Sequence Stratigraphy Flashcards
Walther’s Law
Concept that depositional environments change and migrate through time - shore migrates seaward and the facies with tem
Onlap
Things onlapping onto an existing surface - filling upwards in a landward direction
Downlap
things downlapping onto an existing surface - progressively filling the basin
Transgression
Landward movement of shoreline (the incursion of the sea over the land)
- On lap is a transgressive process
Regression
Seaward movement of the shoreline
- Downlap - regressive process happening
Sediment supply + sea level rise
- sea level is greater than sediment deposition = shoreline transgression
- More sediment than relative sea level rise = regression
- Balance = stable (usually in transgression or regression -more or less pronounced)
Relative sea-level ‘controls’ sedimentation
- In deep parts of the basin, sea-level rise = sediment starvation. Most sediment gets caught on the shelf -> basin will be relatively starved = more space to store sediment
- While sea-level fall = increased sedimentation. Sediment still coming in but ends up nicely down lapping into the basin because shoreline is closer and there is only a certain amount of space to put sediment.
- The opposite is true for shallow parts of the basin because of accommodation
Accommodation Space:
tectonic Subsidence + Rate of eustatic sea-level rise = Rate of sedimentation + rate of water depth increase
Sequence Stratigraphy
A sequence is a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities
Parasequences
- Defined as a relatively conformable succession of genetically related beds or besets bounded by marine flooding surfaces and their correlative surfaces
- what packages of sediment is doing to look whether there are they are transgressing or regressing and what the sea level was doing - compare globally
Relative changes in Sea level (RSL)
RSL is controlled by eustasy, subsidence, tectonics and sedimentation
Sequences:
Sequence boundaries are caused by sea-level falls
- Sequences need no bounding unconformities and are often conformable on basinward margin
- Sequences extend from low sea-level to low sea-level horizons
- Not always caused by massive sea level changes - can be a lot more subtle
- sequence starts from a low sea level
- As sea level rises, get transgression (onlap)
- As sea level falls, sediment accumulates in basin and we get down lap
- ends when sea level is low again
Systems tract:
- Low stand systems tract (sequence boundary)
- Transgressive systems tract
- High stand systems tract
- Falling-stage systems tract (sequence boundary)
Highstand systems tract:
- Relative rate of rise and fall (top left)
- controls what the sediment does rather than the actual positions of the sea level.
- Blue line above black is relative sea level rise
- High stand is top
- brown line is relative rate of sediment supply (positive)
- Sediment regresses basin ward
- More sediment relative sea level rise - shoreline regresses seaward
Falling-stage systems tract:
- Sea level falling (below black line)
- Falls until bottom of curve
- Things ‘drape’ as you go progressively sea ward
Lowstand systems tract
- sea level is just starting to go up again
- as sediment supply, not getting a lot of impact on shoreline
- gradually steep through time but shoreline is still regressing seaward because more sediment than sea level rise
Transgressive systems tract:
- Sea level is rising quite rapidly and then starts to slow
- Sea level moving more rapidly than sediment supply rate there is going to be a distinctive transgression of shoreline
- Transgresses landwards often end up with transgressive surface of erosion as shoreline marches over pre-existing sediments (waves etc)
Maximum Flooding Surface
Most landward position of the shoreline. Not actually when sediment is highest because of sediment accumulation issue
- Basin starved tends to be good for phosphorite and glauconites in a terrigenous system (low sediment accumulation rate - could be from maximum flooding or transgression phase, high stand and start of falling stage (before shoreline reaches basin))
Sequence Boundaries (SBs)
A surface seperating younger from older strata, along which there is evidence of subareila erosional truncation (and, in some areas, correlative submarine erosion) or subaerial exposure, with significant hiatus indicated.
- Transgressive surface (also Transgressive Surface of Erosion (TSE) is when shoreline goes forward
- Regressive Surface (RS) (Also called Regressive Surface of Erosion (RSE) as shoreline goes backwards - seen as unconformity in outcrop
