Module 8, Stratigraphy Flashcards
What is Stratigraphy?
the study of the layered characteristics of sedimentary rocks in outcrop , well logs and core
Why is Stratigraphy used?
in the correlation of petroleum bearing zones
What do stratigraphic sequences describe?
The order in which bodies of sediments were deposited and their geometrical relationship to each other
What two cycles can sediments be of?
Single or multiple and this is found out VIA stratigraphic sequences
What is a vertically stacked association?
A sequence
What are the sedimentary strata related to in the belly river?
the mode of seidment accumulation (deltaic) as well as the changes in sea level (eustasy)
Eustatic/Absolute Sea-Level is based off of what?
A fixed Datum - example: the Centre of the Earth
Relative Sea-Level is based off of what?
A local datum
When do Absolute (eustatic) base -level fluctuations occur
on a global scale and are controlled by major climatic or tectonic events such as glacial episodes and inflation / deflation along spreading centers
What are included in Relative base-level fluctuations?
Both, eustatic and local signals. Major local factors include basin subsidence and sedimentation rates
What is a Base-Level?
The level below which erosion cannot occur and above which deposition does not take place.
What is the ultimate base level?
Sea level
Global sea-level fluctuation occurs on a variety of scales. What are they?
Tectonic and climatic processes
What is Accommodation?
“Space” available for sediment accumulation
How is sediment accumulation limited?
By the amount of space available below base-level. This is referred to as accommodation space.
What determines the amount of accommodation?
a combination of eustatic sea -level fluctuation and basin subsidence and can be either positive or negative
What controls the thickness and type of a sedimentary deposit?
Sediment supply and base level and accommodation space
What is sequence stratigraphy?
The analysis of cyclic patterns in sedimentary successions developed through time
Name the four key principles of stratigraphy?
1) Original Horizontality
2) Stratigraphic superposition
3) Cross-cutting relationship
4) Lateral Continuity
What does an unconformity indicate?
An uplift and erosion of the surface of the older bed before the younger bed was laid down resulting in the geological record
Describe a nonconformity
a surface of erosion that separates younger sedimentary strata above from older igneous or metamorphic rocks below
Describe angular unconformity
A surface of erosion between two groups of sedimentary rocks in which the orientation of older strata, below , are at and an angle to younger strata above
Describe disconformity
a surface of erosion in which the orientation of older strata , below , are parallel to younger strata, above.
Describe a paraconformity
parallel beds and no erosion surface (pinpointed by relative dating method such as fossil succesion)
What is a diastem?
short break or gap between similar depositional events such as floods, storms, etc. They can be pinpointed by radiometric dating.
In a fluvial system the equilibrium state is called what? What is it called in a marine system?
graded profile
base level
How are ichnofacies commonly used as a stratigraphic tool?
To interpret both lateral and vertical changes in depositional environments.
What are 3 examples of ichnofossils?
Tracks
Happy Snails Trails
what well logs are used for correlation?
Gamma Ray
Spontaneous Potential
What 2 methods can be used to correlate stratigraphy?
Lithostratigraphy
Chronostratigraphy
Name 6 reasons why seismic is an essential exploration tool?
Determines:
Tectonic setting adn the depositional environments
Hydrocarbon Traps (structural or stratigraphic)
Structural elevation and dip
Isopach variations
Potential reservoirs and seals
Source rock and reservoir estimation
What factors does accommodation depend ?
The sea level
the movement of sea floor (subsidence)
Changes in rates of sediment supply ( for examples, progradation during sea - level rise)
What has caused the sea level to change over time?
The effects of plate tectonics
Eustacy
Glacial events (isostacy)
What is happening to the sea level during transgression?
The relative sea - level increases
What is happening to the sea level during regression?
The relative sea - level decreases
Explain progradation and what is an example of it
occurs when the sediment accumulation rate is higher than the creating of new accommodation space. The Mississippi River Delta is an example
What is the difference between conformable and unconformable strata?
Conformable is when there is no significant break in deposition (rocks have been deposited in a continuous succession through time).
Unconformable is when an interruption exists - an episode of erosion between deposition (the boundary between the older and younger rocks is unconformable)
