"Toto, I have a feeling that were not in Kansas anymore"(statement borrowed from Fastovsky and Weishampel, 1996). A quote from Dorthy in Wizard of Oz. This would be our feeling if we suddenly stepped back in time to the Mesozoic Era.
Age of Reptiles – 245 to 65 million years ago.
Age of Dinosaurs – approx. 225 to 65 million years ago.
The Earth is dynamic – Everything changes over time. The Earth changed a lot during the Mesozoic and has changed a lot since the Mesozoic. Epicontinental seas have advanced onto the continents and retreated; majestic mountain chains have formed and have been weathered low; continents and oceans basins have changed positions (via Plate Tectonics); new ocean basinshave been created (via Plate Tectonics); animal and plant species have evolved, lived, and become extinct, to be replaced by new species (Organic Evolution), etc.
When did dinosaurs live? How do we know
when they lived? In this course we will focus on the Late Triassic
Period to the end of the Cretaceous Period. That is when the Dinosaurs
roamed the Earth. We know this because we have learned how to date the rocks containing dinosaur fossils.
Triassic Period: 250 to 208 million years ago.
Jurassic Period: 208 to 145 million years ago.
Cretaceous Period: 145 to 65 million years ago.
Dinosaurs lived only on land (terrestrial environments). Therefore dinosaur fossils are found in sedimentary rocks and in sedimentary rocks that were deposited in terrestrial environments (as opposed to marine environments), such as stream and flood plain deposits, lake deposits, desert deposits, and delta deposits.
Stratigraphy is the study of stratified
rocks. All sedimentary rocks are stratified, but stratification is
not as prominent in igneous and metamorphic rocks. Sedimentology
is the study of sedimentary depositional environments. By studying
modern depositional environments, geologists can infer much about ancient
depositional environments by examining how modern depositional processes
ancient stratified sedimentary rocks.
There is a dichotomy when conceptualizing
geolgical history from rocks. Time is continuous. Rocks
and their contained fossils are a record of what happened during particular
time intervals. But rocks at a particular locality do not always
represent continuous depositon, and therefore rocks of a certain time interval
may not be present at that location (i.e. the rocks may not
be conformable -- some key words here: unconformity, hiatus, nondepositon, erosion). Even though rocks of a particular age may be missing, time was continuous at all locations.
Stratigraphers try to correlate rock units that were deposited during the same time interval. They try to correlate all the sedimentary rocks of the world into the correct time sequence, and therefore interpret Earth history in the correct chronologic order.
Stratigraphy is divided into three main categories: Chronostratigraphy, Lithostratigraphy, and Biostratigraphy.
1) Chronostratigraphy- a) Correlation of rock units deposited
at the same time and the placement of rocks into the correct chronologic
order and b) determining the absolute age of rock strata. Therefore
chronostratigraphy involves both relative dating and absolute dating of
rocks. Geologists and paleontologists often speak of relative time
versus absolute time. Thus we may
speak of the Jurassic Period of time (a relative time period that is younger than the Triassic Period, but older than the Cretaceous Period). Or we may speak of the time from 208 million years ago to 144 million years ago (the time span in absolute years of the Jurassic Period).
We also have to remember that in chronostratigraphy
we are correlating rocks rather than time, however, we are correlating
rocks of the same age. Chronostratigraphic (or time-stratigraphic)
units are rock units that were deposited during a particular interval of
time. Thus the Jurassic System of rocks was deposited during the
Jurassic Period of time or the time interval from 208 to 144 million years
ago. Chronostratigraphic Units are thus different in concept than
Time (Chrono) Units. The basic units of time are (in decreasing magnitude)
Eon, Era, Period, Epoch, and Age. The corresponding chronostratigraphic
units are (in decreasing magnitude) Eonothem, Erathem, System, Series,
and Stage. The following example will illustrate the relationship
between Time Units and Chronostratigraphic Units:
Mesozoic Era Mesozoic Erathem
Jurassic Period Jurassic System
Upper Jurassic Epoch Upper Jurassic Series
Oxfordian Age Oxfordian Stage
2) Lithostratigraphy - Lithostratigraphy is the study of
and correlation of rock units. The basic lithostratigraphic unit
(rock unit) is the formation. A formation is a rock unit that
can be easily distinguished from rocks above and below, and is thick enough
and has adequate lateral distribution as to be a practical map unit to
display on geologic maps. A formation may be of a single
lithology (rock type, like sandstone) or it may consist of alternating layers of more than one rock type (like sandstone with interbedded shale and mudstone). An example is the Morrison Formation of Jurassic age that is present over a broad area in Colorado, Utah, Wyoming, New Mexico, and Montana (and small portions of some other states). The Morrison Formation consists of alternating red and brown sandstones and multicolored mudstones.
