Sequence Stratigraphy
History
1748 – Neptunian Theory
Introduced by Benoît de Maillet
-Suggested that eustatic sea-level may be responsible for the topography of Earth (disproven later, but provided a “stepping stone” into sequence stratigraphy
1785 – Theory of the Earth
Introduced by James Hutton
-Introduction of unconformities, erosion/deposition and uplift
-James Hutton is the father of modern Geology
1838 – Uniformitarianism
Introduced by Charles Lyell
-Concepts of “the present is the key to the past”
-Idea that current geological processes can be used to interpret older events
1892 – Walther’s Law of Facies Succession
Introduced by Johannes Walther
-What you observe vertically is what occurs laterally
1906 – Eustasy
Introduced by Eduard Suess
-Global Sea Level and global controls on unconformities
1909 – Geological Time Gaps
Introduced by Eliot Blackwelder
-First use of unconformities as time markers, identification of sedimentary hiatus
1917 – Base Level
Introduced by Joseph Barrell
-Concept of base level and the alternating rise-fall nature
-Introduction of depositional and non-depositional periods linked to baselevel
1916 – Pulsation Theory
Introduced by Amadeus Grabau
-Identification of rhythmic cycles along with the concept of transgression-regression
-Global correlation of unconformities
1951 – Concept of Clinoforms
Introduced by John L. Rich
-Use of seismic reflection geometry to identify sequence packages
-Developed the concept of clinoforms
1958 – Wheeler Diagram
Introduced by Harry Wheeler
-Placing the order of deposition according to the time of deposition
-Introduction of chrono-stratigraphy instead of sequence-stratigraphy
1963 – Major Sequences in North America
Introduced by Lawrence Sloss
-Proved Wheeler diagrams works
-Provided evidence that the eustatic Sea Level controlled 6 major sequences in N. America
1977 – Seismic reflections relation to time lines
Introduced by Peter Vail and Robert Mitchum
-Concept that seismic refection surfaces can be used as “time lines” in stratigraphy
1988 – Slug Model
Introduced by SEPM Publication
-Importance on accommodation space and parasequences
1990 – Well logs correlation
Introduced by Van Wagoner
-Reservoir scale sequence stratigraphy
-Application of well log information to improve sequence stratigraphy
2000– Falling Stage Systems Tract
Introduced by Plint and Nummedal
-FSST is a product of forced regression
Definitions
Allocyclic: Processes that occurs external to the given depositional basin such as global climate, eustasy, etc.
Autocyclic: Processes that are predominately controlled by events within the given basin such as sediment influx, local eustatic uplift, local sea-level, storms/tidal fluctuations, etc.
Sequence: A relatively conformable succession of genetically related strata bounded at their upper surface and base by unconformities and their correlative conformities.
Sequence Boundary (SB): is defined as a laterally extensive (regional scale) unconformity and its correlative conformity and is marked by truncation and toplap below and onlap and downlap above, over which there is an abrupt basinward shift in faces.
Maximum Flooding Surface (or downlap surface) (MFS): is a surface representing maximum landward extent of basinal facies. Defines the top of the transgressive systems tract. Separates retrogradationally stacked parasequences below from aggradationally to progradationally stacked parasequences above.
Progradation: Lateral outbuilding, or progradation, of strata in a sea-ward direction. Progradation can occur as a result of a sea-level rise accompanied by a high sediment flux. Facies are becoming more proximal as sequences stack (Posamentier, 1988; Wilgus, 1988; Emery, and Meyers, 1996). It is characterized by basin-ward (seaward) lateral out building of strata on top of each other (clinoforms).
Aggradation: Vertical buildup of a sedimentary sequence. Usually occurs when there is a relative rise in sea level produced by subsidence and/or eustatic sea-level rise, and the rate of sediment influx is sufficient to maintain the depositional surface at or near sea level. Occurs when sediment flux = rate of sea-level rise, and as sequences stack, facies patterns remain essentially the same (Posamentier, 1988; Wilgus, 1988; Emery, and Meyers, 1996).
