In situ accretion mechanism of concavo‐convex lacustrine oncoids (‘swallow nests’) from the Oligocene of the Mainz Basin, Rhineland, FRG

Abstract

ABSTRACTOligocene oncoids from the lacustrine Süβwasserschichten of the Mainz Basin exhibit a pustular surface and a characteristic cavity on their undersides, which gave rise to their designation as ‘swallow nests’. Oncoids grew in situ, under low‐energy conditions during a pause in sedimentation. Three features indicate that they were never overturned: (1) oncoids can be placed in a stable position on a smooth surface; (2) laminae, which are mostly concentric around the entire oncoids, display polar thickenings on the upper side during all growth stages of oncoids; (3) the depression is always situated on the stable side of oncoids. Development of this cavity is favoured by unionid shells, in the stable position, serving as oncoid nuclei, but occurs also under flat nuclei. It is interpreted as a product of reduced growth of cyanobacteria due to oligophotic conditions, on a fine‐grained substrate, and is considered as diagnostic of the in situ growth of oncoids.Oncoid cortices are mainly formed by a Scytonema‐like cyanobacterium, whose carbonaceous organic remnants are occasionally preserved. Growth stages of individual oncoids were image‐processed and compared by means of shape parameters. Due to optimum growth conditions, cyanobacteria try to occupy the entire available space; hence, oncoids tend to achieve a spherical form even during in situ growth. Renewed background sedimentation triggered the formation of pustular surfaces and finally caused the cessation of cyanobacterial growth.