Sedimentary Rocks

1. Sedimentary Rocks Sedimentology is study of sediments. The Oxford English Dictionary defines sediments as "that material which settles to the base of a liquid." Liquids, in this case may be: water - riverbed or beach deposition air - sand dunes, aeolian dust gas - volcanic ash flows Deposited on or Near Surface of Earth by Mechanical or Chemical Processes

2. Sedimentary Cycle

3. What Rocks Tell Us

4. Sedimentary Rocks are the Principal Repository for Information About the Earth’s Past EnvironmentEARTH HISTORY!

5. Environmental Clues in Sedimentary Rocks • Grain Size - Power of transport medium • Grading - Due to sudden events followed by waning energetics • Rounding • Sorting • Cross-bedding - Wind, wave or current action } Transport, Reworking

6. Environmental Clues in Sedimentary Rocks • Fossils (some examples) • Salt Water - Corals, Echinoderms • Fresh Water - Insects, Amphibians • Terrestrial - Leaves, Land Animals • Color And Chemistry • Red Beds - Often Terrestrial (oxidized iron) • Black Shale - Oxygen Poor, Often Deep Water • Evaporites – Arid Climates

7. Two features characterize sediments and sedimentary rocks: a detrital fabric and the presence of layers, or bedding Sedimentary Fabric Settling produces a detrital fabric i.e. a fabric with point contacts between grains, rather than a fabric of interlocking grains found in igneous or metamorphic rocks.

8. Bedding or Stratification • Almost always present in sedimentary rocks • Originally horizontal • Tilting by Earth forces later • Variations in conditions of deposition • Size of beds (Thickness) • Usually 1-100 cm • Can range from microscopic to 50m

9. Graded Bedding - Often largest particles settle first, followed by successively smaller ones so that the particles are sorted more or less according to size. Normal grading - the coarsest material on bottom, getting finer grained towards the top This suggests an initial energetic event, followed by waning energy (e.g. a flood, turbidity currents)

10. Bedding • Bedding is a series of visible layers within the rock. It is primarily due to episodic nature of sedimentation where very fine particles are laid down slowly between times of more rapid deposition. • Bedding planes are assumed to be originally horizontal or nearly horizontal for water-laid sediments. • For aeolian (wind-laid or sub-aerial) more commonly show cross-bedding, because the sand can support steeper dune surfaces in air than in water.

11. Modern ripples in loose sediment Sedimentary Structures • Often, evidence of the nature of the sedimentary surface are preserved in sedimentary rocks ancient ripples preserved in sandstone

12. Mud cracks on the bottom of a modern puddle. Mud-cracks form because clay minerals may shrink by up to 15% in volume on drying out. Mud-cracks can be preserved and indicate a depositional environment that is near shore and periodically exposed to air. Mud cracks in Cambrian rocks in southwestern Virginia. Even the curls of the flaking mud are preserved.

13. Sorting • Sorting - is the measure of range of particle size of sediments. A sediment having a wide range of particle size is said to be poorly sorted and if the range is small, the sediment is said to be well sorted.

15. Settling is the key process, and is dependent on : size of grain (d) density of grain (rg) fluid velocity and turbulence fluid density (rf) and viscosity (m) Settling Velocity (Stokes Settling Law)

16. Clastic Rocks Made of fragmentary material Deposited by Water (most common) Wind Glacial action Gravity Biochemical Sedimentary Rocks Evaporation Precipitation Biogenic sediments Sedimentary Rocks

17. Clastic Rocks • Clastic sediments are those derived from the weathering, transportation and deposition of older rocks. The term "clastic" comes from the Greek klastos (= broken), indicating the broken nature of the sediment. (An individual particle is a clast) • also known (somewhat confusingly) as detrital sediment, or detritus, which is Latin for "worn down" • As erosion occurs predominantly on the land, they are also termed terrigenous sediments (terra = Earth)

18. Clastic Rocks are classified by: • Grain Size (Sand vs. Gravel vs. Silt, etc.) • Grain Composition (Quartz vs. Feldspar, etc.) • Texture (rounded grains, sorting, etc.)