The Morrison Formation at Dinosaur National Monument, Utah.
Horizontal beds of the Morrison Formation near Cleveland, Utah.
be divided into members or have distinctive lithologies that can
be designated as a member of a particular formation. Members have
all the characteristics that formations have, but may not be as extensive
(cover as large an area). Sometimes similar formations (in terms
of lithology or relationships to unconformities) are lumped to form bigger
units called groups. Groups are often used rather than formations on geologic maps of large regions (such as states).
3) Biostratigraphy – Biostratigraphy attempts to establish units of strata that have distinctive fossils or fossil assemblages and to correctly order the fossil assemblages (based on the Principle of Fossil Succession – fossil assemblages succeed each other in a definite and determinable order). Another goal of biostratigraphy is to correlate units of rock strata that have the same distinctive fossils or fossil assemblages. This implies that the rock strata are of the same approximate relative age and thus were deposited at about the same time. In a sequence of sedimentary strata, the range (first appearance in the strata to last occurrence in the strata) of distinctive fossils or members of a fossil assemblage are referred to as biostratigraphic zones or biozones. The most common biozones are range zones (for an individual fossil type), concurrent range zones (the zone of overlap of two or more fossil types), and assemblage zones (fossil types that commonly occur together). Fossil types are referred to as taxons. The most accurate biozones use the species level taxon (lowest level of classification). Distinctive fossils that are used for biostratigraphic correlation are called index fossils.
CHARACTERISTICS OF INDEX FOSSILS
1. Short geologic time range.
2. Wide geographic distribution (implies ecologic tolerance).
3. Abundant (relatively easy to find in rocks)
4. Easily recognizable to a trained paleontologist (not so hard to identify that only a few experts can identify the fossils)
Some examples of key index fossils and the periods of time that they are associated with. These are all marine index fossils. (From United States Geological Survey Web Site.)
PRINCIPLES OF ABSOLUTE AND RELATIVE DATING
Now let us return to the concepts of stratigraphy and discuss absolute and relative dating of rocks as we have defined them.
Principles of Relative Dating
Steno’s Principles (1669) : These principles are based on the
work of Nicolaus Steno (1638-1686)
1. Principle of Superposition: In a sequence of sedimentary strata, the oldest layer is at the bottom of the sequence and the
strata are progressively younger toward the top of the sequence.
2. Principle of Original Horizontality: Sedimentary strata are originally deposited in a near horizontal manner. Therefore, if sedimentary strata are found to be in a steeply inclined position, some force has altered them from their orginal position.
3. Principle of Lateral Continuity: Sedimentary strata are originally deposited over a laterally extensive area and are continuous until they pinch out at the edge of the ancient depositional basin (or unless removed by subsequent uplift and erosion).
James Hutton (1726-1797)
James Hutton, a Scottish medical doctor by
education, is sometimes referred to as the Father of Geology.
Hutton never practiced medicene, but was very interested in the processes
which formed and shaped the Earth. One of Hutton’s greatest contributions
to geology and stratigraphy was his concept of Uniformitarianism (now referred
to as the Principle of Uniformitarianism). This concept meaning
“the present is the key to the past”, states that by studying geologic
processes in operation today we can infer that such processes operated
in the past. Thus, we can interpret rocks as the result of similar
geologic processes as are in operation today. With some modification,
this concept is still the basis for modern geologic thought. We now
realize that, although the processes themselves have not changed with time,
the rates of some geologic processes may have varied drastically from time
to time (for example, the intensity of world volcanic activity).
We also recognize that catastrophic events can and do occur (for example,
meteorite and asteroid collisions with the Earth have occurred in the past).
Some geologists today prefer the term Actualism for the uniformitarian
concept. The uniformitarian concept implies that the basic principles
laws of nature have not changed with time.
Hutton, of course, recognized that Steno’s Principles of relative dating are valid and he further demonstrated them in his work. He also recognized other principles of relative dating.
(Principles of Relative Dating, continued)
4. Principle of Cross-Cutting Relationships: Any geologic event (igneous intrusion, fault, etc.) that cuts across or truncates sedimentary strata must be younger than the strata affected by the event.
5. Principle of Unconformities: An unconformity is a surface representing a time gap (hiatus) in the rock record, due either to erosion or nondeposition. (Hutton realized that rocks did not represent continuous deposition in some areas.)
William Smith (1769-1839)
William Smith, an English engineer with very little formal education, was the first person to correlate rock units from one area to another over a significant lateral distance. He was the first to determine that fossil assemblages of each age group of strata are characteristic for that particular age of rocks. Thus he was the first to use and state the Principle of Fossil Succession.
