Geologic Time

ï Relative dating

ñ Determine sequence or order that geologic events and processes occurred

ñ No actual dates attached, just the order

ï Absolute dating

ñ Determine actual ages of rocks and events

ñ Utilizes isotopes and radiometric dating

The Stratigraphic Record

ï Sedimentary rocks generally occur in layers reflecting the deposition of original sediments

ï Layering of sedimentary rocks referred to as stratification

Layered Sedimentary Rocks Reflect Original Deposition of Sediments in Horizontal Layers

Sedimentary Rocks Preserve the Horizontal Layering (Stratification) of Original Sediments

Rock units are not continuously exposed, but are separated by great distances where they are either buried or have been eroded away

Correlation involves matching a particular rock layer (formation) in one exposure with its counterpart at a different locality

By correlating various rock exposures separated by great distances, geologic maps can be constructed and the original geographic extent of the rocks canÝ be estimated

Fossils (Relicts of Ancient Life)

Life Evolved and Changed Throughout Earth History.Ý Rocks of Different Ages Therefore Contain Different Types of Fossils

Principle of Fossil Correlation

ï Like or similar assemblages of fossils are the same age and therefore strata containing these fossils are also the same age

ï Fossils are therefore very useful in correlating rock sequences in exposures separated by great distances

Organizing the Rock Record

Stratigraphy is a sub-science of geology that attempts to organize the rock record into some manageable classification scheme

Formations

ï The most basic local unit of stratigraphy is the formation

ï A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics:

ñ Lithology (rock type)

ñ Color

ñ Sedimentary structures

ñ Certain depositional patterns

Lithostratigraphy (formations)

Distinguished by:

mineral compositionÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ color

textural propertiesÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ thickness

organic remainsÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝÝ outcrop character

Unconformities

Unconformities represent erosional surfaces or intervals of missing strata within a rock sequence

Types of Unconformities

ï Angular unconformity

ï Nonconformity

ï Disconformity

Angular Unconformity

An erosional surface separating tilted or folded sedimentary rocks below from flat-lying sedimentary rocks above

Nonconformity

Younger sedimentary rocks lie on older igneous or metamorphic rocks

Disconformity

Rocks above and below the unconformity are flat lying

Principles of Relative Dating

ï Law of superposition

ï Principle of original horizontality

ï Principle of cross-cutting relationships

ï Inclusions

ï Unconformities

Law of Superposition

In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below

Principle of Original Horizontality

ï Layers of sediment are generally deposited in a horizontal position

ï If resulting sedimentary rock is folded or faulted, then the deformation occurred after formation

Principle of Cross-cutting Relationships

ï If a fault cuts rocks or magma intrudes rocksÖ

ñ The fault/magma are younger than the rocks

ñ Faulting and intrusion occurred after the sediments were deposited and lithified

Inclusions

ï Inclusions are fragments of a rock unit enclosed in another rock unit

ï The rock unit that supplied the inclusions must be older than the rock containing the inclusions