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Rudist shells are important archives for palaeoenvironmental reconstructions in the greenhouse world of the Cretaceous. Radiolitids, a family of rudists became very abundant dwellers of the shallow marine carbonate platforms during the Cretaceous. Still, due to the complex structure of their low-Mg calcite outer shell layer, radiolitids are often deemed unsuitable for palaeoenvironmental reconstructions. The aim of this study is to present a combination of petrographic (transmitted light microscopy and cathodoluminescence microscopy) and elemental analyses (high-resolution µXRF and stable isotope) to investigate radiolitid shells for diagenetic alteration and to make them accessible as an archive for environmental conditions during the mid-Cretaceous. Four Eoradiolitites shells were analysed in detail: two right valves (RV), a left valve (LV) with an articulated RV and an isolated LV. Three shell structure types have been identified, including a non-compact, compact and celluloprismatic types. The compact structure type was identified in the LVs while the other two types were observed exclusively in the RVs. Radiolitid shell material composed of the non-compact and the compact structure types can be used as palaeoenvironmental archives as it preserved seasonal signals in both, shell growth structure and isotopic records, while celluloprismatic shell structures are less suitable for reconstructions due to diagenetic alteration. The alternation of dark and light lamellae in both valves, RV and LV, provide information on the intertidal living habitat of the shell-producing organism and its life span.

New insights into the Lower Cretaceous sediments can be gained from a recently drilled core (KB Rehburg-2, Lower Saxony, Germany) covering 180 m of Berriasian strata. Stratigraphic assignment is based on ostracods and indicates Wealden 1-4. Palynofacies analysis was carried out to reconstruct the depositional environment of the sequence studied. This data was complemented by geochemical measurements (TOC, Stot, δ13Corg and RockEval pyrolysis) and sedimentological observations to reconstruct the paleoenvironmental conditions during the earliest Cretaceous in the LSB. The particulate organic matter (OM) shows good to very good preservation and is composed of diverse spores, pollen grains, algae and woody debris; marine particles such as dinoflagellates, acritarchs and prasinophytes occur only sporadically. Data evaluation and integration reveals a predominantly lacustrine, freshwater to brackish depositional system with high continental OM input. Abundance peaks of Botryococcus algae occur throughout the sequence and indicate freshwater to brackish conditions. The rare occurrence of dinoflagellates and acritarchs can be interpreted as indicators of short-term marine ingressions into the LSB. Prasinophytes point to repeated salinity changes. The scarce abundance of dinoflagellate cysts differs significantly from existing studies in the LSB by Schneider et al. (2017), but correlation with two marine transgressive events is tentatively proposed (TE 1-2). This difference may result from a potentially more terrestrially dominated depositional system represented by the Rehburg-2 core. References: Schneider, A.C., Heimhofer, U., Heunisch, C., & Mutterlose, J. 2017. The Jurassic-Cretaceous boundary interval in non-marine strata of northwest Europe – New light on an old problem. Cretaceous Research 87, 42-54.

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.