The Cretaceous-Tertiary boundary biotic crisis in the Basque country

The Zumaya section has been selected as a classic locality for the study of the Cretaceous-Tertiary (K-T) boundary due to its richness in microfaune, macrofaune, and nannoflora. The sections present similar good conditions for the study of the K-T boundary. The sedimentary rocks of the Uppermost Maastrichtian from the Basque Country are purple or pink marls and marls-tones. Above it is found a clayed bed, 40 to 29 cm thick, grey or dark grey in its basal part, of Lowermost Danian age. Above there is alternation of micritic grey-pink limestones and thin clay beds of Dano-Montian age. The average sedimentation is 7 to 8 times higher during the Upper Maastrichtian than in the Dano-Montian. The macrofauna underwent a decrease since the Campanian and was not found in the last 11 m of the Zumaya section; it was associated with changes in paleoceanographic conditions and primary productivity of the oceans. The microfossil assemblages in the K-T transition allows the recognition of several phases of a complex crisis between two well established planktonic ecosystems. In the Mayaroensis Zone there is a stable ecosystem with 45 to 47 planktonic foraminifera species. The disappearance of A. mayaroensis starts a degradation of the ecosystem. The number of planktonic foraminiera species decreases between 20 and 45 percent. The next phase of the crisis was the result of main extinction events in the planktonic calcareous ecosystem. There are several cretaceous planktonic foraminifera species, probably reworked, whose numbers decrease upward. The next and last phase of the biotic crisis shows a diversification of the ecosystem; the number of planktonic foraminifera is 2 to 3 times higher than before and it is noted the first appearance of Tertiary nannoflora species, while Cretaceous species decrease and persisting species are still the main ones

Professor Salvador Reguant and his work on the International Geological Correlation Programme (IGCP)

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Variaciones espaciales y temporales del registro del límite K/T en Haiti: implicaciones acerca del evento o eventos

El registro en la capa del límite K/T (KTB), en diferentes localidades de la Península Meridional de Haití, muestra claras evidencias de disrupciones físicas causadas por el impacto postulado de un bólico al final del Maastrichtiense. Tanto el registro bioestratigráfico como el litológico de la capa KTB, en diferentes localidades, muestra también diversos grados de mezcla, con componentes faunísticos de edades congruentes con los bioeventos característicos de la transición del límite, equivalentes a la parte terminal de la Zona de Abathomphalus mayaroensis y partes de las Zonas de Guembelitria cretacea y de Parvularugoglobigerina eugubina. La nanoflora calcárea también muestra taxones de la transición, concurrentes con los datos de foraminíferos, e indicativos de las Subzonas de Micula murus y Micula prinsii, así como de la Subzona de Cruciplacolithus primus (CP1a), del Paleoceno basal. La capa del límite tiene variaciones en su potencia, con un máximo de 75cm en el estratotipo de la Formación Beloc, y a techo de la capa principal tectítica hay un máximo de iridio. Los análisis geoquímicos y la datación radiométrica han demostrado también que las esférulas son tectitas (Premo e Izett, 1991) que pueden estar relacionadas cronológicamente con el evento de impacto registrado en Chicxulub, Yucatán, México, hace 65 Ma. Aún más, las delicadas estructuras sedimentarias primarias en la capa del límite son constantes en todos los afloramientos, aunque hay diferencias espaciales incluso a corta distancia. Además, en las áreas adyacentes al estratotipo (Platon Piton y Madame Toussaint) se encuentra una capa volcanogénica, bajo el nivel tectítico principal relacionado con el evento de Chicxulub, que muestra una laminación cruzada, tanto conspicua como críptica, indicativa de procesos de flujo complejos, multifásicos y subacuáticos que afectaron a la sedimentación de forma parecida a la del lecho KTB. Se conocen tales estructuras como características de la acción de ondas oscilantes sobre la cohesión de los sedimentos. El movimiento de las aguas asociado con un seiche es el único análogo actual conocido de un flujo subacuático que proporcione un mecanismo plausible para explicar el que varios niveles de la columna de agua, en una gran cuenca, pueden oscilar para desarrollar las estructuras observadas. Dada la magnitud del impacto del bólido, debieron desarrollarse ‘megaseiches’ en los océanos a escala mundial y, subsecuéntemente, se producirían otros ‘megaseiches’ más localizados durante los reajustes principales de la corteza. Esos fenómenos pueden así explicar la heterogeneidad de pautas y discrepancias faunísticas observadas en varias localidades del KTB, de distinta profundidad de depósito, en el mundo. Las estructuras representan un registro de los movimientos del agua y de la resuspensión de los sedimentos en momentos diferentes. Tal como se ha observado en seiches modernos a pequeña escala, la duración y atenuación de los movimientos del agua y la magnitud de la tracción y resuspensión estuvieron controlados por diversos modos oscilatorios que dieron lugar a complejas estructuras sedimentarias y al retrabajamiento de los microfósiles

