The Lower Aptian Orbitolina marl level corresponds to a global event registered in carbonate platforms of tropical latitudes. It has been recognized from South America (Venezuela) to Europe and from the northern margins of the Tethys (e.g. Spain, France) to its southern margins (from Algeria to Oman).
One interesting aspect of this event is the similarity of the sedimentological record and type of the carbonate platform evolution in such a large area and within different tectonostratigraphic settings, such as platforms located on passive margins (e.g. North Africa, France) or in intra platform basins (Spain, Oman). The Lower Orbitolina Marls correspond to the lower part of the first Aptian depositional sequence. This later starts with a sea-level fall close to the Barremian/Aptian boundary. Carbonates deposited during this sea level lowstand, are rich in Charophytes (fresh water algae). The sea level rises very quickly and the transgressive systems tract (TST) is characterized by significant clastic inputs and the environment changes from oligotrophic to mesotrophic. This favored the abundance of marine algae and occurrence of microfauna living in sea-grasses such as Palorbitolina and Choffatella. Two periods of maximum deepening occurred during the transgression: the first at the base of the TST and the second at the end of the TST. These events are indicated by peaks in gamma ray logs. After the transgression, oligotrophic environment were quickly re-established in the platform with the deposition of lagoonal facies containing rudists and corals. The end of this depositional sequence corresponds to a new significant sea level fall (approximately 100m), with the formation of an erosive surface, a well-developed karsts system and incised valleys. During the next transgression, the anoxic event OAE1a is registered.
Erosion of continental areas rather than volcanic activity is considered to be the main source of clastic input to the platform. Tectonic activity may also play an important role in controlling platform development, thus the pull-a-part basin of Organya (Spain) shows a huge subsidence rate (1100m of compacted limestone deposited in only 5Ma).
Within the two intra-shelf basins (Organya in Spain and M’Bab in Oman), the TST is linked to organic rich deposits and displays periods of anoxia. The first gamma ray peak is concomitant with the beginning of organic matter accumulation, and the second one, with the end of anoxia or disoxia. This anoxic-disoxic episode precedes the well-known OEA1a anoxic event and is well registered in intra-shelf basins. On the unrestricted margin, this episode is indicated only by the presence of some algal mats.
Sea-level rise and fall are very rapid and well registered in all basins, even where the subsidence rate is very important. We demonstrated that these rapid eustatic changes did not inhibit registration of maximum floodings and sequence boundaries associated with karst development, weathering and erosion.