160 SEQUENCE STRATIGRAPHY interpreting sequence stratigraphy and facies from seismic-reflection patterns The second major treatise on the subject, SEPM Special Publication 42, was published in 1988 This volume elevated the significance of sequence stratigraphy as a means of understanding Earth history and as a practical tool in petroleum geology Included articles further defined the key elements of sequence stratigraphy, documented the ages of sequences, examined theoretical aspects, and discussed both outcrop and subsurface examples Although these two volumes helped to bring sequence stratigraphy into the mainstream of geological thought, there was notable criticism of the concepts and cycle charts One of the more contentious issues was the hypothesis that eustasy is the primary control on the timing and patterns of deposition of sequences Critics argued that the accuracy and precision of chronostratigraphic control are inadequate to demonstrate synchronicity of sequences from around the world and thereby establish the uniqueness of eustatic control The late 1980s and 1990s saw the evolution of sequence stratigraphy into a tool for investigating increasingly detailed stratigraphic problems Studies examined factors controlling sedimentation in specific basins or time intervals and dealt with increasingly finerscale stratigraphic problems in a variety of depositional environments beyond the marine continental margins This approach can be used for finer-scale reservoir- and aquifer-scale stratigraphy problems integrating well log, core, and outcrop-based datasets Parasequences: The Building Blocks of Sequences The parasequence is the fundamental building block of a sequence A parasequence is a relatively conformable succession of genetically related strata bounded by marine flooding surfaces It is normally a progradational or aggradational package that reflects a shoaling-upwards trend The succession of facies within a parasequence generally follows Walther’s Law, which states that a normal vertical facies succession mirrors the lateral distribution of facies in a sedimentary environment The parasequence boundary is the key to correlation in a sequence-stratigraphic framework It is an approximately planar marine flooding surface commonly characterized by non-deposition or minor erosion It may be marked by significant burrowing by organisms and may have an associated lag deposit of coarse material such as shells, gravel, authigenic minerals, or rip-up clasts formed by erosion and winnowing in the course of the flooding event In shallow-marine successions, parasequences typically coarsen upwards with an increase in sand content and a general increase in the thicknesses of the sand beds Sedimentary facies trace a regular succession of shallower-water sedimentary environments For example, in a river-dominated deltaic environment, facies could reflect shoaling from prodelta to delta front to stream-mouth bar (Figure 1A); for a wave-dominated shoreline, a parallel succession from offshore to lower-shoreface to upper-shoreface environments might be expected (Figure 1B) In some cases, fining-upwards parasequences can be recognized For example, in marginal-marine settings, the base of the parasequence may be marked by the abrupt appearance of marine sand above marginal-marine muds, above which the percentage of sand decreases and the sand beds become thinner The facies trace a succession of shallower-water environments, in this example shoaling from subtidal to intertidal to supratidal non-marine facies (Figure 1C) Parasequence-Stacking Patterns Just as a normal succession of genetically related beds make up a parasequence, so a normal succession of parasequences can be grouped into a unit called a parasequence set The pattern of changes between successive parasequences in a parasequence set is termed the parasequence-stacking pattern The concept of accommodation is fundamental to understanding parasequence-stacking patterns Accommodation is the space available for potential sediment to accumulate and is a function of eustasy and subsidence (Figure 2) Sediment influx controls the rate at which this space is filled The interplay between accommodation and sedimentation rates controls whether the shoreline advances or retreats and the resulting vertical facies changes Three types of parasequence-stacking pattern are progradational, retrogradational, and aggradational A progradational parasequence set is recognized where parasequence stacking reflects overall shoaling and basinwards advance of a depositional system Progradation occurs when the rate of deposition exceeds the rate of accommodation: the lack of vertical space for sediment accumulation forces sedimentation basinwards (Figure 3A) A retrogradational parasequence set reflects the opposite case, in which parasequences are stacked in a pattern that reflects overall deepening and a landwards retreat of the depositional system Retrogradation reflects a sedimentation rate that is lower than the rate of accommodation: the inability of sedimentation to fill the available vertical space shifts sedimentation landwards (Figure 3B) An aggradational stacking