Retrogradation: The movement of coastline land-ward in response to a transgression. This can occur during a sea-level rise with low sediment flux. Retrogradational stacking patterns of sequences refer to patterns in which facies become progressively more distal when traced upward vertically (Posamentier, 1988; Wilgus, 1988; Emery, and Meyers, 1996).
Systems track: Genetically related stratigraphic units that were deposited during specific phase of the relative sea level cycle. It is controlled by the sea level and the sediment influx rate, which is defined by clinoforms. Eg. FSST forms when sea-level falls and the sediments progradates.
Lowsand Systems Track (LST): Sediment accumulation during lowest position of the relative sea-level curve. Overlies the upper surface of FSST and capped by transgressive surface. Prograde to aggradate with onlapping stacking pattern.
Trasgressive Systems Track (TST): Includes deposits that accumulates at the onset of costal transgression until the time of maximum transgression of the coast. Overlies transgressive surface and capped by Maximum Flooding Surface. It has a retrogradtional stacking pattern of clinoforms.
Forced Regression (w/ FSST): Associated with the formation of an unconformity merges into correlative conformity that extends seaward. Stratigraphically sharp base expression of lowstand deposits proximally(Say what?). In conract to gradual expression distally. Normal regression has a conformable continuous sequence.
Base Level: Lowest level, below which sediment accumulation takes place. It is controlled by relative sea level and the sediment influx rate and how sediments are preserved. It controls time-space distribution of deposition and non-deposition. Base level above sediments, accommodation space exists hence potential for sediment accumulation and preservation. If it is below sediments, subaerial exposure will occur resulting erosion.
Eustasy: The global change in sea-level measured relative to the center of Earth (standard datum). It is independent of the local factors. It is a function of ocean basin volume and the fluctuations in the volume of water in the ocean.
Relative (local) sea-level: Measured relative to a local datum such as the seabed. It takes into account local eustatic uplift/local basic subsidence, sediment influx rate and ecstasy.
Accommodation Space: Product of change in eustasy, local sea-level and subsidence/uplift rates. It is the space available for potential sediment accumulation.
Parasequence: Relatively conformable succession of genetically related beds or bed sets bounded by marine flooding surfaces or their correlative surfaces.
Unconformity: Sharp boundary between two stratigraphical units with missing time period of deposition. It is usually characterized by erosional or sedimentary hiatus surfaces.
Wheeler Diagram: Chronostratigraphic representation of sequences. It relates time and space of sediment accumulation.
Uniformitarianism: The concept in which the present geologic processes and conditions can be used to interpret historical geology. In other words, “present is the key to the past”.
Walter’s law: Facies next to each other in a continuous vertical sequence also accumulated adjacent to one another laterally. In other words, “what you see vertically most likely what you will observe laterally”.
Normal Regression: It occurs under constant relative sea-level rise.?????
Forced Regression: It occurs under fall in relative sea-level and often results in formation of FSST. During forced regression, the shoreline migrates seawards regardless of the sediment influx rate.
Regressive surface of marine erosion: A subaqueous surface of marine erosion formed during a relative sea level fall.
Source to sink:This is a method to basically estimate how much sediment from the source ends up at the sink where sediment is accumulated.
Deepwater deposits:They are deep water turbidite deposits that are typically formed from clastic sediments that has been transported to the basin that’s trapped in the basin channel. Deepwater deposits require active tectonics, and shelf break.
Identification
3 ways a Transgressive Surface within a siliciclastic depositional sequence can be identified:
Cores: 1) A shift in facies from shoreface sands to more marine deposits (change from sandy to more muddy deposites).
2) Fauna found in basinward sediments are directly overlaying shoreface sediments.
3) Sharp boundary between facies, but not necessarily erosive (due to rapid back-stepping of shoreline). Facies difference is also highlighted by the grain size change.
Well logs: 1) Increased in GR signature.
2) Sharp and abrupt shift in GR signature (non-gradual).
3) Lower porosity reading as the log passes from sandy to muddy deposits.
