T hroughout geologic history there have been large sub-aerial basins below sea level. There are two times in the plate tectonic cycle when such basins are likely to form: during the rifting of cratons and when old basins are sealed off during collisions. Examples of the former include the Afar Basin at 410 feet below mean sea level (bmsl), the southern North Sea Basin at 750 ft bmsl, and the South Atlantic basins. Examples of the latter include the Mediterranean during Messinian time at 10,000 ft bmsl, the Black Sea at 550 ft bmsl, and the Gulf of Mexico during deposition of the Jurassic Norphlet sands and perhaps the shallow water to sub-aerial Paleocene Wilcox sands at ~6,000 ft bmsl.

Basins that were below sea level but sub-aerial influenced sedimentation and should influence the interpretation of their tectonic histories. The presence of sub-aerial sediments does not necessarily mean basin uplift!

A desiccated sub-aerial basin below sea level may have been the site of extensive desert deposits. Winds pouring across the lip and down into a sub-aerial basin below sea level are heated by compression as they descend. This leads to extreme desiccation, the evaporation of brines, and even the deposition of potassium salts. The same winds can move sand dunes into the deepest portions of the basin. These potential reservoirs are not influenced by distance from shore, as are marine sands.

The most significant event in a sub-aerial sub-sea basin is the sudden flooding upon entry of the sea. Unlike a marine transgression that reworks sediments on gradually submerged land, the sea rises to fill the empty basin in a geological instant. There is little disturbance of the covered terrain. Sand dunes are drowned, preserving their shapes and cross bedding, as in the Permian Rotliegendes of the southern North Sea. Porosity of sandstone may be preserved due to desert conditions that lead to chlorite overgrowths on quartz, as is true of the Norphlet sandstones of the Gulf of Mexico.

Canyons cut to grade with the basin floor are distinctive of former sub-aerial sub-sea basins. They bring coarse clastics to the basin f loor. Such bur ied canyons are found al l around the Mediterranean and western Gulf of Mexico.

Canyons cut to grade are distinctive of subaerial below sea level basins (SABSEL) basins. After the flood of returning water deep water has been established without the erosion of a transgressing sea. The Norphlet sand dunes are preserved intact in the Mobil bay area of the Gulf of Mexico. Draping upon the dunes are deepwater organic rich limestones of the Smackover, a source of hydrocarbons for the reservoirs below and above. At the contact is often a layer of pyrite just like metal rich shales of the Kuperschiffer shale above the Permian Rotliegendes sand dunes of the Southern north sea.

A basin containing a drowned desert environment may have reservoirs that would not be expected if a uniformly marine basin model was used in interpretation and exploration. Realization that one may be dealing with desert sedimentation can result in interpretations that extend successful oil and gas plays and predict locations of new ones.

Figure 1. The Mediterranean Sea dried up and only salt lakes remain.

Figure 2. Air descending is compressed at a temperature lapse rate of 9.8° C per 1000 m (about 18° F per 1000 m).