Unveiling Sedimentary Rocks: Formation, Classification, and Importance

2. What is a sedimentary rock? • Sedimentary rocks result from mechanical and chemical weathering • Comprise ~ 5% of Earth’s upper crust • Contain evidence of past environments • Record how sediment is transported • Often contain fossils

3. Fossil Fish - 50 Million Year Old Lakes in southern Wyoming Sediment = fine-grained mudstone

4. Which of the following would retain the most detailed impressions of fossilized organisms? • very fine-grained volcanic ash deposited in an ancient lake • an ancient, rainforest lateritic soil buried by basalt lava flows • quartz-rich sand deposited in shallow, beachfront environment • gravel and sand stream deposits laid down during flash floods

5. Which of the following would retain the most detailed impressions of fossilized organisms? • very fine-grained volcanic ash deposited in an ancient lake • an ancient, rainforest lateritic soil buried by basalt lava flows • quartz-rich sand deposited in shallow, beachfront environment • gravel and sand stream deposits laid down during flash floods

6. Why do we care about sedimentary rocks? • They are important for economic reasons because they contain • Coal • Petroleum and natural gas • Iron, aluminum, uranium and manganese • We use them to read Earth’s history

7. Changes occur in sediment after it is deposited • Diagenesis – chemical and physical changes that take place • after sediments are deposited • Diagenesis varies along Front Range • (feldspar cement makes Flatirons erode slowly and form cliffs, same rocks north and south, but no cliffs) Turning sediment into rock

8. Turning sediment into rock • Diagenesis • Recrystallization – growth of stable minerals from less stable ones • Lithification – loose sediments is transformed into solid rock by compaction and cementation • Natural cements include calcite, silica, and iron oxide

9. Quartz (new crystal) Quartz (sedimentary grain) Feldspar (altered)

10. How do you know some quartz grains are new while others are original sedimentary grains Grain Shape Grain Size Grain Orientation All of the above

11. Types of sedimentary rocks • Detrital rocks – transported sediment as solid particles • Chemical rocks – sediment that was once in solution

12. Detrital sedimentary rocks • Constituents of detrital rocks can include • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks

13. Detrital sedimentary rocks • Common detrital sedimentary rocks (in order of increasing particle size) • Shale • Mud-sized particles deposited in thin layers called laminae • Most common sedimentary rock

14. Shale containing plant fossils

15. Detrital sedimentary rocks • Sandstone • Made of sand-sized particles • Forms in a variety of environments • Sorting and composition of grains can be used to interpret the rock’s history • Quartz is the predominant mineral (due to its durable nature)

16. Closeup view of sandstone (note large number of qtz grains)

17. Photomicrograph of quartz rich sandstone

18. Detrital sedimentary rocks • Conglomerate and breccia • Both composed of particles > 2mm in diameter • Conglomerate consists largely of rounded clasts • Breccia is composed of large angular particles

19. Conglomerate

20. Outcrop of conglomerate with boulder sized clasts

21. Outcrop of conglomerate with cobble-sized clasts interbedded with sandstone

22. Breccia (note this is not a particularly common sedimentary rock) Note sharp angular clasts

23. Which of the following sedimentary rocks would you expect to have originally been deposited by fast-moving streams? • mudstone • limestone • breccia • conglomerate

24. Which of the following sedimentary rocks would you expect to have originally been deposited by fast-moving streams? • mudstone • limestone • breccia • conglomerate

25. Why are clasts in conglomerate rounded? • Chemical weathering rounds them before they are transported • Chemical weathering rounds them after they are transported and deposited • Mechanical abrasion caused while the clasts are being transported rounds them • All of the above

26. Why are clasts in conglomerate rounded? • Chemical weathering rounds them before they are transported • Chemical weathering rounds them after they are transported and deposited • Mechanical abrasion caused while the clasts are being transported rounds them • All of the above

27. Chemical sedimentary rocks • Precipitated material once in solution • Precipitation of material occurs two ways: • Inorganic processes • Organic processes (biochemical origin)

28. Chemical sedimentary rocks • Common chemical sedimentary rocks • Limestone • Most abundant chemical rock • Made of the mineral calcite • Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) • Inorganic limestones include travertine and oolitic limestone

29. Coquina

30. Note shells and lime matrix Fossiliferous limestone

31. Chalk Outcrops in SE USA Chalk Hand Specimen

32. Ooids under microscope Oolitic Limestone - Bahama Shoals Oolitic Limestone - Hand Specimen

33. 07.03 Oolitic limestone is most likely to form in what type of depositional environment? • quiet, muddy, lagoons and bays • shallow, clear, marine waters with vigorous current activity • deep, marine waters below most wave action • acidic, organic-rich waters in freshwater swamps and bogs

34. 07.03 Oolitic limestone is most likely to form in what type of depositional environment? • quiet, muddy, lagoons and bays • shallow, clear, marine waters with vigorous current activity • deep, marine waters below most wave action • acidic, organic-rich waters in freshwater swamps and bogs

35. Travertine forming in Hot Spring Travertine Hand Specimen

36. Chemical sedimentary rocks • Common chemical sedimentary rocks • Dolostone • Typically formed secondarily from limestone • Common in ancient rocks, rare today • Chert • Made of microcrystalline quartz • Usually deposited as siliceous ooze in deep oceans (can be diatomaceous) chert Diatomaceous chert

37. Chemical sedimentary rocks • Common chemical sedimentary rocks • Evaporites • Evaporation triggers deposition of chemical precipitates • Examples include rock salt and rock gypsum

38. Chemical sedimentary rocks • Common chemical sedimentary rocks • Coal • Different from other rocks because it is composed of organic material • Stages in coal formation (in order) • 1. Plant material • 2. Peat • 3. Lignite • 4. Bituminous

39. Successive stages in coal formation

44. If sediment keepsaccumulating in aplace like this ----->it will ultimately turn into:a. Sandstoneb. Conglomeratec. Limestoned. Coal and/or shale

45. If sediment keepsaccumulating in aplace like this ----->it will ultimately turn into:a. Sandstoneb. Conglomeratec. Limestoned. Coal and/or shale

46. Sedimentary environments • Geographic setting where sediment accumulates • Determines nature of sediment that accumulates (grain size, shape, etc.)

47. Sedimentary environments • Types of sedimentary environments • Continental • Dominated by erosion and deposition associated with streams • Glacial • Wind (eolian) • Marine • Shallow (to about 200 meters) • Deep (seaward of continental shelves)

48. Sedimentary environments • Types of sedimentary environments • Transitional (shoreline) • Tidal flats • Lagoons • Deltas

49. Continental (left) and marine (right) depositional environments

50. Sedimentary environments • Sedimentary facies • Different sediments accumulate next to each other at same time • Each unit (called a facies) possesses a distinctive characteristics reflecting the conditions in a particular environment • The merging of adjacent facies tends to be a gradual transition