1. Sedimentary Rocks

2. What is a sedimentary rock?
Sedimentary rocks result from mechanical and chemical weathering
Comprise ~ 5% of Earth’s upper crust
About 75% of exposed rocks
Contain evidence of past environments
Record how sediment is transported
Often contain fossils

3. What is the economic importance of sedimentary rocks?
They are important for economic reasons because they contain
Coal
Petroleum and natural gas
Iron, aluminum, uranium and manganese
Geologists use them to read Earth’s history
Remember this when we talk about correlation. Note how beds pinch out or are offset by faults

4. How does sediment become rock?
Diagenesis – chemical and physical changes that take place after sediments are deposited
Diagenesis varies with composition
Chesapeake Bay from Skylab
False color image

5. Turning sediment into rock
Diagenesis includes:
Recrystallization – growth of stable minerals from less stable ones
Lithification – loose sediment is transformed into solid rock by compaction and cementation
Natural cements: calcite, silica, and iron oxide. Formed from ions in solution in water.

6. Lithification
Compaction: As more sediments are piled on top, compaction drives out the excess water.
Cementation: Precipitation of chemicals dissolved in water binds grains of a sediment together.
Remember where the dissolved chemicals come from?

7. Types of sedimentary rocks
Chemical rocks – sediment from ions that were once in solution
Detrital rocks –sediment transported as solid particles

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

9. Sediment grains moved to ocean by streams
Ions weather out of rock,
are transported by
groundwater to sediment
layers below
6_11
Sediment grains moved to ocean by streams
Ions transported
to lake or
ocean
Water enters pore spaces between sediment grains
Ion-rich
ground-
water
Dissolved ions precipitate
to form cement between
sediment grains

10. Detrital sedimentary rocks
Mudrocks: less than .06 mm
1. Mud: small particles easily kept in suspension
Settles in quiet water
Includes Shale: mud-sized particles <.004 mm deposited in thin bedding layers called laminae
Most common sedimentary rock
2. Larger mudrock grains called silts
silt-sized particles mm
Gritty grains can be felt

11. Detrital sedimentary rocks
Sandstone
Made of sand-sized particles .064 – 2 mm
Forms in a variety of environments
Sorting, angularity and composition of grains can be used to interpret the rock’s history
Quartz is the predominant mineral (due to its durable nature)

12. Photomicrograph of quartz rich sandstone (Arenite)
Plagioclase grain
Classifying Sandstones
Photomicrograph of quartz rich sandstone (Arenite)
Grains subangular to subrounded, sandstone is poorly sorted
Making thin sections

13. Types of Sandstone Quartz Arenite >90% quartz grains
Beach and dune deposits
Arkoses >25% feldspar, angular, poor sort.
Transform boundaries; exposed granites
Any felsic rock eroded, not transported far
Graywackes Quartz, feldspar, volcanics
Port sorted, angular
Erosion of Island Arcs
Rift Valley Sediments

14. Detrital sedimentary rocks
Conglomerate and breccia
Both composed of particles > 2mm in diameter
Conglomerate consists largely of rounded clasts. Rounded pebbles in high velocity areas
Breccia is composed of large angular particles Breccia is made of shattered rock that accumulates at the base of a cliff

15. Energy
Coarse sediments are deposited in high energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams.
Fine sediments are deposited in low energy environments, e.g. the slow water of deep lagoons, the abyssal plain, etc.

16. Conglomerates are fast-water sediments
Outcrop of conglomerate
with cobble-sized clasts
interbedded with sandstone
Conglomerates are fast-water sediments
“High Energy”
K.E. = 1/2mv2
In fast water, smaller sizes swept away

17. Chemical sedimentary rocks
Precipitated material once in solution
Precipitation of material occurs two ways:
Inorganic processes: the minerals precipitate out of water
Organic processes: animals and plants precipitate the minerals to use as shells or skeletons

18. Common chemical sedimentary rocks Limestone
Most abundant chemical rock
Made of the mineral calcite CaCO3
Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)
Inorganic limestones include travertine (caves) and oolitic limestone (Bahamas)

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

20. Chalk Outcrops in SE USA
Hand Specimen

21. Inorganic Chemical Sediments
Ooids under microscope
Oolitic Limestone - Bahama Shoals
Oolitic Limestone - Hand Specimen

22. Other chemical sedimentary rocks
Chert
Made of microcrystalline quartz
Usually deposited as siliceous ooze in deep oceans (can be diatomaceous)
chert
Diatomaceous chert

