Sub-Aerial Basins below Sea Level Provide Unexpected Reservoirs
Throughout 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: in rifting of cratons, examples being the Afar (- 410ft, below sea level) Southern North Sea (-750ft.), and the South Atlantic basins; and when old basins are sealed off in collisions such as the Mediterranean during Messinian time (-10,000,ft.) the Black Sea (-550ft.). and the Gulf of Mexico (GOM) during deposition of Wilcox sands in basin depths (-6000ft.?).
Basins that were below sea level, but sub-aerial, influenced sedimentation and should influence interpretation of the tectonic history of the basins. Sub-aerial sediments do not necessarily mean basin uplift!
A desiccated basin may be the site of extensive desert deposits. Winds move sand dunes even into the deepest portions of the basin. Those 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 fills the empty basin like filling a bathtub. The sea rises to flood the surface with little disturbance covering the terrain in a geologic moment. Sand dunes are drowned, preserving their shapes and cross bedding, as in Permian Rotliegend of the Southern North Sea. Porosity of sandstone may be preserved due to desert conditions leading to chlorite overgrowths on quartz, as is true of Norphlet Sandstone of the GOM.
Canyons cut to grade with the basin floor are distinctive of former sub-aerial sub-sea basins. They bring coarse clastics to the basin floor. Such buried canyons are found all around the Mediterranean and western Gulf of Mexico
After the flood only fine clay and organic matter settle out of the anoxic water. Rising H2S from rotting vegetation of the suddenly drowned landscape precipitates metals from the ocean water and may cause metal rich shales such as the Kupferschiefer that overlies the sand dunes of the Rotliegend. The fine sediments drape over the preexisting dunes like a blanket of snow, following the curves of the former landscape.
In a basin containing a drowned desert environment can be found reservoirs that might not be expected if a uniformly marine basin model is used in interpretation and exploration.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009