Abstract

There are a few similarities between these two large salt basins, but there are also some striking differences. Both basins experienced two episodes of continental rifting: an initial rift that failed and a later rift that succeeded. The Gulf of Mexico Basin and the Brazilian basins of Santos, Campos, and Espirito Santo (considered one continuous salt basin) underwent a period of syn-rift deposition before any salt was deposited. In the Gulf of Mexico, hypersaline seawater could have entered from the Atlantic-Tethys Ocean via the Cuban Suture ca. 170 Ma, or it could have entered from the Pacific Ocean via Mexico around the same time. Strontium isotope age-dating shows the salt along the margins of the Gulf of Mexico Basin to be 169 Ma. Basinal water depths are estimated to have been less than 2 km and rapid salt deposition occurred over several hundred thousand years to accumulate a formation averaging 2.6 km thick. Gulf of Mexico Louann Salt contains other minerals besides halite, including gypsum, sylvite, anhydrite, and pyrite, but generally in small quantities and randomly distributed within the non-layered salt.

In contrast, the Brazilian salt basin discussed here is much smaller (only 35% the size of the Gulf of Mexico). It is also elongated, and based upon seismic data has a slight tilt from north to south: a drop of 3 degrees over a strike distance of 1304 km, just enough dip to affect salt and source rock deposition, which is thicker in the south. The source of hypersaline seawater (whether from the Tethys Sea in the north or the Atlantic Ocean in the south, or some combination of the two sources) is still debated. Based upon the microfossils found in the sediments below the salt and the limestones above the salt, the water depth was <500 m and salt thickness reached 2.5 km in some locations. The comparatively slow salt deposition occurred between 116 and 110 Ma. This Brazilian salt basin exhibits three distinct episodes of sequential salt deposition easily discernible on seismic data. The first two episodes include a thin base layer of anhydrite followed by a thick layer of halite and then a thin layer of carnallite. In some locations, tachydrite is present on top of the carnallite. The third episode consists of alternating layers of halite with either shales or carbonates, depending upon location within the basin.