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

During the Oxfordian stage, the Lower Saxony Basin (LSB) was covered by a shallow epicontinental sea (Ziegler 1990), in which a thick succession of marine limestones and marls was deposited. Unfortunately, the scarcity of open marine marker fossils and numerous sedimentary gaps make it diffcult to correlate strata from the LSB with Oxfordian deposits elsewhere in the world (Betzler et al. 2007; Zuo et al. 2017). In a first step, this study therefore aims at establishing a high-resolution sequence stratigraphic framework for the Oxfordian Korallenoolith Formation, which is well-exposed in an abandoned quarry located ~40 km SW of Hannover (Bisperode section). The sedimentological characterization of the 126 m thick Oxfordian portion of the Bisperode section is based on a carbonate facies description of 257 rock samples, supported by the analysis of 50 petrographic thin-sections. Microfacies analysis involved the identification of carbonate components following the Dunham classification (skeletal and non-skeletal grains, matrix, cement) and as well as textural and diagenetic features. A total of seven superordinate facies types (FTs) composed of 15 different microfacies types (MF types) were distinguished. A depositional model is presented that is composed of different juxtaposed facies belts, ranging from semi-open lagoonal to open marine settings. Following the accommodation to sediment production (A/S) cycles concept, two orders of cycles were distinguished in the studied section, including small- and medium-scale sequences. A total of 21 small-scale sequences and 11 medium-scale sequences are recognized. Based on the facies type and the sequence stratigraphy, a strong control of sea-level fluctuations on the studied part of the Korallenoolith succession is proposed. Ziegler, P.A. (1990) Geological Atlas of Western and Central Europe. 2nd Edition, Shell Internationale Petroleum Mij. B.V. and Geological Society, London, 239 p. Zuo, F. et al. (2018) Sedimentology and depositional sequences of a Kimmeridgian carbonate ramp system, Lower Saxony Basin, Northern Germany. Facies 64,1-25. Betzler, C. et al. (2017) Facies and stratigraphic architecture of the Korallenoolith Formation in North Germany (Lauensteiner Pass, Ith Mountains), Sedimentary Geology, Volume 194, 61–75.