--> ABSTRACT: Rift Basin Sequence Boundaries and Flooding Surfaces, by D. Nummedal; #91021 (2010)

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Rift Basin Sequence Boundaries and Flooding Surfaces 

NUMMEDAL, DAG

Rift basins contain long-term sequences (typically 2-10 My) that record distinct episodes of tectonic evolution. Sequence boundaries (surfaces recording relative base level fall) and flooding surfaces (related to base level rise) record complex yet well-understood tectonic events.

Typically, the basin-wide pre-rift unconformity records regional uplift. Syn-rift unconformities may record maximum rates of extension and attendant uplift and rotation of footwall blocks. Because extension generally propagates both along and across large rifts, syn-rift unconformities are generally diachronous as well as spatially discontinuous. Post-rift unconformities form on rift margins where they record initial erosion followed by onlap due to regional thermal subsidence. Some rift basin unconformities record isostatic uplift of the lower plate beneath regional detachments, uplifts associated with inversion due to either regional compression or localized transpressional movements, or reverse rotation of fault blocks during the post-rift thermal subsidence phase.

A major regional flooding surface commonly separates fluvial and estuarine strata from overlying deep water facies. In most cases this can be explained as a product of rapid deepening and drainage diversion away from the rift at the onset of the rift climax phase. This flooding surface is most prominent in basinal areas above the hanging wall blocks. It may correlate with or precede the syn-rift unconformity. Major flooding surfaces also form on the footwall blocks during episodes of inversion.

Seismic, well-log, core, and outcrop examples of the range of sequence stratigraphic surfaces discussed above come from basins along the eastern margin of China, the Gulf of Suez, the North Sea, and the southwestern US. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.