19. Rock Type Sediment Grain Size Shale Clay less than 0.004 mm Siltstone Silt .004 - 0.064 mm Sandstone Sand .064 - 2 mm Conglomerate* Pebbles 2mm - 64 mm Conglomerate* Cobbles 64 mm - 256 mm Conglomerate* Boulders > 256 mm Sediment Sizes and Clastic Rock Types Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks. * rocks with rounded clasts are conglomerates. Those with angular clasts are breccias

20. Conglomerate / Breccia (less commonly Rudite / Rudaceous Rocks) Lithified gravel (grains over 2 mm) with rounded grains are called conglomerates. The name can be modified by the size of the predominant particles. Thus we can have a boulder conglomerate, cobble conglomerate, pebble conglomerate or granule conglomerate. Conglomerate Breccia

21. Sandstone (less commonly Arenite / Arenaceous Rocks) Lithified sand (grains between 2 mm and 1/16th mm) is called sandstone, or, less commonly, arenite. The name may be modified by the grain size (eg. very coarse sandstone through to very fine sandstone) or grain composition.

22. A sandstone composed of a significant amount (> 25%) of feldspar is called an arkose (or arkosic sandstone). If feldspar is present in significant quantities, but less than 25%, it may be termed a feldspathic sandstone.

23. A sandstone composed of mainly rock fragments is called a lithic sandstone (or a litharenite). A poorly sorted lithic sandstone with angular to sub-angular grains with a fine, muddy or clay matrix is called a greywacke.

24. Mudstone (Argillaceous Rocks) Mudstones are fine grained clastic rocks, which may be divided into siltstone and claystone. Although mudstones are too fine to easily determine their mineralogy, they are most commonly composed of quartz and clay. If there is a significant proportion of quartz, the rock will be gritty, whereas if it is nearly all clay it will feel slimy. The best way of determining ‘grittiness’ is to scrape the rock on the underside of your teeth! A gritty texture defines a siltstone, whereas a slimy texture is a claystone. The abundance of clay minerals (micas) means that the rock may have a tendency to break along planes that are parallel to bedding, due to the original orientation of the clays when they settled. This property is called fissility. Fissile mudstones are termed shales.

25. SHALES

26. Clastic Sediment CompositionWhat the grains are composed of. Generally this involves determining the amount of quartz grains, feldspar grains and rock fragments. Chemical Sediments mineral aggregates precipitated out of natural bodies of water such as lakes, lagoons and oceans. The components were transported in solution and then chemically precipitated out of solution. Biogenic Sediments sediment composed mainly of fossil remains Residual Sediments products remaining after intensive chemical breakdown of rock exposed to attack by natural agents such as oxygen, water and organic and inorganic acids. Pyroclastic Sediments result from violent volcanic eruptions - typically produces a mixture of angular fragments of mineral grains, volcanic glass fragments, and volcanic rock fragments. When deposited by primary volcanic processes = “pyroclastic.” If reworked by wind or water it can best be called a “volcaniclastic sediment.”

27. Because quartz (SiO2) is abundant and relatively resistant to chemical weathering it makes up the bulk of sand-sized particles. Fine particles (silt and clay) are carried to regions where the water is still (off-shore environments). Clay minerals that are the weathering products of feldspars and ferro-magnesian minerals form the bulk of these particles. Coarse products (boulders and cobbles) require fast moving water (mountain streams) to be moved at all, and so are not transported very far from their sources. These particles may be unweathered and retain their source mineralogy and chemistry. Intermediate-sized particles (sand) are transported by rivers and wind and deposited on river bottoms, at coasts or in deserts.

28. Sediment Maturity • Stability of Minerals (e.g. - feldspar, micas unstable; quartz is very stable) • Rock Fragments (most rock fragments not as durable as individual grains) • Rounding or Angularity (well rounded grains have been through a lot of abrasion, angular ones are “fresher”) • Sorting (well sorted deposits have had processes acting on them longer than poorly sorted ones) Removal of unstable ingredients suggests Mechanical Working

29. Shape - there are two main things to look at here: the grains’ roundness and sphericity Roundness Sphericity

30. Compaction Cementing Quartz Calcite Iron Oxide Clay Glauconite Feldspar Alteration Limestone - Dolomite Plagioclase – Albite Recrystallization Limestone Diagenesis(making a rock from loose sediment)After sediments are deposited, they are commonly compacted by the weight of overlying sediments.

31. After sediments are deposited, they are commonly compacted by the weight of overlying sediments. They may be lithified (solidified) by the deposit of a cement or secondary mineral that fills the pores. They may also be lithified by recrystallization of the primary minerals.

