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

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Presentation on theme: "Rocks, Fossils and Time— Making Sense of the Geologic Record Chapter 5."— Presentation transcript:

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

2 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 Stratigraphy

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

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

5 Stratification in Siamo Slate, in Michigan Stratified Metamorphic Rocks

6 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 Vertical Stratigraphic Relationships

7 Determining the relative ages of lava flows, sills and associated sedimentary rocks uses alteration by heat and inclusions Age of Lava Flows, Sills 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 –A sill will heat the rocks above and below. Sill –The sill might also have inclusions of the rocks above and below, –but neither of these rocks will have inclusions of the sill.

9 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. Unconformities

10 –For 1 million years erosion occurred and removed 2 MY of rocks –and giving rise to a 3 million year hiatus The origin of an unconformity 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

11 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 Types of Unconformities

12 Unconformities of regional extent may change from one type to another They may not represent the same amount of geologic time everywhere Types of Unconformities

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

14 An Angular Unconformity An angular unconformity, Santa Rosa

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

16 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 Lateral Relationships

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 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 Sedimentary Facies

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

20 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 Marine Transgressions

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

22 Marine Transgression

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

25 During a marine regression, sea level falls with respect to the continent Marine Regression –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 Johannes Walther ( ) noticed that the same facies he found laterally were also present in a vertical sequence, now called Walther’s Law 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 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 Extent and Rates of Transgressions and Regressions

29 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 Causes of Transgressions and Regressions

30 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 Fossils

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

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

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 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 Trace Fossils

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

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

37 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 Body Fossil Formation

38 Insects in amber Unaltered Remains 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 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 Altered Remains

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

42 Molds form when buried remains leave a cavity Casts form if material fills in the cavity Molds and Casts

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

45 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 Fossil Record

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

47 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 Fossil Record

48 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 Fossils and Telling Time

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

50 Using superposition, Smith was able to predict the order in which fossils would appear in rocks not previously visited Principle of Fossil Succession –Alexander Brongniart in France also recognized this relationship Their observations lead to the principle of fossil succession

51 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 Principle of Fossil Succession

52 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. Distinct Aspect

53 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 Matching Rocks Using Fossils

54 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 Stratigraphic Terminology

55 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 Lithostratigraphic Units

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

57 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 Biostratigraphic Units

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 Time-Stratigraphic Units

59 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 Time Units

60 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 Correlation

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

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

63 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 Time Equivalence

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

65 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 Biozones

66 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 Guide Fossils

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

68 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 Short Duration Physical Events Absolute ages may be obtained for igneous events using radiometric dating

69 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 Absolute Dates and the Relative Geologic Time Scale

70 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 Absolute Dates for Sedimentary Rocks Are Indirect

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

72 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 Indirect Dating

73 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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