dealing with a coastal deposit, a proximal onlap will
mean that the sea level has risen in relation to the land
surface which forms the top of the unconformity. On
seismic profiles we can see onlaps onto the land,
measure the height range between the lowest and
uppermost onlaps, calculate the difference in seismic
time and convert this into approximate thickness.
However, we must remember that the thickness of
the sediments deposited is due not only to a rise in
eustatic sea level, but also to local subsidence of this
part of the basin. The weight due to increased water
depth will cause further subsidence, and sedimentation
will increase the load, resulting in further subsidence
to attain isostatic equilibrium. Local tectonic subsidence may produce a relative change in coastal onlap
in a seismic profile. Regressions are defined as the
boundary between land and sea being displaced out
into the basin. They may be caused by a fall in sea
level which will shift the coastline to further out on the
shelf or to the edge of the continental slope. Here
unloading of some of the water plus erosion of
sediments leads to isostatic uplift of the area landward
of the coast line, so that the measured regression is
greater than the real lowering of the sea level.
The definition of a transgression is that the sea
encroaches over what was previously land, while a
regression is a situation where the boundary between
sea and land (“shoreline”) moves seaward so that
seabed becomes land.
Transgressions and regressions are not always
directly related to sea level changes. When a delta
builds out into the sea, there is a local regression on
the delta even if the sea level has not fallen. If sedimentation is sufficiently rapid, we can have a local
regression on a delta even with a rising sea level.
Along a coastline we can have transgressions in
some areas and regressions in others at the same
time, depending on the rate of sedimentation or erosion with respect to sea level change.
The term “forced regression” is used to indicate
that there is a primary lowering of the sea level.
Changes in sea level can be due to:
a. Local tectonic movements, for example uplift of a
horst or subsidence of a graben structure.
b. Plate-tectonic movements which can be of great
extent, but are not global.
c. Sea level changes. These are global and are called
eustatic sea level changes.
During the Quaternary, cyclic changes in sea level
of up to 120 m accompanied the growth and decay of
continental ice sheets. These changes were rapid and
of large magnitude and can be traced throughout much
of the world. Nevertheless it is often the local
conditions which count most. In the areas which had
supported ice sheets, such as Scandinavia, the melting
of the ice led to uplift due to unloading, and this
exceeded the rise in sea level so that there was a
regression. A 2,000 m thick ice sheet will cause
about 600–700 m of isostatic depression of the crust
under the ice because the density of ice (0.9 g/cm
3 ) is
about one third of that of rocks (2.7 g/cm
3 ).
When seismic stratigraphy was established (Vail
et al. 1976), it was assumed that most of the variations
in sea level that could be interpreted from seismic
records, were attributable to eustatic changes and
thus could be employed for correlation across great
distances. There was a problem in that one did not
know of any other processes than the accumulation of
ice on the continents capable of producing large and
rapid global sea level changes. However we only know
of such ice ages from the Quaternary and late Tertiary,
the Carboniferous-Permian, late Ordovician and the
end of the Precambrian.
The general consensus now is that the rapid sea
level changes outside the ice ages were mainly caused
by tectonic activity. Nevertheless, transgressions and
regressions can be correlated over greater or smaller
distances, depending on the type of tectonic
movements. In particular large-scale plate tectonic
movements involving changes in seafloor spreading
rates and subduction rates cause global sea level
changes.
In order to prove that a transgression is eustatic, we
need rather accurate age determinations of the transgressive deposits from many parts of the world, but it
is difficult to get high resolution datings. However
seismic reflectors correlated with well data can be
used to obtain good stratigraphic control and a picture
of onlap and offlap can be interpreted in terms of sea
level variation. They may represent relative sea level
changes for a part of a sedimentary basin, or eustatic
(global) transgressions or regressions.
