1. The Stratigraphic Record
First Geologic Map England, 1815
William Smith

2. What is stratigraphy?
The study of sedimentary rock units, usually done by studying sections of rock.
geographic extent
age
classification
characteristics
formation
Does anyone remember who this guy is?
Pull out the book with the dust cover map…
William Smith, English canal builder

3. 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.

4. Units we’ll discuss Lithostratographic – ROCK-ROCK units
Chronostratigraphic – TIME-ROCK units
Biostratigraphic – FOSSIL-ROCK units
Biostratigraphic units will be discussed in an upcoming lecture

5. 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.
Formations usually have to be mappable (sometimes by air), although well data can be used

6. 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.
Supergroup – can be a number of groups or groups and formations
Group – 2 formations with significant features in common
Formation – the fundamental unit of lithostratigraphy; often there is a dominant rock type or combination of types
Member – not necessarily mappable; can be named or unnamed; can extend from one formation to another
Bed –
Type locality -

7. ROCK-ROCK units vs. TIME-ROCK units
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.
Based on physical characteristics, and may not be the same age everywhere it is found.
The actual rocks deposited during a specific time period (like the Jurassic).
Often consists of more than one formation.

8. So, TIME-ROCK units correlate to TIME units
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

9. For example, the Jurassic System describes:
All the ROCKS deposited between 200 – 145 mya
Has

10. Biostratigraphic units (FOSSIL-ROCK units) are based on the occurrence of index fossils

11. 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.
Give some examples of facies: terrestrial vs. marine, near shore vs. off shore. lacustrine

12. 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…

13. 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

14. 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

15. 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.

16. Simple Correlation

17. Physical (lithostratigraphic) Correlation
Another example of simple correlation

18. Correlation over “short” distances: Sedimentary rock sequences (Map of Midwest)
Short distances – meaning on the same continent

20. Fossil (biostratigraphic) Correlation is often used to correlate over LONG distances
In this case LONG distances refers to global correlation

21. 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

22. Lithofacies map
Shows rock facies overlain on a reference map. Allows for inference of features that are no longer visible.