Aptian-Albian Transition Research Articles

Shallow water carbonate platforms (Late Aptian–Early Albian, Southern Apennines) in the context of supraregional to global changes: re-appraisal of palaeoecological events as reflectors of carbonate factory response

Abstract. This paper discusses the palaeoenvironmental significance of the "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits encased in the Aptian/Albian shallow water carbonate platform strata of Monte Tobenna and Monte Faito (Southern Italy). These facies show a peculiar field appearance due to their color and/or fossil content. In the shallow water carbonate strata, the Late Aptian "Orbitolina Level" was formed during a period of decreasing accommodation space. Microbial carbonates occur in different levels in the composite section. They reach their maximum thickness around the sequence boundaries just above the "Orbitolina Level" and close to the Aptian–Albian transition, and were not deposited during maximum flooding. S. dinarica-rich deposits occur in the lower part of the Monte Tobenna-Monte Faito composite section, in both restricted and more open lagoonal sediments. S. dinarica has its maximum abundance below the "Orbitolina Level" and disappears 11 m above this layer. On the basis of δ13C and δ18O values recorded at Tobenna-Faito, the succession has been correlated to global sea-level changes and to the main volcanic and climatic events during the Aptian. Deterioration of the inner lagoon environmental conditions was related to high trophic levels triggered by volcano-tectonic activity. Microbial carbonates were deposited especially in periods of third-order sea level lowering. In such a scenario, periods of increased precipitation during the Gargasian induced the mobilization of clay during flooding of the exposed platform due to high-frequency sea-level changes, with consequent terrigenous input to the lagoon. This and the high nutrient levels made the conditions unsuitable for the principle carbonate producers, and an opportunistic biota rich in orbitolinids (Mesorbitolina texana and M. parva) populated the platform. In the more open marine domain, the increased nutrient input enhanced the production of organic matter and locally led to the formation of black shales (e.g. the Niveau Fallot in the Vocontian Basin). It is argued that the concomitant low Mg/Ca molar ratio and high concentration of calcium in seawater could have favoured the development of the low-Mg calcite skeleton of the S. dinarica green algae. During third-order sea-level rise, no or minor microbial carbonates formed in the shallowlagoonal settings and S. dinarica disappeared. Carbonate neritic ecosystems were not influenced by the environmental changes inferred to have been induced by the mid-Cretaceous volcanism. The "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits in the studied Aptian/Albian shallow water carbonate strata are interpreted to be the response to environmental and oceanographic changes in shallow-water and deeper-marine ecosystems.

Stratigraphic framework and calcareous nannofossil productivity of the Essaouira-Agadir Basin (Morocco) during the Aptian–Early Albian: Comparison with the north-Tethyan margin

In the southern Tethyan margin, the Essaouira-Agadir Basin (EAB), south of Morocco, exhibits well-exposed and fossiliferous sections of Aptian–Albian age. Biostratigraphy by ammonoids and sedimentological analysis have been realized for five sections located along an E-W transect in the EAB. The studied successions were dated from the latest Early Aptian to the Early Albian and are characterized by five major sedimentary discontinuities defining at least four main sedimentary sequences. The Late Aptian–Early Albian succession can be considered a gently westward-dipping ramp, marked by a deepening upward evolution. A quantitative study of calcareous nannofossils and calcium carbonate content has been performed on three of these sections. At this time, the EAB was located in the tropical-equatorial hot arid belt. The decrease in both calcium carbonate content and Nannoconus abundances at the Aptian–Albian transition could be the result of cooler climatic conditions recognized in the EAB, and/or of the associated increasing terrigenous input and nutrients, which hindered carbonate production. In the EAB, the nannofossil productivity is higher below the deposition of dark levels, which are coeval with the Niveau Paquier, recognized as the expression in southern France of the OAE 1b (Early Albian). During the Early Albian, the EAB was characterized by nannofossil fluxes two times lower than the upwelling-influenced Mazagan Plateau (southern Tethyan margin) and eight times lower than the Vocontian Basin (northern Tethyan margin). These results show that, with respect to the northern Tethyan margin, trophic conditions in sea surface waters of the pelagic realm of the southern Tethyan margin were lower. Comparable results obtained by Heldt et al. in the neritic realm of the southern Tethyan margin have been ascribed to more arid climatic conditions.

