Paleo-weathering of different basement rocks along a first-order nonconformity - case study at the post-Variscan nonconformity (Germany)

Abstract

Intracratonic sedimentary basins develop on first-order nonconformities. Although the evolution of such basins has been the focus of many studies, the formation of the basement surfaces has rarely been considered. In this paper, we investigated the physical and chemical paleoweathering of different basement rocks at the post-Variscan nonconformity in Central Europe. We used a comprehensive approach by accounting for different scales and integrating physical and chemical weathering. Physical weathering was qualitatively classified at the macro/mesoscale and quantitatively classified at the microscale using thin sections and through measuring fracture areas by image analysis. Chemical weathering was quantified by distinguishing the mineralogical and geochemical characteristics measured by X-ray diffractometry (XRD), X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Compared to previous studies, common physical and chemical weathering indices were normalized to the rock types to improve the weathering intensity comparison. Clay transformation and carbonate/dolomite precipitation in the paleoweathering sections indicate a temperature interval of 100 to 300 °C for the circulating fluids during burial. The maximum chemical weathering intensity was found at the top surface of the basaltic andesite with a CIAN (normalized chemical index of alteration) value of 83, whereas the IPWN (normalized index of physical weathering) was less than 0.1. The gabbroic diorite and granite had a CIAN value of 78, whereas the physical weathering intensity was approximately 0.6. The lowest chemical weathering intensity was observed at the top of the granodiorite and tonalite at 71, and both rock types showed the highest physical weathering intensity of approximately 4. Because of the normalization procedure, the differences in the ratio of chemical versus physical weathering can be directly linked to aridity-humidity changes in the lower Permian paleoclimate. The concept and results of this work can be used for future studies on continental paleoclimate as well as for reservoir modeling.