Sedimentary Stratigraphy Test 1: (ch. 1,2,3,4,5,6, and part of 15.)
Stratigraphic Principles 3. Superposition
"...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.
Stratigraphic Principles 4. Cross-Cutting Relations
"If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669.
Uniformitarianism:
(James Hutton and Charles Lyell) present-day processes and rates are similar to those of the past.
Munsell Color System: 1.HUE 2.VALUE 3.CHROMA
- Hue: horizontal circle is divided into five principal hues: Red, Yellow, Green, Blue, and Purple, along with 5 intermediate hues halfway between adjacent principal hues. Each hue is given an integer value. - Value: or lightness varies vertically along the color solid, from black (value 0) at the bottom, to white (value 10) at the top. Neutral grays lie along the vertical axis between black and white. - Chroma: measured radially from the center of each slice, represents the "purity" of a color, with lower chroma being less pure (more washed out, as in pastels). Note that there is no intrinsic upper limit to chroma.
Stratigraphic Principles 6.Law of Faunal Succession
- This law was developed by William "Strata" Smith who recognized that fossil groups were succeeded by other fossil groups through time. This allowed geologists to develop a fossil stratigraphy and provided a means to correlate rocks throughout the world.
Stratigraphic Principles 1. Original Horizontality
- all sedimentary rocks are originally deposited horizontally. Sedimentary rocks that are no longer horizontal have been tilted from their original position.
Stratigraphic Principles 2. Lateral Continuity
- sedimentary rocks are laterally continuous over large areas. A useful way for Wisconsinites to consider this law is to think of snowfalls. As snow falls, it is not limited to the intersection of Main and Division streets, nor UWSP campus, but falls over a larger area such as Central Wisconsin. Sediments also "rain" down in a simialr fashion such that sedimentary layers are laterally continuous over an area similar to, or greater than, Central Wisconsin. "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669
Stratigraphic Principles 5. Law of Inclusions
- this law states that rock fragments (in another rock) must be older than the rock containing the fragments. Refer to Inclusions.
IMPORTANT STRATIGRAPHIC QUESTIONS:
--How complete is the geologic record? --What is the nature of the gaps in the geologic record? --How important are they in the interpretation of the record? --In what types of environments are such gaps more likely to occur? --Is there any assurance that the sediment which ties the gaps together represents a continuous record? --What types of events or processes have the greatest amount of influence on the geologic record, i.e., on deposition and preservation of sediment? --In what types of environments is there the greatest likelihood of preserving a complete record?
Importance of sedimentary rocks:
-Cover ~90% of Earths surface -Comprise ~5% of crust. -Reservoirs for groundwater. -Source rocks for oil, gas, other economic minerals -Repositories for the fossil record. -Enable reconstruction of paleoclimate and paleogeography. -Repository of global carbon.
Hydrodynamics and Sediment Transport Quantifying Fluid Flow
1) Reynolds Number (represents the relationship between laminar and turbulent flow) Re = fluid inertial forces /(fluid viscosity forces) Re = DVρ/µ • V = fluid velocity • D = depth (scale) of flow • ρ = fluid density • µ = fluid viscosity A) Laminar Flow (viscosity dominated) --Small Re (range: 500 - 2000) --Viscosity forces greatly exceed inertial forces --Characteristic of unconfined fluids moving across open surfaces --Examples: glaciers, slow-moving streams, tidal currents, highly concentrated mudflows, lahars, heavy oil, poured concrete --Note: even laminar flows often have a turbulent boundary layer B) Turbulent Flow (fluid dominated) --Large Re (>2000) --Inertial forces exceed viscosity forces --Examples: fast-moving streams, turbidity currents 2) Froude Number: represents the tendency of a fluid flow to continue flowing despite the gravitational forces acting to stop the flow. Fr = fluid inertial forces/gravitational forces in flow Fr = [V]/[(gD)^(1/2)] • V = fluid velocity • D = depth (scale) of flow • g = gravitational constant Subcritical (Tranquil) Flow (dominated by gravitational forces) --Fr < 1 --Gravitational forces are greater than flow velocity --Waves can move upstream --Typical of most flowing water bodies Supercritical (Rapid) Flow (dominated by flow velocity) --Fr > 1 --Velocity of gravity waves is less than flow velocity --Waves cannot move upstream
Hydrodynamics and Sediment Transport Sediment Movement by Fluids 1)The ability of fluids to affect sediment particles depends on? 2)Grain-size differences in a sediment deposit are a result of differences in what? and among what?
