Transcript Geologic Time and Stratigraphic Correlation GEOLOGIC TIME Tectonic processes – e.g., sea-floor spreading, subduction, etc. – take place very slowly ...

Geologic Time
and
Stratigraphic
Correlation
GEOLOGIC TIME
Tectonic processes –
e.g., sea-floor
spreading,
subduction, etc. –
take place very
slowly ... about as
fast as your
fingernails grow.
Given enough time,
lots can happen ....
just think of 27 years
in your life .....
And now on to geologic time .......
GEOLOGIC TIME, often called "deep
time," is measured in millions and
billions of years.
The largest subdivisions are the EONS, followed by
ERAS, then PERIODS, and finally EPOCHS.
Each TIME unit has a corresponding ROCK unit:
EON
ERA
PERIOD
EPOCH
* IN




EONOTHEM*
ERATHEM*
SYSTEM
SERIES
EVERYDAY USAGE, EONOTHEM AND
ERATHEM ARE SELDOM USED.
A complete and detailed time scale appears in your text, on p. 31
To review briefly, STRATIGRAPHY will give
us relative ages - older vs. younger:
Principle 1: Sedimentary rocks
are, in general, derived from
sediments originally
deposited horizontally.
Principle 2: In the
sedimentary pile, those
on top are younger than
those beneath.
Principle 3: Any rock unit or structure that cuts across another
MUST be younger than that which it interrupts.
"The result, therefore, of our present enquiry is, that we find no vestige of a beginning,
no prospect of an end." - James Hutton
Siccar Point, Scotland
FAULTS are breaks in the
rocks where one side has
moved relative to the
other side. Obviously,
they follow the same
cross-cutting principle.
This cross-cutting principle also applies to landforms, such as
these glacial moraines on the eastern slopes of the Sierra Nevada.
The Principle of Faunal
Succession means that
a given species of
organism will only be
found in deposits of a
specific age.
These fossils are
important tools in
correlation of strata –
determining which
layers are the same
age, thus helping us to
reconstruct the past
for given time frames.
Why don't we automatically have a
complete section? Because
everything above sea level is
constantly being eroded.
Two rock units are said to
correlate if they are the
same age. (adj.: correlative)
LITHOSTRATIGRAPHIC
CORRELATION is based
on rock units – assuming
the original lateral
continuity.
Sometimes, this can
become quite difficult –
and is inherently
subjective. New data
may require
reinterpretation.
Sequence stratigraphy
correlates units across
broad areas based on
unconformities at the
sequence bases.
Facies changes are lateral
changes in the same bed,
which often reflect
changing sea levels. [
They can also reflect
lateral changes in
depositional setting, as we
saw in the example of
Atkins Bay on Friday. ]
An onlapping or
transgressive sequence
will show a change from
beach-type sediments to
those characteristic of
deeper waters.
An off-lapping or regressive
sequence will show precisely
the opposite trend upwards in
a section.
NOTE that in both cases, the
correlative beds of any one instant
in time are not necessarily of the
same lithology!
UNCONFORMITIES are breaks in the sedimentary record, often
represented by erosional surfaces.
ANGULAR unconformities are
created by deposition of
sedimentary beds over older beds
that have been uplifted, tilted and
eroded.
NONCONFORMITIES
are created by
deposition of
sedimentary beds over
older intrusive igneous
or metamorphic rocks.
DISCONFORMITIES are boundaries between parallel
sedimentary units. When they are linear, as shown here
between two shales in Ohio, they can sometimes be very
difficult to recognize.
For sections with fossils and NO datable materials, we use biostratigraphic
correlation, drawing on other known sites. This relies on the principle
of faunal succession.
To use Olduvai Gorge as an example....
WEDNESDAY:
Dating Techniques in Geology
or Geochronometry
NAH! We just
wanna know
how OLD the
rocks are!