1. Sedimentary Rocks

2. Terminology
Sedimentary Rock - a rock formed through the deposition of sediments from weathering or biologic activity.
Diagenesis- Refers to all the chemical, physical, and biological changes that occur after sediment is deposited and before and after lithification
Lithification - The process by which unconsolidated sediments are transformed into sedimentary rocks.

3. Lithification:
Two Processes control Lithification: Compaction and Cementation
Compaction- As sediment accumulates the weight of the overlying material compresses the lower levels.
This leads to a reduction in pore space (Imagine a garbage compressor)
The size and shape of the grains that make up the sediment will directly determine the amount of compaction that occurs.
Sands and other course-grained sediment are less compressible than fine grained sediments such as clay.

4. Cementation
Cementation- Refers to the glue (cement) that holds the sediments together)
For the most part, cementing materials are the by product of chemical weathering (They are the dissolved components of other rocks).
Important Cementing agents:
Calcite (CaCO3)
Silica (SiO4)
Iron Oxide (Fe2O3, FeO)
These materials precipitate out of solution and become deposited within the pore spaces between the sediment grains

5. Types of Sedimentary Rocks
Detrital- Accumulated material that is transported as solid particles that originated from chemical or mechanical weathering.
Chemical- Formed as previously dissolved minerals precipitate out of solution.

6. Identification of Sedimentary Rocks
Detrital sedimentary rocks are identified by both their composition and their grain size
Chemical Sedimentary rocks are classified by their chemical composition

7. Sedimentary Environments
Sedimentary rocks form in many different depositional environments ie. beaches, oceans, mountains
Each area depositing a similar material is considered a sedimentary facies.
As we move from the Continental Environment to the Marine Environment there will be a gradual transition between the facies due to the matter of transport and other factors affecting deposition.

8. Grain Size vs. Sedimentary Environment
Detrital grain size gives some indication of the energy conditions during transport and deposition
High-energy processes such as swift-flowing streams and waves are needed to transport large grains such as gravel
Conglomerate (composed mostly of gravel) must have been deposited in areas of high energy
Sand transport also requires vigorous currents
Silt and clay are transported by weak currents and and therefore only accumulate under low-energy conditions such as in lakes and lagoons

9. Examples of Sed. Facies

10. Continental Environments
Dominated by erosion and deposition associated with streams
Channel Deposits
Alluvial Fans
Flood Plains
In colder areas glacial movement takes the place of rivers and streams
Morrains
In more arid areas wind is the more dominant factor
Dunes
Playa Lakes

11. Transitional Environments (the shore line)
Beaches
Tidal flats
Lagoons
Spits, bars and barrier islands

12. Marine Divided according to depth Shallow Marine-
reaches to depths of about 200 meters (~700ft)
Extends from the shore to the outer edge of the continental shelf
In some areas can reach up to 1500km, however, average is about 80 km
Types of sediment deposited depend on several factors
Distance from shore
Elevation of adjacent land
Water depth
Water temperature
Climate

13. Deep Marine –
Seaward of the continental shelf and reaches depth greater than 200 m
Deposition in these environments are composed of tiny particles that drift in the ocean current as well as skeletal remains of microscopic animals such as diatoms.

14. Sorting and Rounding
Sorting and rounding are two textural features of detrital sedimentary rocks that aid in determining depositional processes
Sorting refers to the variation in size of particles making up sediment or sedimentary rocks
It results from processes that selectively transport and deposit sediments of particular sizes

15. Sorting
If the size range is not very great, the sediment or rock is well sorted
If they have a wide range of sizes, they are poorly sorted
For example
Wind has a limited ability to transport sediment so dune sand tends to be well sorted
Glaciers can carry any sized particles, because of their transport power, so glacier deposits are poorly sorted

16. Rounding
Rounding is the degree to which detrital particles have their sharp corners and edges warn away by abrasion
Gravel in transport is rounded very quickly as the particles collide with one another
Sand becomes rounded with considerably more transport

17. Rounding and Sorting A deposit of well rounded and well sorted gravel
Angular, poorly sorted gravel

18. Sedimentary Structures
Sedimentary structures are visible features that formed at the time of deposition or shortly thereafter
Represent manifestations of the physical and biological processes that operated in depositional environments
Structures seen in present-day environments or produced in experiments help provide information about depositional environments of rocks with similar structures

19. Bedding Sedimentary rocks generally have bedding or stratification
Individual layers less than 1 cm thick are laminations
common in mudrocks
Beds are thicker than 1 cm
common in rocks with coarser grains

20. Graded Bedding
Some beds show an upward gradual decrease in grain size, known as graded bedding
Graded bedding is common in turbidity current deposits
form when sediment-water mixtures flow along the seafloor
As they slow, the largest particles settle out then smaller ones

21. Cross-Bedding
Cross-bedding forms when layers come to rest at an angle to the surface upon which they accumulate as on the downwind side of a sand dune
Cross-beds result from transport by either water or wind
The beds are inclined or dip downward in the direction of the prevailing current
They indicate ancient current directions, or paleocurrents
useful for relative dating of deformed sedimentary rocks

22. Ripple Marks
Small-scale alternating ridges and troughs known as ripple marks are common on bedding planes, especially in sandstone
Current ripple marks form in response to water or wind currents flowing in one direction
have asymmetric profiles allowing geologists to determine paleocurrent directions
Wave-formed ripple marks result from the to-and-fro motion of waves
tend to be symmetrical
Useful for relative dating of deformed sedimentary rocks

23. Current Ripple Marks Ripples with an asymmetrical shape
In the close-up of one ripple, the internal structure shows small-scale cross-bedding
The photo shows current ripples that formed in a small stream channel with flow from right to left

24. Wave-Formed Ripples
As the waves wash back and forth, symmetrical ripples form
The photo shows wave-formed ripple marks in shallow seawater

25. Cross-Bedding Tabular cross-bedding forms by deposition on sand waves
Typically produce where streams enter large bodies of water (AKA Deltas)
Tabular cross-bedding in the Upper Cretaceous

26. Animation of crossbedding

27. Cross-Bedding Trough cross-bedding formed by migrating dunes
Trough cross-beds in the Pliocene Six Mile Creek Formation, Montana

28. Mud Cracks
When clay-rich sediments dry, they shrink and crack into polygonal patterns bounded by fractures called mud cracks
Mud cracks require wetting and drying to form,
lakeshore
river flood plain
or where mud is exposed at low tide along a seashore

29. Ancient Mud Cracks
Mud cracks in ancient rocks in Glacier National Park, Montana
Mud cracks typically fill in with sediment when they are preserved as seen here

30. Biogenic Sedimentary Structures
Biogenic sedimentary structures include
tracks
burrows
trails
Also called trace fossils
Extensive burrowing by organisms is called bioturbation
may alter sediments so thoroughly that other structures are disrupted or destroyed

31. Sedimentary Structures
Sedimentary rocks form as layer upon layer of
sediment accumulates in various environments.
These layers are known as Strata or Beds.
Each Stratum or bed of a sedimentary rock is unique
Texture and composition of the bed reflects the different conditions under which each layer was deposited.

32. Between the Lines Separating each strata are bedding planes.
Bedding Planes are flat surfaces along which the rock tends to break.
Caused by changes in grain size.
May also occur after pauses in deposition.
In general most deposition occurs due to sediments settling out of water therefore most bedding is horizontal.
In wind driven deposition, layers are not always horizontal. Sand dunes are a prime example. In these cases the bedding is described as Cross Bedding

33. Bedding Planes