1) • Fluid density • Fluid viscosity (resistance of the fluid to shearing) • Flow velocity 2) Grain-size differences in a sediment deposit are a result of differences in fluid density and viscosity among: • Air (low density, low viscosity), • Water (moderate density, moderate viscosity) • Glacial ice (moderate density, high viscosity)
Sedimentary Structures Types of Sedimentary Structures: Primary
1) Bedforms created by unidirectional flow Ripples Sand waves Dunes Plane beds Antidunes 2) Bedforms created by multidirectional flow Herringbone cross-bedding Interference ripples Lenticular bedding Flaser bedding Wavy bedding 3) Structures formed on the surface of sediment bedding planes Sole marks Tool marks Flute casts. 4) Top and bottom indicators (geopetal structures) Cross-bedding Graded bedding
Weathering Types of Chemical Weathering
1) Dissolution: ionization of ionically bonded metal cations (Ca++, Na+, Mg+, K+) by dipolar water molecule. Produces the metal cations common in natural waters H2O + CaCO3 --> (Ca++) + CO3(2-) + H2O 2) Hydration: chemical combination of pre-existing minerals with water, or removal of water from hydrated minerals. CaSO4•2H20 ---> 2 H2O + CaSO4 (gypsum) (water) + (anhydrite) 3) Hydrolysis: hydrogen ion (H+) combines with silicate group; reaction raises pH, and releases silicic acid (a weak acid) Mg2SiO4 + 4H20 ---> 2Mg++ + 4OH- + H4SiO4 (olivine) (hydroxyl) + (silicic acid) 4) Oxidation-Reduction: Loss of an electron with positive increase in valence (charge). Most metals immediately oxidize in the presence of oxygen (the most abundant surface oxidant) especially: Fe++--->Fe+++, Mn++---> Mn+4, S--->S+6 (SO4--)
Weathering Types of Physical Weathering
1) Freeze-thaw: action of water freezing and expanding in fractures or bedding planes 2) Insolation: changing temperature causes expansion and contraction 3) Stress release (unloading): removal of overburden causes expansion cracks and joints, leads to exfoliation 4) Organic activity: plant roots and algae generate on rock surfaces
Hydrodynamics and Sediment Transport Laminar vs. Turbulent Flow
1) Laminar Flow --Low flow velocity, or high viscosity fluid --Fluid particles move in parallel lines down-current --Found in slow-moving streams, glaciers, or mud-supported gravity flows (high viscosity) (also maple syrup and Silly Putty) ****Can move particles only because high viscosity inhibits settling 2)Turbulent Flow --Higher flow velocity --Fluid particles move in random patterns, including upstream and vertically --Characteristic of most natural fluid flows ****Turbulent flows are more effective in eroding and transporting sediment
Sedimentary Rocks Conglomerate and Breccia Characteristics 1)Grain size: 2)Components: 3)Clast Stability: 4)Clast Origin:
1)Grain size: usually bimodal (framework and matrix) -Framework: clasts >2mm -Matrix: smaller clasts < 2 mm filling in the gaps within framework 2)Components: -Minerals: single minerals, such as quartz or feldspar. -Rock fragments: more abundant component; useful in determining provenance (e.g., if a conglomerate contains kimberlite clasts, it is likely that a kimberlite outcrop occurs upstream.) 3)Clast Stability: -Oligomict: >90% of framework clasts consist of resistant rocks and minerals (quartz, quartzite, chert). Result of intense weathering. -Petromict (polymict): >10% clasts of metastable and unstable rocks and minerals (granite, limestone, volcanic rocks, etc.). 4)Clast Origin: -Intraformational: both framework and matrix originate from same rock unit. Product of brief periods of intense weathering which have disrupted normal deposition, such as a storm event. -Extraformational: framework and matrix are derived from source areas outside the depositional basin.
Hydrodynamics and Sediment Transport Forces Acting on Particles During Fluid Flow
1)Inertial Forces: resist particle movement • Gravity • Electrostatic forces • Friction 2)Mobility Forces: enable particle movement • Fluid drag force • Bernoulli (lift) force • Buoyancy
Types of Sedimentary Structures: Secondary
1)Mechanical structures (soft-sediment deformation) Load casts Ball and pillow structures Flame structures Convolute bedding 2)Biogenic structures (trace fossils), formed by: Burrowing Boring Feeding Resting Locomotion
Hydrodynamics and Sediment Transport Sediment Movement by Fluids 1)how are sediments transported? 2)What types of action modes are involved in sediment transport?
1)Sediments are transported by fluids: • Air • Water • Ice 2)The action of fluids on sediment particles consists of: • Entrainment (picking up particles from a surface) • Transport • Deposition
Development of the Geologic Timescale: 1.Classical disciplines of stratigraphy 2.Newer subdisciplines of stratigraphy
1. • Lithostratigraphy: stratigraphy based on lithology (physical properties of the rocks) • Biostratigraphy: stratigraphy based on faunal succession • Chronostratigraphy: stratigraphy based on absolute age 2. • Cyclostratigraphy: stratigraphy based on repeated cycles of facies change • Magnetostratigraphy: stratigraphy based on magnetic reversal chronology • Chemostratigraphy: stratigraphy based on changes in certain sediment stable isotopes over time (O, C, Sr) • Seismic stratigraphy: stratigraphy based on subsurface seismic reflection/refraction profiles • Sequence Stratigraphy: Integration of all disciplines of stratigraphy
Sedimentary rock: Sedimentary rocks cover approximately________ of the Earth's land area, but constitute only about ______of the geologic column.
