Radish concretions grown in mud during compaction

Radish concretions exhibit a typical columnar to pear-shaped, stipe down-ward 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 organo-clastic 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 con- cretions 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 vol- ume of the concretions was still open as suggested by shrinkage experi- ments. 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.