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Ch. 5 Rocks, Fossils, and Time

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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 Origins of 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: 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 ( ) 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;

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 rates increase 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 Skeleton of a 2.3-m-long marine reptile in the museum at Glacier Garden in Lucerne, Switzerland Shells of Mesozoic invertebrate animals known as ammonoids and nautiloids on a rock slab in the Cornstock Rock Shop in Virginia City Nevada 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 , 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
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 oldest The youngest rocks are in column B Whereas the oldest are in column C

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

51 Lithostratigraphic Correlation
Correlation of lithostratigraphic units such as formations traces rocks laterally across gaps

52 Time Equivalence Because most rock units of regional extent are time transgressive we cannot rely on lithostratigraphic correlation to demonstrate time equivalence for example: sandstone in Arizona is correctly correlated with similar rocks in Colorado and South Dakota but the age of these rocks varies from Early Cambrian in the west to middle Cambrian farther east (THAT'S MILLIONS OF YEARS!)

53 Time Equivalence For all organisms now extinct, their existence marks two points in time their time of origin their time of extinction One type of biozone, the range zone, is defined by the geologic range total time of existence of a particular fossil group, a species, or a group of related species called a genus Most useful are fossils that are easily identified geographically widespread had a rather short geologic range

54 Guide Fossils The brachiopod Lingula is not useful because, although it is easily identified and has a wide geographic extent, it has too large a geologic range The brachiopod Atrypa and trilobite Paradoxides are well suited for time-stratigraphic correlation because of their short ranges They are guide fossils

55 Short Duration Physical Events
Some physical events of short duration are also used to demonstrate time equivalence: distinctive lava flow would have formed over a short period of time ash falls take place in a matter of hours or days may cover large areas are not restricted to a specific environment Absolute ages may be obtained for igneous events using radiometric dating

56 Absolute Dates and the Relative Geologic Time Scale
Ordovician rocks are younger than those of the Cambrian and older than Silurian rocks But how old are they? When did the Ordovician begin and end? Since radiometric dating techniques work on igneous and some metamorphic rocks, but not generally on sedimentary rocks, this is not so easy to determine

57 Indirect Dating Absolute ages of sedimentary rocks are most often found by determining radiometric ages of associated igneous or metamorphic rocks

58 Indirect Dating Combining thousands of absolute ages associated with sedimentary rocks of known relative age gives the numbers on the geologic time scale

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