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

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1 Rocks, Fossils and Time— Making Sense of the Geologic Record
Chapter 5 Rocks, Fossils and Time— Making Sense of the Geologic Record

2 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

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

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

5 Stratified Metamorphic Rocks
Stratification in Siamo Slate, in Michigan

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 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.

8 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.

9 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.

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

11 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

12 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

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

14 An Angular Unconformity
An angular unconformity, Santa Rosa

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

16 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

17 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

18 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

19 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

20 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

21 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

22 Marine Transgression

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

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 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 facie sand rock units become younger in the seaward direction

27 Walther’s Law Johannes Walther ( ) 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

28 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

29 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

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 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

31 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

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

33 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

34 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

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

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 altered remains, with some change in composition permineralized recrystallized replaced carbonized

38 Unaltered Remains Insects in amber Preservation in tar

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

40 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

41 Altered Remains Carbon film of a palm frond Carbon film of an insect

42 Molds and Casts Molds form when buried remains leave a cavity
Casts form if material fills 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 Cast of a Turtle Fossil turtle showing some of the original shell material body fossil and a cast

45 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

46 WHY is the fossil record incomplete
WHY is the fossil record incomplete??? Why are there large gaps of time and biological strata?

47 Fossil Record The fossil record is very incomplete because of destruction to organic remains bacterial decay physical processes scavenging metamorphism In spite of this, fossils are quite common

48 Fossils and Telling Time
William Smith , an English civil engineer independently discovered Steno’s principle of superposition Realized that fossils in 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

49 Fossils from Different Areas
To compare the ages of rocks from two different localities Smith used fossils

50 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 lead to the principle of fossil succession

51 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

52 Distinct Aspect An assemblage of fossils
has a distinctive aspect compared with younger or older fossil assemblages Rocks that contain similar fossil assemblages had to have been deposited at about the same time.

53 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

54 Stratigraphic Terminology
Because sedimentary rock units are time transgressive, they may belong to one system in one area and to another system elsewhere At some localities a rock unit straddles the boundary between systems We need terminology that deals with both: rocks—defined by their content lithostratigraphic unit – rock content biostratigraphic unit – fossil content and time—expressing or related to geologic time time-stratigraphic unit – rocks of a certain age time units – referring to time not rocks

55 Lithostratigraphic Units
Lithostratigraphic units are based on rock type with no consideration of time of origin The basic lithostratigraphic element is a formation a mappable rock unit with distinctive upper and lower boundaries It may consist of a single rock type such as the Redwall limestone or a variety of rock types such as the Morrison Formation Formations may be subdivided into members and beds or collected into groups and supergroups

56 Lithostratigraphic Units
Lithostratigraphic units in Zion National Park, Utah For example: The Chinle Formation is divided into Springdale Sandstone Member Petrified Forest Member Shinarump Conglomerate Member

57 Biostratigraphic Units
A body of strata recognized only on the basis of its fossil content is a biostratigraphic unit the boundaries of which do not necessarily correspond to those of lithostratigraphic units The fundamental biostratigraphic unit is the biozone

58 Time-Stratigraphic Units
also called chronostratigraphic units consist of rocks deposited during a particular interval of geologic time The basic time-stratigraphic unit is the system

59 Time Units Time units simply designate certain parts of geologic time
Period is the most commonly used time designation Two or more periods may be designated as an era Two or more eras constitute and eon Periods can be made up of shorter time units epochs, which can be subdivided into ages The time-stratigraphic unit, system, corresponds to the time unit, period

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

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

62 Lithostratigraphic Correlation
We can correlate rock units based on composition position in a sequence and the presence of distinctive key beds

63 Time Equivalence Because most rock units of regional extent are time transgressive we cannot rely on lithostratigraphic correlation to demonstrate time equivalence 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

64 Time Equivalence The most effective way to demonstrate time equivalence is time-stratigraphic correlation using biozones

65 Biozones 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, species, or a group of related species called a genus Most useful are fossils that are easily identified geographically widespread and had a rather short geologic range

66 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

67 Concurrent Range Zones
A concurrent range zone is established by plotting the overlapping ranges of two or more fossils with different geologic ranges This is probably the most accurate method of determining time equivalence

68 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

69 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

70 Absolute Dates for Sedimentary Rocks Are Indirect
Mostly, absolute ages for sedimentary rocks must be determined indirectly by dating associated igneous and metamorphic rocks According to the principle of cross-cutting relationships, a dike must be younger than the rock it cuts, so an absolute age for a dike gives a minimum age for the host rock and a maximum age for any rocks deposited across the dike after it was eroded

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

72 Indirect Dating The absolute dates obtained from regionally metamorphosed rocks give a maximum age for overlying sedimentary rocks Lava flows and ash falls interbedded with sedimentary rocks are the most useful for determining absolute ages Both provide time-equivalent surfaces giving a maximum age for any rocks above and a minimum age for any rocks below

73 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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