Sea‐Level Changes and Extensional Tectonics in the Lower Jurassic (Northern Helvetic Realm, Western Switzerland)

Abstract

Located at the hinge between the stable European continental platform and the northern Alpine Tethys, the Lower Jurassic basin of the northern Helvetic realm (western Switzerland) can be subdivided into several elongate symmetrical WSW-ENE sub-basins. Lateral thickness changes are gradual, and areas of non-deposition act as a major control on facies: Helvetic sandy facies near exposed zones and Dauphine shaley facies in more distal zones.The sediments were deposited in shallow marine offshore and foreshore zones. However, due to Alpine tectonics, preservation of sedimentary structures and fossils is poor. The sections studied display systematic events which can be interpreted as transgressive surfaces, sequence boundaries, condensed sections and shallowing-up sequences. The Lower Jurassic sedimentary record can be subdivided into genetically related strata recording relative base-level changes. Sedimentary structures or facies assemblages related to episodic palaeofault activity are absent. At least two orders of cyclicity (2nd and 3rd) have been recognized, and the same 3rd-order sequences are found in different sub-basins and in the two main facies types. The mechanisms controlling sedimentary sequences are therefore either regional (main-basin scale) or global. However, correlation with global events is recognized only in some situations, and eustasy is therefore not the only control on cyclic sedimentation.In order to investigate the regional processes, the subsidence history has been analysed. The tectonic subsidence curves, with corrections for depositional water depth, compaction, tectonic deformation, erosion, eustasy and Airy compensation, have been compared using three stretching models: uniform extension of the entire lithosphere, crustal and subcrustal extension (both non-uniform or depthdependent discontinuous stretching models). For the study area, the most suitable model is subcrustal stretching, with more extension in the lower lithosphere than in the crust. As predicted using the model, the ratio of initial fault-controlled to thermal subsidence is low and the studied basin is mainly thermally controlled.The results from sequence• and geohistory analyses are integrated. A possible process controlling sequences, and compatible with a mainly thermally controlled basin, is in-plane stress change. This mechanism can explain all observed features, including the asymmetric sedimentary cycles. The stratigraphical record may therefore result from the superposition of both eustasy and in-plane stress changes.The northern Helvetic Lower Jurassic basin was an intracratonic sag-basin and not, at that time, a constituent part of the more fault-controlled North Tethyan margin.

Thermal evolution and dynamic mechanism of the Sichuan Basin during the Early Permian-Middle Triassic

“…In this study, we collected 16 typical wells distributed in different areas of the basin (Figure 1), and de- termined 'the observed' tectonic subsidence during the Early Permian-Middle Triassic by backstripping method [16,17]. Four of them are distributed within the outer zone of Emeishan basalts, which are H1, GK1, NvJ and DS1.…”

早二叠世-中三叠世四川盆地热演化及其动力学机制

“…这里, 我们收集了分别处于盆地不同区域的 16 口代表井(图 图 1 峨眉山玄武岩及四川盆地研究井的分布 Ⅰ, 川西; Ⅱ, 川中; Ⅲ, 川东; Ⅳ, 川南. 修改自文献[3] 1), 采用回剥技术 [16,17] 在构造沉降史恢复时需考虑抬升剥蚀的影响. 据研究 [12] , 峨眉山地幔柱中心的地壳抬升在 1300 m 左右, 中带在百米级, 至外带抬升剥蚀已不明显.…”