Abstract: Role of Subaerial Volcanic Rocks and Major Unconformities in the Creation of South Atlantic Margins

Fonck, J. M. and C. Cramez - Total; M. P. A. Jackson - Bureau of Economic Geology, The University of Texas at Austin

As exploration shifts into deeper water, the nature of the continent-ocean boundary becomes increasingly important. Unconventionally, we argue that (1) some subsalt basins may not be tilted rift basins containing potential source rocks but instead are flood basalts extruded in the initial subaerial phase of sea-floor spreading, and (2) the Aptian salt basins formed partly on oceanic crust after breakup and were never joined to twin basins on the conjugate margin.

Large Igneous Provinces (LIPs) form during pulses of intense magmatism. An underlying mantle plume uplifts a central rift zone, forming a widespread breakup unconformity. Subaerial flood basalts are extruded from volcanoes and fissures parallel to the line of breakup. Flood basalts up to 10 km thick thin landward, forming seaward-dipping reflectors (SDRs). In dip profiles, SDRs are convex upward, with dips and thicknesses increasing basinward because of loading by younger flows emplaced progressively seaward. After the margin drifts off the mantle plume, it cools and subsides below sea level. Flood volcanism is then impossible under water because the lavas freeze too rapidly. Submarine sea-floor spreading thus begins to form true oceanic crust.

The conventional hypothesis for the South Atlantic opening ( A) envisages pre-breakup salt. A first phase of rifting lasted from ~160 Ma to ~135 Ma, when the breakup unconformity (BUU) was created. Between ~135 and 115 Ma, a sag basin formed above the thinned, eroded continental crust and was capped by Aptian salt or time equivalents. Evaporites were thickest in the center of the salt basin over the most extended crust. This salt basin then split during a second phase of rifting. After breakup, the oceanward edge of salt would have been an abrupt, fault- bounded margin severing the thickest part of the original salt basin; any SDRs should overlie the distal salt basin; and these basalts should have sunk into thick salt until they froze and strengthened. None of these features have been observed, including signs of postsalt rifting, though subsalt faults were reactivated in post-salt time.

In our alternative hypothesis ( B) envisages post-breakup salt rested partly on volcanic crust. The presalt rift history is identical to that of the conventional hypothesis above. However, all rifting ended with the formation of the breakup unconformity at ~135 Ma. Sea-floor spreading was initiated with subaerial extrusion at ~135-125 Ma. The thin Neocomian-Aptian ?sag? sequence actually overlay the new continental margins. Aptian evaporate basins formed independently at ~115-100 Ma on conjugate margins. Potash evaporites indicate a hydrothermal origin by interaction with spilitized basalt. The thickest evaporate facies formed in the center of each separate salt basin and pinched out distally over basaltic crust.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil