The Cause of Change
151
The quickest way to change sea level is to fill or to empty an isolated ocean basin.
We know (thanks to Leg 13 of the Deep Sea Drilling Project) that the Mediterranean
dried up intermittently between 5 and 6 million years ago. The water had to go
elsewhere: global sea level was raised by about 10 m whenever the Mediterranean
dried up. Conversely, when ocean water rushed in to fill the empty basin, global sea
level must have dropped by the same amount. Since salt deposits have been found, or
are thought to be present, in other ocean basins also (North Atlantic, South Atlantic),
geologically instantaneous transgressions and regressions may have been quite common after the break-up of Pangaea.
Even a "geologically instantaneous" transgression would not necessarily be spectacular to a human observer. A transgression which is geologically very fast (although
not produced by the hypothetical isolated basin effect) is proceeding right now in
Venice. To this phenomenon we tum in Section 5.6.
5.5.4 Geologic Aspects. Many sedimentary rocks on land were formed in shallow
seas, and depend therefore greatly on tectonically driven sea-level fluctuations,
whether global or regional in origin. The foregoing sections illustrate the complexity
of the processes involved, which geologists face when attempting to interpret the
record. The coastal zone, we have seen, is an extremely active environment, where
there is intense interaction between the forces of land and sea. Even small morphological features - a hill, a trough - become very important in modifying the winddriven waves and currents which shape the bottom and control sedimentation. Climatic variations and hinterland relief dictate sedimentary conditions, as shown in
the overall contrast between low-latitude carbonate shelves and high-latitude
shelves receiving continental debris, or being deeply eroded by ice. Marginal seas,
separated from the main ocean, are even more strongly influenced by regional
climatic and tectonic circumstances. Generally, faunal diversities decrease in these
environments, compared with the open shelf, due to more extreme fluctuations in
living conditions.
In our task to learn from the present for the past, we face an important obstacle.
The present sea floor is far from being an ideal training ground for aspiring geologists, because of the dramatic cycles of geologically rapid sea level changes during
the last half million years, and especially the rise within the last 20 000 years.
Equilibrium between the parameters of the system water/depth/climate/sediment
rarely obtains as already mentioned in Section 2.7. To demonstrate this basic fact
around the world would require a detailed description of a large number of case
histories - which is quite beyond the scope of this book. Our philosophy, instead, is
to stress principles, but to alert the reader to the complications which make their
application difficult.
151
The quickest way to change sea level is to fill or to empty an isolated ocean basin.
We know (thanks to Leg 13 of the Deep Sea Drilling Project) that the Mediterranean
dried up intermittently between 5 and 6 million years ago. The water had to go
elsewhere: global sea level was raised by about 10 m whenever the Mediterranean
dried up. Conversely, when ocean water rushed in to fill the empty basin, global sea
level must have dropped by the same amount. Since salt deposits have been found, or
are thought to be present, in other ocean basins also (North Atlantic, South Atlantic),
geologically instantaneous transgressions and regressions may have been quite common after the break-up of Pangaea.
Even a "geologically instantaneous" transgression would not necessarily be spectacular to a human observer. A transgression which is geologically very fast (although
not produced by the hypothetical isolated basin effect) is proceeding right now in
Venice. To this phenomenon we tum in Section 5.6.
5.5.4 Geologic Aspects. Many sedimentary rocks on land were formed in shallow
seas, and depend therefore greatly on tectonically driven sea-level fluctuations,
whether global or regional in origin. The foregoing sections illustrate the complexity
of the processes involved, which geologists face when attempting to interpret the
record. The coastal zone, we have seen, is an extremely active environment, where
there is intense interaction between the forces of land and sea. Even small morphological features - a hill, a trough - become very important in modifying the winddriven waves and currents which shape the bottom and control sedimentation. Climatic variations and hinterland relief dictate sedimentary conditions, as shown in
the overall contrast between low-latitude carbonate shelves and high-latitude
shelves receiving continental debris, or being deeply eroded by ice. Marginal seas,
separated from the main ocean, are even more strongly influenced by regional
climatic and tectonic circumstances. Generally, faunal diversities decrease in these
environments, compared with the open shelf, due to more extreme fluctuations in
living conditions.
In our task to learn from the present for the past, we face an important obstacle.
The present sea floor is far from being an ideal training ground for aspiring geologists, because of the dramatic cycles of geologically rapid sea level changes during
the last half million years, and especially the rise within the last 20 000 years.
Equilibrium between the parameters of the system water/depth/climate/sediment
rarely obtains as already mentioned in Section 2.7. To demonstrate this basic fact
around the world would require a detailed description of a large number of case
histories - which is quite beyond the scope of this book. Our philosophy, instead, is
to stress principles, but to alert the reader to the complications which make their
application difficult.