Chapter 12 Deep Time
12.2 The concept of geologic time
Uniformitarianism: The formation of rocks and landscapes were a consequence of
processes that he could see happening today; physical process we observe today
also operated in the past at roughly the same rates and these processes were
responsible for the formation of geologic features that we see now in outcrops. “The
present is the key to the past”. Hutton.
Relative age: Age of one feature with respect to another.
Numerical age: The age of a feature given in years.
12.3 Geologic principles used for defining relative age
Geologic principles by Steno, Hutten, Lyell:
- The principle of uniformitarianism
Physical processes we observe operating today also operated in the past at roughly
comparable rates.
- The principle of original horizontality
Sediments on Earth settle out in a gravitational field and on surfaces which are
fairly flat. Therefore layers of sediment when originally deposited are fairly
horizontal.
- The principle of superposition
Each layer of rock must be younger than the one below, for a layer of sediment
cannot accumulate unless there is already a substrate on which it can collect.
- The principle of lateral continuity
Sediments generally accumulate in continuous sheets within a given region.
- The principle of cross cutting relations
If one geologic feature cuts across another, the feature that has been cut is older.
- The principle of baked contacts
Rock that has been backed by an intrusion must be older than the intrusion.
- The principle of inclusions
An inclusion (a fragment of one rock incorporated in another) is always solder than
the rock that contains it. )
The succession of events, in order of relative age, that have produced the rock,
structure and landscape of a region is called the geologic history.
William Smith Canal digging, provided fresh exposures of bedrock. He recognized
distinctive layers of sedimentary rock, and identified the fossil assemblage – the
group of fossil species- that they contained. A particular fossil assemblage can be
found only in a limited interval of strata and not in beds above or below this
interval. principle of fossil succession.
Some species can be found over a broad region, but existed only for a short interval
of geologic time, and thus can be diagnostic of a precise time interval in rocks at
many different locations. The fossils of such species are called index fossils.
, Because of the principle of fossil succession, we can define the relative ages of
strata by looking at fossils.
12.4 Unconformities: Gaps in the record
Unconformity: boundary surface between two units which represents a period of
nondeposition and possibly erosion. The gap in the geologic record that an
unconformity represents is called a hiatus. Geologists recognize three main types
of unconformities:
- Angular unconformity
Represents an unconformity with an “angle”. It happens when rock layers undergo
folding, erosion and new deposition. There is an angle between the side above the
unconformity and below.
-Nonconformity:
A pluton intrudes, erosion cuts down into the crystalline rock, new sedimentary
layers accumulate above the erosion surface.
-Disconformity
No deposition of sediments for a while: In a shallow sea, sediments deposit, the sea
level drops, erosion surface and no new sediment deposition, sea level rises, new
sediment deposition.
-> may be indicated by a gap in fossil succession and/or presence of a surface of
erosion and weathering. If the surface was exposed at the Earth’s surface for a
while, a pebbly layer of debris might occur just above it.
Difference between nonconformity and disconformity:
There are several types of unconformities and of course
each has a specific name (eg. nonconformity,
disconformity, angular unconformity). A disconformity
describes an unconformity at which the same general
layering is present above and below the disconformity.
An disconformity is thus much harder to recognize on
seismic than for example an angular unconformity (in
which layering above and below differs in dip).
Nonconformity is also more easily recognized as it
occurs where sedimentary rocks are deposited on
igneous or metamorphic rocks. These contacts are usually easy to spot since
sedimentary rocks are layered and igneous rocks are not generally layered.
Metamorphic rocks can also have layering, therefore close examination will allow
one to distinguish metamorphic textures and minerals from sedimentary textures
and clasts.
12.2 The concept of geologic time
Uniformitarianism: The formation of rocks and landscapes were a consequence of
processes that he could see happening today; physical process we observe today
also operated in the past at roughly the same rates and these processes were
responsible for the formation of geologic features that we see now in outcrops. “The
present is the key to the past”. Hutton.
Relative age: Age of one feature with respect to another.
Numerical age: The age of a feature given in years.
12.3 Geologic principles used for defining relative age
Geologic principles by Steno, Hutten, Lyell:
- The principle of uniformitarianism
Physical processes we observe operating today also operated in the past at roughly
comparable rates.
- The principle of original horizontality
Sediments on Earth settle out in a gravitational field and on surfaces which are
fairly flat. Therefore layers of sediment when originally deposited are fairly
horizontal.
- The principle of superposition
Each layer of rock must be younger than the one below, for a layer of sediment
cannot accumulate unless there is already a substrate on which it can collect.
- The principle of lateral continuity
Sediments generally accumulate in continuous sheets within a given region.
- The principle of cross cutting relations
If one geologic feature cuts across another, the feature that has been cut is older.
- The principle of baked contacts
Rock that has been backed by an intrusion must be older than the intrusion.
- The principle of inclusions
An inclusion (a fragment of one rock incorporated in another) is always solder than
the rock that contains it. )
The succession of events, in order of relative age, that have produced the rock,
structure and landscape of a region is called the geologic history.
William Smith Canal digging, provided fresh exposures of bedrock. He recognized
distinctive layers of sedimentary rock, and identified the fossil assemblage – the
group of fossil species- that they contained. A particular fossil assemblage can be
found only in a limited interval of strata and not in beds above or below this
interval. principle of fossil succession.
Some species can be found over a broad region, but existed only for a short interval
of geologic time, and thus can be diagnostic of a precise time interval in rocks at
many different locations. The fossils of such species are called index fossils.
, Because of the principle of fossil succession, we can define the relative ages of
strata by looking at fossils.
12.4 Unconformities: Gaps in the record
Unconformity: boundary surface between two units which represents a period of
nondeposition and possibly erosion. The gap in the geologic record that an
unconformity represents is called a hiatus. Geologists recognize three main types
of unconformities:
- Angular unconformity
Represents an unconformity with an “angle”. It happens when rock layers undergo
folding, erosion and new deposition. There is an angle between the side above the
unconformity and below.
-Nonconformity:
A pluton intrudes, erosion cuts down into the crystalline rock, new sedimentary
layers accumulate above the erosion surface.
-Disconformity
No deposition of sediments for a while: In a shallow sea, sediments deposit, the sea
level drops, erosion surface and no new sediment deposition, sea level rises, new
sediment deposition.
-> may be indicated by a gap in fossil succession and/or presence of a surface of
erosion and weathering. If the surface was exposed at the Earth’s surface for a
while, a pebbly layer of debris might occur just above it.
Difference between nonconformity and disconformity:
There are several types of unconformities and of course
each has a specific name (eg. nonconformity,
disconformity, angular unconformity). A disconformity
describes an unconformity at which the same general
layering is present above and below the disconformity.
An disconformity is thus much harder to recognize on
seismic than for example an angular unconformity (in
which layering above and below differs in dip).
Nonconformity is also more easily recognized as it
occurs where sedimentary rocks are deposited on
igneous or metamorphic rocks. These contacts are usually easy to spot since
sedimentary rocks are layered and igneous rocks are not generally layered.
Metamorphic rocks can also have layering, therefore close examination will allow
one to distinguish metamorphic textures and minerals from sedimentary textures
and clasts.
