Puzzling micro-relief (mounds) of a soft-bottomed, semi-enclosed shallow marine environment

Abstract

A dataset comprising geomorphological information (side-scan sonar, bathymetry, samples, drop camera, scuba diving, and a limited number of subbottom profiles) revealed a large number of low-relief buildups occupying almost half of the Gulf of Gera, a shallow land-locked environment. They probably represent mud mounds constructed of biogenic fragments and benthic assemblages consisting mainly of mollusks in a fine-grained matrix, and they are confined to the central part of the gulf. According to their morphometric characteristics and surface distribution, the mounds were separated into three subtypes with the highest (up to 2.5 m), larger coalescing features occurring close to the gulf entrance, whereas the smaller and more circular features appear in an elongated zone at the center of the gulf. This configuration coincides with the gulf’s circulation pattern that disperses and transports nutrients and fine-grained material. The high-resolution subbottom profiles, acquired in a restricted zone of the southeastern gulf, have shown the presence of paleo-mound surfaces in the Holocene highstand surface layer and fluids in the underlying sediments. Similar formations have been rarely reported in the literature, and those other reports are from different environmental settings. Consistent with the literature, they are probably developed due to the interplay of physical and biological processes. However, only an interdisciplinary study could shed light on the specific processes that drive their formation and their unique distribution pattern in the specific environment of the Gulf of Gera.