Sequence stratigraphy:

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.

Sequence stratigraphy was introduced by Larry Sloss as the Sloss cycles.

Definitions:

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:

Because these processes differ, we have two ways of thinking of sea-level:

Thus, accommodation space, the potential space available for sediment to fill is determined from the combined movement of:


Accommodation - the filing of accommodation space with sediment - is a function of:


Resulting patterns reflect accommodation space and rate of sediment influx:

Stratigraphic sequences consist of a succession of genetically related strata, meaning they:

The rise and subsequent fall of sea level within the shallow marine setting generates a characteristic sedimentary wedge with a specific geometry and vertical stacking pattern. Distribution of strata in time and space changes due to changes in accommodation space.

Stratigraphic forcing

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.

Recognizing changes in accommodation space:

Key terms in sequence stratigraphy:

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.