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

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 • 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

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. 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

8. 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 • 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

9. 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

10. 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.

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

12. Unconformities • So far we have discussed vertical relationships among conformablestrata, 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.

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

14. 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

15. 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

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

17. An Angular Unconformity • An angular unconformity, Santa Rosa

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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. MarineTransgression

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

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 • 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 facie sand rock units become younger in the seaward direction

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 • 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 or 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 • Trace fossils are indications of organic activity including • tracks, • trails, • burrows, • nests • 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 Paleocastormade 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 • altered remains, with some change in composition • 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