33. Circulating pore water contains dissolved minerals that may precipitate out of solution, producing a cement. Calcium carbonate is a common cement. Silica is not quite as common, but forms an extremely tough rock when it forms the cement. Circulating pore water may also dissolve minerals from the rock. Such dissolution is particularly common in carbonate rocks and may lead to an increase in pore space. Replacement involves the essentially simultaneous dissolution of existing minerals and the precipitation of a new mineral. Occasionally, delicate carbonate skeletons can be pseudomorphed by microcrystalline quartz crystals.

34. Evaporites -Water Soluble Halite Gypsum Calcite Precipitates Example: Ca(sol'n) + SO4 (Sol'n) = CaSO4 Gypsum Limestone Iron Formations Alteration After Deposition Dolomite Biogenic Sediments Limestone - Shells, Reefs, Etc. Organic Remains Coal Petroleum Chemical Sediments

35. Biogenic Rocks:Limestone is the rock formed by calcite. Calcite (CaCO3) is very near saturation in sea water and so is used as shells material by marine organisms. Most calcite rocks of Phanerozoic age (the last 600 million years) are of biological origin. Dolostones (formed of dolomite (CaMg(CO3)2) may also be formed in this way.

36. Limestone

37. Fossils often form a major component of limestones but not always. Limestones are composed chiefly of calcium carbonate (CaCO3). Calcium carbonate reacts with acids (e.g. - HCl) according to the equation: CaCO3 + 2HCl => CaCl2 + H2O + CO2 (gas) and an audible and visible "fizzing" occurs as the CO2 is released.

38. Limestone is also relatively easy to dissolve compared to other lithologies. When buried, dissolution can be initiated due to pressure. (more soluble under high P) Dissolution occurs along planes that have a jagged appearance to them. Those planes are called stylolites and are frequently black due to the accumulation of insoluble material such as clays and organic detritus.

39. Stylolites

40. Chalk - a limestone composed of the shells of tiny, planktonic one-celled organisms. Coquina - a limestone composed exclusively of large shell fragments. Some special kinds of limestones

41. Oolites are small spherical concretions, commonly formed by calcite that was deposited around a sand grain, shell fragment, or some other foreign particle in shallow, wave-agitated water. Oolites form where gentle or periodic wave action in warm marine waters allow carbonate precipitation on all sides of a grain of sand or shell fragment. Large Oolites are called Pisolites or Peastone.

42. Dissolved material (Ca, Na, K, CO3, SO4, Cl) is carried farthest and deposited where the ocean, sea, or lake is evaporated off. Such rocks are called evaporites. As sea water is evaporated, the sequence of minerals formed is 1) calcite (CaCO3) 2) gypsum (CaSO4.2H2O) 3) halite (NaCl) and 4) sylvite (KCl) Evaporites halite

43. Evaporite Formation

44. Some Weird Chemical Rocks: Lake waters may have different chemistries, depending on the source rocks in the catchment area for the waters. Salts that may precipitate from lake waters include: sodium carbonate (Na2CO3), (for example, lakes in the East Africa Rift Zone) sodium sulfate (Na2SO4) and borax (Na2B4O7.10H2O), which have important commercial applications. Phosphorous Deposits The calcium phosphate, apatite (Ca5(PO4)3(OH,F), can also precipitate from seawater if deep ocean waters, enriched in phosphorous by the decay of marine animals, are brought to the surface by upwelling currents and reach saturation in a shallow basin. Such deposits may be economically valuable sources of phosphorous, which is used as a fertilizer.

45. Chert When silica is precipitated, it forms chert. Chert may form extensive, continuous bands or as nodules in carbonate rocks (flint). (Many banded cherts have formed from the deposition of silica organisms, so are classified as biogenic rocks rather than chemical.)

46. Banded Iron Formations In the past, Fe2+ has formed precipitates in remarkably continuous bands or laminations, known as banded iron formations (BIF). These formed when the atmosphere had very much less oxygen in it than it does today, allowing the dissolution and transportation of Fe2+. Later oxidization of the iron transformed it to Fe3+ which has a much lower degree of solubility and so precipitated out of solution.

47. Banded Iron Formation

48. Coal • A biogenic sedimentary rock composed essentially of lithified plant debris • Delta, continental environments • Carbonized Woody Material • Often fossilized trees, leaves present

49. Coal Seams, Utah

50. Plant Fragments Are Often Visible in Coal