# Sea level cycle order

Series Exploring for Oil and Gas Traps Treatise in Petroleum Geology Critical elements of the petroleum system Sedimentary basin analysis John M. Armentrout Web page AAPG Store

One aspect of basin analysis focuses on mapping specific systems tracts of third-, fourth-, or fifth-order sea level cycles and the relationship that stacked depositional sequences deposited during those cycles have to each other. Knowing the order of a cycle or the phase of a cycle represented by a rock sequence is important for predicting the location and type of reservoir and seal and the location of potential source rocks.

## Procedure

To determine cycle order of a sequence of sediments, we use biostratigraphic data, stratigraphic context (i.e., what part of a systems tract the interval is from), oxygen isotope curves, and published sea level curves. The list below suggests a procedure for determining cycle order of a rock sequence.

1. Determine the time span during which the sequence was deposited and compare to age ranges for cycle orders (see table below).
2. Determine the stratigraphic context of the sequence. What are the cycle orders for similar sequences above or below it?
3. Determine the age of the sequence and compare it to published sea level cycle curves.[1]

## Cycle order from thickness and areal extent

Because rates of sediment accumulation and areas of accommodation space vary, thickness and areal extent are of little use in establishing the order of depositional cycles. Most cycle hierarchies are based on duration. Establishing the duration of a sequence is difficult because of problems in high-resolution dating of rocks. However, with careful work an estimate can be made (see .[2][3][4]

## Table of cycle order

Use the table below to help assess the sea level cycle order of a rock interval.[5]

Cycle order Nomenclature Thickness range (ft) Aerial extent (mi2) Duration (Ma)
Range Mode
1st Megasequence 1000+ Global 50–100+ 80
2nd Supersequence 500–5000+ Regional 5–50 10
3rd Sequence 500–1500 500–50,000 0.5-5 1
4th Parasequence Set 20–800 20–2000 0.1-0.5 0.45
5th Parasequence 10–200 20–2000 0.01-0.1 0.04

## GOM basin example

Figure 1 shows the correlation of the third-order eustatic curve of Haq et al.[1] and the oxygen isotope curve of Williams and Trainor[6] with seven prograding clinoform intervals from the High Island South Addition in the GOM basin (see Figure 2). The correlations were established using the extinction events of the benthic foraminifera Hyalinea balthica (Hyal B) and Trimosina denticulata (Trim A) and the present-day sea floor as chronostratigraphic data. Six of the observed depositional cycles occur during the Tejas supersequence B 3.10 (0.8-0.0 Ma) third-order cycle of Haq et al.[1]). This correlation suggests that the local cycles are fourth-order depositional cycles with a duration of approximately 130,000 years each.[5]

The seven fourth-order cycles occur at approximately the same frequency as the oxygen isotope warm and cold cycles. The oxygen isotope cycles are interpreted as glacial-inter-glacial cycles corresponding with relative high- and lowstands of sea level.[6] The clinoforms generally correlate with trends in upward enrichment in isotope values, suggesting progradation during onset of glacial climates as a consequence of lowering sea level as continental ice formed.