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January 15, 2018 | Author: Anonymous | Category: , Science, Earth Science, Geology
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Chapter 5 Rocks, Fossils and Time— Making Sense of the Geologic Record

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

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

Stratified Sedimentary Rocks

• Stratification in sedimentary rocks consisting of alternating layers of sandstone and shale, in California.

Stratified Metamorphic Rocks

• Stratification in Siamo Slate, in Michigan

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

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.

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.

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.

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

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

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

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

An Angular Unconformity • An angular unconformity, Santa Rosa

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

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

– 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

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

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

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

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

Marine Transgression

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

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

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

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

Walther’s Law • Johannes Walther (1860-1937) 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

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

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

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

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

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

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

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

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

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

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

Unaltered Remains • Insects in amber

• Preservation in tar

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

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

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

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

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

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

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

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

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

Fossils and Telling Time • William Smith • 1769-1839, 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 from Different Areas • To compare the ages of rocks from two different localities • Smith used fossils

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

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

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.

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

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

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

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

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

Time-Stratigraphic Units • 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 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

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

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

Lithostratigraphic Correlation • We can correlate rock units based on

– composition – position in a sequence – and the presence of distinctive key beds

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

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

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

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 timestratigraphic correlation, because of their short ranges • They are guide fossils

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

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

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

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

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

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

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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