23. Other chemical sedimentary rocks
Dolostone (made of mineral Dolomite)
Like Calcite, but some Ca is replaced by Mg
The Dolomites, sediments thrust up when the Alps formed

24. How to make Dolomite Evaporation Limestone Seawater enriched in Mg2
6_19
How to make Dolomite
Limestone
Seawater enriched
in Mg2
Mg2 -rich seawater circulates
through porous limestone
Dolostone
Mg2 replaces some of the
Ca2 in limestone

25. Chemical Sediments: Coal

26. Sedimentary environment determines roundness sorting, mineral diversity
6_5
Character of detrital sediments depends on time, distance, and energy. For example, in streams:
Particles are large and
irregular, and consist of
a variety of lithologies,
including the least
resistant.
Particles are mid-sized
and of intermediate
sphericity, and include
resistant and nonresistant
lithologies.
Particles are small and
nearly spherical, and
consist mainly of the
most resistant lithologies,
such as quartz.
HIGHLANDS
LOWLANDS
NEAR-COASTAL

27. 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

28. Some Facies in an oversimplified drawing
Abyssal Ooze
Nearshore sands
Stillwater muds

29. A picture glossary of sedimentary environments
Turbidite: underwater landslide
= graded bedding
A picture glossary of sedimentary environments

30. Sedimentary structures
Tell us something about past environments
Types of sedimentary structures
Strata, or beds (most characteristic of sedimentary rocks)
-bedding planes that separate strata caused by variation in deposition

31. Strata- Bedding Planes

32. Fine Scale Bedding- Lamellae

33. Floods change the local conditions
Graded bedding
Floods change the local conditions
6_6
Fine-grained sediment
On floodplain
Older sediment 1
Pre-flood
Bounders on bottom, sands and muds suspended
Flood water
Erosion of uppermost
fine-grained sediment 2
Flood stage
Waning flow
Fine-grained above
Coarse-grained
below
Bedding plane 3
Post-flood

34. Graded Beds – grains fine upward
Note: Beds were tilted from horizontal after deposition

35. Recognizable Sedimentary Structures
Ripples
Irregularities in bottom sediment lead to ripples
Asymmetric types indicate flow direction.
Symmetric types formed in tidal areas

36. Slabs of eroding sandstone with ripple marks

37. Cross Beds are ripples in cross section
Irregularities lead to ripples, dunes, sand bars.
In cross section these look like lines at an angle to the horizontal – “cross beds”
Ripples can indicate direction of air or water flow if asymmetrical, a tidal environment if symmetrical. Size and shape indicate fluid velocity.

38. Cross bedding in Sand Dune deposits
Navaho Sandstone
Sandstone deposited
in ancient sand dunes
Frosted Grains, well sorted

39. Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up

40. Sedimentary Environments
Sediments are formed in many different environments
Each have characteristic appearance today, features that allow them to be recognized in the geologic record

41. Fresh Water Facies
Streams (includes big Rivers), with floodplains and levees, called fluviatile. The Point Bar Sequence is typical for meandering streams. Cutoffs generate Oxbow deposits.
High gradient streams with high sediment load are Braided.
Lake deposits called lacustrine, generally still waters, often varved deposits if winters cold

42. Deposits Associated with Meandering Streams
Point-bar Sequence:
Deposits Associated with Meandering Streams
Meandering
Stream
OxBow
Floodplain

43. Point Bar Sequence Fines of Floodplain Erosion Crossbeds of Bar
Gravel of bed
Erosion

44. Terms for Marine (i.e. Ocean) Environments
and some characteristic sediment facies
6_27
Continental
slope
Muds
Continental
shelf
Sands
Shallow
marine
Abyssal
Plain
Turbidite Graded Beds
Ooze
Deep marine
Submarine
volcanoes

45. Comparison of sediments deposited
Facies changes due to rising sea level - water getting deeper everywhere
6_29
River
Direction of migration
of shoreline, and landward
shift of sedimentary facies
Shoreline at
Time B
time B
Sea level
Time A
rising
Shoreline at
time A
Deep
marine
Shallow
marine
Deposited
Deep
Beach
marine
at time A
Shallow
Deposited
marine
at time B
Shallow
River
REMEMBER: the facies follow the shoreline
Beach
marine
Comparison of sediments deposited

46. Fossils are traces of prehistoric life generally preserved in sedimentary rock

47. Dinosaur footprint in mudstone

48. End of Sedimentary Rocks