Microfacies, microtaphonomic traits and foraminiferal assemblages from Upper Jurassic oolitic–coral limestones: stratigraphic fluctuations in a shallowing-upward sequence (French Jura, Middle Oxfordian)

Abstract

The characterization and distribution of the microfacies and the microfossil assemblages of a Middle Oxfordian section from Jura Mountains composed by thick oolitic-coral limestones is analyzed. Six microfacies types (mainly grainstones) are diVerentiated mainly composed by ooids, intraclasts and bioclasts. Foraminiferal assemblages are dominated by agglutinated forms. Benthic microbial communities and sessile foraminifera are the main components of the encrustations. The whole set of microfossil assemblages is typical of shallow subtidal environments rich in "algae" (Cayeuxia, "Solenopora", Thaumatoporella, Bacinella, Girvanella and Terquemella) and foraminifera such as Nautiloculina oolithica, Redmondoides lugeoni, Ammobaculites coprolitiformis, Troglotella incrustans and Rectocyclammina. The increasing upward record of debris of algae and Nautiloculina, and the decrease of serpulids, bryozoans, nodosariids and ophthalmidiids indicate a shallowing-upward trend. The stratigraphic distribution of microfacies and microfossil assemblages lead to diVerentiate two main successive phases. The Wrst is a deeper subtidal environment in an open shelf, while the second is a shallow subtidal environment with evolution from winnowed to more restricted conditions. Microfabrics of radial to concentric ooids upwards in the section correspond to higher energy environments related to an oolitic shoal. This study shows how a very detailed analysis of microfacies, which integrates oolitic features, microfossil assemblages and microtaphonomy is potentially a useful tool for interpreting hydrodynamism and sequence evolution in marine carbonate shallow environments.

Biostratigraphy and tectonic significance of lowermost Cretaceous carbonate rocks of the Circum-Rhodope Belt (Chalkidhiki Peninsula and Thrace region, NE Greece)

“…The presence of corals and microborings of Troglotella incrustans (common within reef and adjacent environments e Wernli and Fookes, 1992) supports the interpretation of shallow marine depositional conditions (Reolid et al, 2007).…”

“…The foraminiferal morphogroup analysis has been developed to evaluate paleobathymetric and paleoenvironmental changes reflected in the formaniniferal assemblages of both deep and shallow waters (Reolid et al, 2010). Studies of modern and ancient assemblages demonstrate that the morphology of the foraminiferal shell (mode of coiling, chamber arrangement, features of aperture, position of perforation) can be directly related to different life-styles and trophic strategies (Nagy, 1992;Tyszka, 1994;Reolid et al, 2007Reolid et al, , 2008aReolid et al, ,b, 2010. According to Nagy (1992) the use of morphological categories in paleoenvironmental analysis, rather than species, can prove advantageous because: 1) the morphological approach allows reliable comparisons of assemblages of different ages, reducing the effect of taxonomic divergences caused by biological evolution; 2) taxonomical determinations at the species level are not required; and 3) this approach simplifies the analysis by reducing the number of variables (as opposed to the use of species) (Reolid et al, 2008a,b).…”

Microfossil assemblages and relative sea-level fluctuations in a lagoon at the Oxfordian/Kimmeridgian boundary (Upper Jurassic) in the eastern part of the Paris Basin

“…The only study on microfossils was done on washed marls (Bure section, Grosheny in Lathuilière et al 2006). In other areas, several papers describe foraminifera and algae for this interval and/ or in similar environments: Ramalho (1971); Septfontaine (1988); Bassoullet et al (1985); Jones and Charnock (1985); Amodio (2006); Reolid et al (2007).…”

Facies characteristic and paleoenvironmental reconstruction of the Fahliyan Formation, Lower Cretaceous, in the Kuh-e Siah area, Zagros Basin, southern Iran

“…Some ooids are diagenetically altered by micritization and cementation (first generation of fibrous and bladed cements followed by equant or blocky cements). The abundance of allochems, grain-supported texture, concentric ooids, good sorting and roundness and subordinate mud content of this facies belt indicate high-energy conditions (Tucker et al 1993;Lucia 1999;Palma et al 2007;Reolid et al 2007;Adabi et al 2010). All facies of this belt have no mud except the aggregate-peloid packstone/grainstone, an environment with lower energy conditions close to the lagoon as is indicated by the abundance of Clypeina sp.…”