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Geology 3120 - Sedimentary Structures. Outline Review the geologic history exercise from last time Contacts, primary structures, and secondary structures.

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Presentation on theme: "Geology 3120 - Sedimentary Structures. Outline Review the geologic history exercise from last time Contacts, primary structures, and secondary structures."— Presentation transcript:

1 Geology Sedimentary Structures

2 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

3 Block model for exercise

4 Geologic History 12 Ma dike 12 Ma dike 15 Ma dike 15 Ma dike Normal fault Normal fault 20 Ma sed 20 Ma sed 22 Ma sed 22 Ma sed Erosion Erosion Thrust fault Thrust fault Folding Folding Layer B Layer B 60 Ma sed 60 Ma sed Layer G Layer G 70 Ma sed 70 Ma sed 80 Ma sed 80 Ma sed

5 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

6 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

7 Whic h way is up? Today Option 1 Overturned syncline Option 2 “up side down” overturned anticline

8 Cross-bedding Checkerboard Mesa, Zion National Park Concave up Flow direction Y Older Younger

9 Graded bedding Y Older Younger Decrease in depositional energy with sedimentation Example: flood deposits, turbidity currents

10 Gradedbedding Y Older Younger Decrease in depositional energy with sedimentation Example: flood deposits, turbidity currents

11 Reverse (Inverse) Graded Bedding Y Older Younger Increase in depositional energy with sedimentation Example: debris flows (a lot less common than normal graded beds)

12 Ripple marks Symmetric ripples indicate bi-modal current Concave = up Asymmetric ripples indicate unidirectional current Y

13 Mud cracks 5 cm Mud cracks Desiccation of muddy sediments

14 Raindrops Y Limited to terrestrial sediments

15 Footprints Y

16 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

17 Geopetal Structures Y A “natural” carpenter’s level MatrixShell or cavity in the rock Infill material (I.e., calcite)

18 Bioturbation Y Habitation burrows Feeding burrows Movement

19 Stromatolites Sharks Bay, Australia Cyanobacteria grow upward toward the surface Y

20

21 Flame structures Less dense material intrudes into material above Y Caused by rapid loading of turbidite sands

22 Pillow lava Y Upper curved surface “V “ notch

23

24 References Slide 8 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., Slide 18 Slide 20


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