This document discusses sedimentary rocks and how they provide clues about past environments. Sedimentary rocks form from the compaction and cementation of sediments like weathered minerals, chemical precipitates, and organic remains. Key clues used to interpret depositional environments include sediment size and shape, mineral composition, sedimentary structures like ripples and cross-bedding, fossils, color, geometry of rock units, and cyclical sequences indicating sea level changes. Together these clues can be used to map facies and reconstruct prehistoric landscapes through the principles of uniformitarianism and lateral continuity.

1. CH. 6 – SEDIMENTARY RX

2. Sediment:
- weathering products (gravel,
sand, clay minerals)
- chemical precipitates
- organic remains

3. Sedimentary Rocks
“sedimentum” = “settling”
Sediment settles out of wind or
water
- forms layers at the surface

4. Importance of Sedimentary Rx
1) Form 75% of exposed rx at
surface (outcrops)
- compose only 5% of total crust
2) Contain clues to reconstruct
past environments on Earth

5. Importance of Sedimentary Rx
3) Fossil record
4) Economic value
- coal, petroleum, construction

6. Sedimentary Rx Classification
1) Detrital = weathering products
(mechanical or chemical)
Named based on particle size

7. Detrital Sedimentary Rocks
Particle Size
Gravel
Name
Breccia
(angular)
Conglomerate
(rounded)

8. Detrital Sedimentary Rocks
Particle Size
Sand
Name
Quartz Sandstone
(quartz sand)
Arkose (K-spar)

9. Detrital Sedimentary Rocks
Particle Size
Sand
Name
Graywacke
(rock frags,
mafic minerals)

10. Detrital Sedimentary Rocks
Particle Size
Clay-size
Name
*Shale
(clay minerals)
*Shale is the most abundant
detrital sedimentary rock

11. Detrital Rock Process
1) Weathering
2) Erosion (transportation)
3) Deposition (sediment settles)
4) Lithification – turning sediment
into rock (“lithos” = rock)
(after sediment is buried)

12. Types of Lithification Processes
a) Compaction
- most effective on fine-grained rx
(ex: shale)
Exception: St. Peters Sandstone
Starved Rock State Park, IL

13. Lithification Processes
b) Cementation
Minerals dissolved in groundwater
precipitate around sediment
Ex: silica, calcite, hematite

14. Sedimentary Rx Classification
2) Chemical – named based on
composition
Ex: calcite = limestone
halite = rock salt
plant remains = coal

15. Chemical Sedimentary Rocks
a) Inorganic – formed by chemical
reactions in environment
Ex: evaporites – rock gypsum,
rock salt
Ex: Travertine (limestone)

16. Inorganic Chemical Sed. Rx
Calcite:
*Limestone = mostly marine origin
Travertine = speleothems
Oolitic Limestone = tidal flats
* Most abundant chemical
sedimentary rock

17. Inorganic Chemical Sed. Rx
Silica (microcrystalline
quartz):
Varieties:
chert
agate

18. Silica (microcrystalline quartz):
Varieties:
Jasper
Petrified Wood

19. Chemical Sedimentary Rx
b) Organic (“biochemical”)
Plant remains = coal
Microscopic sea shells = chalk
Broken sea shells = coquina
limestone

20. Detective Analogy
Geologists are solving a mystery
Scene of
the Crime
Clues
Mystery
Solved

21. Mystery Solved (Sed. Rx)
Depositional environment
- any area on surface where
sediment accumulates
Ex:

22. Principle of Uniformitarianism
“Present is the key to the past”
Study modern depositional
environments for clues
Ex: coquina limestone
Ex: ripple marks

23. Sedimentary Facies
Cumulative physical
characteristics of a sedimentary
rock which indicates its
depositional environment
“Clues” in detective analogy

24. Principle of Lateral Continuity
Nicolaus Steno (1669)
Sediment is accumulated to edge
of its depositional environment

25. Sedimentary Facies
Note: At the same time, each
environment (facies) is
accumulating its own sediment
and characteristics!

