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Rocks, Fossils and Time—Making Sense of the Geologic Record

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  1. Chapter 5 Rocks, Fossils and Time—Making Sense of the Geologic Record -Basic Laws: Superposition, Horizontality, Inclusions, lateral continuity -Unconformities: angular, disconformity, non conformity -Sea Level changes: transgression (rise) or regression (fall) -Sedimentary facies -FOSSILS: what are they? do they form?...importance… -use of fossils and rocks to tell ‘relative time’… -use of fossils and rocks together to correlate rock outcrops.. -importance of ‘guide fossils’

  2. Geologic Record • The fact that Earth has changed through time • is apparent from evidence in the geologic record • The geologic record is the record • of events preserved in rocks • Although all rocks are useful • in deciphering the geologic record, • sedimentary rocks are especially useful • The geologic record is complex • and requires interpretation, which we will try to do • Uniformitarianism is useful for this activity

  3. Stratigraphy • Stratigraphy deals with the study • of any layered (stratified) rock, • but primarily with sedimentary rocks and their • composition • origin • age relationships • geographic extent • Sedimentary rocks are almost all stratified • Many igneous rocks – from volcanoes • such as a succession of lava flows or ash beds • are stratified and obey the principles of stratigraphy • Many metamorphic rocks are stratified • metamorphic rocks- formed by igneous intrusions that heat and change minerals in original rocks

  4. Stratified Igneous Rocks • Stratification in a succession of lava flows in Oregon.

  5. Stratified Sedimentary Rocks • Stratification in sedimentary rocks consisting of alternating layers of sandstone and shale, in California.

  6. Stratified Metamorphic Rocks • Stratification in Siamo Slate, in Michigan

  7. Law of Superposition • Nicolas Steno realized that he could determine • the relative ages of horizontal (undeformed) strata • by their position in a sequence: …youngest rocks are on top, oldest on bottom… • In deformed strata, the task is more difficult • but some sedimentary structures • such as cross-bedding • and some fossils • allow geologists to resolve these kinds of problems • we will discuss the use of sedimentary structures • more fully later in the term

  8. Principle of Inclusions • According to the principle of inclusions, • which also helps to determine relative ages, • inclusions or fragments in a rock • are older than the • rock itself • Light-colored granite • in northern Wisconsin • showing basalt inclusions (dark) • Which rock is older? • Basalt, because the granite includes it

  9. Age of Lava Flows, Sills • Determining the relative ages • of lava flows, sills and associated sedimentary rocks • uses alteration by heat • and inclusions • How can you determine • whether a layer of basalt within a sequence • of sedimentary rocks • is a buried lava flow or a sill? • A lava flow forms in sequence with the sedimentary layers. • Rocks below the lava will have signs of heating but not the rocks above. • The rocks above may have lava inclusions.

  10. Sill • The sill might also have inclusions of the rocks above and below, • but neither of these rocks will have inclusions of the sill. • A sill will heat the rocks above and below.

  11. Unconformities • So far we have discussed vertical relationships • among conformable strata, • which are sequences of rocks • in which deposition was more or less continuous • Unconformities in sequences of strata • represent times of nondeposition and/or erosion • that encompass long periods of geologic time, • perhaps millions or tens of millions of years • The rock record is incomplete. • The interval of time not represented by strata is a hiatus.

  12. The origin of an unconformity • In the process of forming an unconformity, • deposition began 12 million years ago (MYA), • continuing until 4 MYA • For 1 million years erosion occurred • removing 2 MY of rocks • and giving rise to • a 3 million year hiatus • The last column • is the actual stratigraphic record • with an unconformity

  13. Types of Unconformities • Three types of surfaces can be unconformities: • A disconformity is a surface • separating younger from older rocks, • both of which are parallel to one another • A nonconformity is an erosional surface • cut into metamorphic or intrusive rocks • and covered by sedimentary rocks • An angular unconformity is an erosional surface • on tilted or folded strata • over which younger rocks were deposited

  14. Types of Unconformities • Unconformities of regional extent • may change from one type to another • They may not represent the same amount • of geologic time everywhere

