Geologic time, relative/absolute age cont.
Fossil succession and relative age
•
Species originate within geologically brief time frames, few species persist for
more than a few million years, and extinction is permanent
•
Fossils that are, for example, uniquely found within Maastrichtian rocks can be
used to illustrate temporal equivalence of isolated strata that bear these same
fossil assemblages
Thus, physically isolated sedimentary sequences can be temporally correlated in
the absence of datable igneous intrusions
•
Fossil succession
•
•
•
Fossil succession allows relative ages to be produced for many fossiliferous
sedimentary rocks regardless of their spatial contexts
This allows for temporal correlation of widely separated strata on the basis of
fossil content
Overlap ranges (last origination–first extinction) for suites of long-ranging fossil
species provide maximum constraint upon the ages of the strata that contain them
Development of unconformity
•
An unconformity occurs when sedimentary layers are deposited on top of a
surface of erosion or nondeposition
•
Nonconformities arise when sedimentary strata are deposited on top of
crystalline (metamorphic or igneous) rock
•
Disconformities occur when strata are deposited on top of an erosional surface
that was horizontal at the time, so that layers above and below the unconformity
are parallel
Angular conformity
•
Arises when the new strata are deposited on top of older layers that have been
tilted out of horizontality, so that, regardless of future tilting, layers on opposite
sides of the unconformity are not parallel
Geologic column
•
With successful correlation, rock sequences from all parts of the world could be
brought together to form an accurate, complete sequence of relative time, the
geologic column
•
A stratigraphic formation is a recognizable interval of a specific (usually
sedimentary) rock type or types which were deposited within a certain time
interval and which can be traced over a broad region
Methods of correlation
Lithologic (or lithostratigraphic) correlation is the use of physical and chemical
characteristics of rocks to determine that spatially isolated strata were once continuous
(through original lateral continuity)
•
Fossil (or chronostratigraphic) correlation uses common index fossils to
determine the approximate temporal equivalence of two bodies of rock (which
may be of disparate lithologies)
•
Because a single locality does not provide strata that span in age throughout Earth
history, (fossil) correlation was required to provide time equivalence for units that
are geographically isolated
Radioactive decay
•
•
•
•
•
Nuclear breakdown of an unstable isotope by:
(a) ejection of an alpha particle (two protons and two neutrons; this is termed
“alpha decay”)
(b) conversion of a neutron into a proton plus an electron, the latter of which is
expelled out of the nucleus by the weak force (electrons are termed beta particles,
so this process is called “beta decay”)
(c) electron capture by a proton to form a neutron, or (d) through fission of a
isotope into two smaller one (alpha decay being a specific example of this)
The constancy of a half-life for any given unstable isotope implies that
radiometric decay occurs at a constant exponential rather than linear rate