Pre- to early-rift thermal conditions of the Upper Rhine Graben using geological and organic geochemical controls

Abstract

The Toarcian Posidonia Shale and Hettangian/Sinemurian Formation are the two main Mesozoic source rocks of the Upper Rhine Graben (URG). Their thermal maturities are measured and used to reconstruct the thermal history of the URG. However, the thermal maturity of these source rocks was impacted by: 1) the pre-rift burial history which includes a 120 My hiatus from Upper Jurassic to the Paleocene; 2) the syn- and post-rift burial histories; and 3) the current high geothermal gradients and associated hydrothermal convective cells. In this study, the Saverne Fracture Field is considered the pre-rift analogue of the URG and therefore bears witness to the pre-rift thermal history. It was isolated from the graben upon its opening, i.e. in the Priabonian, and contains the same Mesozoic rocks, which, in contrast to the URG, are currently outcropping. As the source rocks in the Saverne Fracture Field were not buried as in the URG, their thermal maturities should be a consequence of the Upper Jurassic/Cretaceous burial only. Organic geochemistry results indicate that they are of pre-oil window thermal maturity. Calibration of the Saverne Fracture Field thermal model for the pre-rift period of the URG allows us to estimate that during the Upper Jurassic/Cretaceous hiatus additional burial did not exceed about 300 m. The additional sediments cannot be attributed specifically to the Upper Jurassic or Cretaceous. However, simulation results indicate that if Cretaceous sediments were deposited, they had to be preserved until the Paleocene, and their maximum cumulative thickness must have been at most 100 m. In addition, thermal modelling indicates that during the very first onset of the URG, Mesozoic sediments could have been buried again by about 250 m of Cenozoic sediments, with negligible effect of the pre-rift thermal signature. These geochemical and geological constraints are key input for further URG thermal modelling. Indeed, such constraints on the pre-rift therma history should allow future assessment of the syn-rift burial and recent hydrothermal convective cells impact on the Mesozoic source rocks maturity.