The filament event near the Cenomanian-Turonian boundary in Tunisia: filament origin and environmental signification

Abstract

Abstract. -Near the Cenomanian-Turonian boundary, the filament event corresponds to the massive occurrence of thin elongate shells, probably bivalves which constitute, in the Bahloul Formation, about 40% of allochems. This biosedimentary event was described in several areas, such as in Tunisia, Algeria, Morocco, North America, Oman and Tibet. In Tunisia, filaments are well represented within well laminated black shales constituting the Bahloul Formation, late Cenomanian -early Turonian in age. These black shales were deposited under anoxic conditions developed in an outer ramp environment. Filaments are particularly abundant in the upper part of the Bahloul Formation, which is topped by a condensation level exhibiting encrusted ammonites associated with frequent burrows, and phosphatic and glauconitic grains. In terms of depositional environments, the filament concentrations announce an evolution towards "normal"-marine conditions, which followed the anoxic conditions favouring black-shale formation. This trend coincided with a deepening that led to maximum flooding at the condensation surface. The petrographic study of samples taken along a N-S transect shows a clear evolution of filament structures from distal to proximal ramp. Limestones deposited in the outer ramp environment mainly contain well preserved "pelagic" bivalve shells showing a thin-shelled structure composed of elongated filaments. Laterally, toward the proximal ramp, the bivalve shells are progressively disintegrated, the filaments being arranged parallel to bedding. In an even more proximal setting, filaments appear finer, highly fragmented and arranged without preferential orientation. The Cenomanian-Turonian "pelagic" bivalves thus probably lived in an outer ramp environment and their thin shells disintegrated progressively into filaments. This happened during high energy conditions generated by storms.The filament event, which is associated to other bio-events expressed by the extinction and the first occurrence of some pelagic species, could be used to establish local to global correlations.When the ammonite Watinoceras, the main early Turonian marker, is rare or absent, the filament-rich facies could indicate the proximity of the Cenomanian-Turonian boundary.At a global scale, the filament event illustrates a sea-level rise in relation with eustatic and climatic changes. The maximum flooding of this transgressive event was identified in several localities and dated about 93 Ma. The very high sea-level conditions are favourable to accumulation and preservation of organic-rich facies such as black shales, which illustrate the anoxic event OAE2.The abundance of filaments near the Cenomanian-Turonian boundary is an indicator of high organic productivity following a climatic change to warmer conditions. The latter have clearly influenced oceanic water-masses circulation, water column stratification and nutrient partitioning. L'événement "à filaments" à la limite Cénomanien -Turonien en Tunisie : origine et milieux de genèse des filamentsMots-c...

Bioevents and redox conditions around the Cenomanian–Turonian anoxic event in Central Mexico

“…Four important bioevents that are commonly recognized within the latest Cenomanian-earliest Turonian transition (Keller et al, 2004;Coccioni and Luciani, 2004;Caron et al, 2006;Robaszynski et al, 2010;Negra et al, 2011;Kędzierski et al, 2012;Bomou et al, 2013;Denne et al, 2014;Lowery et al, 2014;Reolid et al, 2015) were identified within the section studied ( Fig. 2).…”

“…index ammonite Watinoceras (Caron et al, 2006;Negra et al, 2011). Building upon this consideration, we place the C-T boundary below bed 93 (at 15.8 m) in the studied section ( Fig.…”

“…and/or the product of disintegration of pelagic bivalves during high-energy events (Negra et al, 2011). They are related to heightened marine productivity and oxygen-deficient water masses.…”

The Albian to Turonian carbon isotope record from the Shilaif Basin (United Arab Emirates) and its regional and intercontinental correlation

“…Gale et al, 1993;Jenkyns et al, 1994Jenkyns et al, , 2017Tsikos et al, 2004;Scopelliti et al, 2006;Voigt et al, 2007;Gebhardt et al, 2010;Jarvis et al, 2011;Bomou et al, 2013;Dickson et al, 2017;Owens et al, 2017). Other observations associated with OAE 2 include an increase of siliceous plankton in the Umbria-Marche Basin during the event (Mort et al, 2007) or predating it in the Natih Basin (Van Buchem et al, 1996;Wohlwend et al, 2016), and blooms of pelagic bivalve filaments from the earliest Turonian across Northern Africa and Oman to Tibet (Negra et al, 2011).…”

Evidencias petrográficas de estructuras de origen algal/bacteriano en carbonatos de la Formación Agua Nueva (Cenomaniano/Turoniano: Cretácico Superior) en Xilitla, S.L.P. México central

“…Por otra parte, algunos de los filamentos más gruesos presentan un ápice triangular y una estructura interna que consiste en capas o lamelas con diversas orientaciones (Figura 5D). En algunas capas estos filamentos se desprenden de fragmentos calcáreos de mayor tamaño (Figura 5E y F), lo cual es consistente con lo reportado por Scholle et al (1983), Scholle y Ulmer- Scholle (2003), Flügel (2004) y Negra et al (2011) para filamentos traslúcidos de origen metazoario (bivalvos); lo que descarta un origen microbiano para las formas con ápice de Xilitla.…”