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UMD GEOL 342 - Lecture 19: sequence stratigraphy

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Assoc. Prof. A. Jay KaufmanSequence StratigraphyGEOL 342: Sedimentation and StratigraphyLecture 19: sequence stratigraphy28 April 2005Assoc. Prof. A. Jay Kaufman Sequence StratigraphyTransgressive-regressive packages bounded by unconformities make up the sedimentaryrecord of most basins on cratons and continental margins. Some of these unconformities spangreat distances and long periods of time, and these divide the stratigraphic record of continents intodiscrete packages called sequences. These unconformity-bounded packages likely represent largescale tectonic or eustatic events that persist for tens of millions of years. These are naturalintervals of sedimentary history that may cross biostratigraphic period boundaries. Sequencestratigraphy was introduced by Larry Sloss and the Sloss cycles.Global analysis shows that the sequences of North America appear in other regions of theworld. The global similarities in transgressive-regressive cycles are unlikely to have beencontrolled by local tectonics and thus must have a global cause. The only reasonable process isglobal sea level change.Over long time scales (105 to 108 years), sediment accumulation is strongly controlled bychanges in eustatic sea level, tectonic subsidence, and climate. A number of processes can causethe surface of the oceans to move up or down relative to the center of the Earth. Eustatic sea levelis defined by this distance. In addition, the lithosphere can move up or down relative to the center of the Earth, known as upliftor subsidence. The distance between some lithospheric reference horizon (like the sea floor) andthe sea surface is called relative sea level or accommodation space.1Thus, the potential space available for sediment to fill is determined from the combinedmovement of the sea surface and movement of the sea floor, and accommodation is a function ofchanges in relative sea level. It is also a function of rates of sedimentation, for example:a) if the sea level rises and there is a zero or low sediment flux, then transgression results.b) if sea level rises and there is a low rate of sediment flux, then retrogradation of the coastalparasequence results.c) if sea level rises and the rate of sediment flux matches the sea level rise, then aggradation of thecoastal parasequence results.d) if sea level rises and the rate of sediment flux exceeds the sea level rise, then progradation of thecoastal parasequence results.Stratigraphic sequences consist of a succession of genetically related strata,meaning they were deposited during a single cycle of relative sea level change. The riseand subsequent fall of sea level within the shallow marine setting generates a characteristicsedimentary wedge with a specific geometry and vertical stacking pattern.Key terms in sequence stratigraphy include,sequence: A genetically related package of strata bounded by unconformities or their correlativeconformities2parasequence: A genetically related package of strata bounded by flooding surfaces. Mostparasequences are asymmetical shallowing-upward sedimentary cycles. By geneticallyrelated, it is meant that all facies within a parasequence were deposited in lateral continuityto one another, that is, Walther's Law holds true within a parasequence.parasequence set: A genetically related package of parasequences with a characteristic stackingpattern (aggradational, progradational, retrogradational).Note that none of these definitions contains a constraint regarding relative sea level, timeduration, or spatial distribution. This is the essence of sequence stratigraphy – subdividing thestratigraphic record into chronologically distinct units and using their distribution to makepredictions away from your data.Stratigraphic forcingThe distribution of strata in time and space changes due to changes in accommodationspace. As accommodation decreases, stratal packages prograde. As accommodation increases,stratal packages can aggrade or retrograde. Profound increase in accommodation results indramatic flooding. Profound decrease in accommodation can result in truncation, bypass, anddevelopment of unconformities3So what changes accommodation space?Really three things: subsidence, eustacy, and sediment supply. Subsidence is a lowering ofthe subtrate elevation, and is chiefly a tectonic phenomenon (the opposite is uplift). Eustacy refersto actual changes in water height. This usually is driven by climate change (e.g., lake evaporation,glacial storage of ocean water in ice caps). Sediment supply refers to how much sediment isentering a depositional environment and is a function of tectonics and climate. Stratigraphic architecture varies with these parameters.Recognizing changes in accommodation space.The distribution of strata in time and space changes due to changes in accommodationspace. These changes can be recognized by changes in parasequence stacking patterns. Forexample, a change from progradational to retrogradational stacking indicates a change in long-termaccommodation, usually associated with an increase in accommodation space and a rising ofrelative sea level (base level), Important surfaces separate these changes and have special names.transgressive surface: The switch from progradation or aggradation to retrogrdation. Indicatessignificant flooding (transgression) and an increase in accommodation space.maximum flooding surface: The switch from retrogradation to aggradation to progradation.Indicates peak transgression. These surfaces are associated with maximum sediment starvation,often called condensed sections.sequence boundaries: A surface of profound regression. A number of features can be used tocharacterize a sequence boundary.- abnormal subaerial exposure (rooting, deep soils, karst)- a basinward shift in facies (fluvial on offshore, eolian on subtidal carbonates)- a downward shift in coastal onlap (e.g., deposition over an ancestral shelf edge)- abnormal truncation and erosion (deep scour)To see the film on the sedimentary response to sea level go to:http://strata.geol.sc.edu/movies.htmland click on the movie titled “Introduction to basic concepts of sequence stratigraphy.” There aremany other resources on this website that you may want to explore with regards to


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