Abstract: Jurassic Unconformities and Global Sea-Level Changes from Seismic and Biostratigraphy

By

P. R. Vail, J. Hardenbol, and R. G. Todd

Integration of seismic with biostratigraphic data provided the means both to recognize two new types of unconformities and to explain the origin of starved (condensed) intervals of marine section. We use the Jurassic sediments to illustrate these concepts of stratigraphy.

We call the two types of unconformities simply Type 1 and Type 2. Global unconformities which cut both subaerial and submarine strata of the same age are called Type 1 and we attribute them to rapid falls of eustatic sea level. Global unconformities which cut only subaerial strata are called Type 2 and attributed to slow falls of eustatic sea level. 

Figure 1. Chart showing relative changes of coastal onlap, sequence boundaries (including types of unconformities and condensed sections), and eustatic sea level changes for the Jurassic.

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Marine condensed strata are sedimentary sections which are generally quite thin and uninterrupted by unconformities. Such sediments have sometimes been called starved intervals, and are caused by rapid rises of sea level. The transgression moves the depositional site landward thereby preventing significant quantities of sediment from reaching the deeper pans of basins.

Unconformity recognition is locally or regionally enhanced by periodic truncation of folded and faulted strata during sea-level lowstands and onlap onto topographic highs during sea-level highstands, but we find no evidence that the tectonics caused the global unconformities.

Seventeen global unconformities and their correlative conformities (sequence boundaries) subdivide the strata of the Jurassic (Fig.1). These 16 cycles comprise the Jurassic supercycle (Vail et al, 1977, Part 8 AAPG Memoir 26). Eight of the global unconformities are both subaerial and submarine (Type 1); the remaining nine unconformities are subaerial only (Type 2). In addition, over 12 marine-condensed (starved) intervals have been identified. The 16 cycles of the Jurassic supercycle are chronostratigraphic intervals that subdivide the Jurassic into a series of genetic depositional sequences, which are ideal for facies analysis.

The Jurassic unconformities and the stratigraphic and facies patterns between them are caused by the interaction of basement subsidence, eustatic sea-level changes and varying sediment supply. Detailed analysis of the sediments with seismic stratigraphy and well data permit quantification of the subsidence history and reconstruction of paleoenvironment and sea-level changes through time.

The integrated use of seismic stratigraphy and biostratigraphy provides a better geologic age history than could be obtained by either method alone. Paleobathymetry, sediment facies, and relative changes of sea level can be interpreted from seismic data and confirmed or improved by well control. Geohistory analysis provides a quantitative analysis of basin subsidence. When this subsidence is corrected for compaction and sediment loading, the tectonic subsidence and long-term eustatic changes may be determined. Short-term, rapid changes of sea level can be demonstrated from seismic and well data. The stratigraphic resolution of these changes rarely allows exact quantification of their magnitude, but a minimum rate of change of sea level often can be determined. We shall use examples to illustrate the application of these procedures.

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