2 TerminologySedimentary 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 lithificationLithification - The process by which unconsolidated sediments are transformed into sedimentary rocks.
3 Lithification:Two Processes control Lithification: Compaction and CementationCompaction- 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 CementationCementation- 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 sizeChemical Sedimentary rocks are classified by their chemical composition
7 Sedimentary Environments Sedimentary rocks form in many different depositional environments ie. beaches, oceans, mountainsEach 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 depositionHigh-energy processes such as swift-flowing streams and waves are needed to transport large grains such as gravelConglomerate (composed mostly of gravel) must have been deposited in areas of high energySand transport also requires vigorous currentsSilt and clay are transported by weak currents and and therefore only accumulate under low-energy conditions such as in lakes and lagoons
10 Continental Environments Dominated by erosion and deposition associated with streamsChannel DepositsAlluvial FansFlood PlainsIn colder areas glacial movement takes the place of rivers and streamsMorrainsIn more arid areas wind is the more dominant factorDunesPlaya Lakes
11 Transitional Environments (the shore line) BeachesTidal flatsLagoonsSpits, 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 shelfIn some areas can reach up to 1500km, however, average is about 80 kmTypes of sediment deposited depend on several factorsDistance from shoreElevation of adjacent landWater depthWater temperatureClimate
13 Deep Marine –Seaward of the continental shelf and reaches depth greater than 200 mDeposition 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 RoundingSorting and rounding are two textural features of detrital sedimentary rocks that aid in determining depositional processesSorting refers to the variation in size of particles making up sediment or sedimentary rocksIt results from processes that selectively transport and deposit sediments of particular sizes
15 SortingIf the size range is not very great, the sediment or rock is well sortedIf they have a wide range of sizes, they are poorly sortedFor exampleWind has a limited ability to transport sediment so dune sand tends to be well sortedGlaciers can carry any sized particles, because of their transport power, so glacier deposits are poorly sorted
16 RoundingRounding is the degree to which detrital particles have their sharp corners and edges warn away by abrasionGravel in transport is rounded very quickly as the particles collide with one anotherSand 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 thereafterRepresent manifestations of the physical and biological processes that operated in depositional environmentsStructures 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 laminationscommon in mudrocksBeds are thicker than 1 cmcommon in rocks with coarser grains
20 Graded BeddingSome beds show an upward gradual decrease in grain size, known as graded beddingGraded bedding is common in turbidity current depositsform when sediment-water mixtures flow along the seafloorAs they slow, the largest particles settle out then smaller ones
21 Cross-BeddingCross-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 duneCross-beds result from transport by either water or windThe beds are inclined or dip downward in the direction of the prevailing currentThey indicate ancient current directions, or paleocurrentsuseful for relative dating of deformed sedimentary rocks
22 Ripple MarksSmall-scale alternating ridges and troughs known as ripple marks are common on bedding planes, especially in sandstoneCurrent ripple marks form in response to water or wind currents flowing in one directionhave asymmetric profiles allowing geologists to determine paleocurrent directionsWave-formed ripple marks result from the to-and-fro motion of wavestend to be symmetricalUseful 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-beddingThe photo shows current ripples that formed in a small stream channel with flow from right to left
24 Wave-Formed RipplesAs the waves wash back and forth, symmetrical ripples formThe 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
27 Cross-Bedding Trough cross-bedding formed by migrating dunes Trough cross-beds in the Pliocene Six Mile Creek Formation, Montana
28 Mud CracksWhen clay-rich sediments dry, they shrink and crack into polygonal patterns bounded by fractures called mud cracksMud cracks require wetting and drying to form,lakeshoreriver flood plainor where mud is exposed at low tide along a seashore
29 Ancient Mud CracksMud cracks in ancient rocks in Glacier National Park, MontanaMud cracks typically fill in with sediment when they are preserved as seen here
30 Biogenic Sedimentary Structures Biogenic sedimentary structures includetracksburrowstrailsAlso called trace fossilsExtensive burrowing by organisms is called bioturbationmay alter sediments so thoroughly that other structures are disrupted or destroyed
31 Sedimentary Structures Sedimentary rocks form as layer upon layer ofsediment accumulates in various environments.These layers are known as Strata or Beds.Each Stratum or bed of a sedimentary rock is uniqueTexture 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