Author(s): P R Vail
Seventeen global unconformities and their correlative conformities (sequence boundaries) subdivide the strata of the Jurassic and earliest Cretaceous into genetic depositional sequences produced by 16 eustatic cycles. These 16 cycles make up the Jurassic supercycle. Eight of the global unconformities 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 enh nced 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 depositional sequences, which are recognizable from seismic, well, and outcrop data.