Parasequences and stacking patterns[edit]A parasequence is a relatively conformable, genetically related succession of beds and bedsets bounded by marine flooding surfaces and their correlative surfaces. The flooding surfaces bounding parasequences are not of the same scale as the regional transgressive surface that is associated with a sequence boundary.The parasequences are the separated into stacking patterns: Aggradational Progradational RetrogradationalEach stacking pattern will give different information on the behaviour of accommodation space, a major control of which is relative level. So a rapidly progradational pattern will be indicative of falling sea level, rapidly retrogradational is evidence for rapidly transgressing sea level and aggradational will be indicative of gently rising sea level.Sea level through geologic time[edit]

Comparison of two sea level reconstructions during the last 500 Myr. The black bar shows the magnitude of sea level change during the Quaternary glaciations; this is for the past few million years, but the bar is offset further in the past for readability.Sea level changes overgeologic time. The graph on the right illustrates two recent interpretations of sea level changes during thePhanerozoic. The modern age is depicted on the left side, labeled N forNeogene. The blue spikes near date zero represent the sea level changes associated with themost recent glacial period, which reached its maximum extent about 20,000 yearsBefore Present(BP). During this glaciation event, the world's sea level was about 320feet(98meters) lower than today, due to the large amount ofsea waterthat had evaporated and been deposited assnowandiceinNorthern Hemisphereglaciers. When the world's sea level was at this "low stand", former sea bed sediments were subjected tosubaerialweathering (erosionby 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 theQuaternaryglacial cycles because of rapid end-Pleistoceneand early-Holocenedeglaciation. 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 theCretaceous(labeled K on the graph), sea level was so high that aseawayextended across the center ofNorth AmericafromTexasto theArctic 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 theEarth's rotational axis (seeMilankovitch 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 theSunvaries (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 fromplate tectonicevents like the opening of new ocean basins by splitting continental masses.Hundreds of similar glacial cycles have occurred throughout theEarth'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 levelis the sea level with reference to a fixed point, the centre of the Earth.Relativesea 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 localsedimentary 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 adjacentbasinswhere these sediments are being supplied. For example when the basinward (oceanward) shift with progradation of shorelines was occurring in theBook Cliffsarea ofUtahthe shorelines were receding or transgressing northwards inWyoming. These sedimentary cycles are representative of the amount of supply of sediment to the basin. In atransgression, less sediment is being supplied than the rate of increase in the depth of water, and thus the shoreline migrates landward. In aregression, 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.Economic significance[edit]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".