1 / 186

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

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

Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.


Presentation Transcript

  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?...how 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

More Related