1. 75% 2. 5%
Basin (definition):
1. A region of potential sediment accumulation generally caused by subsidence. 2. The largest possible body of related and once-contiguous strata; e.g., the Cretaceous Western Interior basin.
Completeness of the Geologic Record Types of Missing Sedimentary Records
1. Bedding planes: small-scale pause in deposition 2. Diastems: relatively short break, with little or no erosion 3. Hardgrounds: surface of non-deposition; corrosion, dissolution, alteration 4. Condensed sections: apparently uninterrupted deposition, but thin section relative to time span 5. Hiatuses: periods of non-deposition; occur even in the deep sea; may be globally correlated 6. Unconformity: A geological surface separating older from younger rocks and representing a gap in the geologic record. Such a surface might result from a hiatus in deposition of sediments, possibly in combination with erosion, or deformation such as faulting. The study and interpretation of unconformities locally, regionally and globally is the basis of sequence stratigraphy.
Development of the Geologic Timescale. chronology
1. LYELL (1797-1875) "Principles of Geology" published 1833, subdivided Tertiary (now Cenozoic Era) into 4 epochs based on molluscan Fauna. *ignored mass extinction 2. Terminology The terminology of the geologic timescale was established primarily during the 19th century. EON, ERA, PERIOD, EPOCH -tertiary and quaternary (terms held over from Werner) -Cambrian, Ordovician, Silurian Periods (defined by british and named after ancient tribes) -Devonian (named after English coutnry "DEVON" -Carboniferous ("coal measures", old british term) -Permian (named after Perm, Russia) -Triassic (named by german after three distinct layers, red beds, chalk caps, follwed by black shales) -Jurrassic (named by frenchman, Jura mountain marine lmst outcrops -Cretaceous (creta-means chalk) 3.PERIODS AND EPOCHS -naming occured in the 19th century based on field observations. -eocene-divided into paleocene &oligocene -Permian (based on faunal succession in Ural Mnts, Russia) -Mssp & Penn (subdivide cretac.) -Devonian- split off originally from carbonif -sulurian- based on strat/fossils -cambrian -Ordovician (camb/silurian boundary dispute was settled, based on trilobite and graptolite fauna) 4. ERA Geologic Eras: Mass extinctions have resulted in major change in Earths biota. severe extinction events were: --the Cretaceous-Paleogene (K-P) (MESO/CENOZOIC boundary) --the Permo-Triassic event (PALEO/MESOZOIC boundary) ERAS Paleozoic, Mesozoic and Cenozoic. The eras were defined & named before the mass extinctions were recognized. " Paleozoic: named by Adam Sedgwick in 1838. " Mesozoic and Cenozoic: named by John Philips in 1840. 5. Precambrian - representing 85% of geologic time -The eon subdivisions Archean and Proterozoic have been adopted by the International Union of Geological Sciences (IUGS) -Precambrian stratigraphy has been based on 20th century radiometric ages
GUIDING PRINCIPLES OF STRATIGRAPHY : important persons, in chronological order of importance
1. Nicholas Steno(1638-1686): Steno's Laws-Superposition, Original horizontality, Lateral continuity 2.Giovanni Arduino (1714-1795): Contributed to the development of basic lithostratigraphic classification of rocks and mountain building. (neptunism) 3. James Hutton (1726-1797): Proponent of uniformitarianism, angular unconformities, cross cutting relationships (Plutonism) 4. Abraham Gottlob Werner (1750-1817) :Used superposition to develop relative ages of geologic formations. 5.William "Strata" Smith(1769-1839) -Developed principle of faunal succession -First modern geologic map 6.Charles Lyell (1797-1875) -First Geologic Timescale -Popularised Uniformitarianism -argued "gradualism" 7.Johannes Walther (1860-1937) -Walthers Law of Facies Succession: Facies adjacent to one another in a continuous vertical sequence also accumulated adjacent to one another laterally.
Stratigraphic Principles
1. Original Horizontality 2. Lateral Continuity 3. Superposition 4. Cross-Cutting Relations 5. Law of Inclusions 6.Law of Faunal Succession
Stratigraphic Principles
1. Original Horizontality- all sedimentary rocks are originally deposited horizontally. Sedimentary rocks that are no longer horizontal have been tilted from their original position. "Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669 2. Lateral Continuity- sedimentary rocks are laterally continuous over large areas. A useful way for Wisconsinites to consider this law is to think of snowfalls. As snow falls, it is not limited to the intersection of Main and Division streets, nor UWSP campus, but falls over a larger area such as Central Wisconsin. Sediments also "rain" down in a simialr fashion such that sedimentary layers are laterally continuous over an area similar to, or greater than, Central Wisconsin. "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669 3. Superposition "...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669. 4. Cross-Cutting Relations "If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669. 5. Law of Inclusions- this law states that rock fragments (in another rock) must be older than the rock containing the fragments. Refer to Inclusions. 6.Law of Faunal Succession- This law was developed by William "Strata" Smith who recognized that fossil groups were succeeded by other fossil groups through time. This allowed geologists to develop a fossil stratigraphy and provided a means to correlate rocks throughout the world.
