Schematic illustration showing four parasequences stacked to form parasequence sets, and their well log characteristics. For the progradational parasquence set, each parasequence progrades seaward to a greater extent than did the preceding parasequence, giving rise to a coarsening- and thickening-upward well log pattern. Such a parasequence set forms when the rate of deposition exceeds the rate of accommodation over the time interval represented by the parasequence set. The overall progradation is punctuated periodically by a rise in relative sea level, which results in a flooding surface upon which the next younger parasequence is deposited. For the retrogradational parasequence set, each parasequence progrades seaward to a lesser extent than did the preceding parasequence, giving rise to a fining- and thinning-upward well log pattern. Such a parasequence set forms when the rate of deposition is less than the rate of accommodation, over the time interval represented by the parasequence set. The overall retrogradation is punctuated periodically by a stillstand in relative sea level, which results in progradation of the next younger parasequence. For the aggradational parasequence set, each parasequence progrades seaward the same distance as did the preceding parasequence, giving rise to a uniform well log pattern for the four parasequences. Such a parasequence set forms when the rate of deposition is equal to the rate of accommodation, over the time interval represented by the parasequence set. After Van Wagoner et al. (1990).
Reprinted with permission of AAPG, whose permission is required for further use.
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