GEOL 342: Sedimentation and Stratigraphy
10 April 2006
Assoc. Prof. A. Jay Kaufman
Sequence Stratigraphy
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. Sequence
stratigraphy was introduced by Larry Sloss and the Sloss cycles.
Global analysis shows that the sequences of North America appear 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 is
global sea level change.
Over long time scales (105 to 108 years), sediment accumulation is strongly controlled by
changes in eustatic sea level, tectonic subsidence, and climate. A number of processes can cause
the surface of the oceans to move up or down relative to the center of the Earth. Eustatic sea
level is defined by this distance.
In addition, the lithosphere can move up or down relative to the center of the Earth, known as
uplift or subsidence. The distance between some lithospheric reference horizon (like the sea
floor) and the sea surface is called relative sea level or accommodation space.
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Thus, the potential space available for sediment to fill is determined from the combined
movement of the sea surface and movement of the sea floor, and accommodation is a function of
changes in relative sea level. It is also a function of rates of sedimentation, for example:
a) if the sea level rises and there is a zero or low sediment flux, then transgression results.
b) if sea level rises and there is a low rate of sediment flux, then retrogradation of the coastal
parasequence results.
c) if sea level rises and the rate of sediment flux matches the sea level rise, then aggradation of
the coastal parasequence results.
d) if sea level rises and the rate of sediment flux exceeds the sea level rise, then progradation of
the coastal parasequence results.
Stratigraphic sequences consist of a succession of genetically related strata,
meaning they were deposited during a single cycle of relative sea level change. 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.
Key terms in sequence stratigraphy include,
sequence: A genetically related package of strata bounded by unconformities or their correlative
conformities
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parasequence: A 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 and using their distribution to make
predictions away from your data.
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. Profound increase in accommodation results in
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dramatic flooding. Profound decrease in accommodation can result in truncation, bypass, and
development of unconformities
So what changes accommodation space?
Really three things: subsidence, eustacy, and sediment supply. Subsidence is a lowering
of the subtrate elevation, and is chiefly a tectonic phenomenon (the opposite is uplift). Eustacy
refers to actual changes in water height. This usually is driven by climate change (e.g., lake
evaporation, glacial storage of ocean water in ice caps). Sediment supply refers to how much
sediment is entering a depositional environment and is a function of tectonics and climate.
Stratigraphic architecture varies with these parameters.
Recognizing changes in accommodation space.
The distribution of strata in time and space changes due to changes in accommodation
space. These changes can be recognized by changes in parasequence stacking patterns. For
example, a change from progradational to retrogradational stacking indicates a change in longterm accommodation, usually associated with an increase in accommodation space and a rising of
relative sea level (base level), Important surfaces separate these changes and have special names.
transgressive surface: The switch from progradation or aggradation to retrogrdation. Indicates
significant flooding (transgression) and an increase in accommodation space.
maximum flooding surface: The switch from retrogradation to aggradation to progradation.
Indicates peak transgression. These surfaces are associated with maximum sediment starvation,
often called condensed sections.
sequence boundaries: A surface of profound regression. A number of features can be used to
characterize a sequence boundary.
 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)
To see the film on the sedimentary response to sea level go to:
http://strata.geol.sc.edu/movies.html
and 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.
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