • Contacts, primary structures, and secondary structures • How to determine which “way is up”… • Cross-bedding, graded-bedding, reverse graded-bedding • Determining way up using top surface features - ripples, mudcracks, raindrops, footprints • Determining way up using bottom surface features - load casts • Determining way up using features within rocks geopetal, bioturbation, stromatolites, flame structures, pillow lavas Block model for exercise 15 Ma Dike 12 Ma Dike Normal Fault 22Ma 20 Ma Fold 60 Ma Layer G Fold 50 Ma 70 Ma 70 Ma Layer B 80 Ma 80 Ma 70 Ma Thrust Fault 60 Ma 70 Ma Geologic History 15 Ma Dike 12 Ma Dike • 12 Ma dike • 15 Ma dike • Normal Fault 22Ma 20 Ma Fold 60 Ma Layer G Fold 50 Ma 70 Ma 70 Ma • 20 Ma sed • 22 Ma sed • Erosion • Thrust fault • Folding • Layer B • 60 Ma sed • Layer G • 70 Ma sed • 80 Ma sed Layer B 80 Ma 80 Ma 70 Ma Thrust Fault Normal fault 60 Ma 70 Ma Contacts, Primary Structures & Secondary Structures • Contact - a boundary between rock units • Primary structure - structures that form during lithification • Secondary structure - structures that form after lithificaiton Contacts, Primary Structures & Secondary Structures • Contact - a boundary between rock units • Primary structure - structures that form during lithification • Secondary structure - structures that form after lithificaiton Whic h way is up? Today Option Overturned syncline Option “up side down” overturned anticline Cross-bedding Checkerboard Mesa, Zion National Park Concave up Younger Y Flow direction Older Graded bedding Younger Y Older • Decrease in depositional energy with sedimentation • Example: flood deposits, turbidity currents Graded bedding Younger Y Older • Decrease in depositional energy with sedimentation • Example: flood deposits, turbidity currents Reverse (Inverse) Graded Bedding Younger Y Older • Increase in depositional energy with sedimentation • Example: debris flows (a lot less common than normal graded beds) Ripple marks • Symmetric ripples indicate bi-modal current •Concave Y • Asymmetric ripples indicate unidirectional current = up Mud • • Desiccation of cracksmuddy sediments Mud cracks cm Raindrops Y • Limited to terrestrial sediments Footprints Y • Limited to terrestrial sediments Load casts • Protrusion of material into a layer below • Load casts indicate the base of a layer, not the top of a layer • Determining the current direction may be possible Geopetal Structures • A “natural” carpenter’s level Shell or cavity in the rock Matrix Y Infill material (I.e., calcite) Bioturbation • Habitation burrows • Feeding burrows • Movement Y Stromatolites Y • Sharks Bay, Australia • Cyanobacteria grow upward toward the surface Flame structures Y • Less dense material intrudes into material above • Caused by rapid loading of turbidite sands Pillow lava Upper curved surface Y “V “ notch References Slide http://www.utahpictures.com/Checkerboard.html Slides 9-15, 17 Busch, R M and D Tasa, Laboratory Manual in Physical Geology, 3rd Ed., American Geological Institute and National Association of Geology Teachers, 260 p., 1990 Slide 18 http://www.discoverwest.com.au/hablin.html Slide 20 http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html [...]... normal graded beds) Ripple marks • Symmetric ripples indicate bi-modal current •Concave Y • Asymmetric ripples indicate unidirectional current = up Mud • • Desiccation of cracksmuddy sediments Mud cracks 5 cm Raindrops Y • Limited to terrestrial sediments Footprints Y • Limited to terrestrial sediments Load casts • Protrusion of material into a layer below • Load casts indicate the base of a layer, not... material intrudes into material above • Caused by rapid loading of turbidite sands Pillow lava Upper curved surface Y “V “ notch References Slide 8 http://www.utahpictures.com/Checkerboard.html Slides 9- 15, 17 Busch, R M and D Tasa, Laboratory Manual in Physical Geology, 3rd Ed., American Geological Institute and National Association of Geology Teachers, 260 p., 1990 Slide 18 http://www.discoverwest.com.au/hablin.html
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