  15. A Disconformity • A disconformity between sedimentary rocks • in California, with conglomerate deposited upon • an erosion surface in the underlying rocks

  16. An Angular Unconformity • An angular unconformity in Colorado • between steeply dipping Pennsylvanian rocks • and overlying Cenozoic-aged conglomerate

  17. A Nonconformity • A nonconformity in South Dakota separating • Precambrian metamorphic rocks from • the overlying Cambrian-aged Deadwood Formation

  18. Sea Level Change- Marine Transgressions • A marine transgression • occurs when sea level rises • with respect to the land • During a marine transgression, • the shoreline migrates landward • the environments paralleling the shoreline • migrate landward as the sea progressively covers • more and more of a continent

  19. Marine Transgressions • Each laterally adjacent depositional environment • produces a sedimentary facies • During a transgression, • the facies forming offshore • become superposed • upon facies deposited • in nearshore environments

  20. Marine Transgression • The rocks of each facies become younger • in a landward direction during a marine transgression • One body of rock with the same attributes • (a facies) was deposited gradually at different times • in different places so it is time transgressive • meaning the ages vary from place to place younger shale older shale

  21. A Marine Transgression in the Grand Canyon • Three formations deposited • in a widespread marine transgression • exposed in the walls of the Grand Canyon, Arizona

  22. Marine Regression • During a marine regression, • sea level falls • with respect • to the continent • and the environments paralleling the shoreline • migrate seaward

  23. Walther’s Law • Johannes Walther (1860-1937) noticed that • the same facies he found laterally • were also present in a vertical sequence, • now called Walther’s Law • which holds that • the facies seen in a conformable vertical sequence • will also replace one another laterally • Walther’s law applies • to marine transgressions and regressions

  24. Extent and Rates of Transgressions and Regressions • Since the Late Precambrian, • 6 major marine transgressions followed • by regressions have occurred in North America • These produce rock sequences, • bounded by unconformities, • that provide the structure • for U.S. Paleozoic and Mesozoic geologic history • Shoreline movements • are a few centimeters per year • Transgression or regressions • with small reversals produce intertonging

  25. Causes of Transgressions and Regressions • Uplift of continents causes regression • Subsidence causes transgression • Widespread glaciation causes regression • due to the amount of water frozen in glaciers • Rapid seafloor spreading, • expands the mid-ocean ridge system, • displacing seawater onto the continents • Diminishing seafloor-spreading rates • increases the volume of the ocean basins • and causes regression

  26. Fossils • Fossils are the remains or traces of prehistoric organisms • They are most common in sedimentary rocks • and in some accumulations • of pyroclastic materials, especially ash • They are extremely useful for determining relative ages of strata • but geologists also use them to ascertain • environments of deposition • Fossils provide some of the evidence for organic evolution • and many fossils are of organisms now extinct

  27. How do Fossils Form? • Remains of organisms are called body fossils. • and consist mostly of durable skeletal elements • such as bones, teeth and shells • rarely we might find entire animals preserved by freezing or mummification

  28. Body Fossil • Skeleton of a 2.3-m-long marine reptile • in the museum at Glacier Garden in Lucerne, Switzerland

  29. Trace Fossils • Indications of organic activity • including tracks, trails, burrows, and nests • are called trace fossils • A coprolite is a type of trace fossil • consisting of fossilized feces • which may provide information about the size • and diet of the animal that produced it

  30. Trace Fossils • Fossilized feces (coprolite) • of a carnivorous mammal • Specimen measures about 5 cm long • and contains small fragments of bones

  31. Body Fossil Formation • The most favorable conditions for preservation • of body fossils occurs when the organism • possesses a durable skeleton of some kind • and lives in an area where burial is likely • Body fossils may be preserved as • unaltered remains, • meaning they retain • their original composition and structure, • by freezing, mummification, in amber, in tar • or altered remains, • with some change in composition or structure • permineralized, recrystallized, replaced, carbonized

