cGOL 106 LAB 4 FOSSILS AND THEIR LIVNG RELATIVES Group ________ Member Names (1)_________________________________________________ (2)_________________________________________________ (3)_________________________________________________ Page Question(s) 103 2, 4 104 5 105 1 107 All 108 3, 5 109 7 a, b, c, d Chapter 5 Rocks, Fossils, and Time 1 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 – sedimentary rocks are especially useful in deciphering the geologic record • The geologic record is complex – and requires interpretation • Uniformitarianism offers a useful approach 2 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 • Almost all sedimentary rocks are stratified • Many volcanic rocks – such as lava flows or ash beds as well as many metamorphic rocks are stratified and obey the principles of stratigraphy 3 3 Stratified Sedimentary Rocks • Although these rocks in South Dakota are deeply eroded stratification is still clearly visible 4 Stratified Rocks • Stratified rocks in California are deformed so that they are no longer in their original position 5 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 non-deposition 6 6 Superposition • Nicolas Steno realized that he could determine – the relative ages of horizontal (undeformed) strata by their position in a sequence Youngest Oldest • In deformed strata, the task is more difficult – but some sedimentary structures and some fossils allow geologists to resolve these kinds of problems 7 7 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 8 Age of Lava Flows & Sills • Determining the relative ages of lava flows, sills and associated sedimentary rocks uses contact metamorphism effects 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. 9 • Rocks below the lava will have signs of heating but not the rocks above. • The rocks above may have lava inclusions. 9 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. 10 Unconformities • 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 at this location – The interval of time not represented by strata is a hiatus. 11 Types of Unconformities • Three types of surfaces can be unconformities: – A disconformity is a surface in sedimentary rocks 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 strataover which younger rocks were deposited 12 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 13 Sedimentary Facies • A sedimentary facies is a body of sediment with distinctive physical, chemical, & biological characteristics from the sediments deposited around it. • These distinctive characteristics help in inferring the environment of deposition of the sediment • Both inter-tonguing and lateral gradation indicate simultaneous deposition in adjacent environments 14 14 Marine Transgressions • A marine transgression occurs when sea level rises with respect to the land • During a transgression, the shoreline moves landward as the sea progressively covers more and more of a continent 15 Marine Transgressions • Each laterally adjacent depositional environment produces a sedimentary facies • The facies forming offshore become superposed upon facies deposited in nearshore environments 16 Marine Regression • During a marine regression – sea level falls with respect to the continent & the environments parallel to the shoreline migrate seaward • It yields a vertical sequence with nearshore facies overlying offshore facies & rock units become younger in the seaward direction 17 Walther’s Law • Johannes Walther (1860-1937) noticed that the same facies he found laterally were also present in a vertical sequence. – Walter’s law states that • the facies seen in a conformable vertical sequence will also replace one another laterally – Walther’s law applies • to marine transgressions and regressions 18 Causes of Transgressions and Regressions • Uplift of continents causes regression • Subsidence causes transgression • Widespread glaciation causes regression – because of the amount of water frozen in glaciers • Rapid seafloor spreading, – expands the mid-ocean ridge system, displacing seawater and causing transgression • Slow seafloor-spreading – increases the volume of the ocean basins and causes regression 19 19 Fossils • Fossils are the remains or traces of past life forms • They are most common in sedimentary rocks but can be found in volcanic ash & volcanic mudflows • 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 • Remains of organisms are called body fossils. They consist mostly of durable skeletal elements such as bones, teeth & shells 20 Trace Fossils (or Ichno-Fossils) • Indicate activity of the organism that produced them – They include tracks, trails, burrows, and nests • 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 excreted it 21 21 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 – or altered remains, • with some changes in composition or structure • permineralization, replacement, carbonization • Insects in amber 22 22 Unaltered Remains • Frozen baby mammoth found in Russia in 1989 23 23 Altered Remains • The bones of this mammoth – on display at the Museum of Geology and Paleontology in Florence, Italy • have been permineralized – with minerals added to the pores and cavities of the bones 24 24 Altered Remains • Carbon film of a palm frond • Carbon film of an insect 25 Molds and Casts • Molds form when buried remains dissolve and leave a cavity • Casts form if minerals or sediments fill in the cavity Step a: burial of a shell Step b: dissolution leaving a cavity, a mold Step c: the mold is filled by sediment 26 forming a cast 26 Fossil Record • The fossil record is the record of ancient lives 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, ammonoids, and dinosaurs 27 27 Fossils and Time • 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 he discovered a method – whereby relative ages of sedimentary rocks at different locations could be determined 28 28 Fossils from Different Areas • William Smith (1769-183) used fossils to compare the ages of rocks from two different localities 29 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 – led to the principle of fossil succession 30 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 • An assemblage of fossils has a distinctive aspect compared with younger or older fossil assemblages31 31 Matching Rocks Using Fossils youngest oldest • 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. 32 32 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 33 33 Geologic Column and the Relative Geologic Time Scale Absolute ages (the numbers) were added much later. 