Cement stratigraphy in Triassic and Jurassic limestones of the Weserbergland (northwestern Germany)

Abstract

Cement stratigraphy of the Korallenoolith (Oxfordian) and Trochitenkalk (Upper Muschelkalk) Formations, southern Lower Saxony Hills of Germany, is based on investigation of 232 carbonate samples by cathodoluminescence (CL). This enables subdivision of cements into four main generations: Generation 1, consisting of fine columnar, equant and syntaxial cements with blotchy CL and/or microdolomite inclusions, that is interpreted as originally submarine Mg calcitic precipitate. Generation 2, found in Jurassic samples only. These cements show intrinsic to non-luminescent CL with thin bright orange subzones and they are interpreted as meteoric phreatic calcites. Generation 3.1, characterized by zoned calcites with variable CL colours. These cements precipitated under shallow burial conditions, in a phreatic environment during an incipient stage of late diagenesis. Generation 3.2, with relatively uniform orange CL, indicative of precipitation in a stable chemical environment. This cement generation is interpreted as late diagenetic deep burial calcites. Generation 3.3, partially strongly zoned and with a characteristically intense CL contrast. It has been identified in only 17 of the 82 Trochitenkalk samples. Deformation twins end within this generation, indicating a post-tectonic genesis for the subsequent younger cement zones. The cements were probably precipitated in a near-surface environment characterized by reducing or variable Eh conditions. Generation 4, consisting of calcites that show intrinsic CL with bright orange subzones comparable to generation 2, but lacking deformation twins. These cements are interpreted as post-tectonic late diagenetic products of a meteoric phreatic environment. Based on detailed CL petrography, fifteen diagenetic events (three calcite cements, three dolomite generations, three fissure generations, two stages of dedolomitization, aragonite dissolution, matrix recrystallization and two stages of HMC → LMC transformation) can be distinguished and timed relative to cement zonation. The progressively decreasing δ 18O values for cement generations 1 to 3.2 likely reflect increasing temperatures caused by burial. The low δ 18C values of cement generation 4 are probably a reflection of a meteoric diagenetic environment. A time—burial—cementation pathway can be modelled combining CL patterns, cement isotopic data and the subsidence history of the study area. The main features of this model are: 1. (1) Generation 1 cements are submarine precipitates and their age therefore coincides with sedimentation. 2. (2) Cement generation 3.1 of the Trochitenkalk commenced precipitation at a depth of about 120 m, reached during Late Triassic time. In case this minimum depth is also the formation depth of generation 3.1 in the Korallenoolith Formation, then precipitation of these cements started during Late Jurassic time. 3. (3) Cement generation 3.2 of both the Trochitenkalk and the Korallenoolith Formation, precipitated at assumed depths > 1000 m, reached during the Doggerian and at the time of Jurassic/Cretaceous transition, respectively. 4. (4) Cement generation 4 precipitated after the sediments had undergone telogenesis, and must thus be younger than the Intra-Senonian (Sub-Hercynian) tectonic phase and possibly even younger than the Paleocene tectonic pulse (Laramian Phase).