Carbonate Sequence Stratigraphy

Although much of the previous discussions have drawn on siliciclastic margins as examples, sequence analysis can be readily applied to carbonate systems as well. Carbonates have several unusual features relative to siliciclastics that make their response to relative sea-level changes and their expression of sequence stratigraphic elements somewhat different.

First, when subaerially exposed, carbonates are much more prone to dissolution than erosion like siliciclastics. Consequently, sequence boundaries in carbonates are more commonly expressed as karst surfaces with solution relief, collapsed breccias, paleosols, and silicification. Many additional exposure features are visible petrographically (pendant and meniscus cements, vadose silt, grain dissolution, neomorphism, etc.) or isotopically (negative shifts in delta C-13).

Second, carbonate sediment production is largely in situ rather than transported from outside the basin as in siliciclastics. Whereas relative sea-level rise can trap siliciclastic sediments in estuaries and coastal lagoons, moderate rates of relative sea-level rise allow the carbonate factory to produce at much higher rates. Consequently, transgressive systems tracts in carbonate settings can be extremely thick. Likewise, highstand systems tracts in carbonate settings can be much thinner, because, unlike siliciclastic settings, much of the accommodation space generated during the transgressive systems tract is continually filled. Consequently, highstand sediments only fill space generated during the highstand systems tract and not unfilled space generated during the previous transgressive systems tract. Extremely rapid rates of relative sea-level rise can cause a total shutdown in carbonate production, leading to the formation of spectacular condensed sections with extensive hardground formation as well as pyrite and phosphate mineralization.

Because carbonate production can often keep pace with moderate rates of relative sea-level rise, some carbonate settings are characterized by extremely thick sections of peritidal cycles, carbonate parasequences that shallow upwards to supratidal depths. Identifying depth trends over hundreds of peritidal cycles can be difficult or impossible, so stacking patterns can alternatively be recognized by vertical trends in the thickness of peritidal cycles. Upward thickening of successive parasequences without a net water depth change would indicate progressively greater rates of relative sea-level rise while sediment production was always able to keep up with the rise. Such upward thickening of cycles would be interpreted as retrogradational stacking. Upward thinning of peritidal cycles would indicate slowing rates of relative sea-level rise and would be interpreted as progradational stacking.

Except for these differences, application of sequence stratigraphic principles in terms of intepreting beds and bedsets, recognizing parasequences, stacking patterns and parasequence sets, and identifying significant stratal surfaces and systems tracts is nearly identical in approach for carbonates and siliciclastics.

Next . . . Recommended Readings in Sequence Stratigraphy

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