  32. Unaltered Remains • Insects in amber • Preservation in tar

  33. Altered Remains • Petrified tree stump • in Florissant Fossil Beds National Monument, Colorado • Volcanic mudflows • 3 to 6 m deep • covered the lower parts • of many trees at this site

  34. Altered Remains • Carbon film of a palm frond • Carbon film of an insect

  35. Molds and Casts • Molds form • when buried remains leave a cavity • Casts form • if material fills in the cavity

  36. Mold and Cast Step a: burial of a shell Step b: dissolution leaving a cavity, a mold Step c: the mold is filled by sediment forming a cast

  37. Molds and Casts

  38. Cast of a Turtle • Fossil turtle • showing some of the original shell material • body fossil • and a cast

  39. Fossil Record • The fossil record is the record of ancient life • preserved as fossils in rocks • Just as the geologic record • must be analyzed and interpreted, • so too must the fossil record • The fossil record • is a repository of prehistoric organisms • that provides our only knowledge • of such extinct animals as trilobites and dinosaurs

  40. Fossil Record • The fossil record is very incomplete because • bacterial decay, • physical processes, • scavenging, • and metamorphism • destroy organic remains • In spite of this, fossils are quite common

  41. Rocks, Fossils and Time—Making Sense of the Geologic Record Fossils have many uses- a. Give an indication of relative time in comparison to other rocks and fossils above, below and laterally to a particular layer.. b. Some fossils can be used as indicators of paleoenvironment- i.e. they are indicative of certain environments that they lived in. Benthic forams =bottom dwellers, different forams lived in different water depths. Planktonic forams- float near surface in ocean. Recognize differences between low to mid to high latitude forms (morphology). Pollen- indicative of swamps, forests, humid vs dry environments. c. Preservation of calcareous vs siliceous fossils are indicative of certain depositional or post depositional processes.

  42. Fossils and Telling Time • William Smith • 1769-1839, an English civil engineer • independently discovered • Steno’s principle of superposition • He also realized • that fossils in the rocks followed the same principle • He discovered that sequences of fossils, • especially groups of fossils • are consistent from area to area • Thereby discovering a method • of relatively dating sedimentary rocks at different locations

  43. Fossils from Different Areas • Smith used fossils • To compare the ages of rocks from two different localities

  44. Principle of Fossil Succession • Using superposition, Smith was able to predict • the order in which fossils • would appear in rocks • not previously visited • Alexander Brongniart in France • also recognized this relationship • Their observations • lead to the principle of fossil succession

  45. Principle of Fossil Succession • Principle of fossil succession • holds that fossil assemblages (groups of fossils) • succeed one another through time • in a regular and determinable order • Why not simply match up similar rocks types? • Because the same kind of rock • has formed repeatedly through time • Fossils also formed through time, • but because different organisms • existed at different times, • fossil assemblages are unique

  46. Geologic Column and the Relative Geologic Time Scale Absolute ages (the numbers) were added much later.

  47. Stratigraphic Terminology • Because sedimentary rock units • are time transgressive, • they may belong to one system in one area • and to another system elsewhere • At some localities a rock unit • straddles the boundary between systems • We need terminology that deals with both • rocks—defined by their content • lithostratigraphic unit – rock content • biostratigraphic unit – fossil content • and time—expressing or related to geologic time • time-stratigraphic unit – rocks of a certain age • time units – referring to time not rocks

  48. Lithostratigraphic Units • Lithostratigraphic units are based on rock type • with no consideration of time of origin • The basic lithostratigraphic element is a Formation • which is a mappable rock unit • with distinctive upper and lower boundaries • It may consist of a single rock type • such as the Redwall limestone • or a variety of rock types • such as the Morrison Formation • Formations may be subdivided • into members and beds • or collected into groups and supergroups

  49. Lithostratigraphic Units • Lithostratigraphic units in Zion National Park, Utah • For example: The Chinle Formation is divided into • Springdale Sandstone Member • Petrified Forest Member • Shinarump Conglomerate Member

  50. Lithostratigraphic Correlation • Correlation of lithostratigraphic units such as formations • traces rocks laterally across gaps