1 / 43

Today’s Lecture:

Today’s Lecture:. Sedimentary structures: Inferring depositional processes from sedimentary rocks Sea-level changes & the facies concept. Chapter 7: Sedimentary Rocks. Sedimentary structures: Features observed within a single bed.

shawna
Download Presentation

Today’s Lecture:

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Today’s Lecture: • Sedimentary structures: • Inferring depositional processes from sedimentary rocks • Sea-level changes & the facies concept Chapter 7: Sedimentary Rocks

  2. Sedimentary structures: Features observed within a single bed. Within sedimentary beds, distinctive structures can usually be seen. These include systematic variations in grain size and sorting, internal bedding features, etc. that are diagnostic of particular depositional processes.

  3. Sedimentary Structures Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1

  4. Fig. 7.26a Stephen Marshak

  5. Sedimentary Structures Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1 Bottoms of beds: Coarser

  6. Sedimentary Structures Tops of beds: Finer Graded beds: Show a gradual change in particle size as you move from the bottom of a bed to the top. Bed 2 Bed 1

  7. Graded Bedding Finer The bed to the right shows a change from large grains at the bottom, to small at the top. This is called “normal” grading. Coarser

  8. Graded Bedding Lower Velocity As transport velocity declines, coarser particles settle out first (see video on turbidity currents). Higher Velocity

  9. Graded Bedding Lower Velocity Thus, graded beds tell us how flow velocity changed during deposition! Higher Velocity

  10. Sedimentary structures: Cross-Bedding Cross-bedding is internal bedding that is tilted at an angle to the primary bedding. Cross beds are formed by a scour and fill transport process involving either wind or water (see ripple movie).

  11. First we need to distinguish between primary beddingvs. internal layering.

  12. Primary bedding vs. internal layering Primary Bed Contacts between sedimentary beds

  13. Primary bedding vs. internal layering Internal, inclined layers Contacts between sedimentary beds

  14. Primary bedding vs. internal layering Internal inclined layers Contacts between sedimentary beds

  15. More cross-bedding Bed contacts”

  16. Fig. 7.25abc W. W. Norton

  17. More cross-bedding Cross beds Bed contacts”

  18. Cross-bedding Transport direction Tilt-direction of cross beds indicates the direction of transport (e.g., wind direction or direction of water flow).

  19. Large-scale cross-beds like these are formed by sand dune migration

  20. What a geologist sees. Which way did the wind blow?

  21. What a geologist sees. Paleowinds

  22. Sedimentary Structures: Ripple Marks Ripple marks form when moving wind or water causes sedimentary grains to “hop” along the bottom.

  23. Fig. 7.27a Stephen Marshak Ripple marks can be either symmetrical (formed by waves sloshing back and forth), or symmetrical (formed by water or wind flowing in one direction).

  24. Transport and Deposition in Running Water Ripple formation movie

  25. Sedimentary Structures: Ripple Marks Look closely at the ripples on this surface. Are they symmetrical, or asymmetrical? Which way did the water flow?

  26. Sedimentary Structures: Ripple Marks Look closely at the ripples on this surface. Are they symmetrical, or asymmetrical? Which way did the water flow? Paleocurrent direction

  27. Sedimentary Structures: Ripple Marks Study these ripple carefully. Are they symmetrical or asymmetrical? What do they suggest about paleocurrent direction? Look at these!

  28. Sedimentary structures: Ripple Marks Oscillation ripples (back and forth) Interpretation: Paleoshoreline

  29. Sedimentary Structures: Mud Cracks

  30. Ancient mud cracks (cross-sectional view) Fig. 7.27d Stephen Marhsak

  31. Sedimentary Structures: Mud Cracks Mud cracks form when mud-covered shorelines or lake bottoms, dry up. This produces an irregularly- cracked surface. Margin of a dry lake with mud cracks. Note ripple-marked sand dunes at top of picture.

  32. Sedimentary Structures Note ripple-marked sand dunes at top of picture.

  33. Sedimentary structures: Mud Cracks

  34. Say you find mud cracks in an ancient sedimentary rock. What does that suggest about the environment where the rock formed? Fig. 7.27c Stephen Marhsak

  35. What a geologist sees.

  36. Describe what you see in this outcrop and interpret the geologic history and conditions of deposition. In class exercise:

  37. Contains “clasts” of older rocks. Igneous clast Metamorphic clast

  38. Fine layering Wavy basal contact Ripple cross- bedding Lower contacts cut into units underneath. Erosional! Wavy basal contact

  39. Bed shows size grading. Finer top Single bed Coarser base

  40. Uplift of a deep-seated igneous pluton (granitic), with subsequent erosion by running water which transported igneous and metamorphic clasts to a river which then carried them some distance from the source area, to a site of deposition (stream channel). Transport by running is inferred by the rounding of the clasts, size grading, and sorting. Erosional contacts at the bases of beds indicate initial turbulent transport, followed by declining flow velocity (flood event?). What a geologist sees:

  41. Sedimentary environments Concept of Sedimentary “Facies” • “Facies” are representations of sedimentary environments defined by the overall association of features preserved in rocks.

More Related