Defining a hierarchy of cyclicity is considered a basic step for constructing a stratigraphic framework. The construction of this hierarchy must be done as a conscious effort and must include data that many times is considered non-standard for reservoir characterization. Establishment of the longer-term signal requires incorporation of data on the regional stratigraphic framework. For example, many carbonate core descriptions recognize cycle tops only where tidal-flat lithofacies are present, leaving all subtidal cycles uninterpreted. This can create an inconsistent view of the cycle hierarchy, with a few very thick cycles and a few very thin cycles and no transitional cycles. Establishing a stratigraphic hierarchy must entail looking at both big picture and detailed data Building a robust sequence framework does not necessarily save time, but instead gives a more predictive and useful end result.
The terminology advocated for cyclostratigraphy is a relative one. In two-dimensional sequence parlance, the terms applied here are composite sequence, high-frequency sequence, and high-frequency cycles, discussed above. In the one-dimensional world of cyclostratigraphy this hierarchy tends to be a numbered order system (1st = longest term, 5th = shortest term). In most Precambrian through early Cenozoic data sets it is not possible to constrain the time element sufficiently to determine the average cycle duration. Still, most workers find that a first-pass evaluation of their cycle hierarchy in terms of this stratigraphic ordering is a useful exercise. As better resolution techniques for absolute dating of stratigraphic successions become available, it will be possible to improve the current technique of finding upper and lower bounding surfaces that are loosely constrained radiometrically and dividing the elapsed time implied by the number of cycles in the interval to arrive at a average cycle duration that is then matched to the closest Milankovitch frequency.

1st-2nd Order Cycles

Standard terminology of cyclicity developed by Haq et al. (1987) and Goldhammer et al. (1990) is still in a state of flux. The terminology of cyclicity is extremely simple. The application of this terminology to the real world, and the normalization of this terminology is more difficult. A more important scale of cyclicity for regional exploration and for thorough understanding of the stratigraphic hierarchy of the reservoir setting is the 2nd order or 10-100 my event. These packages include stacks of seismically resolvable depositional sequences in an Exxon sense, and have predictable changes within each. The Exxon term for these packages is supersequence. Typically the condensed section at the supersequence scale forms the key regional hydrocarbon source bed.

3rd Order Cycles (Composite Sequences)

The 3rd-order scale of stratigraphic packaging and it's use in interpretation of seismic datasets is important and forms the basis of the original Exxon global cycle chart (Vail et al. 1977). Third-order cycles are 1-3 my (Haq et al. 1987) or 1-10 my (Goldhammer et al. 1991) units that are representative of the classic Exxon-type depositional sequences.
The origin of these cycles and their global synchroneity are problematic. Whereas a general consensus exists regarding the longer term units of clear tectono-eustatic origin, and the higher-frequency units that are more defensibly of glacio-eustatic origin, no such agreement exists for third-order units (Miall 1986, Miall and Tyler 1991). The ability to demonstrate global synchroneity with biostratigraphic techniques is typically not available, and in cases where precise dating is available, significant variations exist in timing and character of sequences in different basins as a result of syndepositional tectonism and flux in sediment supply. Possible mechanisms are changing rates of sea-floor spreading and long-term climatic/glacio-eustatic variations. Regardless of origin, our experience working with 40 Hz regional seismic data illustrates that, in comparison with our outcrop-defined sequence hierarchy, the 3rd-order or composite sequence scale is that which is resolvable on this quality of data.

4th-5th Order Cycles (High-frequency Sequences, Cycle Sets, Cycles) (after van Wagoner, 1990)

Sea-level cycles in the 20-400 ky duration range are the "bread and butter" of carbonate reservoir characterization. General consensus is that most of these cycles are forced by Milankovitch-band glacio-eustacy (e.g. Koershner and Read 1989; Goldhammer et al. 1990) or productivity/oxygenation (Fischer and Bottjer, 1991). Milankovitch-band changes are generated by cyclic variations in the shape of the earth's orbit, and in the tilt and wobble of the axis. The earth cycles are precession (19-23 ky), obliquity (41 ky), and eccentricity (100-400 ky). The Milankovitch signal must be viewed in the context of the icehouse/greenhouse cycles of the Phanerozoic (Sandberg 1983), which are roughly in-sync with first-order tectono-eustatic cycles.

Milankovitch icehouse-greenhouse settings are fundamental to the way that cycles are developed in carbonate platforms. Major differences in lithofacies continuity, preservation of depositional topography, and formation of diagenetically enhanced porosity can be tied to these changes. During icehouse periods, including the late Proterozoic, late Ordovician, the later half of the Carboniferous through Permian, and the Cenozoic, major ice sheets existed and melting and reformation of these sheets, largely tied to eccentricity cycles, yielded 100-400 ky (4th order) cycles with up to 100 m sea level amplitudes. During greenhouse periods, including the Late Cambrian through Devonian, and middle Triassic through Cretaceous, warmer climates prevailed, ice caps were minimal, and a 20 ky sea-level cycle with a low-amplitude (10 m), high-frequency signal dominated. Read (1994) notes that the Milankovitch band glacio-eustatic signal may be enhanced during third-order lowstands, which are presumably periods of greater ice volume overall. These relationships are summarized in Figures 1.13 and 1.14.

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