Reconstructing time and diagenesis of limestone-marl alternations from the selective compaction of colonies of the tabulate coral Halysites

Limestone-marl alternations are often interpreted to reflect cyclic changes in the depositional environment, with time spans for the deposition of a limestone-marl couplet between thousands and tens of thousands of years. Data from halysitid coral colonies from Gotland (Silurian), Sweden, indicate a diagenetic origin of limestones and marls indifferent to original sedimentary differences. The specimens crosscut several layers of their surrounding limestone-marl alternation, which means that both limestone and marl layers are among corallites and surrounding the coral colony. We analysed thin sections, SEM samples and micro-CT scans from Halysites catenularius from Gotland to document (1) the spatial extent of corals, limestone, and marl, and (2) the chronological order of sedimentation and subsequent diagenesis. Our results indicate a sedimentary infill of the coral frame while the coral was still alive. Growth banding indicates growth rates of 3.8 mm per year. Early diagenetic mass loss by aragonite dissolution as well as sedimentary overburden during progressive burial caused the compaction in marl beds fragmenting the coral, whereas it is well preserved in the limestone beds. The time span of a limestone-marl couplet penetrating the colony lies in the order of a few decades. Thus, the missing time in the depositional record is rather reflected by hiatuses than condensed sedimentary cycles. The mismatch of environmental changes recorded in the coral colonies with the changes in lithology, and the spatial heterogeneity of sedimentation rates within one bed imply constraints for applying lithological changes in cyclostratigraphy.