Sequence Stratigraphy - Sea Level Through Geologic Time

Sea Level Through Geologic Time

Sea level changes over geologic time. The graph on the right illustrates two recent interpretations of sea level changes during the Phanerozoic. Today's date is on the far left side, labeled N for Neogene. The blue spikes near date zero represent the sea level changes associated with the most recent glacial period, which reached its maximum extent about 20,000 years Before Present (BP). During this glaciation event, the world's sea level was about 320 feet (98 meters) lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice in Northern Hemisphere glaciers. When the world's sea level was at this "low stand", former sea bed sediments were subjected to subaerial weathering (erosion by rain, frost, rivers, etc.) and a new shoreline was established at the new level, sometimes miles basinward of the former shoreline if the sea floor was shallowly inclined.

Today, sea level is at a relative "high stand" within the Quaternary glacial cycles because of rapid end-Pleistocene and early-Holocene deglaciation. The ancient shoreline of the last glacial period is now under approximately 390 feet (120 meters) of water. Although there is debate among earth scientists whether we are currently experiencing a "high stand" it is generally accepted that the eustatic sea level is rising.

In the distant past, sea level has been significantly higher than today. During the Cretaceous (labeled K on the graph), sea level was so high that a seaway extended across the center of North America from Texas to the Arctic Ocean.

These alternating high and low sea level stands repeat at several time scales. The smallest of these cycles is approximately 20,000 years, and corresponds to the rate of precession of the Earth's rotational axis (see Milankovitch cycles) and are commonly referred to as '5th order' cycles. The next larger cycle ('4th order') is about 40,000 years and approximately matches the rate at which the Earth's inclination to the Sun varies (again explained by Milankovitch). The next larger cycle ('3rd order') is about 110,000 years and corresponds to the rate at which the Earth's orbit oscillates from elliptical to circular. Lower order cycles are recognized, which seem to result from plate tectonic events like the opening of new ocean basins by splitting continental masses.

Hundreds of similar glacial cycles have occurred throughout the Earth's history. The earth scientists who study the positions of coastal sediment deposits through time ("sequence stratigraphers") have noted dozens of similar basinward shifts of shorelines associated with a later recovery. The largest of these sedimentary cycles can in some cases be correlated around the world with great confidence.

The three controls on stratigraphic architecture and sedimentary cycle development are:

• Eustatic sea level changes
• Subsidence rate of the basin
• Sediment supply.

Eustatic sea level is the sea level with reference to a fixed point, the centre of the Earth. Relative sea level is measured with reference to the base level, above which erosion can occur and below which deposition can occur. Both eustatic sea level changes and subsidence rates tend to be longer cycles. Sediment supply is largely thought to be controlled by local climatic conditions and can vary rapidly. These variations in local sediment supply affect the local and relative sea level which causes local sedimentary cycles.

Smaller and localised sedimentary cycles are not related to world wide (eustatic) sea level changes but more to the supply of sediment to the adjacent basins where these sediments are being supplied. For example when the basinward (oceanward) shift with progradation of shorelines was occurring in the Book Cliffs area of Utah the shorelines were receding or transgressing northwards in Wyoming. These sedimentary cycles are representative of the amount of supply of sediment to the basin. In a transgression, less sediment is being supplied than the rate of increase in the depth of water, and thus the shoreline migrates landward. In a regression, if the water depth is decreasing, the shoreline migrates seaward (basinward) and the previous shoreline is eroded. A regression of the shoreline also occurs if more sediment is being supplied than the shoreline can erode, causing the shoreline to migrate seaward. The latter is called progradation.