Transcript Relative Dating of Strata

Relative Dating of
Strata
Relative Dating
• Determining relative ages of rocks or strata
compared to another rock or strata.
• Can say which layer is older and younger but can’t
say absolute age.
• For example, Rock Layer A is older than rock Layer
B.
• Cannot say that Rock layer A is 32 million years old
and rock layer B is 30 million years old.
Law of Superposition
• Each bed in a sequence of sedimentary rocks (or
layered volcanic rocks) is younger than the bed
below it and older than the bed above it.
• Based on two assumptions:
• 1.) Principle of Original Horizontality
• 2.) The beds of strata are not overturned
Cross Cutting Relationships
What happened first in the picture on
the right? Second? Third?
That which cuts through is younger than the
Object that is cut
dike cuts through
granite is cut
Principle of Faunal Succession
• States that fossils aren’t randomly placed in strata,
but rather follow a succession where they succeed
each other in a definite and determinable order.
• Organisms live for a certain amount of time and
then go extinct, so we can use them to tell us age of
rocks.
• Index Fossils: A good index fossil is is widespread
but short-lived.
• Certain Trilobites are good index fossils.
Fossil Correlation
• Fossils can be used to
correlate strata hundreds of
miles apart.
• If an index fossil is found in
two different rock layers, we
know they must be the same
age.
• We can then correlate or
connect them together.
Relative Ages of Lava Flows and Sills
Principle of Inclusions
• Inclusions (one rock type contained in another rock type) are
older than the rock they are embedded in. That is, the younger
rock contains the inclusions
Principle of Inclusions
Example:
Storm beds can include rip-up clasts.
Which is younger in each case?
Unconformities
• Gaps in the geologic record, called unconformities,
are common where deposition stopped and erosion
removed the previously deposited material.
• Drawn as wavy lines.
3 Types of Unconformities
• Angular Unconformity
• Disconformity
• Nonconformity
Angular Unconformity
• Where tilted beds have been
eroded and horizontal beds
have been deposited on top.
• Gap in time between tilting
of beds and deposition of
horizontal beds.
Disconformity
• Erosion surface (gap in
time) between two
horizontal beds of
sedimentary rocks.
• For example:
• Limestone is deposited
• Limestone is eroded
• Sandstone deposited on top
of Limestone.
Nonconformity
• Unconformities (gap in
time) that separate igneous
or metamorphic rocks from
overlying sedimentary
rocks.
• They usually indicate that a
long period of erosion
occurred prior to deposition
of the sediments.
• Example: Sandstone
overlying granite.