2D seismic: 1) TS occur between a LST and TST. TS overlie LST and capped by TST.
2) First landward shift of clinoforms. It is the first significant flooding surface.
3) Beginning of onlaps and very thin clinoform marking on top of back-stepping of shoreline.
3 ways a Maximum Flooding Surface within a siliciclastic depositional sequence can be identified:
Cores: 1) Maximum deposition of marine deposits.
2) Appearance of dense shallow marine funna.
3) Very high concentration of shale at the MFS point.
Well logs: 1) MFS is typically max shale content which kicks far right on Gamma Ray (GR).
2) MFS separates between the coarsening and fining upward sequences.
3) Maximum shale content.
2D seismic: 1) Down lapping of parasequences.
2) Forms between HST and TST.
3) Seen as the most landward shift of clinoforms on a seismic section.
3 ways a Highstand Systems Tract within a siliciclastic depositional sequence can be identified:
Cores: 1) Transition from muddy to more clean sand deposits.
2) Grain-size increase upwards as core move up.
3) Within HST, early highstand is composed of more muddy/shaly deposits while late highstand is composed of more shoreface sand.
Well logs: 1) Shows a coarsening upward sequence, indicating shoreface deposits.
2) Increased in channel clustering in late highstand.
3) Deltaic successions may be observed indicating highstand. But it also possible (rarely) to observe deltaic deposits in LST.
2D seismic: 1) HST is above MFS, below sequence boundary but below FSST for genetic sequence model.
2) It has aggradating to progradating stacking parasequence pattern, indicating a change to RSL regression.
3) Parasequence downlap onto the maximum flooding surface.
3 ways a Lowstand Systems Tract within a siliciclastic depositional sequence can be identified:
Cores: 1) Very clean sand or sand channels (typically fluvial or estuarine)
2) May include coals (often coals are formed in the incised valley as a transgressive incised valley fill).
3) Fluvial deposits with carbonate cementation.
Well logs: 1) Lowest Gamma Ray (GR) signature indicating very clean sand channels and even carbonates.
2) Abnormal high porosity kick may be observed indicating coal. But this is not always the case.
3) May also represented by a large interval of low Gamma Ray (GR) signature indicating fluvial deposit.
2D seismic: 1) It overlies the upper surface of FSST or SB and capped by transgressive surface.
2) Prograde to aggradate with onlapping stacking pattern.
3) LST clinforms show on lapping and downlapping onto SB.
3 ways a Sequence Boundary within a siliciclastic depositional sequence can be identified:
Cores: 1) Erosional surface
2) Abrupt change in facies.
3) Situated right after a coarsening upward sequence.
Well logs: 1) Abrupt facies change indicated by a sharp change in the Gamma Ray (GR) log.
2) Coal above marine shale.
3) Situated below your lowstand where the lower Gamma Ray (GR) kick occurs.
2D seismic: 1) SB overlies HST and capped by LST in 3-stage systems track.
2) It forms bellow FSST in 4-stage systems track and in T-R model, TS is the SB.
3) LST downlap and onlaps onto SB.
Type 1 verses Type 2 Sequence Boundaries
1) Type 1 SB forms during a period of relative sea level fall (higher eustatic sea level fall than the rate of subsidence). As a result of relative sea level fall, the accommodation space will decrease. Type 2 SB however, demonstrates a slowdown in relative sea level rise, but not a fall in relative sea level rise. In Type 2 SB the subsidence is greater than the eustatic sea level fall.
2) Type 1 SB is characterized by widespread erosion. It is described as an unconformable sequence. Type 2 SB, in contrast, characterized by a conformable and continuous sequence, in which minimal erosion occurs. Unconformable Type 1 SB can grade into a conformable Type 2 SB, which is a correlative conformity.
3) Type 1 SB is characterized by a forced regression, in which the shorelines drops regardless of the sediment influx rate. Type 2 SB is characterized by a normal shoreline regression, in which we can observe the progradational stacking patterns where shoreline migrates basinward.