The Stratigraphic Record
First Geologic Map England, 1815William 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 UnitsFormation – 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 formationsFormation – 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.
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.
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
ErathemSystem
SeriesStage
EraPeriodEpochAge
For example, the Jurassic System describes:
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 fossilsBiofacies – a facies change based on fossils contained within the rocksLithofacies – 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
ShaleSandstone
Sea-level Change Regression creates an offlap sequence
As sea level becomes lower, a coarsening upward sequence of rocks is deposited:
Regressive Sea
SandstoneShale
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.