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.