Weathering Factors in Weathering
1. Source rock: composition (mineralogy, texture) 2. Climate: (temperature, precipitation, freeze-thaw) 3. Drainage: (good vs. poor; good drainage removes reaction products) 4. Topographic relief and slope steepness: (high relief and high slope favors mass wasting) 5. Chemical and physical weathering relative rates: (determined by climate, relief and latitude)
Completeness of the Geologic Record Sixth Great Extinction ??
1.$ Fossil-fuel burning and agriculture have caused substantial increases in the concentrations of 'greenhouse' gases — carbon dioxide by 40% and methane by 150% 2.The development of agriculture 8,000 years ago, and the deforestation that resulted, led to an increase in atmospheric CO2 just large enough to stave off what otherwise would have been the start of a new ice age. 3.Human biomass represents roughly 40 megatons of carbon 4.Machines now need more carbon every year than humans do. 5. Approximately 50% of all ice-free land surface has been fully or partially converted to human use 6.The fixation of atmospheric nitrogen for fertilizer (160-170 megatons per year) now exceeds fixation by natural processes (150-190 megatons per year). 7. Between 25% and 40% of the earth's total net primary productivity has been appropriated for human use 8. Energy use has grown 16-fold during the twentieth century, releasing 160 million tonnes of atmospheric sulphur dioxide emissions per year, more than twice the sum of earth's natural emissions 9.Nitric oxide production by the burning of fossil fuel and biomass now exceeds natural emissions 10.Human-made impoundments (dams, reservoirs) hold back about 6500 km3 of water, or about 15% of the total annual river runoff globally. 11.The sixth great extinction is underway. One species goes extinct every 20 minutes. One fifth of all species will be gone by 2030 if the present rate continues.
Completeness of the Geologic Record Gaps in the Geologic Record
1.Catastrophic Uniformitarianism --Continuous sedimentation is a myth --Periodic catastrophic events (e.g., storms, turbidity flows) may have more effect than long periods of gradual sedimentation. --Much of the stratigraphic record may be the result of catastrophic deposition, such as major storm events. 2.Episodic Sedimentation --Even normal fairweather processes are inherently discontinuous and episodic, such as a prograding shoreline, or a submarine fan.
Completeness of the Geologic Record Importance of Catastrophic Events
1.Continental environments: Major earthquakes, supervolcano eruptions, and mega-floods can be catastrophic, and significantly influence regional stratigraphy. 2.Nearshore marine environments: storms may have windspeeds of over 120 m/hr; storm surge can pile up water onshore, eroding nearshore; on the Gulf coast, local areas experience a major one every 100 yr or so 3.Continental shelf: can be impacted by tsunamis (seismic sea waves). Tsunami runup can reach up to 30 m high along coasts 4. Continental slopes: instantaneous mass wasting, including events triggered by earthquakes, landslides, rockfalls, slumps, slides
CATASTROPHIC EVENTS
1.Cretaceous-Paleogene (K/P) Boundary Impact Event and Mega-tsunami Deposits - Unit appears to be have been deposited in a single event, deposited shortly after the Chicxulub impact event 2.Grand Banks Earthquake (1929) and Turbidity Currents -An earthquake that occurred south of Newfoundland caused the loose sediment on the continental slope to move in a submarine landslide. 3.Megafloods in the Channeled Scablands -20,000 years ago, as the glaciers began retreating across the Pacific Northwest. Giant ripples up to 10 m high were formed in gravel. Peak flow velocities reached 130 km/hr (20x that of an average stream, and equal to a 1000 or 10,000-year flood). 4.Yellowstone Caldera - recalculating the volume of erupted volcanic ash and pumice in terms of the original volume of molten rock (magma) released (shown in this diagram by orange -On this basis, the 2450 cubic kilometers (km3) of magma that was erupted from Yellowstone 2.1 million years ago (Ma) was nearly 6,000 times greater than the volume released in the 1980 eruption of Mount St. Helens -Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years 5. Sumatra Earthquake and Tsunami, 12/26/04 • Magnitude 9.0 • 200,000+ killed • $14 billion losses • 50-ft tsunami waves
Types of Sedimentary Structures: Secondary Biogenic structures (trace fossils), formed by:
1.Cruziana trace fossils (A) appear as bilobate convex structures with parallel scratch marks from the legs of the burrowing trilobite 2.Thalassinoides burrows are complex, three-dimensional networks of traces at multiple levels, which usually collapse into a jackstraw-like web of burrows when viewed in a two-dimensional bedding plane. 3.Typical trace fossils of Zoophycos facies; (B) and (C ) Typical Zoophycos traces, complex arcuate feeding traces in three dimensions 4.(A) Typical deep-water trace fossils of Nerites facies; (b) Two different meandering traces, Spirophycus (larger burrows) and Phycosiphon (smaller burrows), Oquirrh Fm.,Wasatch Mtns., Utah; (C ) Paleodictyon, a netlike trace, from Mid-Jurassic, Ziz Valley, Morocco.
