Post-Thinning Salt Basins on Passive Margins

Rowan, Mark G.
[email protected]

Some of the largest and most hydrocarbon-prolific salt basins are in paired passive-margin settings such as the South Atlantic and Gulf of Mexico (GoM). The evaporite sequences were deposited just prior to accretion of normal oceanic crust yet are relatively unaffected by crustal extension, with only local and minor offset of the base salt, and are thus characterized as post-thinning. Here we examine some other unusual aspects of these salt basins.

First, post-thinning salt basins are flanked along strike by magma-rich margins with seaward-dipping reflectors (SDRs), and it is commonly assumed that volcanic crust extends beneath the salt. However, recent data and interpretations in the South and Central Atlantic and GoM suggest that SDRs die out into the salt basins, just as prominent magnetic anomalies gradually disappear, and that the most distal salt is underlain by exhumed subcontinental mantle. The southern barriers to the Atlantic salt basins were not formed by local volcanic ridges but by large, elevated magmatic provinces with subaerial basalts that were onlapped by salt. The CaSO₄-poor evaporites are not related to deposition over volcanics so much as to seepage of ocean water through the bounding igneous highs.

Second, post-thinning salt basins have anomalous uplift and subsidence histories manifested by presalt unconformities and ‘sag’ sequences. Modern analyses suggest that so-called breakup unconformities actually occur during mantle exhumation and possibly earlier, most likely due to thinning of cold lithospheric mantle in proximal positions, lateral emplacement toward more distal areas, and replacement by hot asthenospheric mantle. Mantle infiltration and serpentinization may also contribute to uplift. Subsequent regional thermal subsidence explains in part the development of sag basins, including the evaporite level, and postsalt sequences. Some proximal to distal diachronicity in uplift and subsidence is likely as subcontinental mantle moves basinward. Third, post-thinning salt basins have remarkably pure layered evaporite sequences. Whereas many salt basins are characterized by the presence of interbedded carbonates, siliciclastics, and volcanics, such non-evaporite lithologies are rare to absent in the South Atlantic and GoM. This might be explained by a combination of regional subsidence and low relief in areas of mantle exhumation and complete basin isolation provided by the adjacent subaerial magmatic provinces.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013