Rheotactic Macaronichnus, and Human and Cattle Trackways in Holocene Beachrock, Greece: Reconstruction of Paleoshoreline Orientation

摘要

Macaronichnus segregatis occurs in an upper foreshore conglomeratic to sandy beachrock dating from Roman times, on the east coast of Rhodes, Greece. In some laminae, M. segregatis is strongly oriented perpendicularly to the Recent and Holocene seashore. In other laminae, this trace fossil displays a winding to spiral course. These forms are introduced as new ichnosubspecies, M. segregatis lineiformis, M. segregatis maeandriformis and M. segregatis spiriformis, respectively. It is suggested that these middle-tier trace fossils were produced under conditions of different porewater flow. During higher energy periods, predominantly in winter, water movement perpendicular to the coastline caused M. segregatis to be oriented rheotactically. During periods of more or less stationary pore-water, predominantly in summer, M. segregatis assumed a spiral form. The winding form represents intermediate conditions. Closely beneath the Macaronichnus horizons a large burrow fill was found, probably referable to Psilonichnus isp. In the same beachrock are undertracks attributed to artiodactyls, associated with tracks and a trackway of human origin. The anatomy of the tracks indicates the direction of travel of the trackmakers, eastward and westward along-shore in all cases but one. The morphology of the tracks has been influenced by the gravitational effect of beach slope. Orientation, morphology and structure of trace fossils provide clear evidence that the sandstone and gravel unit is a beachrock and reveal the precise orientation of the paleoshoreline.