|University of Georgia||Geology Department||Stratigraphy Lab||Steven Holland|
Parasequences are defined as a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces and their correlative surfaces. In addition to these defining characteristics, most parasequences are asymmetical shallowing-upward sedimentary cycles.
By genetically related, it is meant that all facies within a parasequence were deposited in lateral continuity to one another, that is, Walther's Law holds true within a parasequence. So, for a typical siliciclastic wave-dominated shoreline, a particular suite of facies should occur in a predictable order. A parasequence that spanned all of these facies would begin with bioturbated offshore mudstones, pass through the storm beds of the transition zone or lower shoreface, continue through the trough crossbedding of the shoreface, pass upwards into the seaward inclined laminae of the foreshore, and be capped by a backshore or coastal plain coal bed. In reality, a single parasequence at a single outcrop rarely passes through all of these facies, but instead includes only a portion of this facies succession; however, all of the facies that do occur appear in the correct order as predicted by Walther's Law. For example, a typical sandy wave-dominated parasequence in an outcrop might include only offshore and transition zone facies, or only shoreface, foreshore, and coastal plain facies, but offshore facies would not be overlain by coastal plain facies within a single parasequence. A parasequence along a deltaic coastline would show a similar coarsening-upward succession, although it would differ in the sedimentary structures developed.
A parasequence developed on a muddy siliciclastic shoreline would have a different suite of facies, but they would also be arrayed vertically in a shallowing upward order and facies relationships would obey Walther's Law. A typical muddy shoreline parasequence would start with cross-bedded subtidal sands, continue with interbedded bioturbated mudstones and rippled sands of the intertidal, and pass upwards into entirely bioturbated and possibly coaly mudstones of the supratidal.
The flooding surfaces that define the top and base of a parasequence display abrupt contacts of relatively deeper-water facies lying directly on top of relatively shallow-water facies. Rocks lying above and below a flooding surface commonly represent non-adjacent facies, such as offshore shales directly overlying foreshore sands or basinal shales directly overlying mid-fan turbidites. Thus, Walther's Law cannot be applied across flooding surfaces. Given that many parasequences are meters to tens of meters thick, this radically reduces the scale at which Walther's Law can be applied. Cases where Walther's Law has been applied to sections hundreds to thousands of meters thick are nearly always incorrect.
Flooding surfaces may also exhibit small scale erosion, usually of a meter or less. Flooding surfaces may be mantled by a transgressive lag composed of shells, shale intraclasts, calcareous nodules, or siliciclastic gravel; such lags are usually thin, less than a meter thick. Flooding surfaces may display evidence of firmgrounds, such as Glossifungites ichnofacies, or hardgrounds that may be bored, encrusted, and possibly mineralized.
Origin and Scale
A parasequence represents a single episode of progradation, that is, the seaward movement of a shoreline. This seaward shoreline movement produces the familiar shallowing-upward succession seen within parasequences. The shallowing-upward succession indicates that accommodation space is being filled more rapidly than it is being created, and some evidence suggests that in some cases, accommodation space is created only at flooding surfaces and not during the bulk of a parasequence.
Flooding surfaces represent a relative rise in sea level, such that accommodation space is being created at a faster rate than it is being filled with sediment. Although these rapid rises in accommodation space are commonly attributed to eustatic sea-level rise, some flooding surfaces are clearly attributable to earthquake-induced subsidence or to delta switching or similar autocyclic mechanisms.
Scale is not part of the definition of a parasequence. However, parasequences are commonly meters to tens of meters thick and they commonly represent durations of tens to hundreds of thousands of years. Many authors confuse these typical scales with the definition of a parasequence, and erroneously assume that any small cycle must be a parasequence and that any long or thick cycle cannot be a parasequence. This is not the case as some meter-thick cycles clearly do not have a parasequence structure and some hundred to thousand meter-thick cycles do display a parasequence structure.
Lateral and Vertical Relationships within a Parasequence
One of the most powerful aspects to recognizing parasequences is understanding and applying the predictable vertical and lateral facies relationships within parasequences. As stated earlier, facies reflect increasingly shallower environments upwards within a parasequence. Although a complete vertical succession of facies can be compiled from a suite of parasequences, most parasequences will display only a portion of the entire shallowing-upward succession of facies.
figure adapted from Van Wagoner et al. (1990)
Because shallow water facies within a parasequence will pinch out laterally in a downdip direction and deeper water facies within a parasequence will pinch out in an updip direction, the facies composition of a single parasequence changes predictably updip and downdip. Thus, a single parasequence will not be composed of the same facies everywhere, but will be composed of deeper water facies downdip and shallower water facies updip, as would be expected. Because parasequence boundaries represent a primary depositional surface, that is, topography at the time of deposition, flooding surfaces will tend to be relatively flat but dip slightly seaward at angles typical of continental shelves. Finally, parasequence boundaries may become obscure in coastal plain settings and in deep marine settings because of a lack of facies contrast necessary to make flooding surfaces visible.
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