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 • 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. Geologic Record • for nearly 14 million years of Earth history • preserved at Sheep Rock • in John Day Fossil Beds National Monument, Oregon • Fossils in these rocks • provide a record • of climate change • and biological events

4. 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 • such as a succession of lava flows or ash beds • are stratified and obey the principles of stratigraphy • Many metamorphic rocks are stratified

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

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

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

8. 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

9. 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 • 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

10. 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

11. 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.

12. 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.

13. 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.

14. 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

15. 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

16. 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

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

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

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

20. Lateral Relationships • In 1669, Nicolas Steno proposed • his 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 eroded • where truncated by faults

21. 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 intertonging • where a rock unit changes • by lateral gradation • as its composition and/or texture • becomes increasingly different

22. Sedimentary Facies • Both intertonging 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

23. Sedimentary Facies • On a continental shelf, sand may accumulate • in the high-energy nearshore environment • while mud and carbonate deposition takes place • at the same time • in offshore low-energy environments

24. 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

25. 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

26. 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

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

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

29. 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

30. 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

31. 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

32. 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

33. 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, one needs more information • to determine the ages of rocks • in one section relative to • those in the other

34. Relative Ages between Separate Areas • Rocks in A may be • younger than those in B, • the same age as in B • older than in B • Fossils could solve this problem

35. 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

36. 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

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

38. Body Fossils • Shells of Mesozoic invertebrate animals • known as ammonoids and nautiloids • on a rock slab • in the Cornstock Rock Shop in Virginia City Nevada

39. 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

40. Trace Fossils • Paleontologists think • that a land-dwelling beaver • called Paleocastor • made this spiral burrow in Nebraska

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

42. 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

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

44. Unaltered Remains • 40,000-year-old frozen baby mammoth • found in Siberia in 1971 • It is 1.15 m long and 1.0 m tall • and it had a hairy coat • Hair around the feet is still visible

45. 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

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

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

48. 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

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

50. 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