Transcript lecture19y
Interpreting the Past Part 1 More Chapter 17 plus Chapter 18 To interpret the geologic past • History of Earth and its life can be read as a sequence of events • Geological record events can be interpreted by a study of the present – The principle of uniformitarianism: • Developed by James Hutton • “the present is the key to the past” • Championed by Charles Lyell in his book Principles of Geology • Changes can be gradual or sudden Sometimes, however, catastrophes occur. Beginnings of relative dating To make sense of the data, geologists constructed the geologic time scale – It records major geologic events – It notes appearances, and disappearances, of varied forms of life The sedimentary record • Formed by familiar surface processes • Sediments 75% of surface-exposed rock • Sediment sequences record environments where they accumulated • Different sequences represent different times and places: – Evidence of past mountain belts and coastlines, seas, rivers, lakes, etc. – Some give information about past climates Sedimentary Facies • Individual rock types are not specific to just a single environment of deposition • Example: Sand that eventually becomes sandstone may be deposited in river channels, sand dunes, beaches, a shallow ocean shelf, or a deep ocean landslide (a turbidite), etc. Sedimentary Rock Interpretation • To identify the depositional environment for a sedimentary rock body, we must consider: • Sedimentary structures • Fossils & the “niche” of modern counterparts - define • The relationships to other sedimentary rock units both above and below and laterally i.e. the sedimentary facies Stratigraphy • The study of sedimentary rock sequences • Layering of sedimentary rock is the result of grain size or grain composition changes during deposition • The changes record: (a) Tectonic activity (b) Rising or falling sea level (c) Climate changes (d) Variations in sediment type deposited Division of stratigraphic sequences • We need discrete units to define lateral and vertical rock relationships “lithostratigraphic” • The most commonly used stratigraphic unit is the FORMATION: – A rock body distinguishable from rocks above and below it – A rock body large enough to be shown on a map • It may contain one sedimentary facies over a wide area, for example a widespread limestone • OR it may contain several time related facies: alluvial fan conglomerate, point bar stream deposits, lake mudstones. Names of rock formations Bloomsburg Formation named for Bloomsburg, Pennsylvania • “Formation” as second part of name if rock types are diverse • Correlated with similar rocks in, for example, the Delaware Water Gap area, there called High Falls Formation. You will hear me call it “High Falls Bloomsburg” Names of rock formations • Or, a rock formation may be named for a single rock type • Navajo Sandstone • Redwall Limestone Correlation of strata in southwestern United States Some are named “Formation” Others sandstone (“Ss”) or Limestone (“Ls”) Subdivision of formations • A formation may be divided into members • For example, the red “Passaic Formation*” outside can be divided into members, e.g. the Perkasie member, Graters member, etc. *Formerly called the Brunswick Formation Groups of Formations Two or more vertically adjacent formations can be combined into a group. For example, the Late Triassic and Early Jurassic rocks around here form the Newark Group. • Included are the Passaic Formation, the Lockatong Formation, the Stockton Sandstone. • Related groups can be combined into a Supergroup • These formed in a rift valley, eventually flooded, opening the Atlantic Role of Tectonic Forces - 1 • Uplift may control distribution of sedimentary facies: • Uplift increases stream gradients and velocity, rate of erosion of sediments into basin. Streams faster, erosion greater. • Example: Continent-Continent collision • Causes thick sedimentary clastic wedges Clastic wedges Thick and coarse-grained close to source area, thinner and more finegrained further from source area Shallow water deposition in a basin Crust deforms as weight increases. Basin floor stays at about the same depth. Transgression and Regression • • • Changing sea level greatly influences distribution of depositional facies The process of transgression ( rising sea level): the ocean rises and covers the continental margin Shoreline moves towards continental interior •Global sea level changes are called eustatic Regression: The process of regression produces lowering of sea level Shoreline migrates away from the continental interior Very useful for correlation – widespread erosion causes unconformities – Sequence Stratigraphy Eustatic sea-level changes for the Phanerozoic Eon Note how often period boundaries correspond to regressions These same boundaries often correspond to extinctions REGRESSION