Seismic Characteristics of Paleo-volcanics Relative to Diapiric Salt in Rifted Basins

Dunlap, Dallas B.¹; Wood, Lesli ¹
(1)Bureau of Economic Geo, Univ of Texas at Austin, Austin, TX.

Detailed mapping of a large, 1,064-km², high-resolution 3D seismic dataset acquired in the Safi Haute Mer seismic block of offshore Morocco’s Atlantic margin revealed the existence of five distinct salt diapirs above the Paleozoic rifted basement. Triassic-age salt mobilizing from basement north-south-trending half-grabens produced a range of salt morphologies. These diapirs are expressed as deep-seated tongues, canopies, and shallow salt sheets exhibiting evidence of periodic movement during the Cretaceous through the modern. A sixth conical feature was identified and was initially thought to be produced from two converging salt pillows. Further examination determined this feature to be a buried paleo-volcano having 1,800 ms (~2,200 m) of vertical relief and lateral dimensions of 3,000 × 2,800 m at its base. The possibility of a salt core could not be fully discounted because the cone’s upper boundary displayed a seismic response similar to that of the local diapiric top-of-salt, and magnetic and gravity surveys proved inconclusive.

The seismic distinction between buried paleo-volcanoes and salt diapirs has broad implications for prospectivity, hydrocarbon potential, trapping styles and mechanisms, and proximity to source rock. This study focused on the seismic characteristics that can be used to differentiate deeply buried paleo-volcanoes from diapirs in salt basins. Attributes such as internal character, proximal debris flows, adjacent stratal character, and structural mechanics were evaluated. The conical feature was determined to be a paleo-volcano from (1) the lack of a basal reflector, (2) a slightly rectangular base uncharacteristic of those in mobile salt, (3) uniformly stacked onlapping reflectors from base Jurassic through its peak in the Lower Cretaceous, and (4) lack of structural and stratigraphic deformation indicative of salt tectonics.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.