Evolution of the Deseado Massif in Argentina, and its relationship with San Julian Offshore Area in the South Atlantic Ocean

Juan F. Homovc, Luis A. Constantini, Rudy F. Ferreira, and Aloisio Pellon de Miranda

The San Julian offshore area, located in the southern margin of the South American plate, records a tectonic history that correlates with the evolution of the Deseado Massif and the fragmentation of the Gondwana supercontinent. Upon an old mosaic of continental crust, a Devonian sequence was deposited, composed of low grade metamorphic rocks of La Modesta Formation. During the Middle Permian, a rift developed followed by a continuous filling of the basin in a sag-like thermal Triasic subsidence mode. A Jurassic episode of extension produced a framework of graben and halfgrabens filled with thick proximal volcanic and volcaniclastic both basic and acidic in composition. During the early sag tectonic period, a shifting of the depocenters took place in coincidence with the major opening f the South Atlantic Ocean Faulting and thermal subsidence contributed to the growth of the basin. A strong compressional event affected the sequences and the area remained as a positive region throughout the Cretaceous. Only a restricted zone in the inner part of the Deseado Massif received sedimentation in the Aptian - Turonian interval. Maastrichtian lapilli tuffs and Tertiary marine rocks were deposited over the entire area. Acquisition of new data in recent years in the Deseado and San Julian offshore areas, greatly enhanced the knowledge of tectonics and sedimentation in this frontier exploration area.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California