Tectonically Driven Sea Level Fluctuations
147
5.5 Tectonically Driven Sea Level Fluctuations
5.5.1 Sea Level and Sediment Bodies. Sea level fluctuated considerably all through
the Phanerozoic, even during periods when apparently no ice was present. As to the
present sea floor, fluctuations since the Jurassic are of special interest: essentially
they determined the sequence of sedimentary layers within the continental margins.
The thick sediment stacks in the "passive" continental margins have been much
studied for economic reasons. When a margin sinks more or less continuously, coastal sediment bodies must reach great thickness, provided the sediment supply keeps
pace with subsidence, and deposition remains locked in to the sea level. Tertiary
sandy beach deposits can attain up to 1500 m thickness in the northwestem Gulf of
Mexico, for example, also widths of up to 40 km and lengths of several 100 km
(Chap. 5.32).
The significance of the sand bodies in economic geology lies in their porosity and
permeability. Thus, they can retain (and deliver) great quantities of water, petroleum,
or gas. It is for this reason that the origin, dimensions, and properties of coastal sand
bodies have received much attention from marine geologists as well as oil geologists.
Both drilling and seismic exploration helps define their extent in the coastal areas of
interest.
To interpret the sequences of marine sediments on land, in the margin, and on the
deep-sea floor, and for economic reasons as well, we would like to know how sea
level fluctuated over the last 150 million years. However, in as much as the sea level
variations within this geologic period were not driven by the growth and decay of ice
caps, they were not reflected in the isotopic composition of seawater. We cannot,
therefore, find them in the isotopic composition of the foraminifera in the manner
indicated earlier.
How then can we measure these fluctuations?
5.5.2 Reconstruction of Sea Level Changes. The intensive world-wide exploration
of continental margins by seismic profiling has recently led to the realization that the
sediment-stacking patterns in margins of different ocean basins are quite similar -
hence, it is assumed, they must be due to global sea level variation. Using this
hypothesis, P. Vail, R. M. Mitchum, B. U. Haq, and their associates developed a
method to derive sea-level fluctuations from the geometry of sediment layers, as
recognized on seismic reflection records. For the time since the beginning of the
Triassic they have found more than 100 major global sea-level changes, about one for
every 2 million years, on average. The basic idea is this: during a relative rise of sea
level (transgression), sediment layers expand into shallower water, and they become
wider as they build up. During a fall of sea level (regression) the reverse occurs, and
erosion sets in on the shelf. Erosion, of course, produces a hiatus: a surface which
joins older and younger sediments in a discontinuous way. Hence, the course of
regression is poorly documented, and it looks as though it happened rapidly. It is
much like cutting a section out of a movie: the change between "before" and "after"
becomes very sudden.
147
5.5 Tectonically Driven Sea Level Fluctuations
5.5.1 Sea Level and Sediment Bodies. Sea level fluctuated considerably all through
the Phanerozoic, even during periods when apparently no ice was present. As to the
present sea floor, fluctuations since the Jurassic are of special interest: essentially
they determined the sequence of sedimentary layers within the continental margins.
The thick sediment stacks in the "passive" continental margins have been much
studied for economic reasons. When a margin sinks more or less continuously, coastal sediment bodies must reach great thickness, provided the sediment supply keeps
pace with subsidence, and deposition remains locked in to the sea level. Tertiary
sandy beach deposits can attain up to 1500 m thickness in the northwestem Gulf of
Mexico, for example, also widths of up to 40 km and lengths of several 100 km
(Chap. 5.32).
The significance of the sand bodies in economic geology lies in their porosity and
permeability. Thus, they can retain (and deliver) great quantities of water, petroleum,
or gas. It is for this reason that the origin, dimensions, and properties of coastal sand
bodies have received much attention from marine geologists as well as oil geologists.
Both drilling and seismic exploration helps define their extent in the coastal areas of
interest.
To interpret the sequences of marine sediments on land, in the margin, and on the
deep-sea floor, and for economic reasons as well, we would like to know how sea
level fluctuated over the last 150 million years. However, in as much as the sea level
variations within this geologic period were not driven by the growth and decay of ice
caps, they were not reflected in the isotopic composition of seawater. We cannot,
therefore, find them in the isotopic composition of the foraminifera in the manner
indicated earlier.
How then can we measure these fluctuations?
5.5.2 Reconstruction of Sea Level Changes. The intensive world-wide exploration
of continental margins by seismic profiling has recently led to the realization that the
sediment-stacking patterns in margins of different ocean basins are quite similar -
hence, it is assumed, they must be due to global sea level variation. Using this
hypothesis, P. Vail, R. M. Mitchum, B. U. Haq, and their associates developed a
method to derive sea-level fluctuations from the geometry of sediment layers, as
recognized on seismic reflection records. For the time since the beginning of the
Triassic they have found more than 100 major global sea-level changes, about one for
every 2 million years, on average. The basic idea is this: during a relative rise of sea
level (transgression), sediment layers expand into shallower water, and they become
wider as they build up. During a fall of sea level (regression) the reverse occurs, and
erosion sets in on the shelf. Erosion, of course, produces a hiatus: a surface which
joins older and younger sediments in a discontinuous way. Hence, the course of
regression is poorly documented, and it looks as though it happened rapidly. It is
much like cutting a section out of a movie: the change between "before" and "after"
becomes very sudden.