Sea level cycles and carbonate sequences
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Cycles and sequences
Sea level cycles interact with subsidence, sedimentation rate, and climate to create the stratigraphy of carbonate sequences. The chart below lists five orders of sea level cycles and defines them by duration.
| Order | Duration, m.y. | Stratigraphic name | Typical thickness, m |
|---|---|---|---|
| 1st | 50–350 | Megasequence | |
| 2nd | 5–50 | Supersequence | |
| 3rd | 0.5–5 | Sequence | 100–1000 |
| 4th | 0.1–0.5 | Parasequence | 1–10 |
| 5th | 0.5–0.01 | Parasequence | 1–10 |
Parasequence sets or systems tracts
Sets of parasequences generally stack in retrogradational, aggradational, or progradational patterns. A parasequence set approximately corresponds to a systems tract and is categorized by its position within third-order sequences (i.e., highstand, lowstand, and transgressive).
Superimposition of cycles
Fourth- and fifth-order cycles combine with third-order cycles to create complex composite curves. The diagram in Figure 1 shows third-, fourth-, and fifth-order sea level cycles and a composite curve of all three.
Relative changes of sea level
Combining eustatic sea level change with tectonic subsidence produces relative changes of sea level. Relative changes of sea level create space for sediment accumulation (called accommodation space).
Tectonic subsidence primarily controls sediment thickness; as sea level cycles up and down, tectonic subsidence creates permanent space. Sea level cycles control lithofacies distribution and stratal patterns.
Interpreting parasequence facies deposition
A simple, effective approach to interpreting facies deposition in carbonate parasequences or sequences is to assume the following:
- Tectonic subsidence is constant.
- Carbonate sediment accumulation rates are greater than subsidence rates.
- The major causes of changes in carbonate facies patterns are cyclic eustatic sea level changes and climatic changes.
Example of interpreting parasequences
Figure 2 shows the correlation of eustatic sea level change with parasequence deposition.
Depositional order of example
If subsidence and sediment production were constant and sea level cyclic, the lithofacies in the parasequence highlighted in the above diagram could have been deposited as follows.
| Step | Action | Description |
|---|---|---|
| 1 | Intraclastic transgressive lag formed. | Transgressions occurred rapidly because the surface of the platform was wide, flat, and had a very gentle dip. A relative sea level rise of only a few meters inundated large areas of the platform. During this start-up phase,[3] the carbonate factory could not go into full production until sea level rose enough to allow efficient circulation on the platform. |
| 2 | Thrombolite bioherms with wackestone deposited. | Water depth may have been 2 or length::3 m initially and sediment quickly built up to sea level as the carbonate factory went into full production. This was the “catch-up” phase. |
| 3 | Ribbon rock and cryptalgal laminite formed. | During the “keep-up” phase, the sediment accumulation rate closely matched sea level rise and subsidence rate. |
| 4 | Sheet floods deposited thin, green, mud-cracked, silty marl and some or most of the mud-cracked cryptalgal laminate. | This occurred across the tops of supratidal flats during highstand conditions. The thin marl and mud-cracked laminites indicate little available accommodation space was available because of slowing sea level rise. |
| 5 | Vadose diagenetic features formed. | Pendant and meniscus cements formed in the upper part of the sequence as a result of subaerial exposure during a sea level fall. |
Correlation of cycles with sequences
Figure 3 shows schematics of carbonate lithofacies portrayed both in depth and time. In the lower part, the composite third- and fourth-order sea level curve shows the correlation of third- and fourth-order sea level change with the sequences and parasequences of the diagram.
See also
- Predicting carbonate porosity and permeability
- Carbonate facies
- Carbonate diagenesis
- Carbonate diagenetic stages
- Early carbonate diagenesis
- Basics of carbonate porosity formation and preservation
- Sea level cycles and carbonate diagenesis
- Sea level cycles and climate
- Sequences during low-amplitude, high-frequency cycles
- Sequences during moderate-amplitude, high-frequency cycles
- Sequences during high-amplitude, high-frequency cycles
- Predicting carbonate reservoir location and quality
References
- ↑ Van Wagoner, J. C., Mitchum, R. M., Campion, K. M., Rahmanian, V. D., 1990, Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops: AAPG Methods in Exploration Series 7, 55 p.
- ↑ Montañez, I. P., Osleger, D. A., 1993, Parasequence stacking patterns, third-order accommodation events, and sequence stratigraphy of Middle to Upper Cambrian platform carbonates, Bonanza King Formation, southern Great Basin, in R. G. Loucks and J. F. Sarg, eds., Carbonate Sequence Stratigraphy—Recent Developments and Applications: AAPG Memoir 57, p. 305–326.
- ↑ 3.0 3.1 Sarg, J. F., 1988, Carbonate sequence stratigraphy, in Wilgus, C. K., Hastings, B. S., Kendall, C. G. St. C., Posamentier, H. W., Ross, C. A., and Van Wagoner, J. C., eds., Sea Level Changes: An Integrated Approach: SEPM Special Publication 42, p. 155–182.
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