Radish concretions grown in mud during compaction

Abstract

AbstractRadish concretions exhibit a typical columnar to pear‐shaped, stipe downward geometry. In Middle Jurassic mudrock in south‐west Germany, radish concretions started to form around an iron‐sulphide lined tube by pervasive cementation constituting an ellipsoidal parent domain in uncompacted sediment at burial depths of ≤5 to 8 m as recorded by 75 to 80% minus‐cement porosity. Thereafter, the concretions grew vertically in compacting sediment as evidenced by laminae within the concretions being increasingly inclined towards the tips, and concomitantly decreasing minus‐cement porosity. During early diagenesis, prior to septarian crack formation, bicarbonate generating the microbial cement originated within the sulphate reduction zone chiefly from anaerobic oxidation of methane and to a lesser degree of organoclastic material. Later, at 50 to 70 m burial depth, septarian cracks formed as evaluated by sedimentation–compaction analysis based on minus‐cement porosity data and compressibility of similarly composed sediments. The outward‐narrowing septarian cracks indicate that they formed when the concretions were still in a plastic state but already cemented sufficiently to be resistant against compaction. In this stage, up to one‐quarter of the pore volume of the concretions was still open as suggested by shrinkage experiments. This pore volume, and the septarian cracks, were filled with cement termed late diagenetic. In the study area, the decompacted net‐sedimentation rate was low, about 2 to 3 cm kyr−1, for ca 2.5 Myr, allowing the concretions to reside for a long time within the sulphate reduction zone and to grow. Radish concretions formed within the transition zone from thick, rapidly deposited to long‐term, slowly accumulating sediment.