The problem with marker beds is that lithostratigraphic units that are unambiguously not time-transgressive are rare and hard to identify. What about the unconformities - the layers where rocks aren't?
Transgressive-regressive packages bounded by unconformities make up the sedimentary record of most basins on cratons and continental margins. Some of these unconformities span great distances and long periods of time, and these divide the stratigraphic record of continents into discrete packages called sequences. These unconformity-bounded packages likely represent large scale tectonic or eustatic events that persist for tens of millions of years. These are natural intervals of sedimentary history that may cross biostratigraphic period boundaries. These form the basis of sequence stratigraphy.
Sequence: discrete package of sediments bounded by the unconformities that divide transgressive-regressive cycles from one another. Note: These unconformities generally form through erosion of exposed sediments at lowstand.
- Some of the unconformities span great distances and long time periods
- Usually represents large scale tectonic or eustatic events that persist for up to tens of millions of years
- Natural intervals of sedimentary history that may cross bio/lithostratigraphic boundaries
- Transgression: ocean moves towards shore; result of sea level rise
- Regression: ocean moves away from shore; result of sea level fall
- Progradational: shore and nearshore deposits move outward into the ocean and overlie deeper water deposits
- Retrogradational: deeper water deposits move towards land and overlie shallow water deposits
- Aggradational: facies remain in the same general location and stack atop others of the same facies
Global analysis shows that the major sequences of North America appear (to varying degrees) in other regions of the world. The global similarities in transgressive-regressive cycles are unlikely to have been controlled by local tectonics and thus must have a global cause. The only reasonable process that could be responsible is global sea level change.
Why does this happen?
Over long time scales (105 to 108 years), sediment accumulation is strongly controlled by changes in Eustatic sea level - The depth of the ocean with respect to the center of the Earth. Related to:
- Tectonic activity: uplift or subsidence of the lithosphere relative to the center of the Earth. E.G Cretaceous highstand.
- Climate. E.G. Quaternary ice ages.
Because these processes differ, we have two ways of thinking of sea-level:
- Eustatic sea level: Depth of the ocean with respect to the center of the Earth. Tends to vary with mid-ocean ridge activity
- Relative sea level: The distance between some lithospheric reference horizon (like the sea floor bedrock) and the sea surface is called relative sea level or accommodation space. Can vary with climate even when tectonic activity is constant.
Thus, accommodation space, the potential space available for sediment to fill is determined from the combined movement of:
- the sea surface
- the sea floor
Accommodation - the filing of accommodation space with sediment - is a function of:
- changes in relative sea level
- rates of sedimentation
Resulting patterns reflect accommodation space and rate of sediment influx:
- if the sea level rises and there is a zero or low sediment flux, then transgression results.
- if sea level rises and there is a low rate of sediment flux, then retrogradation results.
- if sea level rises and the rate of sediment flux matches the sea level rise, then aggradation results.
- if sea level rises and the rate of sediment flux exceeds the sea level rise, then progradation results. If sea level is constant, progradation is extreme.
Stratigraphic sequences consist of a succession of genetically related strata, meaning they:
- were deposited during a single cycle of relative sea level change
- obeyed Walther's law.
The distribution of strata in time and space changes due to changes in accommodation space. As accommodation decreases, stratal packages prograde. As accommodation increases, stratal packages can aggrade or retrograde.
- Profound increase in accommodation results in dramatic flooding and the development of erosional maximum flooding surfaces
- Profound decrease in accommodation can result in truncation, bypass, and development of lowstand unconformities constituting primary sequence boundaries.
Recognizing changes in accommodation space:
- Transgressive surface: switch from progradation or aggradation to retrogradation. Indicates significant flooding (transgression) and an increase in accommodation space.
- Maximum flooding surface: switch from retrogradation to aggradation to progradation. Indicates peak transgression. Surfaces are associated with max sediment starvation because there is less continent exposed to erode. Often called condensed sections.
- Sequence boundaries: surface of profound regression.
- abnormal subaerial exposure (rooting, deep soils, karst)
- a basinward shift in facies (fluvial on offshore, eolian on subtidal carbonates)
- a downward shift in coastal onlap (e.g., deposition over an ancestral shelf edge)
- abnormal truncation and erosion (deep scour)
Key terms in sequence stratigraphy:
- Sequence: Consists of succession of genetically related strata deposited during a single cycle of relative sea level change. Characteristic sedimentary wedge with specific geometry and vertical stacking pattern.
- Parasequence: Remember that trnsgressions typically yield much less sediment than regressions. Thus, it is often useful to identify packages of sediment between maximum flooding surfaces, that mostly consist of regressional deposits. A parasequence genetically related package of strata bounded by flooding surfaces. Most parasequences are asymmetical shallowing-upward sedimentary cycles. By genetically related, it is meant that all facies within a parasequence were deposited in lateral continuity to one another, that is, Walther's Law holds true within a parasequence.
- Parasequence set: A genetically related package of parasequences with a characteristic stacking pattern (aggradational, progradational, retrogradational).
Note that none of these definitions contains a constraint regarding relative sea level, time duration, or spatial distribution. This is the essence of sequence stratigraphy - subdividing the stratigraphic record into chronologically distinct units using erosional and maximum flooding surfaces as time-specific marker beds.
Link to film on the sedimentary response to sea level. Click on the movie titled "Introduction to basic concepts of sequence stratigraphy." There are many other resources on this website that you may want to explore with regards to physical stratigraphic architecture.