CLOCKS IN ROCKS
Timing the Geologic Record
The Stratigraphic Record
Important Principles
1. Original horizontality—sediments were deposited
originally as horizontal beds.
2. Lateral continuity—beds thin and pinch out laterally.
3. Superposition—overlying beds are younger than
underlying beds in a succession of sedimentary strata
that are undisturbed by tectonic forces.
4. Cross cutting relationships—a feature that disrupts a
previously formed feature is younger than the one that is
disrupted.
For example, the folding event post-dates the deposition of the
sediment, or the dyke is younger than the rocks it cross-cuts.
Geologic time
• geology’s greatest contribution to science (deep time).
• emphasizes that geology is a historical science. Rocks
record evolution of the earth system and its biotic
inhabitants.
• earth scientists use observation, and the principles and
tools of physics, chemistry, mathematics and biology to
piece together the story from the clues left behind in rocks.
Relative Dating
• relies on principles of original horizontality, superposition and
cross cutting relationships
• can only determine that some event is older or younger than
another
• cannot determine the duration of an event.
Sedimentation in
lake or sea
Sedimentation in
lake or sea
Sediments are deposited in horizontal
layers and slowly change into rock.
Principle of original horizontality
Younger
Older
If left undisturbed, the youngest layers
remain above the oldest.
Correlation of Sedimentary Rocks
Question
Can we correlate by establishing the physical
continuity of the sedimentary rock record; i.e.,
walking out bed contacts between one area and
another?
Answer
Difficult to do in practice because we can’t correlate
across oceans, continents, or even across highways in
some instances due to: (1) tectonic disruption of the
beds (2) non-deposition, and (3) erosion.
Lithostratigraphy— correlation of sedimentary rock units through
a comparison of the stratigraphic succession of rock types, bed
thicknesses, and structures. Very unreliable. Missing strata not
accounted for (unconformities). Also changes in bed thicknesses and
sedimentary facies changes. Sedimentary facies are the rock
attributes that reflect the characteristics of the depositional
environment (e.g., near shore sands, offshore muds)
offshore
Facies change in bed
thickness
nearshore
Shoreward pinchout of limestone
facies, basinward pinchout of sand
facies.
Unconformities— missing (time) in a stratigraphic succession of
sedimentary rocks.
Nonconformity–sedimentary rocks overlying
igneous or metamorphic rocks
Disconformity–missing time in a conformable
succession of sedimentary rocks. Often only
detectable using principle of faunal succession.
Angular unconformity–sedimentary rocks overlying
igneous or metamorphic rocks
Unconformities are a major source of error in lithostratigrphic
correlation.
Biostratigraphy— correlation of sedimentary rock units
through a comparison of their fossil types and the pattern of
their succession. Very reliable.
Biostratigraphic correlation is based on the principle of faunal
succession, which states that fossils succeed one another in a
definite and recognizable order (biotic evolution). The fossil
record is a record of faunal succession. Fossils have limited
stratigraphic (time) range within the sedimentary rock record.
Index fossils and biostratigraphy
•The shorter the time-stratigraphic range of the
fossil, the more precise are the correlations that can
be made.
•Fossils representing free swimming or drifting
organisms can be found world wide and deposited in
a wide variety of rock type (different depositional
environments). These are the most useful for global
correlation of the stratigraphic record. They are called
index fossils.
Conodonts
Graptolites
Conodonts
Graptolites
Trilobites
Brachiopods
An application of biostratigraphy
Outcrop A
I
Outcrop B
II
II
III
An application of biostratigraphy
Some of the fossils found in
outcrop A are the same as fossils
found in outcrop B, some distance
away. (Law of Faunal Succession)
Outcrop A
Outcrop B
I
II
II
Layers with the
same fossils are
the same age.
III
A example of stratigraphic correlation
D
C
B
A
TIME 1
Beneath the sea, sediments
accumulated in beds.
D
C
B
A
TIME 2
Tectonic forces caused
uplift, exposing the beds
to erosion.
Uplift
C
B
A
TIME 3
Erosion stripped away
bed D and part of C.
E
C
B
A
TIME 4
Subsidence allowed a new
layer, E, to be deposited.
Subsidence
Unconformity
TIME 1
Beneath the sea,
sediments accumulated in beds.
Compression
TIME 2
Tectonic forces caused
uplift, folding, and deformation.
Uplift
TIME 3
Erosion stripped away the
tops of the folded layers, leaving
portions of several layers exposed.
Angular
unconformity
TIME 4
Subsidence allowed new
sediments to be deposited.
Subsidence
TIME 1
Beneath the sea,
sediments accumulated
in beds.
TIME 2
Tectonic forces
caused uplift, folding,
and deformation.
Dike
TIME 3
A dike from molten
magma intruded the
folded layers.
Pluton
TIME 4
Faulting displaced the
layers and the intruding
dike.
Fault
Sandstones containing
land fossils
E
C
B
A
F
D
Angular unconformity E
Sandstones, limestones,
and shales containing
marine fossils
Unconformity C
Granite pluton
intrusion
Deformed metamorphosed
sedimentary rocks
Walther’s Law
Sediments from depositional environments occurring beside
each other at the present day will be found on top on each
other in the stratigraphic record due to sea level changes.
Laterally
distributed
environments of
deposition
Transgression
denotes sea
level rise
Vertical
arrangement of
sedimentary
facies
Walther’s Law
Sediments from depositional environments occurring beside
each other at the present day will be found on top on each
other in the stratigraphic record due to sea level changes.
Laterally
distributed
environments of
deposition
Regression
denotes sea
level fall
Vertical
arrangement of
sedimentary
facies
Transgression/Regression
Click here to view the Flash animation in your web browser
Seismic technology can be used
to create seismic profiles,…
…which allow geologists to see
individual beds in a sequence.
Seismic sequence
Younger strata
Seismic profile
Younger strata
Sequence C
Sequence C
Sequence B
Sequence A
Sequence B
Sequence A
Older strata
Older strata
The seismic sequence reveals
changes in sedimentation.
C
Delta
B
A
A sequence of delta
sediments, B,
accumulates over
previous sediments, A.
B
Sediment
The sea level rises,
and the shoreline
recedes inland.
A
Another sedimentary
sequence, C, accumulates
over sequence B.
The relative geologic timescale
…what came before…what came after…exquisitely pieced
together
Dating transforms the relative timescale into an absolute one. It allows us to
know the duration of events.
Clocks in rocks: absolute dating
Clocks in rocks: absolute dating
Rubidium-87 nucleus
Neutrons
Protons
Electron
Rubidium-87 nucleus
Neutrons
Protons
Electron
A neutron decays,
ejecting an electron…
Rubidium-87 nucleus
Neutrons
Strontium-87 nucleus
Protons
Electron
A neutron decays,
ejecting an electron…
Absolute Dating
Rubidium-87 nucleus
Neutrons
Strontium-87 nucleus
Protons
Electron
A neutron decays,
ejecting an electron…
…and producing a proton,
which changes the atom.
Radiometric Dating
D* = number of daughter isotopes produced from decay of parent N