26. Sedimentary Facies

27. Clues
1) Sediment size
Distance deposited from source
area
Large = (closer to, further from)
Small = (closer to, further from)

28. Clues (sediment size)
High vs. low energy environment
Large particles = (high, low)
Ex: ___________
Small particles = (high, low)
Ex: ___________

29. Clues
2) Sediment shape
Distance deposited from source
area: close to or far from?
angular = ______
rounded = ______

30. Clues
3) Composition – minerals present
Two most common minerals found
in sedimentary rocks are _____
and ______ .

31. Variety of minerals:
Ex: quartz, K-spar, plagioclase
feldspar, mafic minerals, rock
fragments
- very little chemical weathering
- indicates rapid deposition close
to source area
Ex: arkose in alluvial fan

32. One mineral composition:
Well-sorted
- sediment traveled far from
source area
Ex: Quartz sandstone, shale

33. Clues
4) Sedimentary Structures
- features produced in sediment
BEFORE it’s lithified

34. Sedimentary Structures
a) Bedding – layering w/in rock
Bedding plane – flat surfaces
along which rx tend to break or
separate
- separates different episodes of
deposition

35. Bedding
i) Laminar bedding – horizontal
layers
- usually deposited in calm energy
environment
Ex: shale, some sandstones

36. Bedding
ii) Graded bedding – sorted by
size
- occurs w/sudden decrease in
velocity
Ex: turbidity currents, alluvial fans

37. Bedding
iii) Cross-bedding (Figure 6.22)
- layers are angled in sets
- occurs with change of current
direction (either wind or water)

38. Sedimentary Structures
b) Ripple marks
- wave action moves sediment
- shallow water

39. Types of Ripple Marks
i) Asymmetrical - also called “current ripple
marks”
- used to determine paleocurrent directions

40. Types of Ripple Marks
ii) Oscillation (symmetrical)
- wave action is back and forth
- usually indicates shallow lagoon
environment

41. Sedimentary Structures
c) Mudcracks
Environmental conditions:
a)
b)
Ex:

42. Sedimentary Structures
d) Trace Fossils
Any evidence an organism was in
the environment but no physical
remains of the organism exist

43. Trace Fossils
Examples:
i) Footprints
ii) Coprolites
iii) Bioturbation

44. Clues
5) Fossils – evidence of
prehistoric life
Fossil record is incomplete
- shows remarkable pattern of
change from simple to complex
life forms

45. Steps to becoming a fossil
Step 1: Death
Step 2: Hard parts – bones, teeth,
shells (Who you are!)
Step 3: Rapid burial (Where you
die!)
Step 4: Time (prehistoric)

46. Fossils
Fossil record biased towards
marine environment
- abundant life in oceans
- many critters have hard parts
- lots of sediment for quick burial

47. Fossils
Good evidence for reconstructing
past environments
Ex: Beach sandstone vs. desert
sandstone

48. Clues
6) Color
Black or green = unoxidized iron
- lack of oxygen in environment
Ex: deep ocean, deep lake,
swamp

49. Color
Red = oxidized iron
Ex: river floodplain, tidal flats,
desert deposits
Green/purple = volcanic ash
mixed in with sediment

50. Clues
7) Geometry of rock unit
- shape & thickness of rock unit
Ex: beach sandstone vs. desert
sandstone

51. Depositional Environments
Handout

52. Lithofacies map
For a particular moment in
geologic time, rocks indicate the
landscape
1) Map out the areas where each
rock type is found & draw
boundaries

53. Lithofacies map
2) Look at the lithologies (rock
types) & compare them to:
- nearby rx
- geometry of rock unit
3) Determine the original
depositional environment

54. Cyclothems
Cyclical alternations of lithologies
deposited as shorelines shift

55. Sedimentary Facies

56. Stratigraphic Sequences
1) Transgressions - sea level rises
Strat column changes vertically
from coarser sediment to finer
sediment
“Fining-upward” sequence

57. Stratigraphic Sequences
2) Regressions – sea level drops
Strat column changes vertically
from finer sediment to coarser
sediment
“Coarsening-upward” sequence

58. Color
Red = oxidized iron
Ex: river floodplain, tidal flats,
desert deposits
Green/purple = volcanic ash
mixed in with sediment