OMNIPRESENCE OF COCCOLITHS IN MIDDLE-UPPER JURASSIC LAGOONAL LIMESTONES IN FRANCE

We emphasized in a former paper the genetic importance of coccoliths in lithographic limestones s.s. sampled in Bavaria (Solnhofen area), the French Jura (Cerin) and South-East of France (Canjuers). Here we have studied numerous sublithographic, pure limestones from different, Middle and Upper Jurassic lagoons : 1) Bathonian lagoon with the Comblanchian Formation sampled in the French Jura near Besancon (Calcaire de la Citadelle) and in Burgundy at Plombieres-les-Dijon, Chaumont, and Buisson and Ladoix quarries, both near Ladoix-Serrigny; 2) Middle Oxfordian lagoon with the so-called Calcaire de Clerval sampled at Avrigney and Sainte Ylie near Dole; 3) Kimmeridgian (Upper part of Pterocerian) lagoonal deposit sampled at Audincourt (Jura); 4) Tithonian sublithographic limestone collected in Mas de Pegourdy quarry (Dordogne). Besides the geological grounds, the lagoonal nature of these calcareous deposits is proved by on the one hand, a scanty marine fauna without normal and diversified assemblages, next to no benthic fauna and on the other hand, no lacustrine biota. Furthermore these deposits were generated in very shallow waters and drying phases can even be observed in the sequences. In all the studied samples, the SEM examination shows that the only constitutive elements which can be identified are minute-sized, oligospecific and cuff-link shaped coccoliths. Other fauna and flora are very scarce. Major diagenetic alteration occurs : the only remains of coccoliths are the holes related to the central wall which links the distal and the proximal shields of the coccoliths and the peculiar structure of the area surrounding the holes. The strong diagenesis effects explain both the hardness and the compactness of sublithographic and lithographic limestones. Our interpretation of these deposits is not very different from the one we formerly proposed for lithographic limestones s.s. In lagoons where the supply of continent-derived organic debris was low and the waters less saline than normal marine waters, Coccolithophorids bloomed to such an extent that their coccoliths could fill up the space resulting from subsidence. Abnormal salinities may have inhibited the other groups of the biota, Furthermore the coccolithic blooms may have played an antibiotic role as do the red tides in present-day marine environments or eutrophication in lacustrine environments. In Jurassic sediments where Coccolithophorids show anomalic features such as low diversity, minute-size, etc., they can no longer be considered as representative of the only true marine environment as it is unanimously admitted.