Deposition rates vary widely, due to:
1.Environmental differences 2.Transgressions-regressions 3.Uplift and erosion 4.Metamorphism and subduction
Development of the Geologic Timescale : Stratigraphic rock successions are clear, although _______. They form the basis for a well-defined geologic column. Stratigraphy is a mature science. Most of the remaining questions in stratigraphy revolve around the ___________ assigned to the boundaries.
1.INCOMPLETE 2.ABSOLUTE NUMERICAL DATES
Development of the Geologic Timescale 1.Absolute Age of the Earth: Mythologic Ideas 2.Absolute Age of the Earth: Religious Ideas 3.Absolute Age of the Earth: Scientific Ideas 4.Absolute Dating
1.Mythologic Ideas -Buddhist tradition: infinite age (cycles of creation, destruction and and rebirth of cosmological systems) Han Chinese tradition (2nd century BC): 23 million-year cycles 2.Religious Ideas -James Ussher (1581-1656), Having established the first day of creation as Sunday, 23 October, 4004 BC, 9:00 am, Calculation of the age of the Earth, using the begat method (from Cooper's Chronicle, 1560) 3.Scientific Ideas a) Natural processes b) Occurring at a constant rate c) Leaving a continuous geologic record -Age (time) =(Amount of change)/(Rate of change) -Uniformitarianism: Charles Lyell (mid-1800's) (80million 4 cenoz) -John Phillips (1800-1874) divided the total thickness of the sedimentary record by average sedimentation rates (in mm/yr) earth found to be 3 million and 1,500 million years old. -Salt Content of the Ocean rate of delivery of salt to the ocean, 90million yrs old -Thermodynamics: In 1897, Lord Kelvin, assuming that the Earth was originally molten, calculated a date based on cooling through conduction of heat to the surface and radiation of heat into space, 100 million yrs old 4. ABSOLUTE DATING -ages of the boundaries in the geologic timescale remained to be defined after the development of radioactivity and absolute geologic time, beginning at the end of the 19th century - Henri Becquerel, Rutherford, Marie & Pierre Curie (1896) found that Earth has its own heat source, with the discovery of natural radioactivity -Bertram Boltwood (1905, American): first dates (Uranium-lead dating). Rock ages 250 Ma to 1.3 Ga -Arthur Holmes (1913, British ) The Age of the Earth - makes a conclusive case for the radiometric method. -Law of Radioactive Decay The rate of decay of radioactive nuclide is proportional to the number of atoms of that nuclide remaining at any time 5.Absolute Dating Methods A) ISOTOPIC TECHNIQUES (-238U 206Pb) 4.5 x 109 10 Ma - 4.6 Ga {zircon, apatite} (235U 207Pb) 0.71 x 109 10 Ma - 4.6 Ga {zircon, apatite} (40K 40Ar) 1.25 x 109 50 ka - 4.6 Ga {volcanic ash, lava} (87Rb 87Sr) 47 x 109 10 Ma - 4.6 Ga {igneous and metamorphic rocks} (14C 14N) 5730 100 - 70 ka {wood, charcoal, peat, bone, tissue, shell} B)RADIATION EXPOSURE TECHNIQUES Fission track: 0.5 Ma - 1 Ga Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL): 0 - 500 ka Electron Spin Resonance (ESR): 1 ka - 1 Ma C)OTHER TECHNIQUES Amino acid racemization 500 - 300 ka Obsidian hydration 500 -- 200 ka Dendrochronology 0 -- 12 ka Magnetic polarity reversal chronology 780 ka - 200 Ma
Although sedimentary rocks cover ~75% of the Earths land area, they (are/are not) evenly distributed through geologic time. Their distribution (left panel) is heavily weighted toward the (past/present) due to weathering and recycling of existing sediments.
1.are not 2. present
For a given environment (e.g., fluvial), rates in modern environments are often orders of magnitude _____ than in ancient rocks Why?
1.higher 2. -Most of the stratigraphic record is missing for much of ancient intervals, relative to modern products we observe forming from active processes. -Ancient sedimentary rocks are generally dealt with in packages of 1 to 10 million years' worth of rock. -Recent sediments are largely post-ice age deposits (last ~10,000 yr)
There have been five major extinctions over the past _______, and about a dozen lesser ones. The major ones are:
600 MYA 1. End-Ordovician (~440 ma): ~60% of all marine genera died off due to sea-level change (?). 2. Late Devonian (~360 ma): more than half of all marine genera went extinct due to global cooling. 3. End-Permian (~250 ma): ~90% of all species died off, and multicellular life nearly ended, due to a comet impact or volcanic outpouring (?). 4. End-Triassic (~200 ma): ~50% of marine genera went extinct due to volcanism accompanying the breakup of Pangaea. 5. End-Cretaceous (~65 ma): nearly 50% of marine genera, most major reptiles, ammonites and the majority of mammal species went extinct.