The Cenomanian-Turonian passage of Menoyo (Alava): Calcareous nannofossil changes

The study of calcareous nannofossil assemblages of the Cenomanian-Turonian passage (Menoyo section, N Spain) has allowed us to characterize the Lithraphidites acutus and Quadrum gartneri zones. The first appearance datum of Quadrum gartneri Prins and Perch-Nie/sen, seems us a good index to mark out the Cenomanian-Turonian boundary. The species Watznaueria barnesae presents high percentages (40% or more) in all samples, then nannofossil assemb/ages shou/d have underwent importan taphonctll ical process, e.g. less number of species with higher percentages of W. barnesae (poor preservati0i l). On the Other hand, the nannofossi/ abundance could be related to productivity of the OA

Cretaceous environmental change in E &S. Asia: The Cenomanian-Turonian transition in Southern Tibet

Fossil assemblages consist on foraminifera and rare ostracodes; all samples have been quantitativelystudied. Planktonic foraminifera allow us to characterise the Rotalipora cushmani Whiteinella archaeocretacea and Helvetoglobotruncana helvetica zones. Dominant species belong to genera Rotalipora and Dorothia. Planispiral rounded and tapered rounded morphotypes are dominant in benthic assemblages from the upper part of the Rotalipora cushmani Zone, which shows an oxygen deficiency in the bottom waters. The scenery is completely different during the W. archaeocretacea Zone. All species of the genus Rotalipora became extinguished and other species temporally disappeared; both diversity and abundance are very low in these samples. The keeled/non-keeled planktonic foraminifera ratio felt from 1 00% to a mere 2%, then increasing until similar figures at the top of this zone. Genus Ammodiscus and other agglutinant foraminifera species are the most frequent specimens. This is probably caused by very low oxygen bottom waters. During the H. helvetica Zone, biotic recovery is well settled with the appearance of Helvetoglobotruncana helvetica (Bolli) and Marginotruncana spp. in the plankton, but it is not so clear in the benthos, which remains poor and with low diversity assemblage

The Cretaceous-Tertiary transition at Beloc, Haiti: micropaleontologic record and blostratigraphic Implications

The section studied is located immediately south of the village of Beloc, in the Southern Peninsula of Haiti. Rock sequences of that locality have the distinctive stratigrapbic characteristic of yielding a series that spans the transition from the Cretaceous to the Tertiary (K/T boundary), which is distinguished by a conspicuous marker bed composed of spherulites. The micropaleontologic data based on foraminifers indicate the presence of several important biozones charateristies of the boundary zone, namely the uppermost part of ihe Abathomphalus mayaroensis Zone, part ? of the Guembelitria cretacea Zone, and the Parvularugoglobigerina eugublna Zone, respectively. The calcareous nannoplanktons also yield transitional taxa that concur with the foramihiferal data, as they are indicative of the Micula prinsii Zone (uppermost Maastrichtian), as well as the lower part of NP1 or basal Danian. The absolute frequency of the different groups greatly diminish at the level that is assigned to the lowest Danian, which is coincident with the biotic crisis of the K/T boundary event. The dramatic decrease is particularly evident in the nannofos.sils, which are reduced by 7/7 00 to 1/400 of the assemblages recorded toward the end of the Maastrichtian. The relatively high frequency ofThoracosphaera spp.(up to 8 percent total), the predominance of heterohelicids, and the remarkable abundance of juvenile globotruncanids (about 50 percent) in the Cretaceous materials seem to also indicate a very unstable planktonic ecosystem toward the end of the Maastrichtia