Sequence Stratigraphy

Question 1

Walther’s Law

Answer 1

Concept that depositional environments change and migrate through time - shore migrates seaward and the facies with tem

Question 2

Onlap

Answer 2

Things onlapping onto an existing surface - filling upwards in a landward direction

Question 3

Downlap

Answer 3

things downlapping onto an existing surface - progressively filling the basin

Question 4

Transgression

Answer 4

Landward movement of shoreline (the incursion of the sea over the land)
- On lap is a transgressive process

Question 5

Regression

Answer 5

Seaward movement of the shoreline

- Downlap - regressive process happening

Question 6

Sediment supply + sea level rise

Answer 6

• 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)

Question 7

Relative sea-level ‘controls’ sedimentation

Answer 7

• 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

Question 8

Accommodation Space:

Answer 8

tectonic Subsidence + Rate of eustatic sea-level rise = Rate of sedimentation + rate of water depth increase

Question 9

Sequence Stratigraphy

Answer 9

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

Question 10

Parasequences

Answer 10

• 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

Question 11

Relative changes in Sea level (RSL)

Answer 11

RSL is controlled by eustasy, subsidence, tectonics and sedimentation

Question 12

Sequences:

Answer 12

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

Question 13

Systems tract:

Answer 13

• Low stand systems tract (sequence boundary)
• Transgressive systems tract
• High stand systems tract
• Falling-stage systems tract (sequence boundary)

Question 14

Highstand systems tract:

Answer 14

• 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

Question 15

Falling-stage systems tract:

Answer 15

• Sea level falling (below black line)
• Falls until bottom of curve
• Things ‘drape’ as you go progressively sea ward

Question 16

Lowstand systems tract

Answer 16

• 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

Question 17

Transgressive systems tract:

Answer 17

• 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)

Question 18

Maximum Flooding Surface

Answer 18

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))

Question 19

Sequence Boundaries (SBs)

Answer 19

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