Sedimentary Petrology:
Branch of geology dealing with the origins, features, classification, nomenclature, and history of sedimentary rocks and Earth's stratigraphic record.
Breccia:
Breccia: Lithified rubble made up of angular clasts coarser than 2 mm. breccia-rubble (granule,pebble,cobble)
CONGLOMERATES/BRECCIA: OROGENY FLOWCHART
CORDILLERAN OROGENY (Cont-edge subduction zone) Mixed Igneous/metamorphic sourceland 1.LITHIC BRECCIA (alluvial fan) rounding 2.LITHIC CONGLOMERATE (proximal braided river) BLOCK FAULTED CONTINENT Continental (granitic) sourceland 1.ARKOSE BRECCIA (alluvial fan) rounding 2.ARKOSE CONGLOMERATE (proximal braided river)
SEDIMENTARY ROCKS Carbonate Rocks
Carbonate Rocks Limestones Dolostones Carbonate environments
Sediment Properties Clast
Clay <1/256 mm silt 1/256 mm - 1/16 mm sand 1/16 mm - 2 mm granule 2 mm - 4 mm pebble 4 mm - 64 mm cobble 64 mm - 256 mm Boulder >256 mm
Sediment Properties Color
Color is a function of sedimentary rock composition, reflecting the bulk mineralogical components. Color can be quantified using the Munsell Color System: 1.HUE 2.VALUE 3.CHROMA
Conglomerate
Conglomerate: lithified gravel made up of rounded to subangular clasts with diameters greater than 2 mm conglomerate-gravel (granule,pebble,cobble)
Sediment Properties sedimentary rock
Conglomerate: rounded, subrounded, subangular clasts Breccia: Angular clasts Sandstone (clast roundness variable) mudrock: siltstone, mudstone, claystone
SEDIMENTARY ROCKS Siliclastic
Conglomerates Sandstones Mudrocks Diagenesis of siliclastic rocks
Completeness of the Geologic Record Sediment Deposition Rates vs. Time Rates vary over more than ____ orders of magnitude, due to:
Deposition rates vary between different sedimentary environments, and vary depending on the time span over which they are measured. Rates vary over more than 12 orders of magnitude, due to: • Environmental differences • Dewatering and compaction • Uplift • Erosion • Metamorphism and subduction
Sedimentary Petrography:
Detailed description of sedimentary rocks.
Sedimentary Structures Types of Sedimentary Structures: Primary Structures formed on the surface of sediment bedding planes
Flute casts on the bottom of a turbidite bed, Ordovician Normanskill Fm., NY. Tapered ends point downstream. Current from lower right to upper left. tool Marks include circular skip casts from spool-shaped fish vertebrae, shallow brush marks, and deeper drag marks.
Rudite:
Generic term for very coarse clastic rocks (gravels). --Rudites constitute approximately 1% - 2% of all sedimentary rocks.
Completeness of the Geologic Record Great Extinctions: who proposed? -at least __?__times over the past 600 million years, life has experienced "mass extinctions" why?
Georges Cuvier ((1769-1832)) : The fossil evidence led him to propose that periodically the Earth went through sudden changes, each of which could wipe out a number of species, a mass extinction. - Cuvier is the originator of the extinction theory. Darwin held that individual species gradually became extinct as part of the natural process of evolution. Cuvier believed that species go extinct due to catastrophes -at least five times over the past 600 million years, life has experienced "mass extinctions", in which nearly half of all species alive at the time disappeared in fewer than two million years—a blink of a geological eye. The causes may include -asteroids, -volcanoes, -or relatively fast changes in sea level
Sediment Properties unconsolidated
Gravel- rounded, subrounded, subangular clasts Rubble- angular clasts sand, silt, mud, clay (great to least)
Hydrodynamics and Sediment Transport Forces Acting on Particles During Fluid Flow: Inertial Forces
Inertial Forces a) Gravity: function of grain diameter (d) and mineral (grain) density (ρ) • Hydraulic equivalence: ρ1 *v1= ρ2*v2 [ρ=grain density; v=grain volume] if • ρ1 > ρ2 and v2> v1; • Heavy mineral concentrates, placer deposits b) Electrostatics: surface area to volume relationships • Fine-grained particles have substantial surface electrostatic attractive/repulsive properties c) Friction
Sedimentary Structures Types of Sedimentary Structures: Primary 2) Bedforms created by multidirectional flow Herringbone cross-bedding Interference ripples Lenticular bedding Flaser bedding Wavy bedding
Lenticular bedding: small lenses of sand trapped in troughs in mud. Flaser bedding: minor mud layers in a sandy substrate Wavy bedding: equal mixture of sand and mud
Types of Sedimentary Structures: Secondary1)Mechanical structures (soft-sediment deformation)
Load casts Ball and pillow structures Flame structures Convolute bedding
Hydrodynamics and Sediment Transport Forces Acting on Particles During Fluid Flow: Mobility Forces
Mobility Forces a) Fluid drag: frictional interaction and orientation of fluid motion vectors • Fluid drag force is higher for turbulent versus laminar flow b) Bernoulli (lift) force: high velocity = low pressure c) Buoyancy: • Buoyancy lift force is relative to density contrast of grain versus the continuous fluid • High density sediment-charged water or gas has a smaller density contrast and greater grain mobility
Hydrodynamics and Sediment Transport Critical Threshold for Particle Entrainment
