Jurassic Unconformities, Chronostratigraphy and Sea-Level Changes from Seismic Stratigraphy and Biostratigraphy

Abstract

Abstract Seventeen global unconformities and their correlative conformities (sequence boundaries) subdivide the strata of the Jurassic and earliest Cretaceous into genetic sequences produced by 16 eustatic cycles. These 16 cycles make up the Jurassic supercycle. Eight of the global unformities are both subaerial and submarine (Type 1), and are believed to have been caused by rapid eustatic falls of sea level. Nine of the unconformities are subaerial only (Type 2), and are believed to be related to slow eustatic falls of sea level. In addition, 16 marine condensed sections (starved intervals) have been identified. These condensed sections are interpreted to be related to rapid eustatic rises of sea level. 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. The 16 eustatic cycles that make up the Jurassic supercycle correspond to 16 global chronostratigraphic intervals that subdivide Jurassic strata into a series of genetic sequences, which are recognizable from seismic, well and outcrop data. The Jurassic unconformities and the stratal and facies patterns between them are caused by the interaction of basement subsidence, eustatic changes of sea level, and differing sediment supply. Detailed analyses 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 intergrated 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 on by well control. Geohistory analysis based on geologic time-depth diagrams provides a quantitative analysis of total 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, well and outcrop 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.