basins indicate that there have been simultaneous
transgressions and regressions in completely different
parts of the world. This has been documented by
correlating seismic profiles with oil wells in the same
area where the age of seismic unconformities and
depositional sequences can be dated by means of biostratigraphy. It turns out that characteristic seismic
reflectors which represent falls in sea level are of
approximately the same age, for example in the
North Sea, South China Sea, Mexican Gulf and
Alaska. It has become clear that many areas, especially continental margins, have a rather similar tectonic history which is ascribable to global seafloor
spreading.
Much of geological time is not represented by
deposits, though; there are usually greater or lesser
breaks in deposition (hiatuses). This becomes very
clear when one looks closely at the continental
sequences. Ocean floor sequences are more continuous, but also there one finds clear breaks in deposition.
The time represented by breaks (hiatuses) varies
greatly because there will always be some sedimentation somewhere, and correlation with sequences with
continuous sedimentation entails trying to find signs of
rapid changes in facies and deposition depth.
Microfossils can be helpful in indicating water depth,
even though they are not always reliable.
Mapping unconformities in the field can be difficult
in the absence of abundant exposures. Many of the
best examples have therefore been found from desert
areas in the USA and elsewhere.
8.7
Changes in the Volume of the
Ocean Basins
As we have seen, large-scale ocean floor topography is
a function of the age of the seafloor, i.e. how long it
has had to cool down since it was formed, and of the
overlying sediment thickness. Periods with rapid seafloor spreading will result in relatively broad spreading ridges which cause the volume of the ocean basins
to decrease, and seawater then will spread further onto
continent margins. If all seafloor spreading ceased, the
spreading ridges would slowly sink and within about
100 million years would have disappeared almost
completely, The volume of the oceans basins would
then be greater, as there would be a sea level drop
corresponding to the volume of the ocean ridges. This
mechanism can explain the great fluctuations in sea
level through geological time. Note too that periods
with major transgressions can be correlated with
periods of rapid seafloor spreading, for example in
the Cretaceous and Carboniferous periods. The deep
channels formed in connection with subduction are, in
fact, small in relation to the width of the spreading
ridge. In Permian and Triassic times we had one big
supercontinent and little seafloor spreading. This was a
regressive period with a large land area.
Drying out of cut-off ocean basins may also lead to
eustatic changes in sea level. There is much to indicate
that the Mediterranean Sea was cut off from the Atlantic and dried up in Upper Miocene (Messinian) times.
This reduced the world’s total volume of ocean basins,
increasing the sea level by about 5–6 m.
Crustal thickening and thinning. An increase in the
depth to the Moho (seismic discontinuity separating
the Earth’s crust and mantle) will lead to elevation of
the land. The greatest land elevation results from continental collision, when the thickness of the continental crust may be doubled (to 70–80 km), as in the
Himalayas.
Stretching and thinning of the continental crust will
move heavy mantle rocks upwards and increase the
average density of the rocks down to a compensation
depth of 100 km. This will lead to subsidence and
more low density sediments can be accumulated. We
see this at the transition between continental and oceanic crust, and where we have rift formation the continental crust thins below the rift, causing graben
formation.
Variations in the temperature gradient affect the
density of the rocks and thereby the isostatic equilibrium. Rifting causes elevation of the areas along the
margin of the rift where the crust is not thinned (e.g.
East Africa) and subsidence of the whole area when
the rifting ceases and the crust cools (e.g. the North
Sea).
