UGA Stratigraphy Lab

The data is in the strata

Parasequence Sets and Stacking Patterns

In most cases, there will not be simply one parasequence by itself, but there will be a series of parasequences. Sets of successive parasequences may display consistent trends in thickness and facies composition and these sets may be retrogradational, aggradational, progradational, or degradational.

Retrogradational Stacking

In a retrogradational set of parasequences, each parasequence is shifted landwards relative to the the preceding parasequence. This net facies shift produces an overall deepening-upward trend within the entire parasequence set, and the set is referred to as retrogradational parasequence set or is said to display retrogradational stacking. Retrogradational stacking is also commonly called backstepping. In a single outcrop, a retrogradational parasequence set can be recognized by the progressive appearance of deeper water facies upwards within the parasequence set as well as the progressive loss of shallower water facies upwards in the parasequence set. For example, in a set of retrogradationally stacked parasequences, offshore facies might be present in only the uppermost parasequences, and coastal plain coals might be present in only the lowermost parasequences.

retrogradational parasequence set

figure adapted from Van Wagoner et al. (1990)

In a cross-section, a retrogradational parasequence set can be recognized by the landward movement of a particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to move farther landward in each successive parasequence.

Retrogradational stacking results when the long-term rate of accommodation exceeds the long-term rate of sedimentation. In this way, accommodation space is created more rapidly than it is filled, water depth becomes deeper, and facies increasingly move farther landward. Although each parasequence is shallowing-upward, the amount of deepening at the flooding surface exceeds the amount of shallowing in the following parasequence, producing a net overall deepening within the parasequence set.

Aggradational Stacking

In an aggradational set of parasequences, each parasequence is placed in roughly the same lateral position as the previous parasequence. As a result, there is no net vertical trend in water depth. Such a set is called an aggradational parasequence set or is said to display aggradational stacking. In a single outcrop, an aggradational parasequence set can be recognized by the similarity of facies composition in each successive parasequence. No new deeper or shallower water facies will tend to appear near the top or base of the parasequence set.

aggradational parasequence set

figure adapted from Van Wagoner et al. (1990)

In a cross-section, an aggradational parasequence set can be recognized by the relative stability of any particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to stay at essentially the same position in each successive parasequence. Facies contacts rarely remain at exactly the same position, so aggradational parasequence sets are commonly characterized by relatively minor facies shifts that display no clear long-term trend.

Aggradational stacking results when the long-term rate of accommodation closely matches the long-term rate of sedimentation. In this way, accommodation space is filled about as rapidly as it is created, water depth remains constant from one parasequence to the next, and facies show no net landward or seaward movement. Although each parasequence is shallowing-upward and is bounded by a flooding surface, the shallowing in each parasequence closely balances the deepening at the underlying flooding surface, resulting in no net shift of facies from one parasequence to the next.

Progradational Stacking

In a progradational set of parasequences, each parasequence is shifted somewhat farther seaward than the parasequence before. This produces an overall shallowing-upward trend within the entire parasequence set and the set is referred to as a progradational parasequence set or is said to display progradational stacking. In a single outcrop, a progradational parasequence set can be recognized by the progressive appearance of shallower-water facies upward in the parasequence set as well as the progressive loss of deeper-water facies upward in the parasequence set. For example, in a set of progradationally stacked parasequences, perhaps all of the parasequences contain shoreface and foreshore facies, but only the uppermost parasequences may contain the coastal plain coal, and only the lowermost parasequences may contain offshore and transition zone facies.

progradational parasequence set

figure adapted from Van Wagoner et al. (1990)

In a cross-section, a progradational parasequence set can be recognized by the seaward movement of a particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to move farther basinward in each successive parasequence. Likewise, the same contact at the base of each parasequence will appear to move farther basinward in each successive parasequence.

Progradational stacking results when the long-term rate of accommodation is exceeded by the long-term rate of sedimentation. In this way, accommodation space is filled more rapidly than it is created, water depth becomes shallower, and facies increasingly move farther seaward over time. Each parasequence is shallowing-upward and is bounded by a flooding surface, across which water depth abruptly increases. However, the shallowing gained in one parasequence overshadows any deepening across the underlying flooding surface, resulting in a net seaward movement of facies relative to the previous parasequence.

Degradational Stacking

In a degradational set, each unit is not only shifted farther seaward (as is also true for progradational stacking), each unit is also shifted so far seaward that each cycle starts farther seaward than the end of the previous cycle. In other words, the cycles are no longer bounded by flooding surfaces, but by surface that record abrupt basinward shifts of facies. In outcrop, these are recognized as surfaces of abrupt shallowing, called surfaces of forced regression. In degradational stacking, successive cycles step not only seawards, but also downwards (as in down the depositional profile), unlike progradational stacking, in which successive cycles step seawards and upwards. Like progradational stacking, degradational stacking produces a net upward-shallowing.

degradational parasequence set

In a cross-section, a degradational parasequence set can be recognized by the seaward and downward (that is, down the depositional slope) movement of a particular facies contact at an equivalent position in a parasequence. For example, the contact between the shoreline sands and the coastal plain facies at the top of each parasequence will appear to move farther basinward and downward in each successive parasequence. Likewise, the same contact at the base of each parasequence will appear to move farther basinward in each successive parasequence.

Degradational stacking results when the long-term rate of accommodation is negative, that is accommodation space is being lost through some combination of eustatic fall and uplift.

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