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.