The major transgressions in the Cambrian, Ordovician and Cretaceous, can be explained fairly satisfactorily by means of plate tectonic models. A relatively
low sea level at the end of the Palaeozoic
(Carboniferous-Permian) and the beginning of the
Mesozoic (Triassic) can be explained as being due to
limited seafloor spreading, for example along the
Atlantic Ocean. With fewer and smaller spreading
ridges the oceans could accommodate more water
and consequently less seawater would flood the
262
K. Bjørlykke
transgressions and regressions in completely different
parts of the world. This has been documented by
correlating seismic profiles with oil wells in the same
area where the age of seismic unconformities and
depositional sequences can be dated by means of biostratigraphy. It turns out that characteristic seismic
reflectors which represent falls in sea level are of
approximately the same age, for example in the
North Sea, South China Sea, Mexican Gulf and
Alaska. It has become clear that many areas, especially continental margins, have a rather similar tectonic history which is ascribable to global seafloor
spreading.
Much of geological time is not represented by
deposits, though; there are usually greater or lesser
breaks in deposition (hiatuses). This becomes very
clear when one looks closely at the continental
sequences. Ocean floor sequences are more continuous, but also there one finds clear breaks in deposition.
The time represented by breaks (hiatuses) varies
greatly because there will always be some sedimentation somewhere, and correlation with sequences with
continuous sedimentation entails trying to find signs of
rapid changes in facies and deposition depth.
Microfossils can be helpful in indicating water depth,
even though they are not always reliable.
Mapping unconformities in the field can be difficult
in the absence of abundant exposures. Many of the
best examples have therefore been found from desert
areas in the USA and elsewhere.
8.7
Changes in the Volume of the
Ocean Basins
As we have seen, large-scale ocean floor topography is
a function of the age of the seafloor, i.e. how long it
has had to cool down since it was formed, and of the
overlying sediment thickness. Periods with rapid seafloor spreading will result in relatively broad spreading ridges which cause the volume of the ocean basins
to decrease, and seawater then will spread further onto
continent margins. If all seafloor spreading ceased, the
spreading ridges would slowly sink and within about
100 million years would have disappeared almost
completely, The volume of the oceans basins would
then be greater, as there would be a sea level drop
corresponding to the volume of the ocean ridges. This
mechanism can explain the great fluctuations in sea
level through geological time. Note too that periods
with major transgressions can be correlated with
periods of rapid seafloor spreading, for example in
the Cretaceous and Carboniferous periods. The deep
channels formed in connection with subduction are, in
fact, small in relation to the width of the spreading
ridge. In Permian and Triassic times we had one big
supercontinent and little seafloor spreading. This was a
regressive period with a large land area.
Drying out of cut-off ocean basins may also lead to
eustatic changes in sea level. There is much to indicate
that the Mediterranean Sea was cut off from the Atlantic and dried up in Upper Miocene (Messinian) times.
This reduced the world’s total volume of ocean basins,
increasing the sea level by about 5–6 m.
Crustal thickening and thinning. An increase in the
depth to the Moho (seismic discontinuity separating
the Earth’s crust and mantle) will lead to elevation of
the land. The greatest land elevation results from continental collision, when the thickness of the continental crust may be doubled (to 70–80 km), as in the
Himalayas.
Stretching and thinning of the continental crust will
move heavy mantle rocks upwards and increase the
average density of the rocks down to a compensation
depth of 100 km. This will lead to subsidence and
more low density sediments can be accumulated. We
see this at the transition between continental and oceanic crust, and where we have rift formation the continental crust thins below the rift, causing graben
formation.
Variations in the temperature gradient affect the
density of the rocks and thereby the isostatic equilibrium. Rifting causes elevation of the areas along the
margin of the rift where the crust is not thinned (e.g.
East Africa) and subsidence of the whole area when
the rifting ceases and the crust cools (e.g. the North
Sea).
The major transgressions in the Cambrian, Ordovician and Cretaceous, can be explained fairly satisfactorily by means of plate tectonic models. A relatively
low sea level at the end of the Palaeozoic
(Carboniferous-Permian) and the beginning of the
Mesozoic (Triassic) can be explained as being due to
limited seafloor spreading, for example along the
Atlantic Ocean. With fewer and smaller spreading
ridges the oceans could accommodate more water
and consequently less seawater would flood the
262
K. Bjørlykke