GEOL 104 Dinosaurs: A Natural History
Fall Semester 2000
Deep Time: Dating the fossil record
“Deep Time”: analogy to “deep space”; the vast expanse of time in the
(geologically ancient) past.
Many attempts at calculating age of the Earth:
- Different cultures using religious texts/beliefs come up with different ages (thousands of years to hundreds of billions of years)
- Early attempts by physicists and geologists predicted a few million to hundreds of millions of years
Two different aspects of time to consider:
- Relative Time: sequence of events without consideration of amount of time (A came before B, B before C, etc.)
- Numerical Time: (sometimes called “absolute time), dates or durations of events in terms of seconds, years, millions of years, etc.
- “The Apollo 11 moon landing came after the Signing of the Declaration of Independence, but before the release of Star Wars: The Phantom Menace” is a statement of relative time.
- “The Signing of the Declaration was in 1776, the Apollo 11 landing was in 1969, and the release of SW:TPM was in 1999” is a statement of numerical time.
In the history of geology and paleontology, relative time was determined LONG before absolute time.
Sedimentary rocks, because they are deposited, naturally form horizontal layers (strata, singular stratum). Because of their layered form, strata allow geologists to determine relative time (that is, sequence of deposition of each layer, and thus the relative age of the fossils in each layer):
- Principle of Original Horizontality: because strata are deposited under gravity, they form horizontal layers. If the strata are no longer horizontal, something has disturbed the sediments AFTER they became rocks.
- Principle of Superposition: unless they have been disturbed, the strata at the bottom of a stack were deposited first, the ones on top of that are next oldest, and so on, with the youngest strata being the ones on top.
- Principle of Cross-cutting Relationships: any structure (fold, fault, weathering surface, igneous rock intrusion, etc.) which cuts across or otherwise deforms strata is necessarily younger than the rocks and structures it cuts across or deforms.
Using these principles, early geologists were able to figure out the sequence of
events of deposition, the changing local environments, and the folding, faulting,
igneous intrusions, etc. for any particular section of rock. However, how could they
extrapolate the sequence at one section with the sequence at another?
In some cases, the particular rock type, color, sedimentary structures, and so on
were the same in strata in nearby sections. These groups of strata were named
formations, which represent units of rock produced by the same conditions
(environment) and having the same history (produced over a particular sequence of time.
Formations are given formal names (e.g., the Morrison Formation, the Hell Creek
Formation, the Solnhofen Limestone, etc.). Sometimes groups of formations which
lie directly on top of or next to each other are catalogued together as formal Groups,
and sometimes groups which lie directly on top of or next to each other are placed
into formal Supergroups.
By mapping out formations, groups, and supergroups, geologists could connect sequences
of rocks across regions. But what about across continents and oceans?
Needed a method of correlation. Rock type doesn't work, because the same
environment will produce the same rock type regardless of relative or absolute time.
Fossils, however, were useful:
- Principle of Fossil Succession: there is a unique, non-repeating pattern
(history) of fossils through stratigraphic time. All rocks containing fossils of the
same species were deposited during the duration of that species on Earth.
Fossils allowed correlation from continent to continent. Only certain types of fossils
(called index fossils) were useful for correlation. To be a good index fossil,
the species should:
- Have been VERY common, so chances of individuals being buried is good
- Have hard parts, so chances of fossilization are good
- Have a wide geographic range, so that correlation over wide region is possible
- Lived in (or could be deposited in) different environments, so can be found in different formations
- Have some distinctive features, so it can be recognized from closely related forms
- Have a short geological duration (a few million years at most), so finding a
fossil of the species in a rock means it had to be deposited in those few million years
Using index fossils, geologists were able to correlate across Europe, and then to other
continents. Created a global sequence of events (based on the sequence of (mostly
European) formations and the succession of fossils) termed the Geologic Column.