(Principles of Relative Dating, continued)
6. Principle of Fossil Succession: Fossil assemblages succeed one another in a definite and determinable order and the relative age of sedimentary strata can be determined by their fossil content.
The Principle of Fossil Succession is the basis for:
7. the Principle of Biostratigraphic Correlation.
Therefore, using index fossils in rocks and the Principle of Fossil Succession,
rocks in different regions of the world can be correctly ordered and correlated
in a time-rock context (i.e., rocks of similar age in different regions
can be matched-up).
Charles Lyell (1797-1875)
Charles Lyell published the first textbook
of geology in 1833 (actually three volumes, the first in 1830 and the last
in 1833; the book went through eleven editions). In his text, Lyell
preached the Huttonian concept of uniformitarianism. Lyell was also
a close friend of Charles Darwin, and no doubt the ideas of each
influenced the thought of the other. Both Lyell and Darwin
thought that the Earth must be hundreds of millions of years old; they believed that it would take that long to form the rock and fossil record as found on Earth.
Using the principles of relative dating (as outlined here), Lyell and others during the later part of the nineteenth century were able to place the rocks of the Earth in correct chronologic order. By 1900, The Relative Geologic Time Column was basically in its modern form.
Charles Darwin (1811-1882)
The C. Warren Irvin, Jr., Collection of Charles Darwin and Darwiniana
Charles Darwin and Alfred Russel Wallace are jointly given credit as the originators of our modern theory of organic evolution. They presented independent papers during the summer of 1858 at the same scientific meeting in London that initiated our modern view of organic evolution.
Darwin had completed years of research on evolution, beginning with the Voyage of the HMS Beagle (1832-1836), and is given (rightfully so) the largest amount of credit as the founder of modern evolutionary theory.
Darwin wrote his famous book “On the Origin of Species by Means of Natural Selection” in 1859. Charles Darwin - The Origin of Species (The book caused a real stir in Victorian England.) Darwin believed that vast amounts of time would be needed to get the evolutionary record that we see from fossils embedded in the rocks. He basically took a uniformitarian view of how evolution occurs (slow, gradual change over eons of time). However, some modern evolutionists (Gould and Eldridge, xxxx) have proposed that evolutionary change (at the species level) may occur rapidly, with long periods of little change (stasis) puntuated by rapid evolutionary change (this is referred to as the Puncturated Equilibrium Theory of Evolution). Even so, it would still take vast amounts of time to get the fossil record that we observe in the rocks.
The theory of evolution as proposed by Darwin
and Wallace explains why the Principle of Fossil Succession works.
Principles of Absolute Dating of Rocks
There were several attempts during the middle
to late 1800’s to determine the age of rocks and the age of the Earth in
1. Rates of Sedimentation – ages ranged from a few million to about 1.5 billion.
2. Salinity of the Oceans – age of 90 million years for the Earth.
3. Lord Kelvin (William Thomson) – based on the cooling of a molten ball (the Earth). Age of no more than 100 million years, probably about 40 million years, and life could have inhabitated the Earth for only 20-25 million years. Geologists of the mid to late 1800's thought the Earth was much older, but could not overcome the reasoning of this well known physicist. Darwin also thought that life had existed on Earth much longer than this.
There are flaws with all of the above methods!
Can you reason out what some of the flaws are?
Radiometric Dating of Rocks
1. Henri Becquerel discovered radioactivity in 1896 (Pitchblende,
ore of Uranium, gives off some kind of energy that darkens photographic
2. Marie and Pierre Currie (1903) determine that the element radium gives of heat energy. The fact that certain elements, like radium, are radioactive and liberate heat killed Kelvin’s basic premise.
3. Ernest Rutherford and coworkers about 1904 explained how radioactive decay works based on new concepts in atomic theory.
4. Bertram Boltwood of Yale (1907) used Uranium-Lead radiometric methods to measure the age of several old rocks and got ages ranging from 400 million years to 2 billion years.
5. Arthur Holmes (from about 1910 to 1927) using Uranium-Lead and Uranium-Helium methods produced the first calibrated (with absolute ages) geologic time scale.
6. Many thousands of samples have been radiometrically dated since the early 1900’s and the geologic time scale has been revised many times. The absolute dates are continually being revised as more radiometric dates are obtained.
Radiometric Dating of Rocks and Minerals
Review of Atomic Theory
Protons, electrons, neutrons, ions, isotopes, etc.
What is Radioactive Decay?
Radioactive decay is the emission of particles and energy from the nucleus of an atom, whereby one element is converted to another element. The three main types of radioactive decay are alpha decay, beta decay, and electron capture.
The three major types of radioactive decay.
