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Lecture 8

ENVIRSC 1G03 Lecture Notes - Lecture 8: Unconformity, Stratigraphic Column, Geologic Time Scale


Department
Environmental Science
Course Code
ENVIRSC 1G03
Professor
Luc Bernier
Lecture
8

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Chap 10
November 25, 2014
3:38 PM
10.1 Intro
Geologic time - span of time since earth's formation
Tools used to determine numerical age of earth and features in years:
o Isotopic (radiometric) dating and geologic time scale.
10.2 Concept of geologic time
Setting the Stage for studying the Past
Hutton speculated that formation of rocks/landscapes were consequence of processes that he
could see happen today.
Uniformitarianism - processes that operate in modern world also operated in the past at roughly
same rates, these processes were responsible for forming geologic features preserved in outcrops.
o Principle can be stated as: the present is key to the past
Hutton realized that:
o DEVELOPMEN OF INDIVIDUAL GEOLOGIC GEATURES TOOK A LONG TIME, NOT ALL
FEATURES FORMED AT THE SAME TIME, thus earth must have history that includes a
succession of slow geologic events
o Since no one has seen the process of sediment turning into rock, then mountains, he
realized that It must have been a long time before human history began.
Relative versus numerical age
Relative Age - age of ones feature with respect to another in a sequence
Numerical Age (absolute age) - Age of a feature given in numbers
Geologists learned how to determine relative age before numerical age
10.3 Geologic principles for defining relative age
Principle of Uniformitarianism - States that physical processes we observe today, also operated in
the past at roughly comparable rates. Present is key to past
Principle of Original horizontality - states that layers of sediment when first deposited are fairly
horizontal as sediments accumulate in low relief in a gravitational field.
o If they were deposited on a slope, it would slide down
o Concluded that folds and tilted beds represent consequences of deformation after
deposition
Principle of superposition - states that in a sequence of sedimentary rocks layers, each layer must
be younger than the one below. Top youngest, bottom oldest.
Principle of lateral continuity - states that sediments generally accumulate in continuous sheets
within a given region.
o If you find a sedimentary layer by buy a canyon, u assume that layer once spanned the area
that was eroded by a river that formed the canyon
Principle of cross-cutting relations - States that if one geologic feature cuts across another, the
feature that has been cut is older.
o Eg. If dike cuts across a bed, the dike is younger, later of sediment buries a fault, sediment is
younger, etc
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Principle of baked contacts - states that igneous intrusion (bakes) surrounding rocks so that the
rock has been baked must be older than the intrusion
Principle of inclusions - states that rock containing an inclusion (fragment of another rock) but
must younger than the inclusion.
These principles are applied to find the relative age of rocks, structures, and other geological features at
a location.
Geologic history of a region - succession of events in order of relative age that produced the rock,
structure and landscape of a region.
Adding fossils to the story: Fossil succession
William smith learned to recognize distinctive layers of sedimentary rocks and to identify the
fossil assemblage ( group of fossil species) that they contained
Also realized that particular assemblage can be found only in a limited interval of strata, not above
or below this interval.
Principle of Succession - Once a fossil species disappears at a horizon in a sequence of strata, it
never reappears higher than the sequence. Extinction is forever.
o This observation is repeated in million locations all over the world
Index fossils (guide fossils) - Some fossil species are widespread but survived for a relatively short
interval of geologic time
o They can be used by geologists to associate strata with the specific time interval
10.4 Unconformities: Gaps in the Record
o Gray sandstone shale had been deposited turned to rock, tilted and truncated by erosion
before red sandstone had been deposited
o Hutton deduced that surface between gray and red rock sequences represented a time
interval during new strata that had not been deposited.
o Unconformity - Surface representing a period of non deposition and possibly erosion
o Three kinds of unconformities
1. Angular unconformity - rocks below angular unconformity were tilted or folded before
unconformity developed. Thus, angular unconformity cuts across underlying layers
and orientation of layers below an unconformity differs from layers above.
2. Nonconformity - Type of unconformity where sedimentary rocks overlie generally much
older inrusive igneous rocks and/or metamorphic rocks.
Igneous or metamorphic rocks underwent cooling, uplift and erosion prior to
becoming the substrate or basement, on which new sediments accumulate
3.
Disconformity- imagine a sequence of sedimentary beds deposited beneath a shallow
sea. Sea level drops exposing beds for some time.
No new sediment accumulates, some preexisting sediment gets eroded away.
When sea level raises, new sequence of sediment accumulates over the old.
Boundary between these two sequences is a disconformity.
Even tho the sequences are parallel, contact between them is a disruption in
deposition
o Because of unconformities, the record preserved in rock layers is incomplete. There is no
single location on earth that has a complete record of earth history.
10.5 Stratigraphic Formations and their correlation
Stratigraphic column - Information about sequence of sedimentary strata at a location that is
drawn
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