34 Example of the Development of Systems • Cambrian System – Sedgwick studied rocks in northern Wales – and described the Cambrian System without paying much attention to the fossils – His system could not be recognized beyond the area • Silurian System – Murchinson described the Silurian System in South Wales – and carefully described the fossils – His system could be identified elsewhere 35 Dispute of Systems • The two systems partially overlapped! 36 36 System Dispute • The dispute was settled in 1879 – when Lapworth proposed the Ordovician 37 37 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 38 38 Lithostratigraphic Units • Lithostratigraphic units are based on rock type – with no consideration of time of origin • The basic lithostratigraphic element is the formation – which is a mappable rock body 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 (LST, Sst, Sh, Siltstone) • Formations may be subdivided – into members and beds – or collected into groups and supergroups 39 39 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 40 Time-Stratigraphic Units • Time-stratigraphic units • also called chronostratigraphic units – consist of rocks deposited during a particular interval of geologic time • The basic timestratigraphic unit – is the system 41 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 an 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 42 42 Classification of Stratigraphic Units Lithostratigraphic Units • Supergroup – Group • Formation – Member » Bed TimeTimestratigraphic Units Units • Eonothem • Eon – Erathem • System – Series » Stage – Era • Period – Epoch » Age 43 43 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 44 44 Lithostratigraphic Correlation • Correlation of lithostratigraphic units such as formations – traces rocks laterally across gaps 45 Lithostratigraphic Correlation • We can correlate rock units based on – composition – position in a sequence – and the presence of distinctive key beds 46 46 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 further east • The most effective way to demonstrate time equivalence is time-stratigraphic correlation using biozones • But several other methods are useful as well 47 47 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 • a 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 48 48 Guide Fossils (Index 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 or index fossils 49 49 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 50 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 • may cover large areas • are not restricted to a specific environment • Absolute ages may be obtained for igneous events – using radiometric dating 51 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? • Absolute ages determined for minerals – – – – in sedimentary rocks give only the ages of the source that supplied the minerals and not the age of the rock itself 52 52 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 53 53 Indirect Dating • Absolute ages of sedimentary rocks – are most often found – by determining radiometric ages – of associated igneous or metamorphic rocks 54 54 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 55 Indirect Dating • These sedimentary rocks – are bracketed by metamorphic and – igneous rocks for which – absolute ages are known 56 56 Indirect Dating • Accurate radiometric dates are now available – for many ash falls, plutons, lava flows and metamorphic rocks – with associated fossil-bearing sedimentary rocks • These absolute ages have been added to the geologic time scale • Additionally, we know when a particular organism lived 57 57 Indirect Dating • Baculites reesidei – biostratigraphic zone – in the Bearpaw Formation – Saskatchewan, Canada – is about 72-73 million years old – because absolute ages have been determined – for associated ash layers 58 58 QUESTIONS? 59 QUIZ 1. Record of events preserved in rocks is called _____ a) Geologic record b) Rock record c) Event record d) Earth’s record 2. The study of layered rocks is known as _________ a) Sedimentology b) Paleontology c) Palynology d) Stratigraphy 3. ____ shows period of non-deposition or erosion in rock sequence a) Gap b) Erosive time c) Unconformity d) Parasequence 4. The missing interval of time in strata of rocks is called ______ a) Erosive surface b) Hiatus c) Bedding plane d) Fault 5. What type of unconformity is shown in the diagram below? a) Non-conformity b) Disconformity c) Angular Unconformity 60 4:16 (9 Figure 10.16 Range chart. Stratigraphic distribution of some genera of the foraminiferida Genera Periods Quaternary Tertiary Cretaceous Jurassic Triassic Permian Pennsylvanian Mississippian Devonian 7. c. What is the advantage, if any, of having more than one index fossil when making age determinations? On Figure 10.16, show by means of vertical bars the geologic ranges for Ammodiscus, Endothyra, Fusulina, Ammobaculites, Globigerina, Bolivina, and Quinqueloculina. What is the maximum possible geologic range for rock containing only Endothyra? d. Which is the better index fossil, Fusulina or Ammodiscus? Why? b. What is the age of a formation containing both Robulus and Endothyra? Chapter 10 109 Share… Add to Photos Copy 8 4:15 CS Figure 10.12 The planktonic foraminiferida Globigerinoides. Figure 10.13 Thin section of a fusulinid showing complex internal structure. Length of specimen is 6 mm. (For image of a fusulinid before thin-sectioning, see Schwagerina in Fig. 10.11.) final chamber aperture ப் suture pores b. Assume that the shale and sandstone have not changed in thickness, and complete the rock column for well B. 2. In Figure 10.15 a distinctive foraminiferida assemblage is encountered at a depth of 410 m in well #34. In well #62, an identical assemblage is encountered at 400 m and again at 1200 m. In well #71, the assemblage occurs at 1200 m. On the diagram, draw in a fault to indicate how faulting may have caused the repeated assemblage in well #62. a. At what depth will the oil zone be reached at the second location? State the assumptions you used to reach your conclusion. a. b. What is the name given to this type of fault? (See Chapter 14 for a summary of fault terminology.) Figure 10.14 Use of foraminiferida in predicting drilling depth. B pse 300 m 600 900 1200 1500 1800 2100 2400 2700 Oil-bearing sandstone Chapter 10 107 Share… Add to Photos Copy

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