3-DIMENSIONAL ACCOMMODATION ANALYSIS OF THE TRIASSIC IN THE PARIS BASIN - A NEW APPROACH IN UNRAVELING THE BASIN EVOLUTION WITH TIME

The mechanisms governing the development of the Paris Basin throughout the Triassic are regarded as being the result of superimposed and suc cessive processes. In this study, the Triassic succession of the Paris Basin was re-interpreted in a sequence stratigraphic context, using e ssentially wireline log data. From this, a series of isopach maps, lit hofacies maps and palaeobathymetric maps was produced for each sequenc e. Three-dimensional accommodation analysis was then carried out seque nce by sequence, over the entire basin to produce a precise, detailed accommodation history for the entire Triassic succession. Previous stu dies have proposed that the Triassic was deposited during a rift perio d in a transtensional stress regime, with the formation of a trough su perposed onto three fault systems derived from the Variscan structural framework. In this study, Scythian to Ladinian sediments (Buntsandste in and Muschelkalk) record the stress regime that prevailed over much of NW Europe. The basin architecture at this time is in continuity wit h the neighbouring Germanic Basin. Our three-dimensional accommodation modelling shows that the stress regime changed during the Carnian and the late Norian (Keuper). The Camian events are marked by (1) the cre ation of a large depocentre infilled with halite, and (2) a northwest migration of this depocentre during the mid-late Carnian along with de position of the Gres-a-Roseaux. an extensive fluvial deposit. This doc uments renewed strike-slip movement along the Bray fault. The Norian e vents involved major tectonic uplift on the basin margins, producing f an delta progradation into the basin. Rotation of the previous depocen tre axis occurs on the downthrown side of the Bray fault. This may be viewed as a consequence of sinistral strike-slip displacement along th e Bray fault, forming a local transpressive stress regime. The followi ng Liassic cycle commenced with the Rhaetic sequences and illustrates a complete change in the stress regime. This corresponds with the new stress regime which prevailed over northwest Europe during the Liassic cycle. Accommodation curves from the basin complement this interpreta tion recognising two major accommodation phases which are separated by a significant unconformity. Both phases record periods of acceleratin g accommodation followed by a more uniform phase of decelerating accom modation. The accelerating phases correspond to periods of rapid accom modation space creation and result in thick evaporite deposits. They c orrespond to 'rift pulses' which occurred in the remote North Atlantic and Tethyan domain. Each accommodation phase is proposed to correspon d to Lithospheric stretching pulses followed by a relaxation period. T his study illustrates the importance of sequence stratigraphy coupled with three-dimensional accommodation analysis in refining important st ages in the basin evolution with time.