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Abstract:

Sedimentological studies of Cretaceous deep–sea sediments deposited in the western Tethys allow at least rough analyses of the Earth Systems processes involved. The deep–sea evidence shows that the change from Lower Cretaceous pelagic carbonates to mid–Cretaceous black shales was not a sudden event, but took 15 Myr for a complete transition. This change was a response to thermal subsidence of oceanic crust in the western Tethys. The mid–Cretaceous black–shale development was forced by high deposition of organic matter in a response to a humid, warm climate, with the latter in turn resulting in decreased rate of deep water formation, poor ventilation, and lower content of dissolved oxygen in deep waters. However, the bottom waters remained oxic and/or variably dysoxic, as indicated by presence of mid–Cretaceous greenish–gray claystones in the Moroccan basin, while elsewhere black shales were deposited, and by local bioturbation and intercalated reddish, oxic horizons.

Various processes were involved in the origin of oceanic anoxic events (OAE1a and OAE2). Sediments deposited during these events were laid down during onset of major transgressions in the central North Atlantic and thus can be viewed as condensed horizons related to flooding events. The high content of organic matter is most probably a phytoplankton response to ocean surface water fertilization by iron during extensive Aptian volcanic activity along North Atlantic continental margins. Less certain is the influence of the Caribbean igneous plateau (93–89 Ma) and Madagascar flood basalts on the development of OAE2.

The reddish colors of the overlying Upper Cretaceous pelagic claystones and shales are of early diagenetic origin and not a depositional feature. Sediment oxidation was forced by low sedimentation rate of 20 mm/kyr, driven by eustatic sea–level rise and increased ventilation of the basin. The red beds intercalated in mid–Cretaceous strata in western Tethys have variable origins, such as 1) an inflow of higher–salinity surface waters from the southern Atlantic when the equatorial seaway began to open; 2) reduction of sedimentation rate as a result of local tectonics; and 3) sea–level changes. Less certain is whether a temporary inflow of colder, more oxygenated bottom water into small tectonic sub–basins of the Mediterranean region could have resulted in bottom–sediment oxidation, because the validity of such a hypothesis cannot be confirmed by studies of modern marine sediment.

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