Sequence‐stratigraphic concepts applied to the identification of depositional basins and global tectonic cycles

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Abstract
Division of thousands of metres of strata into genetic units that are tied to depositional and tectonic origins is a problem common to all stratigraphic successions. Sequence stratigraphy, which defines large‐scale genetic units by a hierarchy of unconformity‐bounded sequences, resolves that problem. However, there are no physical‐recognition criteria for the genetic rank of sequence boundaries. Therefore, genetic‐stratigraphic division by unconformity‐bounded sequences alone is not possible, because all successions are composites of sequences of different origins. Three orders of unconformity‐bounded sequences are defined: third‐order sequences are basin‐filling rhythms; second‐order sequences are depositional basins or tectonic stages; and first‐order sequences are global tectonic cycles. Sequence mapping must identify these orders by genetic‐interpretive and basin‐analysis techniques. Depositional basins (second‐order sequences) are classified primarily, by subsidence mechanism, into extensional, thermal relaxation and compressional types. Each type is identified by a particular type of stacking pattern between third‐order (depositional) sequences: extension by upward‐deepening; thermal relaxation by upward‐deepening to upward‐shallowing; and compression by upward‐shallowing. Depositional‐sequence analysis is, therefore, a prerequisite for depositional‐basin recognition. Further classification is based on geotectonic setting. Tectonic forces determine the geometry of second‐order sequences. In contrast, second‐order eustatic cycles are caused by pulses of sea‐floor spreading and are often masked (or enhanced) in the rock record by the signals from tectonic forces. There is no evidence that the interplay between tectonic and eustatic factors has changed throughout Earth history, which implies that actualistic tectonic models are relevant to Precambrian geology. Global tectonic cycles (first‐order sequences) are determined by the expansion and the contraction of oceans that are internal and external to a supercontinent. Supercontinent rifting and internal‐ocean formation are genetically related to external‐ocean contraction, whereas external‐ocean expansion causes internal‐ocean closure. Supercontinents break up, disperse and assemble as the consequence of ocean expansion and contraction. Ocean rhythms produce a continuum of depositional basins at continental margins that is recorded by the stacking of second‐order sequences into first‐order sequences. There is no evidence that the spatial and temporal scales of the interplay between the continents and the oceans has changed throughout Earth history. Therefore, the ages of first‐order, and probably also second‐order, sequences may define global chronostratigraphic intervals for the entire stratigraphic rock record. Universal application of sequence stratigraphy to Precambrian successions has the potential to revolutionise Precambrian geology.