In alpha decay an alpha particle (just
like a helium nucleus), consisting of two protons and two neutrons, is
ejected from the nucleus of a radioactive parent isotope, thus the atomic
number decreases by two and the atomic mass number decreases by four for
the daughter isotope. Beta decay involves the emission of
a high speed electron from the nucleus resulting in the
conversion of a neutron to a proton, thereby increasing the atomic number by one without changing the atomic mass number for the daughter isotope. Electron capture decay takes place when the nucleus captures an electron from the inner electron orbits of the atom; thereby converting a proton to a neutron; the atomic number daughter isotope decreases by one, but the atomic mass number
How do we relate radioactive decay to geologic time?
When speaking of a single radioactive atom, the decay process is spontaneous and unpredictable. However, it is statistically possible to predict how many atoms of a given element will decay in a given amount of time. Thus, we cannot say which atoms will decay, but knowing how many atoms we start with, we can predict how many will decay in a certain amount of time.
We can use an analogy of radioactive decay
to a Life Insurance Company. The Life Insurance Company can predict
(based on past statistics) how many people of a certain age group will
die in a certain time interval, but not which individuals will die.
However, the analogy breaks down when referring to different age groups
of people. Younger age groups will have a smaller
percentage of individuals dying for a particular time period. This is not true of radioactive decay.
The number of atoms of a radioactive isotope (radioisotope) that decay during a given time interval is directly proportional only to the number of atoms of the radioisotope present in the sample (i.e., radioactive decay is proportional to the remaining concentration of radioisotope). Thus the rate of decay, proportional to the remaining amount of radioisotope, is independent of time. This is the basis for the concept of HALF-LIFE. The half-life of a radioisotope is defined as “the time taken for any initial amount of atoms of a particular radioisotope to be reduced by one half (50%) through decay”.
Two implications of the half-life concept are obvious. If we know the ratio of parent radioisotope to stable daughter isotope in the mineral or rock sample and if we know the half-life, then we can determine the age of the sample. The relationship can be expressed as follows:
Age = value of half-life for that particular radioisotope times number of half-lives elapsed. Or, in equation form:
T = (T1/2)n
where T is the age, T1/2 is the half-life in years, and n is the number of half-lives elapsed.
Most methods of radiometric dating are based
on measurements of accumulation of stable daughter atoms produced by a
parent radioisotope. When a mineral crystallizes, there are no radiogenic
daughter atoms of the radioisotope contained within the mineral composition,
i.e., the daughter to parent ratio is equal to zero (D/P=0). Knowing
the decay constant (k) (which can
be determined by measurement) of the parent radioisotope, it is possible to measure the ratio of daughter and parent isotopes (D/P) in a mineral sample (this is done with a mass spectrometer) and compute the age of the sample by the following relationship:
If we assume that one half-life has elapsed, then the above equation reduces to:
Possible Sources of Error in Radiomentric Dating
The measurement of parent radioisotopes and
stable daughter isotopes in minerals and rocks involves laboratory procedures
and mass spectrometer analyses, thus there is always some error in measurement.
However, with careful laboratory procedures, this error is minimal (in
the 1 to 2% range). Some of the problems that must be considered
in the determination of
radiometric dating are listed below:
1. One of the basic premises in radiometric dating is that the
minerals and rocks used as samples have remained in a closed system since
crystallization. If parent or daughter isotopes have been added or
removed from the system, the radiometrically calculated age will not be
the true age of the mineral or rock. Samples used for radiometric
dating must be chosen that represent unaltered igneous rock (only rarely
can sedimentary rocks be dated directly). For example, Argon leakage
common problem with the Potassium-Argon method. This is because Argon (an inert gas) is easily driven out of the crystalline structure due to weathering of the rock or any degree of metamorphism.
2. Any initial daughter product that was present at crystallization must be corrected for. This is routinely done in the laboratory.
3. Because, typically, only igneous minerals and rocks are being dated, inferences about the age of sedimentary rocks are based on field geologic interpretations. Normally, bracketed ages of sedimentary strata are inferred using principles of cross-cutting relationships, superposition, etc.
Absolute Age Dating of Sedimentary Rocks
With a few exceptions (such as the K-Ar dating of sedimentary formed glauconite or volcanic ash deposites mixed with other sediment), radiometric dates are typically determined for igneous rocks and minerals. So, how do we date sedimentary rocks in an absolute way? The relative ages of the geologic events in a particular area must be worked out before radiometric ages can be applied. Once the relative sequence of events in a particular field situation is determined, radiometric dates can be obtained for igneous intrusions and/or extrusions (volcanic rocks) that bracket certain sedimentary strata, thus giving an age range for the sedimentary strata.