Carbonate microfacies and transgressive-regressive sequences of Oxfordian shallow-water limestones (Korallenoolith, Lower Saxony Basin)

Abstract

This study represents a detailed analysis of a stratigraphic section of Oxfordian (Late Jurassic) age with the aim to reconstruct the facies pattern and sequence-stratigraphic evolution, followed by a discussion of the dominant controls on shallow marine carbonate platform sedimentation in the Lower Saxony Basin (LSB). During the Oxfordian, the LSB was covered by a shallow epicontinental sea, in which a thick succession of marine limestones and marls was deposited. The stratiform Oxfordian carbonate bodies hold significant economic potential due to their ability to form hydrocarbon reservoirs or targets for future geothermal exploration. The scarcity of open-marine biostratigraphic marker fossils and the presence of numerous sedimentary gaps pose significant challenges for establishing reliable correlations with Oxfordian deposits globally. This study focuses on the Oxfordian Korallenoolith Formation, which is well exposed in the Bisperode section located ~40 km SW of Hannover. The ~126 m thick section was described bed-by-bed during fieldwork and complemented by 73 petrographic thin sections that were analysed for carbonate microfacies. Based on differences in grain and mud composition and primary sedimentary fabrics, an integrated log of the Bisperode quarry was constructed. Seven facies types (FTs) and 13 microfacies types (MFs) are characterised upon which a reef-bearing carbonate ramp model was built. Following the magnitude of the transgressive/regressive trends concept, three low-order sequences are recognised, which are composed of eight higher order sequences. The low-order sequences correlate well to the established sequence-stratigraphic model and the higher order sequences point to a possible control of their stacking pattern by Milankovitch long eccentricity cycles. This study is important because it helps to better understand the link between the stacking pattern of Oxfordian strata in the LSB and allocyclic processes. It thus provides a foundation for basinwide and inter-basin sequence-stratigraphic correlation, allowing future global chemostratigraphic comparisons and providing valuable insights into the synchronicity of geological events.