Development and eustatic control of an Upper Jurassic reef complex (Saint Germain-de-Joux,Eastern France)

Summary This study consists of a sedimentological and diagenetical analysis of reef facies from the Upper Kimmeridgian (sensu gallico). The investigated deposits are situated in eastern France, about fifty kilometres west of the city of Geneva (Switzerland). The reef complex is a fine example of vertical development and facies differentiation. It is subdivided into two distinct sequences by a perforated hardground horizon and sand shoals. The onset of the first reef sequence is characterized by a pioneer growth stage followed by up to 20 m of reef-core and-flank facies. Corals forming the reef-core are typically the ramose variety ofCalamophylliopsis flabellum. The second reef sequence has a reef-core with an average thickness of about 5 m. Corals, however, display much more varied morphologies, and in some areas massive rudist (Heterodiceras) build-ups occur. Development of the second reef sequence was seriously weakened by a storm which produced a 2 m thick accumulation of coral rubble. A shallowing-upwards trend gradually leads to the formation of beach deposits, followed by a newly detected black-pebble horizon. Diagenesis is an important aspect of the reef complex. Especially noteworthy is the dolomitization of certain horizons. At the base of the reef formation, the passage of the phreatic mixing zone provoked invasive dolomitization in large irregular patches (probably deposits richer in Mg-calcite). Some of the beds above the black-pebble horizon, in particular a deposit of accumulated microbial mats, are also dolomitized. In this case, dolomitization is stratiform and is interpreted as having precipitated under conditions of evaporative pumping. The sedimentary record clearly shows the imprint of eustasy. The reef complex was initiated during a transgressive cycle and the hardground found between the two reef sequences is interpreted as a maximum flooding surface (mfs). At the top of the sequence, the horizon overlain by the black-pebble conglomerate is believed to represent the new sequence boundary SB140. Other significant features identified from the St. Germain-de-Joux deposits include the discovery of a new foraminifera,Troglotella incrustans, which is only marginally covered here but is the topic of another paper (Wernli & Fookes, 1992); the subdivision of the first coralligenous level defined byPelletier (1953) into two reef sequences; and a proposition to redefine the ‘Calcaires de la Semine’ (Bernier, 1984). The investigations carried out in the past on the Kimmeridgian deposits in the area of St. Germain-de-Joux were mostly based on stratigraphy and palaeontology. These reefs are among the finest known in the Jura Mountains, but no thorough study had been made on their sedimentological aspects. The aim of this study is to fill this void and also to clarify the more confusing aspects of local stratigraphy (paper based onFookes, 1991).