Ch. 5 Rocks, Fossils, and Time

1. Ch. 5 Rocks, Fossils, and Time ESCI 102

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 • We will learn to interpret the geologic record using uniformitarianism

3. Geologic Record • Fossils in these rocks provide a record of climate change and biological events • The rocks themselves help reconstruct the environment John Day Fossil Beds National Monument, Oregon

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 and metamorphic rocks are also stratified

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

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

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

8. Vertical Stratigraphic Relationships • Surfaces known as bedding planes • separate individual strata from one 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 • sedimentary structures, such as cross-bedding, and fossils • allow geologists to resolve these kinds of problems • more later in term

10. Principle of Inclusions • According to the principle of inclusions • inclusions or fragments in a rock are older than the rock itself • Light-colored granite showing basalt inclusions (dark) • Which rock is older? – basalt, because the granite includes it northern Wisconsin

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 • How can you determine whether a layer of basalt within a sequence of sedimentary rocks is a buried lava flow or a sill? • sill will heat the rocks above and below • sill might also have inclusions of the rocks above and below • but neither of these rocks will have inclusions of the sill

13. Unconformities • So far we have discussed vertical relationships among conformable strata • 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 • millions to hundreds of millions of years • The rock record is incomplete • interval of time not represented by strata is a hiatus

14. Deposition began 12 million years ago (MYA) • Continuing until 4 MYA Origins of an Unconformity • 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: • disconformity • separates younger from older rocks • both of which are parallel to one another (implies sed rx) • nonconformity • cuts into metamorphic or intrusive rocks • is covered by sedimentary rocks • angular unconformity • 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. Lateral Relationships • In 1669, Nicolas Steno proposed the principle of lateral continuity • layers of sediment extend outward in all directions until they terminate • terminations may be abrupt • at the edge of a depositional basin, and… • where eroded • where truncated by faults

18. 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

19. 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

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

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 • Each laterally adjacent depositional environment produces a sedimentary facies • During a transgression, the facies forming offshore become superposed upon facies deposited in nearshore environments

22. Marine Transgression • 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 • ages vary from place to place younger shale older shale

23. A Marine Transgression in the Grand Canyon • Three formations deposited in a widespread marine transgression are exposed in the walls of the Grand Canyon • What is the sea level history recorded?

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

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

26. Walther’s Law • Johannes Walther (1860-1937) noticed that the same facies he found laterally were also present in a vertical sequence • Walther’s Law: the facies seen in a conformable vertical sequence will also replace one another laterally • Walther’s law applies to marine transgressions and regressions adapted from Van Wagoner et al., 1990; http://www.uga.edu/~strata/sequence/parasequences.html

27. 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 sequence, 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

28. Causes of Transgressions and Regressions

29. Causes of Transgressions and Regressions • Uplift of continents causes local regression • Subsidence causes local transgression • Widespread glaciation causes regression • – due to the amount of water frozen in glaciers • • Rapid seafloor spreading causes transgression • – expands the mid-ocean ridge system, displacing • seawater onto the continents • • Diminishing seafloor-spreading ratesincrease the • volume of the ocean basins and causes regression

30. 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 • geologists also use them to ascertain environments of deposition • Fossils provide some of the evidence for organic evolution • many fossils are of organisms now extinct

31. How do Fossils Form? • Remains of organisms are called body fossils • mostly durable skeletal elements such as bones, teeth and shells • rarely we might find entire animals preserved by freezing or mummification

32. 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

33. Trace Fossils • A land-dwelling beaver, Paleocastor, made this spiral burrow in Nebraska

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

35. 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 or structure by being permineralized, recrystallized, replaced, carbonized

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

37. 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

38. 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

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

40. Molds and Casts • Molds form when buried remains leave a cavity • Casts form if material fills in the cavity – fossil turtle showing some of the original shell material – body fossil and a cast

41. 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

42. Fossil Record • The fossil record is the record of ancient life preserved as fossils in rocks • The fossil record is very incomplete because of: • bacterial decay • physical processes • scavenging • metamorphism • In spite of this, fossils are quite common

43. 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

44. Fossils from Different Areas • Compare the ages of rocks from different localities

45. Principle of Fossil Succession • Using superposition, Smith was able to predict the order in which fossils would appear in rocks not previously visited • lead to the principle of fossil succession

46. 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

47. Matching Rocks Using Fossils youngest • The youngest rocks are in column B • Whereas the oldest are in column C oldest

48. Relative Geologic Time Scale • Investigations of rocks by naturalists between 1830 and 1842 based on superposition and fossil succession • resulted in the recognition of rock bodies called systems • and the construction of a composite geologic column that is the basis for the relative geologic time scale

49. Geologic Column and the Relative Geologic Time Scale Absolute ages (the numbers) were added much later.

50. Correlation • Correlation is the process of matching up rocks in different areas • There are two types of correlation: • lithostratigraphic correlation • simply matches up the same rock units over a larger area with no regard for time • time-stratigraphic correlation • demonstrates time-equivalence of events