exposes shelf K\T > Regression at boundary Pm\TR largest > Absaroka extinction, no regression, definite ash layer Kaskasia Tippecanoe Sauk Long-term sea level change • They last tens of millions of years • Controlled by size of MORs • Warm, active, buoyant ridges displace ocean waters onto continental margins (transgression) • Cold, inactive, dense ridges are smaller and basins have more room for water. (regression) Rapid sea-floor spreading Growing MOR takes up basin volume, sea level rises - transgression Cessation of sea-floor spreading MOR cools and shrinks, sea level drops - regression Very short Sea-level Cycles • Associated with growth and shrinkage of ice sheets • Last hundreds of thousands of years • Result from removal of water by storage as glacial ice • Exposed much of continental shelf surrounding North America • Sea level as much as 140 meters lower than present Recall Lowered Sea-level –Pleistocene Caused exposed shelf Land Bridge Recall Sediments controlled by Energy • Sand deposited at shoreline and in adjacent shallow water High Kinetic Energy – surf, longshore drift • Mud deposited in calm, deeper water Low Kinetic Energy lakes in winter, lagoons, bays, very deep ocean beneath the surface Some time-equivalent sediments Differ in energy and distance from source Facies changes with Transgression • Rising sea level = shoreward migration of sedimentary facies • Deeper water sediment over shallower • Mud facies is superimposed over sandy beach facies • Carbonate Ooze facies is superimposed over Mud facies • “FINE-ING” UPWARD Effect of transgression on Note the wedge sedimentary facies Deeper water sediment over shallower (see center block) Mud facies is superimposed over sandy beach facies Carbonate Ooze facies is superimposed over Mud facies Regression • Falling sea level causes landward facies to be superimposed on the seaward facies • Sandy beach facies is superimposed over muddy lagoon facies • Shallower water sediment (coarse) over deeper (fine) • COARSENING UPWARD sequence OR a DISCONFORMITY due to “subaerial exposure” i.e. shelf or inland sea sediments were exposed to erosion when sea-level dropped Coarsening upward sequence Regression Coarse Sands of Mesa Verde Group Fine Muds of Mancos Shale Regression: from deepwater to shallow water over any spot Walther’s Law • Vertical sequences of sedimentary facies result from superposition of laterally adjacent depositional environments. • Used to recognize changes in sea-level t-r • Used to recognize neighboring facies Walther’s Law Transgression fining (deeper) upward Vertical sequences of sedimentary facies result from superposition of laterally adjacent depositional environments Paleogeography • Reconstruction of past landscapes • Example: Asymmetrical stream ripples indicate flow directions of rivers • Gives location of high topography • Paleocurrent indicators in delta deposits: Size of ripples and their form • Continent positions from Paleomagnetics Paleoecology • The reconstruction of past ecosystems • Example: • Type and thickness of vegetation near pond, oxbow, lake, or floodplain can be determined from fossil leaves and pollen, and fossil soil layers • Niche of modern animals can be related to fossil animals similar in skeleton, teeth. Sedimentology • Interpret sediments deposited in past by comparison to sediments in modern environments • Energy decreases with depth • Fine-grained sediment indicates deep water • So: Fining-upward sequence of sediments indicates decreasing energy (deeper water - Transgression) • And: Coarsening-upward sequence of sediments indicates increasing energy (shallower water - Regression) Paleoclimatology • Certain types of sedimentary rocks are good indicators of paleoclimate • Evaporites indicate dry regions • Coals indicate swamp conditions • Dunes and desert pavement semi-arid to arid regions • Tillites, striated rock surfaces, loess etc. record very cold glacial conditions End of Part 1 Importance of Fossils • Many early philosophers (including Aristotle) recognized fossils are remains of ancient life • Importance of fossils to geology realized later • Wm. Smith – Mapping in Wales and England developed Principle of faunal succession – Different-aged rock layers contained different fossils – Allowed formulation of the geologic time scale Fossil Wooly Mammoth Pleistocene and Cold Part 2 • Mostly Chapter 18 Index fossils vs. Long Ranging Long range useless for correlation Short range “index fossil” Wm. Smith: used fossils to correlate rocks far apart and establish relative age John Ray (1680) Grouped organisms according to similarity Domains of life Carl von Linne’ ,1760, called Linnaeus Binomial Nomenclature (Genus and species) Similar species grouped together in a Genus (capitalized) each with a unique species name (lower case). Example lion Felis leo, African wild