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 Does anyone remember who this guy is? Pull out the book with the dust cover map… 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 Biostratigraphic units will be discussed in an upcoming lecture

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

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 -

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.

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

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

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

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 Another example of simple correlation

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

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

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.