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

Chapter 5. Rocks, Fossils, and Time— Making Sense of the Geologic Record. 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

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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

  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 • sedimentary rocks are especially useful • in deciphering the geologic record, • The geologic record is complex • and requires interpretation • Uniformitarianism offers a useful approach

  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 • Almost all sedimentary rocks are stratified • Many volcanic rocks • such as lava flows or ash beds • as well as many metamorphic rocks • are stratified and obey the principles of stratigraphy

  4. Stratified Sedimentary Rocks • Although these rocks in South Dakota • are deeply eroded • stratification is still clearly visible

  5. Stratified Rocks • Stratified rocks in California are • deformed so that they are no longer in their original position

  6. Vertical Stratigraphic Relationships • Surfaces known as bedding planes • separate individual strata from one another • or the strata grade vertically • from one rock type to another • Rocks above and below a bedding plane differ • in composition, texture, color • or a combination of these features • The bedding plane signifies • a rapid change in sedimentation • or perhaps a period of nondeposition

  7. Superposition • Nicolas Steno realized that he could determine • the relative ages of horizontal (undeformed) strata • by their position in a sequence • In deformed strata, the task is more difficult • but some sedimentary structures • and some fossils • allow geologists to resolve these kinds of problems

  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 contact metamorphism effects • 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 at this location • 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 in sedimentary rocks • 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 Montana, Jurassic-age rocks rest unconformably on top of Mississippian-age strata • an erosion surface separates the two.

  16. A Nonconformity • A nonconformity between Precambrian granite • and the Cambrian Formation • in Bighorn Mountains, Wyoming

  17. An Angular Unconformity • An angular unconformity between the flat-lying Medial Jurassic Entrada Sandstone and underlying Upper Jurassic red beds in New Mexico.

  18. Lateral Relationships • In 1669, Nicolas Steno proposed • the principle of lateral continuity, • meaning that layers of sediment extend outward • in all directions until they terminate • Terminations may be abrupt • at the edge of a depositional basin • where they are eroded • where they are truncated by faults

  19. Gradual Terminations • or they may be gradual • where a rock unit • becomes progressively thinner • until it pinches out • or where it splits into • thinner units • each of which pinches out, • called intertonguing • where a rock unit changes • by lateral gradation • as its composition and/or texture • becomes increasingly different

  20. Sedimentary Facies • Both intertonguing and lateral gradation • indicate simultaneous deposition • in adjacent environments • A sedimentary facies is a body of sediment • with distinctive • physical, chemical, and biological attributes • deposited side-by-side • with other sediments • in different environments

  21. 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

  22. 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

  23. 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

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

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

  26. Marine Regression • A marine regression • is the opposite of a marine transgression • It yields a vertical sequence • with nearshore facies • overlying offshore facies • and rock units become younger • in the seaward direction older shale younger shale

  27. 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

  28. Extent, 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 intertonguing

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

  30. Relative Ages between Separate Areas • Using relative dating techniques, • it is easy to determine • the relative ages of rocks • in Column A • and of rocks in Column B • However, you need more information • to determine the ages of rocks • in one section relative to • those in the other

  31. Relative Ages between Separate Areas • Rocks in A may be • younger than those in B, • the same age as in B • or older than in B • Fossils can help to solve this problem

  32. Fossils • Fossils are the remains or traces of past life forms • They are most common in sedimentary rocks • but can be found • iIn volcanic ash and volcanic mudflows • 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

  33. 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

  34. 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 • that may provide information about the size • and diet of the animal that produced it

  35. Trace Fossils • This slab of rock • formed over the actual tracks of birds, • so it is a cast of the tracks

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

  37. 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 • permineralization, replacement, carbonization

  38. Unaltered Remains • Insects in amber

  39. Unaltered Remains • Frozen baby mammoth • found in Russia in 1989

  40. Altered Remains • The bones of this mammoth • on display at the Museum of Geology and Paleontology in Florence, Italy • have been permineralized • with minerals added to the pores and cavities of the bones

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

  42. Molds and Casts • Molds form • when buried remains dissolve and leave a cavity • Casts form • if minerals or sediments fill in the cavity

  43. 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

  44. 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

  45. 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 he discovered a method • whereby relative ages of sedimentary rocks at different locations could be determined

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

  47. 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 • led to the principle of fossil succession

  48. 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

  49. Distinct Aspect • An assemblage of fossils • has a distinctive aspect • compared with younger • or older fossil assemblages

  50. Matching Rocks Using Fossils • Geologists use the principle of fossil succession • to match ages of distant rock sequences • Dashed lines indicate rocks with similar fossils • thus having the same age

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