Depositional model of a Cenomanian-Turonian extensional basin (Sopeira Basin, NE Spain): Interplay between tectonics, eustasy and biological productivity

The Sopeira Basin is a small extensional basin with a continuous record of sedimentation from the Late Albian to the Late Turonian. In its early stages (Late Albian Middle Cenomanian), it formed a half-graben, bounded to the south by a low-angle normal fault. During this time, sedimentation was limited to the downfaulted block until the emerged footwall was flooded, resulting in the deposition of shallow-water limestones over a vast platform. The platform was exposed during the Middle Late Cenomanian by a relative sea-level fall. The sedimentation up to this point was controlled by local factors (extensional tectonics), and influenced by eustatic sea-level change. Subsequently, the entire area was flooded by the Late Cenomanian transgression. During the Cenomanian-Turonian boundary interval, the basinal sedimentation was characterized by calcisphaerid limestones with black cherts and organic-rich shales, while on the platform a hiatus was produced. These sedimentary features are interpreted as having been caused by global mechanisms: changes in atmospheric CO2 and temperature, along with an increased rate of precipitation that caused an intense primary productivity at the water surface. In the basinal sea, eutrophication led initially to an unusual development of calcisphaerids and later, to the development of opportunistic planktonic foraminifera, whereas in the platform, it affected the life of carbonate-producing organisms dramatically and sedimentation ceased. Eutrophication of marine water is thus interpreted to be the main cause of the extinction of K-strategist foraminifera and associated fauna at the end of the Cenomanian.