Science Fair Project Encyclopedia
The 'sequence' part of the name refers to cyclic sedimentary deposits. The term 'stratigraphy' refers to the geologic knowledge about the processes by which sedimentary deposits form and how those deposits change through time and space on the Earth's surface.
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 ice age, 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" because the majority of the glaciers had melted by about 10,000 BP and minor glacial melting has slowly continued (with occasional reversals) throughout recorded human history. The ancient shoreline of the last ice age is now under approximately 390 feet (120 meters) of water. For this reason, most early civilization seaport cities are currently under water (this may be the historic origin of the biblical Noah story).
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 (see reconstruction here).
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.
These events have economic significance because these changes in sea level cause large lateral shifts in the depositional patterns of seafloor sediments. These lateral shifts in deposition create alternating layers of good reservoir quality rock (porous and permeable sands) and poorer-quality mudstones (capable of providing a reservoir "seal" to prevent the leakage of any accumulated hydrocarbons that may have migrated into the sandstones). Hydrocarbon prospectors look for places in the world where porous and permeable sands are overlain by low permeability rocks, and where conditions are right for hydrocarbons to be generated and migrate into these "traps".
- A chart of sea level for the past 140,000 years (The different orders of cyclicity can be seen as higher frequency chatter on an overall asymmetric cycle. Today's date is on the right side of this chart.)
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