Microbial carbonates and corals on the marginal French Jura platform (Late Oxfordian, Molinges section)

Abstract

International audienceMolinges was located on an Upper Jurassic ramp system of low-energy regime that developed at the southern margin of the French Jura platform. The sedimentary succession is characterized by the transition from a mixed siliciclastic-carbonate to a carbonate depositional setting that occurred during a long-term shallowing-upward trend. The disappearance of siliciclastics is explained by a climatic change, from humid and cold to drier and warmer conditions, previously identified in Late Oxfordian adjacent basins. The base of the section shows marl-limestone alternations of outer ramp. In its middle part, the section displays oncolitic marls, coral-microbialite beds and oncolitic limestones that deposited in a mid ramp position. Finally, the upper section part is made of oolitic limestones of inner ramp. In outer- to mid-ramp settings submitted to terrigenous inputs, the stacking pattern of deposits and facies evolution allow the identification of elementary, small-, medium-, and large-scale sequences. Small amplitudes of sea-level variations probably controlled rapid shifts of facies belts and reef window occurrences. In small-scale sequences, the coral beds developed during periods of sea-level rise. The decreasing rate of sea-level rise is marked by the downramp shift of the oncolitic limestone belt that led to the demise of coral-microbialite beds. These bioconstructions are mainly represented by thin biostromes in which corals never reach great sizes. The coral assemblages mainly include the genera Enallhelia, Dimorpharaea, Thamnasteria, and some solitary forms (Montlivaltia and Epistreptophyllum). They suggest relatively low-mesotrophic conditions in marine waters during the edification of the primary framework. Relatively cold water temperatures and periods of more elevated nutrient contents are probably responsible of the reduced coral development and the formation of a large amount of microbialites

Late Jurassic facies architecture of the Złoczew Graben: implications for evolution of the tectonic-controlled northern peri-Tethyan shelf (Upper Oxfordian–Lower Kimmeridgian, Poland)

“…Marly facies with oncoids were described from low-energy, mixed, carbonate-siliciclastic mid-ramp setting (e.g., Olivier et al 2011). The appearance of oncoids in marly sediments along with the traces of bioerosion on their surfaces indicate nutrient-rich and turbid waters.…”

“…FT 1 represents typical Upper Oxfordian facies, widely distributed in Europe (e.g., Aurell et al 1995;Leinfelder et al 1996;Matyszkiewicz 1997b;Olóriz et al 2003;Olivier et al 2004Olivier et al , 2011Reolid et al 2005). The development of FT 1 can be related to a more distal mid-ramp setting.…”

“…This event can be observed in successions from the Kraków-Częstochowa and the Wieluń uplands as well as from the southwestern margin of the Holy Cross Mountains and from the Miechów Depression (e.g., Kutek 1994;Matyszkiewicz 1996;Matyja and Wierzbowski 2006;Złonkiewicz 2009;Krajewski et al 2014). Initially, oncolitic-marly facies appeared, related to a more distal setting of the mid ramp (e.g., Olivier et al 2011). These were followed by the marly-limestone alternations representing a deep-water, mixed carbonate-siliciclastic outer-ramp facies (Fig.…”

“…It is suggested that in the Złoczew area as well as in the southern part of the Kraków-Częstochowa Upland the equivalents of coral facies are the microbial-Crescentiella (FT 2) facies deposited in a midramp setting. Indirect support of this concept is provided by the results of studies on Upper Oxfordian coral facies from central and southern Poland (Roniewicz and Roniewicz 1971;Roniewicz 2004), where coral facies are dominated by platy coral colonies of Microsolena, which grew in a mid-ramp, low-light and low-energy setting, below fair-weather wave base (Insalaco 1996;Leinfelder et al 1996;Olivier et al 2011). Corals described from the sites in the Kraków-Częstochowa Upland most commonly occur together with the Crescentiella-dominated facies (e.g., Olchowy 2011).…”

Middle Jurassic (Bathonian) encrusted oncoids from the Polish Jura, southern Poland

“…They may have formed in both marine and freshwater environments (e.g., Brand and Veizer 1983;Nickel 1983). Usually, oncoids are reported from shallow-water (under photic and wave agitation conditions) environments being formed by photosynthetic cyanobacteria and algae (e.g., Peryt 1981Peryt , 1983Riding 1983;Tucker and Wright 1990;Védrine et al 2007;Schlagintweit and Gawlick 2009;Bádenas and Aurell 2010;Olivier et al 2011). Riding (1983, see also Herbig 1994) even called such oncoids 'cyanoids', a term that provided a more genetic sound to the general term 'oncoids'.…”