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

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