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Terra Nova, 24, 333–338, 2012 Abstract A widespread marine transgression, which began at the very end of the Early Aptian, is well recorded in Mexico and Spain. In Mexico, this transgression was the most important Aptian transgressive event and its record begins in the Dufrenoyia justinae Zone, whereas in Spain the corresponding transgression is registered in the uppermost part of the Dufrenoyia furcata Zone. The basal age of this Tethyan transgression does not correspond exactly to any of the Lower Aptian third‐order sequences as reported in the literature. In Spain, the most important Aptian transgressive event was earlier, corresponding to the Tethyan sequence Ap3, which is well‐defined below this transgression reported herein. Consequently, it is possible to correlate this later transgressive event with the third‐order Ap4 sequence, which has commonly been attributed to the Upper Aptian. The available ammonoid data allow us to correct the basal age of the transgression, and to correlate the start of the ammonite record in Mexico with the Tethyan sequence Ap4.

A detailed bed-by-bed sampled stratigraphic section of the Guasasa Formation in the Rancho San Vicente area of the "Sierra de los Órganos", western Cuba, provides well-supported evidence about facies and calpionellid distribution across the Jurassic/Cretaceous boundary. These new data allowed the definition of an updated and sound calpionellid biozonation scheme for the section. In this scheme, the drowning event of a carbonate platform displayed by the facies of the San Vicente Member, the lowermost unit of the section, is dated as Late Tithonian, Boneti Subzone. The Jurassic/Cretaceous boundary was recognized within the facies of the overlying El Americano Member on the basis of the acme of Calpionella alpina Lorenz. The boundary is placed nearly six meters above the contact between the San Vicente and the El Americano Members, in a facies linked to a sea-level drop. The recorded calpionellid bioevents should allow correlations of the Cuban biozonation scheme herein proposed, with other previously published schemes from distant areas of the Tethyan Domain.

The Xilitla section of central Mexico, proto-North Atlantic, is characterized by pelagic sediments enriched in marine organic matter. Using biostratigraphic and radiometric data, it was dated at the latest Cenomanian-earliest Turonian transition. We identified an interval coeval with the turnover of Oceanic Anoxic Event 2, recording the Heterohelix shift and the "filament event" for the first time in Mexico. An integral analysis of sedimentary facies, pyrite and geochemical proxies reveals vertically variable redox conditions, with prevailing anoxic to dysoxic bottom waters. Along with phosphorous and manganese depletion, the highest content of total organic carbon and certain redox-sensitive trace elements (RSTEs) is found during part of the anoxic event, confirming more uniform and constant oxygen-depleted conditions. This interval is also characterized by a significant enrichment in biogenic barium and elevated TOC/N TOT ratios, suggesting a link between productivity and anoxia. Sulfur isotope fractionation has a maximum value within the anoxic event, favored by both the increase in the flux of organic matter and