Three-dimensional accommodation analysis of the Triassic in the Paris Basin: a new approach in unravelling the basin evolution with time

Abstract

The mechanisms governing the development of the Paris Basin throughout the Triassic are regarded as being the result of superimposed and successive processes. In this study, the Triassic succession of the Paris Basin was re-interpreted in a sequence stratigraphic context, using essentially wireline log data. From this, a series of isopach maps, lithofacies maps and palaeobathymetric maps was produced for each sequence. Three-dimensional accommodation analysis was then carried out sequence by sequence, over the entire basin to produce a precise, detailed accommodation history for the entire Triassic succession. Previous studies have proposed that the Triassic was deposited during a rift period in a transtensional stress regime, with the formation of a trough superposed onto three fault systems derived from the Variscan structural framework. In this study, Scythian to Ladinian sediments (Buntsandstein and Muschelkalk) record the stress regime that prevailed over much of NW Europe. The basin architecture at this time is in continuity with the neighbouring Germanic Basin. Our three-dimensional accommodation modelling shows that the stress regime changed during the Carnian and the late Norian (Keuper). The Carnian events are marked by (1) the creation of a large depocentre infilled with halite, and (2) a northwest migration of this depocentre during the mid-late Carnian along with deposition of the Grès-à-Roseaux, an extensive fluvial deposit. This documents renewed strike-slip movement along the Bray fault. The Norian events involved major tectonic uplift on the basin margins, producting fan delta progradation into the basin. Rotation of the previous depocentre axis occurs on the downthrown side of the Bray fault. This may be viewed as a consequence of sinistral strike-slip displacement along the Bray fault, forming a local transpressive stress regime. The following 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 interpretation recognising two major accommodation phases which are separated by a significant unconformity. Both phases record periods of accelerating accommodation followed by a more uniform phase of decelerating accommodation. The accelerating phases correspond to periods of rapid accommodation space creation and result in thick evaporite deposits. They correspond to ‘rift pulses’ which occurred in the remote North Atlantic and Tethyan domain. Each accommodation phase is proposed to correspond to lithospheric stretching pulses followed by a relaxation period. This study illustrates the importance of sequence stratigraphy coupled with three-dimensional accommodation analysis in refining important stages in the basin evolution with time.