Sequence stratigraphy
Sequence stratigraphy is a useful alternative to a purely lithostratigraphic approach, which emphasizes solely based on the compositional similarity of the lithology of rock units rather than time significance.
This in turn led to sequence stratigraphy becoming systematized and understood to have widespread application to stratigraphic study of rock outcrops on the earth's surface as well.
During the 1980s this ushered in a revolution in stratigraphy based on the delineation of regional physical surfaces that separate the sedimentary rock into packages representing discrete and sequential periods of time and predictable patterns of sediment depositional history.
For example, multi-story fluvial sandstone packages often infill incised valleys formed by the sea level drop associated with sequence boundaries.
There are four criteria distinguishing incised valley fills from other types of multi-story sandstone deposits: a widespread correlation with a regional, high relief erosional surface that is more widespread than the erosional bases of individual channels within the valley; facies associations reflect a basinward shift in facies when compared with underlying units; erosional base of the valley removes preceding systems tracts and marine bands producing a time gap, the removed units will be preserved beneath the interfluves; increasing channel fill and fine grained units upwards or changes in the character of the fluvial systems reflecting increasing accommodation space.
After production begins the parasequences act as separate drainage units with the flooding surfaces, which are overlain by shales or carbonate-cemented horizons, forming a barrier to vertical reservoir communications.
Different kinds of systems tracts are assigned on the basis of stratal stacking pattern, position in a sequence, and in the sea level curve and types of bounding surfaces.
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).
Today, sea level is at a relative "high stand" within the Quaternary glacial cycles because of rapid end-Pleistocene and early-Holocene deglaciation.
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.
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.
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).
Lower order cycles are recognized, which seem to result from plate tectonic events like the opening of new ocean basins by splitting continental masses.
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.
[3][4] Sequence boundaries have economic significance because these changes in sea level cause large lateral shifts in the depositional patterns of seafloor sediments.
