Abstract
Following a sedimentary crisis which begins in the late Lower Callovian and spans all the early Oxfordian, the settlement down patterns of a platform with carbonated sedimentation are analysed in a southeastern area of the Paris Basin (fig. 1).
Ten lithostratigraphic units (reefal formations, associated bioclastic facies and marly distal lateral facies ; fig. 2) are defined (fig. 3). New ammonite and brachiopod faunas, collected in situ, allow to date accurately the sedimentary units with a precision matching an ammonite subzone of the standard bio-chronostratigraphic scale (fig. 4) of the middle-late Oxfordian (from the Parandieri Subzone, at the base of the Transversarium Zone, to the Planula Subzone, at the top of the Planula Zone).
The sedimentologic analysis coupled with the study of the benthic and pelagic faunal communities allow to define twelve type-facies (tabl. I and II). Regrouped into three associations, these characterise depositional environments which occur in succession, following three platform models (fig. 5).
As witnesses of the evolution of the accomodation/sedimentation ratio, the resulting time succession of sedimentary bodies shows a depositional dynamics organised into three sequence tracks (fig. 5 and 6) :
– a retrogradation phase is characterised by a moderately deep and open platform, dominated by low energy and marly sedimentation, which ranges from the Middle Oxfordian (Plicatilis Zone and Transversarium Zone) to the lowermost late Oxfordian (Bifurcatus Zone) ; – during the late Oxfordian (Bimammatum Zone, from the Semimammatum Subzone to the Bimammatum Subzone) an aggradation phase corresponds to the installation of three successive shallow platforms with contrasted morphology. Indicating the re-initiation of carbonated production, these platforms are well limited and represent high energy shallows with reef buildings, which lateraly grade into dismantling bioclastic faciès, then secondly and more laterally again into low energy and medium deep marly facies ; – the upper part of the late Oxfordian (Hauffianum Subzone, in the uppermost Bimammatum Zone, and Planula Zone) shows the wide extent of a low energy and morphologically very little contrasted distal platform. This one has a high potential of carbonated production characterised by bioclastic, oolitic and micritic facies which illustrate a progradation phase ; such a phase proceeds in the early Kimmeridgian.
The collected data and the corresponding phenomenons pointed out on this area of the southeastern border of the Paris Basin are discussed and replaced in the general framework of the sedimentary, biologic, palaeogeographic and palaeoclimatic events henceforth recognised at the middle-late Jurassic boundary on the peri-Tethyan intracratonic domains of western Europe. The demise of the carbonate production seems to be correlated with a global cooling of both marine waters and atmosphere, which is considered as a limiting factor. During the Middle Oxfordian, the re-initiation of carbonate production with the developement of reef buildings should correspond to a large scale warming of the marine waters still observed elswhere on the Russian Platform, in the North Sea and in the Paris Basin. However, the geographic distribution and the chronologic succession of the facies and deduced palaeoenvironments is probably equally related to a synsedimentary tectonic activity which operates as a favorable factor at both a local and regional scale.