Mobility Forces > Inertial Forces Hjulstrom Diagram - Empirical relationship between grain size (quartz grains) and current velocity (using standard temperature, clear water) - Defines critical flow velocity threshold for entrainment - As grain size increases entrainment velocity increases (sand size and larger particles) - For clay-size particles electrostatics requires increased flow velocity for entrainment
Modern View on Uniformitarianism
Modern View: "Geologic processes remain similar, but rates and intensities vary significantly over time. " -For example, weathering rates have varied with the concentration of oxygen in the Earths atmosphere (originally zero; today 21%). -Also some infrequent processes may be inactive for long periods of time (e.g., major bolide impacts). Infrequent processes, such as major storms, tsunamis, great earthquakes, major volcanic eruptions, meteorite impacts, etc., may have a greater impact on the geologic record than day-to-day processes. -Recent fossil evidence indicates that major extinctions have periodically occurred, causing a majority of species to die off.
examples of depositional system:
Modern example: succession of facies in a prograding coastal barrier island depositional system Ancient example: Book Cliffs, Utah - Cretaceous prograding coastal barrier depositional system
Types of Unconformities (major changes which delete a significant part of the record)
Nonconformity- two different rock types in contact, representing a large time gap; e.g., sedimentary rocks deposited on top of igneous rocks) Angular unconformity- set of sedimentary rocks overlying another set that is tectonically deformed, indicating a large time gap Disconformity or Erosional Unconformity- successive rock layers are essentially parallel, but erosional surface is physically identifiable Paraconformity- successive rock layers are parallel, with no obvious erosional surface, but sediment layers are absent, as attested, for example, by missing parts of the fossil record
Phi Scale: Phi = - log2 (diameter in mm)
Phi = - log(base2) (diameter in mm) Log(base2)N=(Log(base10)[N])/(Log(base10)[2]) The phi scale was introduced by Krumbein (1934) as a convenient means of visualizing and statistically analyzing distributions over a wide range of sediment grain sizes.
Completeness of the Geologic Record Relationship Between the Preserved Sedimentary Record & Age
Preservation of the geologic record deteriorates with time. Outcrops of older deposits are systematically less common. As a result, older reconstructions of the continents, climates, etc., are inherently less reliable. The Precambrian era (85% of geologic time) is represented by only ~5% of the sedimentary record.
Powerpoint #1B Outline
Sediment Properties Weathering Hydrodynamics and Sediment Transport Sedimentary Structures
Hydrodynamics and Sediment Transport Development of Bedforms
Sediment bedforms observed in flume experiments over a range of velocities (left), vs. ideal turbidite facies model (right).
SEDIMENTARY ROCKS
Siliclastic Carbonate
Sediment:
Solid bits and pieces of material produced by weathering, transported by various agents like wind, ice, running water and mass movement, and either deposited or precipitated in layers on, at, or near Earth's surface, normally as loose, unconsolidated material.
Sedimentology:
Study of sedimentary rocks, their classification, origin, and interpretation, and the processes by which sediment is produced, transported and deposited.
Stratigraphy:
Subdiscipline of geology that examines the complex distribution of the sedimentary rock record in time and space in order to understand Earth's history.
Hydrodynamics and Sediment Transport Particle Settling: Stokes' Law
Three forces act on a particle moving through a fluid: 1)The external force (gravitational or centrifugal) 2) The buoyant force, which acts parallel with the external force but in the opposite direction 3) The drag force, which appears whenever there is relative motion between the particle and the fluid [Drag: the force in the direction of flow exerted by the fluid on the solid.]
Sediments -------->Sedimentary Rocks
Weathering --->Sediment Particles--->Sediment Transport--->Deposition, sedimentary structures--->Sedimentary Rocks
Weathering
Weathering-->Transport--> Deposition--> Diagenesis--> Clastic sedimentary rock
Sedimentary rock:
a consolidated body formed from sediments or solutes that are transported and deposited by gravity, biologic activity, or a fluid and then lithified by the combined effects of compaction and cementation
Depositional system:
assemblage of multiple process-related sedimentary facies assemblages, commonly identified by the type of geographic location in which deposition occurs. (Examples: coastal depositional system; nearshore depositional system; deep marine depositional system; glacial depositional system; etc.) NOTE: depositional systems are modern features, which are used to interpret ancient sedimentary successions
Completeness of the Geologic Record Relationship of Unconformities to Base-Level Change
base level change up = preservation, base leven down= erosion and hiatus in gap produced
Completeness of the Geologic Record: Relationship Between the Preserved Sedimentary Record and Base Level
because base level fluctuates up and down, most geologic time isnt represented by the sedimentary record.
