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Abstract

Detailed stratigraphic and sedimentological analyses on Late Triassic-early Liassic sections in the internal and external western Alps, the southeast French basin and the southeastern part of the French Massif Central provides the opportunity to study the distribution of sedimentary bodies which were deposited during a long-term transgressive trend at the beginning of a continental encroachment cycle that extended onto the weakly differentiated Late Triassic European platform. Four short-term sequence boundaries can be identified: Late Norian or early Rhaetian, latest Rhaetian, end of early Hettangian and late Hettangian. They bound three sequences (SR, SH1 and SH2) whose duration is compatible with Vail's (1992) "sequence cycles". In some places, the first sequence (SR) can be divides into two minor sequences (SR1 and SR2), which define an additional, upper Rhaetian sequence boundary. The second sequence (SH1) is also composed locally (subalpine domain) of two minor cycles. Evidence for rapid relative sea-level fall is lacking because of overall subsidence and moderate sediment supply. The maximum flooding intervals of sequences SR1 and SR2 are of Rhaetian age, but they cannot be dated more accurately due to restricted platform facies. Two other maximum flooding intervals are dated with ammonites from the early Hettangian (upper planorbis subzone) and the middle Hettangian (upper portlocki or lower laqueus subzone), respectively The deposition of these sequences is coeval with an increase in extensional tectonic activity whose culmination, during the middle and late Hettangian time, represents the earliest synrift event of Ligurian Tethys rifting in the study area. Differential subsidence, synsedimentary faults and paleostress indicators show that a new tectonic regime was developed, starting at the Triassic/Jurassic boundary. This can be correlated to the plate-tectonic reorganization observed, i.e., along the central Atlantic margins and may account for the long-term transgressive trend. As in many other Tethyan or Atlantic areas, this event is marked locally by short-lived intracontinental rift volcanism. The short-term sequences observed indicate some differences to Haq et al.'s (1987) chart, but the transgressive events are known in many areas inside and outside the European realm. The corresponding changes in accommodation space thus cannot be ascribed to local tectonics only. They may have been controlled by eustasy, but the widespread extent and the synchronism of some short events (volcanism around the Triassic/Jurassic boundary; middle-late Hettangian time breakup and drowning) suggest that subsidence, possibly controlled by fluctuations of intraplate stresses, may also have forced the short-term signal.

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