Sedimentary Rocks

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 tothe 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 particlesthat originated from chemical or mechanical weathering. • Chemical- Formedas 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-beddingforms 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