Radish concretions grown in mud during compaction

Abstract

Radish 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.

THE TRACE FOSSILTISOA SIPHONALISIN ITS TYPE AREA—CHARACTERISTICS AND ENVIRONMENTAL SIGNIFICANCE

“…Therefore, the funnel feature described by Knaust (2019) could be of concretionary origin that reflects locally enhanced permeability related to somewhat coarser host sediment (see Fig. 4B, Appendix 2; Wetzel and Bojanowski 2022). Nonetheless, passive ventilation of a U-tube would have been possible if the lining of one U-tube limb extended like a chimney above the seafloor.…”

“…The minus-cement porosity of ∼ 50% and the telescoping of the pyritic lining implies that the Tisoa- encasing concretion started to form after abandonment of the burrow but still during early diagenesis somewhat below the seafloor (e.g., Wetzel and Bojanowski 2022). This deduction is supported by the following evidence of differential compaction of the host sediment relative to the already rigidly behaving incipient concretions: (1) striae on concretions' surface; (2) reverse faults dissecting the concretions but not the host rock; and (3) the concretions' possible reorientation (Figs.…”

Dynamic environmental conditions recorded by the trace fossil Teichichnus and event beds during deposition of the basal Vaca Muerta Formation in the central Neuquén Basin (Argentina)

“…The concretion ʽhalvesʼ above the T-bed also record times of lowered or interrupted sedimentation; to grow they must have resided for at least some hundreds of years in the zone of carbonate supersaturation (e.g., Raiswell and Fisher, 2004;Lash, 2018;Blouet et al, 2021). Consequently, the concretions document discontinuous sedimentation characteristic of a sedimentary hiatus (e.g., Wetzel and Allia, 2000;Zatoń et al, 2011;Wetzel and Bojanowski, 2022).…”