Determining geological ages

Determining geological ages

◊ Relative age dates – placing rocks and events in their proper sequence of formation

◊ Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating, radiometric dating)

◊ Determining age is difficult

◊ Relative age dates – placing rocks and events in their proper sequence of formation

◊ Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating, radiometric dating)

◊ Determining age is difficult

Principles of relative dating

Principles of relative dating

◊ Law of superposition◊Developed by Nicolaus Steno in

1669◊ In an undeformed or altered

sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

◊ Law of superposition◊Developed by Nicolaus Steno in

1669◊ In an undeformed or altered

sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom

Principles of relative dating - The law of superposition

Principles of relative dating - The law of superposition

Principles of relative dating

Principles of relative dating

◊ Principle of original horizontality◊ Layers of sediment are generally

deposited in a horizontal position◊ Rock layers that are flat have not been

disturbed

◊ Principle of original horizontality◊ Layers of sediment are generally

deposited in a horizontal position◊ Rock layers that are flat have not been

disturbed

Principles of relative dating

Principles of relative dating

◊ Principle of cross-cutting relationships• Younger features cut across older feature

◊ Inclusions - a piece of rock that is enclosed within another rock

• Fault - A break in a rock mass along which movement has occurred

• Dike - A tubular-shaped intrusive igneous feature that cuts through the surrounding rock.

◊ Principle of cross-cutting relationships• Younger features cut across older feature

◊ Inclusions - a piece of rock that is enclosed within another rock

• Fault - A break in a rock mass along which movement has occurred

• Dike - A tubular-shaped intrusive igneous feature that cuts through the surrounding rock.

UNCONFORMITIESUNCONFORMITIES◊ An unconformity is a break in the rock record

produced by erosion and/or nondeposition of rock units.

These time gaps in the rock layers, are typically on the order of tens of millions of years or more.

◊ Nondeposition - means that no sediments were deposited for an interval of time.

◊ uplift and erosion can remove layers deposited at an earlier time.

◊ Unconformities indicate:• Major sea level changes• Major tectonic events have occurred

◊ An unconformity is a break in the rock record produced by erosion and/or nondeposition of rock units.

These time gaps in the rock layers, are typically on the order of tens of millions of years or more.

◊ Nondeposition - means that no sediments were deposited for an interval of time.

◊ uplift and erosion can remove layers deposited at an earlier time.

◊ Unconformities indicate:• Major sea level changes• Major tectonic events have occurred

◊Types of unconformities

◊ Angular unconformity – tilted rocks are overlain by flat-lying rocks

◊ Disconformity – strata on either side of the unconformity are parallel

◊ Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

◊Types of unconformities

◊ Angular unconformity – tilted rocks are overlain by flat-lying rocks

◊ Disconformity – strata on either side of the unconformity are parallel

◊ Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata

Formation of an angular unconformity

Formation of an angular unconformity

NonconformityNonconformity◊ A nonconformity is a place where younger sedimentary layers

lie on top of older igneous or metamorphic rocks.

◊ The older igneous or metamorphic rocks indicate active igneous and/or tectonic period, followed by uplift and erosion

◊ The overlying sedimentary rocks indicate a time of deposition

of sedimentary layers

◊ A nonconformity is a place where younger sedimentary layers lie on top of older igneous or metamorphic rocks.

◊ The older igneous or metamorphic rocks indicate active igneous and/or tectonic period, followed by uplift and erosion

◊ The overlying sedimentary rocks indicate a time of deposition

of sedimentary layers

NonconformityNonconformity

DISCONFORMITYDISCONFORMITY

◊ A disconformity is a type of gap in the rock layers where the above and below an erosional boundary have the same orientation (often horizontal)

◊ Disconformities may be difficult to recognize ◊ However, as with all unconformities,

disconformities involve a significant time gap-- typically on the order of tens of millions of years!

◊ A disconformity is a type of gap in the rock layers where the above and below an erosional boundary have the same orientation (often horizontal)

◊ Disconformities may be difficult to recognize ◊ However, as with all unconformities,

disconformities involve a significant time gap-- typically on the order of tens of millions of years!

Example of missing layers from a disconformity Example of missing layers from a disconformity

DISCONFORMITYDISCONFORMITY◊ All of the units above the Grand Canyon Series are

horizontal.

◊ Yet, a significant disconformity exists between the Cambrian Muav Limestone and the Mississippian Redwall Limestone.

◊ All of the units above the Grand Canyon Series are horizontal.

◊ Yet, a significant disconformity exists between the Cambrian Muav Limestone and the Mississippian Redwall Limestone.