Integrated biostratigraphy and carbon isotope stratigraphy of the Lower Cretaceous (Barremian to Albian) Adriatic-Dinaridic carbonate platform deposits in Istria, Croatia

The Adriatic-Dinaridic carbonate platform (ADCP) was one of the largest and relatively well preserved Mesozoic platforms in the Mediterranean region (central Tethys). The peninsula Istria, in the northwestern part of the ADCP, is built up predominantly of shallow-water carbonates of the Middle Jurassic (Dogger) to Eocene age and, to a lesser extent, of Paleogene clastic deposits (flysch and calcareous breccia). This study focuses on a Lower Cretaceous (Barremian to Albian) succession of strata at five localities in western Istria. Stratigraphic determinations are based on identification of nine microfossil assemblages (benthic foraminifera and calcareous algae Dasycladales) and on using their taxa as index fossils. The age of strata with these microfossil assemblages, however, is questionable. Most of the age uncertainties are associated with a regional emersion, which occurred on the ADCP during the Aptian or close to the Aptian–Albian transition. It is unclear what portions of the Upper Aptian and/or Lower Albian are missing along this unconformity. A stable isotope study was conducted on homogenous micritic matrix samples in an attempt to resolve some of these uncertainties. Variations in carbon isotope compositions proved useful for stratigraphic correlation between the examined successions of strata, for improving their age determination, and for relating them to other coeval successions that span an important time interval of major oceanographic changes and carbon-cycle perturbations associated with the Early Aptian oceanic anoxic event (OAE 1a).

Découverte de charophytes de l’Albien dans la Formation Kebar (Tunisie centrale) : implications paléoécologiques et paléobiogéographiques

In Tunisia, in the platform domain, the Aptian-Albian transition is associated with a major stratigraphic gap not precisely dated, which is related to the subaerial exposure of the Aptian Orbata platform. Locally, in central Tunisia (Jebel Kebar), this gap is substituted by the sedimentary records of the Kebar Formation. This formation provided a rich association of fossil charophytes, formed by Atopochara trivolvis trivolvis, Clavator harrisii zavialensis and Sphaerochara verticillata which allows attributing it to the late Aptian–early Albian interval (most probably early Albian). In addition, a new variety, Sphaerochara verticillata var. kebariensis, is described. The charophyte assemblage shows significant biogeographic affinities with a coeval flora found in the Iberian Peninsula. The Kebar Formation was previously regarded as exclusively nonmarine, but the presence of the assemblage of charophytes and associated ostracods described rather indicates a fluctuation between margino-littoral (lagoonal) and continental environments.

A new species of Gervillia Defrance (Bivalvia, Bakevelliidae) from the Aptian-Albian transition of Sergipe, Brazil

A new species of Gervillia Defrance (Bivalvia, Bakevelliidae) from the Aptian-Albian transition of Sergipe, Brazil

Age of the Los Ranchos Formation, Dominican Republic: Timing and tectonic setting of primitive island arc volcanism in the Caribbean region

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island arc sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formation (Quita Sueno Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Sueno sample, formed at either 113.9 ± 0.8 or 118.6 ± 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 ± 0.8 Ma. The Cotui stock (RD-73-601) was emplaced ca. 111.8 ± 0.6 Ma or 112.9 ± 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calcalkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions.

A BACIA DE CAMPOS NO MESOCRETÁCEO: UMA CONTRIBUIÇÃO À PALEOCEANOGRAFIA DO ATLÂNTICO SUL PRIMITIVO

This work integrates paJeoeco logica1 research data obtained in Campos Basin and others areas of the Brazilian continental margins with those from different regions of the South Atlantic. A general model is proposed for the oceanographic evolution of the South Atlantic in the Mid-Cretaceous. Since the Early-Middle Albian, probably since the Aptian-Albian transition, the area extending from the equatorial region to the Sao Paulo Plateau-Walvis Ridge complex was simultaneously invaded by waters from the North (Tethys Sea) and South. The sea then formed constituted a long, continuous, and narrow epicontinental sea-way. Its waters were warm , physic-chemically unstable, and probably hypersaline. ln the Late Albian, the South Proto-Atlantic had its salinit y decreased to more normalmarine values. It was after a significant anoxic event in Cenomanian-Turonian times that truly oceanic deposition phase started.

O NEO-ALAGOAS NAS BACIAS DO CEARÁ, ARARIPE E POTIGUAR (BRASIL): CARACTERIZAÇÃO ESTRATIGRÁFICA E PALEOAMBIENTAL

The stratigraphic analysis of the Alagoas Stage (local stage) in Ceara Basin allows the investigated interval to be divided into two parts, with distinctive tectono-sedimentary and paleobiological features. The lower part shows non-marine environmental character and the upper part, inforrnally named Aptian-Albian transition, contains marine organisms in certain levels. These records would represent the first phases of a primitive equatorial Atlantic Ocean in the area. The Trairi Beds at the top of the continental section were deposited in an environmental complex which includes a marginal sabkha, reflecting cyclical conditions of extreme aridity or humidity. Similar events have occurred in the Potiguar Basin with the Ponta do Tubarao Beds and in Araripe Basin with the deposition of thinny calcareous and shale layers, here informally named Batateira Beds. These environmental conditions provided an excellent setting for preservation of organic matter due to cyclical anoxic processes. The observation that the organic extracts from shales of all these strata are similar supports the hypothesis of correspondence of these events.