Became a “calendar” for events in the ancient past: used to divide up time as well as
The Geologic Column is divided into a series of units. Each unit may contain smaller
units, and may be part of larger units. The largest units are Eons: animal and
plant fossils are mostly restricted to the last (most recent) Phanerozoic Eon
(“visible life eon”). The Phanerozoic Eon is comprised of three Eras:
- The Paleozoic Era (“ancient life era”)
- The Mesozoic Era (“middle life era”): the Age of Dinosaurs
- The Cenozoic Era (“recent life era”): the Age of Mammals. We are still in the
When these terms were named, no one had any idea how long (in numerical time) each
era was. The boundaries between eras were defined by the disappearance
(mass extinction) of many different groups of marine invertebrates.
Eras are divided into multiple Periods; periods are divided into multiple
Epochs; and epochs are divided into multiple Ages.
The Mesozoic Era is divided into three periods:
- The oldest (furthest from us in time) is the Triassic Period
(“three-fold period”), comprised of the Early Triassic, Middle Triassic,
and Late Triassic Epochs
- The middle one is the Jurassic Period (“Jura mountain period”), comprised of
the Early Jurassic, Middle Jurassic, and Late Jurassic Epochs
- The youngest (closest to us in time) is the Cretaceous Period (“chalk period”),
comprised of only the Early Cretaceous and Late Cretaceous Epochs.
Although the Geologic Column was developed as a relative time scale, geologists wanted
to figure out the numerical age dates for Era-Era boundaries and other events.
Discovered radiometric dating.
- Radioactive materials decay at predictable rate, known as the half-life
- Atoms decay from one form (parent) to another (daughter product), releasing energy
- After one half-life has passed, half the original parents in the material will have
decayed into the daughter product; after two half-lives, only one-quarter of the parent
material remains, with three quarters daughter product; after three half-lives, 1/8 to
7/8; after four half-lives, 1/16 to 15/16; and so on
- Can thus date rocks:
- Compare the ratio of parent product to daughter product
- Only works accurately if daughter product cannot be produced except by radioactive
- Only works accurately if all material (parent and daughter) remain trapped in rock
- Radiometric dates will only be effective for igneous rocks
- In sedimentary rocks, can date the individual grains of sediment: tells you age of
source rock, but not deposition
- In metamorphic rock, chemical alteration of rock can obscure signal
- Since only igneous can best be dated radiometrically, use principles of superposition,
cross-cutting relationships, etc., to determine ages of sedimentary rocks (and their
fossils) relative to numerical dates, and tie dates into Geologic Column by correlating
with index fossils
- Note: radiocarbon (carbon 14) dating cannot be used for Mesozoic fossils!
- Half-life is WAY too short; only useful for tens-of-thousands-of-years scale.
Radiometric dates reveal the Paleozoic-Mesozoic boundary is 251±0.1 Ma (million years ago);
the Triassic-Jurassic boundary is 200±0.4 Ma, the Jurassic-Cretaceous boundary is 142±2.0 Ma,
and the Mesozoic-Cenozoic boundary is 65±0.1 Ma.
Other techniques are also used in global correlation:
- Marker Beds: some large geologic events (major volcanic eruptions,
asteroid impacts, etc) leave a characteristic thin layer of rock across wide regions
- Magnetostratigraphy: the magnetic (but NOT the geographic) poles have
“flip-flopped” throughout geologic time, so that sometimes a magnet's north pole points
towards geographic North, and sometimes toward geographic South.
- Magnetic polarity can be recovered by some iron-bearing rocks (sedimentary
- Because based on the Earth's magnetic field, the changes occur everywhere on the
planet at the same time.
- Can use the particular “bar code”-like pattern of flip-flops to match any section to
known global pattern (based on continuous record in ocean rocks).
Most effective approach in getting age dates for a fossil bed is to combine multiple
techniques: get relative age relationships between local units, find index fossil ages
for the sedimentary rocks, and radiometric and magnetic dates where possible.
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