cat Felis silvestris Note plural of species is species. Plural of genus is genera. Similar genera grouped into families, families into orders, orders into classes, etc. Mnemonic Species based hierarchy of relationships Darwin’s Role • Knew of similarities among diverse organisms, Naturalist on Beagle 1831-1836, saw changes in fossils through time. • Knew of Artificial Selection by farmers • Was aware of Malthus (1798) essays on population, “more individuals are born than food supply can support.” • Read Lyell’s Principles of Geology- supports Hutton’s uniformitarianism/gradual change • Suggested Natural Selection http://www.aboutdarwin.com/voyage/voyage01.html Evidence: Homologous structures Homology: Same anatomy to make different structures. Why, if not related? Darwin’s Idea: Natural Selection • Organisms produce more young than 2 • There is competition for food and mates • There is variation in characteristics (types) essential to survival and reproduction • Some individuals (types) survive to reproductive age, mate and have progeny more frequently than others. This is “success”. • “Successful” individuals pass on their characteristics to the next generation http://www.ucmp.berkeley.edu /history/lamarck.html Inheritance Individuals vary, but we look like our parents How does variation get passed on? • Jean Baptiste de Lamarck (pub.1801) – Acquired Traits are inherited - WRONG – Giraffe strains to reach high leaves, offspring have longer necks – failed idea • Johann Gregor Mendel (pub.1866) Established rules of inheritance in peas. – Traits controlled by genes – Genetic traits are inherited - RIGHT http://anthro.palomar.edu/mendel/mendel_1.htm Mendel 1865 -1866 Prevailing idea was that the characteristics of an organism were due to the blending of the traits from each parent (blending inheritance). Mendel proposed instead that an “element” determined a particular characteristic of an organism. Called particulate inheritance. The element (now called ‘gene’) is the fundamental unit of heredity. Mendel’s work was not noticed at the time 1865-66 Rediscovered by Hugo deVries, Carl Correns, and Erik von Tschermak in the early 1900’s. Mendel’s (1865 -1866) Discoveries Some physical traits are caused by inheritable particles, now called genes May occur in two forms: one is dominant (capital letter, e.g. R, the other recessive (lower case letter, e.g. r). Every individual gets two copies of each gene, one from each parent. Only need one dominant to get normal appearance. Ex: Pea flower color, normal is Red Can get RR, Rr, rR, (all red) or rr (white) Double recessive rr red gene is broken, no red, flower white Mendel’s experiments with peas Flower color Suppose you cross a RR plant with a white plant Parents Some traits (flower color) occur in two forms. One (Red flower R) is dominant. The other (white r) is recessive First generation all Rr If Red (R) and white (r) parents 3/4 of next generation 1/4 are Red (dominant) 1/4 1/4 + 1/4 = 1/2 Where are the genes? • Early 1900s growing suspicion that the inheritance material is in the chromosomes • Evidence from microscope studies of cell division (Mitosis) versus gamete formation (Meiosis) Mitosis – cell division In Mitosis (cell division) the chromosomes duplicate and separate, then the cell divides, so each daughter cell again has 2 pairs of chrosomes. Two copies of each per cell. One from mother, one from father. Diploid But in Meiosis – formation of gametes: But in Meiosis, gamete formation, an extra division makes cells with one copy of each chromosome, so many possible combinations Each gamete has, for each chromosome, either version, not both Sexual reproduction One from each parent How does the genetic material make copies? Chromosomes contain lots of DNA deoxyribose nucleic acid Chargaff’’s Rules on DNA components weighed nucleotides The amount of Guanine (G) equals Cytosine (C) The amount of Adenine (A) equals Thymine (T) Adenine and Guanine are Purines,Thymine and Cytocine are Pyrimidines How does the genetic material make copies? http://www.pbs.org/wgbh/nova/photo51/ Francis Crick, James Watson (models) Maurice Wilkins, Rosalind Franklin (x-rays) of DNA The Double Helix by James Watson DNA molecule Francis Crick, James Watson, Maurice Wilkins, Rosalind Franklin It has not escaped our notice … • Chargaff’s Rules make sense immediately • Cytosine only fits Guanine (3 H bonds) • Thymine only fits Adenine (2 H bonds) • They will bond only to each other, making a perfect copy on the opposite strand • Parents can pass a copy of their DNA in their gametes Breaking the Genetic Code • RNA • The genetic code UUU (Uracils) => Phenylalanine • One gene, one polypeptide chain, enzymes Marshall Warren Nirenberg Speciation – Geographic Isolation Separated – populations eventually unable to interbreed - definition of “distinct species”