The Stratigraphic Record
First Geologic Map
England, 1815
William Smith
What is stratigraphy?
The study of sedimentary
rock units, usually done
by studying sections of
rock.
• geographic extent
• age
• classification
• characteristics
• formation
William Smith, English canal builder
So, what is a section?
In general it is a representation of an actual
column of rock drawn to show rock type,
thickness, deformation, inclusions, and used
for correlation across geographic areas.
Units we’ll discuss
Lithostratographic – ROCK-ROCK units
Chronostratigraphic – TIME-ROCK units
Biostratigraphic – FOSSIL-ROCK units
ROCK-ROCK Units
Formation – basic unit; A laterally continuous
rock unit with a distinctive set of characteristics
that make it possible to recognize and map from
one outcrop (or well) to another.
More Lithostatigraphic Units…
Supergroup – two or more related groups (may include
formations) with significant features in common
Group – two or more formations
Formation – the basic unit (previous slide)
Member - a subunit of a formation
Bed – distinctive layer 1cm – 2m thick that is
distinguishable from layers above & below
These units are typically named for a geographic location
and a type section is described.
The difference between lithostratographic units
and chronostratigraphic units, or…
ROCK-ROCK units vs. TIME-ROCK units
A body of rock with
distinctive properties
without regard to
time.
The actual rocks
deposited during a
specific time period
(like the Jurassic).
Based on physical
characteristics, and
may not be the same
age everywhere it is
found.
Often consists of more
than one formation.
Chronostratigraphic Units (or TIME-ROCK units)
have the same boundaries as time units, but
represent the rocks deposited at that time.
So, TIME-ROCK units correlate to TIME units
Erathem
System
Series
Stage
Era
Period
Epoch
Age
For example, the Jurassic System describes:
Eon
Era
Period
Tertiary
Neogene
Paleogene
Mesozoic
Cretaceous
Age
Holocene
Pleistocene
Pliocene
Miocene
2.6 mya
Oligocene
Eocene
Paleocene
23 mya
65.5 mya
Jurassic
Triassic
251 mya
Carboniferous
Permian
Paleozoic
Phanerozoic Eon
Cenozoic
Quaternary
Epoch
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
542 mya
Proterozoic
Eon
2500 mya
Archean Eon
3950 mya
All the ROCKS deposited
between 200 – 145 mya
Biostratigraphic units (FOSSIL-ROCK units) are
based on the occurrence of index fossils
Facies Concept
Rocks include characteristics that help identify
their depositional environment, or…
Every rock is the product of the environment in
which it was deposited.
Based on rock type or fossils
Biofacies – a facies change based on fossils
contained within the rocks
Lithofacies – a facies change based on rock type
We can then use facies to see how environments have
changed over time, lets see how…
Sea-level Change
Transgression creates an onlap sequence
Transgressive Sea
As sea level rises, a fining upward sequence of
rocks is deposited: Limestone
Shale
Sandstone
Sea-level Change
Regression creates an offlap sequence
Regressive Sea
As sea level becomes lower, a coarsening upward
sequence of rocks is deposited: Sandstone
Shale
Limestone
Putting it All Together:
Correlation of Rock Layers
• Simplest correlation, if possible, would be to walk along
outcrop ledges. Almost never possible - rocks concealed
by soil, vegetation, and other rocks.
• Correlation over short distances - noting a position of a
bed in the strata, or, by noting uncommon or distinctive
mineralogy or structures.
• For large distances (widely separated distances or
between continents) - fossils are used.
Simple Correlation
Physical (lithostratigraphic) Correlation
Correlation over “short” distances: Sedimentary rock
sequences (Map of Midwest)
Fossil (biostratigraphic) Correlation is often
used to correlate over LONG distances
Unconformities: a break in the
stratigraphic record
They represent an unknown amount of time, or
HIATUS, unless the rocks immediately below
AND above the unconformity can be dated.
What’s missing?
•Time of deposition of the missing rock
•time it took to erode that rock away
•Any additional deposition/erosion cycles
•Subaerial exposure
Lithofacies map
Shows rock
facies overlain
on a reference
map. Allows
for inference
of features
that are no
longer visible.