• Facies assemblage:
collection of multiple facies resulting from genetically related accumulation and modification. (Example: lenticularly bedded stratified pebble conglomerate with subordinate planar cross-stratified sandstone)
Completeness of the Geologic Record: Development of Bedding Planes
erosion of dewatered & compacted beds by storms : *animated GIF
Strata:
layers of (usually sedimentary) rock
Weathering Physical vs. Chemical Weathering
low moisture and temp = physical weathering high moisture and temp= chemical weathering
Unconformities
major changes which delete a significant part of the record
sedimentary rocks relative abundance increases during times of _________?
orogenies (mountain-building episodes)
Angular unconformity
set of sedimentary rocks overlying another set that is tectonically deformed, indicating a large time gap
Hydrodynamics and Sediment Transport Particle Settling: Stokes' Law EQN
settling velocity = [(ρs - ρf)g(d^2)]/[18µ] VS = settling velocity ρs = sediment particle density ρf = fluid density µ = fluid viscosity d = grain diameter (mm) • Simplified Stoke's "law" , C is a constant VS (cm/sec) = Cd2 • Ruby's Curve: Stoke's + Newton's curves - Newton's Curve considers only turbulent drag on all grains - Stoke's settling considers only laminar flow only; therefore invalid for sand > 1 phi (.5mm)
respective order of: facies assemblage, depositional system, facies
smallest to largest: facies, facies assemblage, depositional system
Disconformity or Erosional Unconformity
successive rock layers are essentially parallel, but erosional surface is physically identifiable
Paraconformity
successive rock layers are parallel, with no obvious erosional surface, but sediment layers are absent, as attested, for example, by missing parts of the fossil record
Global Sea-Level Change, Based on Seismic Stratigraphy
the major oscillations are primarily global in origin, associated with continental breakup and the formation of new ocean ridge systems.
Facies:
the total textural, compositional and structural characteristics of a sedimentary deposit, resulting from accumulation and modification in a particular environment. These characteristics might include sediment texture (grain size), lithology (composition), sedimentary structures, and bedding type. (Example: well-sorted, arkosic, cross-stratified, coarse-grained sandstone)
Nonconformity
two different rock types in contact, representing a large time gap; e.g., sedimentary rocks deposited on top of igneous rocks)
Sedimentary Structures Types of Sedimentary Structures: Primary Bedforms created by unidirectional flow Variation in ripple forms and stratification caused by changes in:
velocity, grain size, depth, rate of sediment supply, and flow direction
Average deposition rates for Phanerozoic sequences:
~(10^-3) m/(10^3)yr.
Sedimentary Rocks Conglomerate and Breccia Characteristics
Ø Products of a high-energy environment (e.g., alluvial fan, braided river) Ø Short distance of transport Ø Often the product of tectonic activity Ø Poorly sorted Ø Useful in determining provenance of the source area
Sediment Properties Texture: Sorting, Roundness, Sphericity
• Sorting: measure of the diversity of grain size - A function of grain origin and transport history • Roundness: surface irregularity - Due to prolonged agitation during transport and reworking • Sphericity: degree to which grain approximates a sphere - A function of mineralogy and hardness
Hydrodynamics and Sediment Transport: Entrainment, Transport and Deposition
• Terrigenous sediments strongly reflect their source and are transported to the sea by rivers, wind, and glaciers. • Rate of erosion is important in determining nature of sediments. • Average grain size reflects the energy of the depositional environment. • Hjulstrom Diagram: relationship between particle size and flow velocity (energy available for entrainment, transportation and deposition).
Hydrodynamics and Sediment Transport Traction
• With a grain at rest, as flow velocity increases Fmobility remains less than < Finertial , but fluid drag causes grain rolling • Grain Traction: for large grains (typically pebble size and larger) - Normal surface (water) currents have too low a U for grain entrainment - Bedload Transport Mode
Hydrodynamics and Sediment Transport saltation
• With a grain at rest, as flow velocity increases, Fmobility becomes greater than Finertial and particle motion is initiated • Grain Saltation: for larger grains (sand size and larger) - When Fm > Fi -- U > VS but through time/space U < VS - Intermittent Suspension + Bedload Transport Mode
Hydrodynamics and Sediment Transport Suspension
• With a grain at rest, as flow velocity increases, Fmobility becomes greater than Finertial and particle motion is initiated • Grain Suspension (for small particle sizes, fine silt <0.01mm) - When Fm > Fi -- U (flow velocity) >>> VS (settling velocity) - Constant grain Suspension occurs at relatively low U (flow velocity) - Wash Load Transport Mode (suspension)
