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Sedimentary Rocks. Terminology ► Sedimentary Rock - a rock formed through the deposition of sediments from weathering or biologic activity. ► Diagenesis-

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Presentation on theme: "Sedimentary Rocks. Terminology ► Sedimentary Rock - a rock formed through the deposition of sediments from weathering or biologic activity. ► Diagenesis-"— Presentation transcript:

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 ► 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 Grain Size vs. Sedimentary Environment

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 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 Sorting and Rounding

15 ► 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 Sorting

16 ► 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 Rounding

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

18 ► 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 Sedimentary Structures

19 ► Sedimentary rocks generally have bedding or stratification Bedding  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 ► Some beds show an upward gradual decrease in grain size, known as graded bedding 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 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 Cross-Bedding

22 ► 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 Ripple Marks

23 ► 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 Current Ripple Marks

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

25 ► Tabular cross-bedding forms by deposition on sand waves ► Typically produce where streams enter large bodies of water (AKA Deltas) Cross-Bedding ► 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 ► 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, Mud Cracks  lakeshore  river flood plain  or where mud is exposed at low tide along a seashore

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

30 ► 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 Biogenic Sedimentary Structures

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

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