Although one might perhaps expect that the sedimentation within a basin would primarily be
characterised by local tectonic conditions, drainage,
depositional conditions etc., studies of thousands of
seismic profiles and wells from many sedimentary
8 Seismic Stratigraphy, Sequence Stratigraphy and Basin Analysis
261
mean that the sea level has risen in relation to the land
surface which forms the top of the unconformity. On
seismic profiles we can see onlaps onto the land,
measure the height range between the lowest and
uppermost onlaps, calculate the difference in seismic
time and convert this into approximate thickness.
However, we must remember that the thickness of
the sediments deposited is due not only to a rise in
eustatic sea level, but also to local subsidence of this
part of the basin. The weight due to increased water
depth will cause further subsidence, and sedimentation
will increase the load, resulting in further subsidence
to attain isostatic equilibrium. Local tectonic subsidence may produce a relative change in coastal onlap
in a seismic profile. Regressions are defined as the
boundary between land and sea being displaced out
into the basin. They may be caused by a fall in sea
level which will shift the coastline to further out on the
shelf or to the edge of the continental slope. Here
unloading of some of the water plus erosion of
sediments leads to isostatic uplift of the area landward
of the coast line, so that the measured regression is
greater than the real lowering of the sea level.
The definition of a transgression is that the sea
encroaches over what was previously land, while a
regression is a situation where the boundary between
sea and land (“shoreline”) moves seaward so that
seabed becomes land.
Transgressions and regressions are not always
directly related to sea level changes. When a delta
builds out into the sea, there is a local regression on
the delta even if the sea level has not fallen. If sedimentation is sufficiently rapid, we can have a local
regression on a delta even with a rising sea level.
Along a coastline we can have transgressions in
some areas and regressions in others at the same
time, depending on the rate of sedimentation or erosion with respect to sea level change.
The term “forced regression” is used to indicate
that there is a primary lowering of the sea level.
Changes in sea level can be due to:
a. Local tectonic movements, for example uplift of a
horst or subsidence of a graben structure.
b. Plate-tectonic movements which can be of great
extent, but are not global.
c. Sea level changes. These are global and are called
eustatic sea level changes.
During the Quaternary, cyclic changes in sea level
of up to 120 m accompanied the growth and decay of
continental ice sheets. These changes were rapid and
of large magnitude and can be traced throughout much
of the world. Nevertheless it is often the local
conditions which count most. In the areas which had
supported ice sheets, such as Scandinavia, the melting
of the ice led to uplift due to unloading, and this
exceeded the rise in sea level so that there was a
regression. A 2,000 m thick ice sheet will cause
about 600–700 m of isostatic depression of the crust
under the ice because the density of ice (0.9 g/cm
3 ) is
about one third of that of rocks (2.7 g/cm
3 ).
When seismic stratigraphy was established (Vail
et al. 1976), it was assumed that most of the variations
in sea level that could be interpreted from seismic
records, were attributable to eustatic changes and
thus could be employed for correlation across great
distances. There was a problem in that one did not
know of any other processes than the accumulation of
ice on the continents capable of producing large and
rapid global sea level changes. However we only know
of such ice ages from the Quaternary and late Tertiary,
the Carboniferous-Permian, late Ordovician and the
end of the Precambrian.
The general consensus now is that the rapid sea
level changes outside the ice ages were mainly caused
by tectonic activity. Nevertheless, transgressions and
regressions can be correlated over greater or smaller
distances, depending on the type of tectonic
movements. In particular large-scale plate tectonic
movements involving changes in seafloor spreading
rates and subduction rates cause global sea level
changes.
In order to prove that a transgression is eustatic, we
need rather accurate age determinations of the transgressive deposits from many parts of the world, but it
is difficult to get high resolution datings. However
seismic reflectors correlated with well data can be
used to obtain good stratigraphic control and a picture
of onlap and offlap can be interpreted in terms of sea
level variation. They may represent relative sea level
changes for a part of a sedimentary basin, or eustatic
(global) transgressions or regressions.
Although one might perhaps expect that the sedimentation within a basin would primarily be
characterised by local tectonic conditions, drainage,
depositional conditions etc., studies of thousands of
seismic profiles and wells from many sedimentary
8 Seismic Stratigraphy, Sequence Stratigraphy and Basin Analysis
261