An Online Guide to Sequence Stratigraphy

An Online Guide to Sequence Stratigraphy

This online guide is primarily aimed at the application of sequence
stratigraphy to outcrops. As a result, none of the examples deal with
topics related specifically to cores, well logs, or most significantly,
seismic. Perhaps the best way to work through this online guide is to start
with accommodation and to continue down the list of topics from there. Many
of the illustrations in this online introduction to sequence stratigraphy
are modified from the figures in Van Wagoner et al.'s Siliciclastic
Sequence Stratigraphy in Well Logs, Cores, and Outcrops (AAPG Methods in
Exploration #7). Interested readers should study that reference and other
references in the Further Reading section for a fuller explanation of the
concepts introduced here.

Accommodation


The Accommodation Space Equation

Over long time scales (105 - 108 years), sediment accumulation is strongly
controlled by changes in eustatic sea level, tectonic subsidence rates, and
climatic effects on the production of sedimentary grains. Several of these
factors are linked to one another through the accommodation space equation.
This balance of terms is most easily explained for marine sediments, but
can be easily modified to include terrestrial sedimentation. A number of
processes can cause the surface of the oceans to move up and down relative
to the center of the earth. The distance from the sea surface to the center
of the earth is eustatic sea level. In addition, the lithosphere can also
move up and down relative to the center of the earth, and changes in the
distance from some arbitrarily chosen reference horizon and the center of
the earth are called uplift or subsidence. The distance between this
reference horizon and the sea surface is called relative sea level or
accommodation space.
[pic]

Acommmodation space can be filled with sediments or water. The distance
between the sediment/water interface and the sea surface is known as water
depth. The accommodation space not filled with water is filled with
sediment. The rates of change of tectonic subsidence, eustatic sea level,
sediment thickness and water depth are linked to one another through the
accommodation space equation:
T + E = S + W
where T is the rate of tectonic subsidence, E is the rate of eustatic sea-
level rise, S is the rate of sedimentation, and W is the rate of water
depth increase (or deepening). These four variables are defined such that
positive values correspond to tectonic subsidence and eustatic sea-level
rise (factors that increase accommodation space) and sediment accumulation
and water depth increase (factors that reflect filling of accommodation
space). Reversing the signs of these variables accommodates tectonic
uplift, eustatic sea-level fall, erosion, and shallowing of water depth,
respectively.
The accommodation space equation represents a simple volume balance, with
the terms on the left controlling the amount of space that can be occupied
by sediments and water and the terms on the right describing how much water
or sediment fills the accommodation space. As written, the equation is an
approximation. In reality, sediment thickness and water depth must be
corrected for compaction of sediments and for the isostatic effects of
newly deposited sediment.
Through section measurement, changes in sediment thickness can be known,
and through facies analysis, changes in water depth can be known or
approximated. However, without outside information, the rates of eustatic
sea-level change and tectonic subsidence cannot be isolated, nor can their
effects be distinguished from one another for a single outcrop. In other
words, there is no unique solution to this equation as it has two unknowns.
Thus, it is impossible in most cases to ascribe water depth or
sedimentation changes to eustasy or tectonics without having regional
control or outside information. Backstripping is a method of analysis that
iteratively solves the accommodation space to measure changes in relative
sealevel through time. Although as pointed out earlier that no unique
solution exists for this equation, solving it for relative sea level can
provide useful insights into eustasy and tectonics. These data may then be
used to date the timing of rifting and orogeny, to constrain estimates of
lithospheric thickness, and to understand global CO2 cycles and global
patterns of sedimentation.

Causes of Eustatic Sea-Level Change

Changes in eustatic sea level arise from either changes in the volume of
ocean basins or changes in the volume of water within those basins. The
volume of ocean basins is controlled primarily by the rate of seafloor
spreading and secondarily by sedimentation in ocean basins. Because hot and
young oceanic lithosphere is relatively buoyant, it floats higher on the
asthenosphere and displaces oceanic waters upwards and onto continents.
Older and colder oceanic lithosphere is denser, floats lower on the
asthenosphere, and allows oceanic waters to stay within ocean basins. Long-
term (102 k.y. - 105 k.y.) changes in the global rate of seafloor spreading
can change the global average age and density of oceanic lithosphere,
resulting in tens to a couple hundred meters of eustatic change. Filling of
ocean basins with sediments derived from continental weathering is a
relatively slow and minor way of changing ocean basin volumes and is
capable of meters to tens of meters of eustatic change over tens to
hundreds of millions of years.
The three most important controls on the volume of seawater are glaciation,
ocean temperature, and the volume of groundwater. Continental and mountain
glaciation is perhaps the most efficient and rapid means of storing and
releasing ocean water. Due to Archimede's principle, ice caps over polar
oceans do not affect eustatic sea level, so frozen seawater must be placed
on a landmass to lower eustatic sea-level. Continental glaciation is
capable of driving high amplitude (10 - 100 m) and high frequency (1 - 100
k.y.) eustatic changes. Because water expands at temperatures higher and
lower than 4 degrees C, and because the depths of the oceans average around
5 km, small changes in the temperature of seawater can lead to significant
changes in ocean water volume. Changes in water temperature can drive a few
meters of eustatic change over short time scales (0.1 - 10 k.y.). Ocean
water is continuously being recycled through continents as groundwater and
surface water, such as rivers and lakes. Over relatively short time scales
(0.1 - 100 k.y.), changes in the amount of water sequestered on the
continents can cause up to a few meters of eustatic change.

Causes of Tectonic Subsidence

Tectonic subsidence is also called driving subsidence and is distinguished
from the isostatic effects of sediment and water loads. Tectonic
subsidence, as its name implies, is driven by tectonic forces that affect
how continental lithosphere floats on the asthenosphere. Three main
mechanisms that affect this isostatic balance and therefore drive tectonic
subsidence include stretching, cooling, and loading.
Stretching of continental lithosphere in most situations results in the
replacement of relatively light continental lithosphere with denser
asthenosphere. The resulting stretched and thinned lithosphere sinks,
causing tectonic subsidence. Stretching occurs in several types of
sedimentary basins including rifts, aulacogens, backarc basins, and
cratonic basins.
Cooling commonly goes hand-in-hand with stretching. During stretching,
continental lithosphere is heated, becomes less dense, and tends to uplift
from its decreased density (the net effect in a stretched and heated basin
may result either in uplift or in subsidence). As continental lithosphere
cools, it becomes denser and subsides. Cooling subsidence decreases
exponentially with time yet can cause a significant amount of subsidence
hundreds of millions of years following initial cooling. Cooling subsidence
is especially important on passive margins and in cratonic basins.
Tectonic loading can also produce subsidence. The additional weight of
tectonic loads such as accretionary wedges or fold and thrust belts causes
continental lithosphere to sink, leading to tectonic subsidence. Because
the lithosphere responds flexurally, the subsidence occurs not only
immediately underneath the load, but in broad region surrounding the load.
Tectonic loading is particularly important in orogenic regions such as
foreland basins.

Parasequences


Expression

Parasequences are defined as a relatively conformable succession of
genetically related beds or bedsets bounded by marine flooding surfaces and
their correlative surfaces. In addition to these defining characteristics,
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. So, for a typical siliciclastic wave-
dominated shoreline, a particular suite of facies should occur in a
predictable order. A parasequence that spanned all of these facies would
begin with bioturbated offshore mudstones, pass through the storm beds of
the transition zone or lower shoreface, continue through the trough
crossbedding of the shoreface, pass upwards into the seaward inclined
laminae of the foreshore, and be capped by a backshore or coastal pl