The Influence of Synrift Salt on Deformation During and After Rifting: Examples from the Orpheus Rift Basin, Offshore Nova Scotia and Newfoundland, Canada

Hanafi, Bari R.; Withjack, Martha O.; Schlische, Roy W.; Syamsir, Zulfitriadi; Durcanin, Michael A.

The Orpheus rift basin is part of the eastern North American (ENA) rift system that formed prior to the opening of the Atlantic Ocean. Like many of the northern ENA rift basins, a considerable thickness of salt (i.e., the Late Triassic/Early Jurassic Argo Formation) accumulated within the Orpheus basin during rifting. We have used a dense grid of public and proprietary 2-D seismic data, industry well data, and information from the adjacent Fundy rift basin to better understand how the presence of synrift salt influenced rift-basin development. Our work shows that the stratigraphic distribution, thickness, and composition of the synrift salt significantly influenced deformation patterns during and after rifting. The lower Argo Formation consists of massive halite. Salt deposition was widespread, but accumulation varied, controlled by basement-involved faulting. Generally, the basal halite is thin or absent above shallow fault blocks and thick above deep fault blocks. The upper Argo Formation consists of halite and interbedded clastic sedimentary rocks. In parts of the basin, the upper Argo Formation is predominantly halite with few shale beds. In other parts of the basin, however, the halite is interbedded with numerous, thick shale beds. Growth beds in the upper Argo Formation associated with extensional fault-propagation folds reflect continued activity on basement-involved faults below the salt during the deposition of this unit. During the later phases of rifting, focused deposition near the northern border-fault zone caused the underlying salt to move. Paired minibasins and salt walls/columns preferentially formed where the lower Argo salt was thick and/or where the upper Argo Formation had a high proportion of halite. Immediately after rifting, shortening associated with basin inversion reactivated the basement-involved faults and produced broad sub-salt folds similar to those in the adjacent Fundy basin, which shares its border-fault zone with the Orpheus basin. In response, detached structures formed above thin salt, and the existing salt walls/columns narrowed to accommodate the shortening. Additional postrift deformation during the Late Jurassic/Early Cretaceous and Oligocene/Miocene again reactivated basement-involved faults and shortened the buried salt walls/columns, producing domes in the sedimentary cover above them.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013