Shapes of organic walled dinoflagellate cysts in early diagenetic concretions—markers for mechanical compaction

•Dinoflagellate cysts are proposed as markers for mechanical compaction of the rocks.•Near original shape of the cysts are obtained from early diagenetic concretions.•A light microscopy examination of the dinoflagellate cyst height is presented.•Compaction ratios revealed herein are about 64%–67% for mudstones.

Two species of organic walled dinoflagellate cysts: Pareodinia ceratophora Eisenack 1938 (Jurassic) and Deflandrea phosphoritica Deflandre 1947 (Palaeogene) are proposed as markers for determining the mechanical compaction ratio of fine-grained rocks. The near original shapes of these species are obtained from specimens preserved in siderite and calcareous concretions occurring in mudstone host rocks of various ages. An efficient and simple light microscopy examination method of the dinoflagellate cyst height along the microscope optical axis is presented and quantitatively tested on the available material. The differences of the cyst height measurements between specimens preserved in concretions and specimens preserved in the host rock deposits reflect the compression of dinoflagellate cysts most likely due to mechanical compaction of the rocks studied. The mechanical compaction ratios revealed are about 67% for Jurassic mudstones and about 64% for Palaeogene mudstones. Further marker investigations of samples from different outcrops and strata of different ages are recommended.