CONICET | Buscador de Institutos y Recursos Humanos

This study documents the processes involved in forming flaser and wavy beddinggoverned by microbial activity in sediments. It focuses on a modern marginal-tidal system providing evidence of the role that biofilms play in thestabilization of ripples and their potential preservation. A combination ofdetailed field work, analysis of water level records and microscopic petrographicinspection were used to reply to the question: how fine-grained andcoarse-grained sediments can sequentially be deposited and preserved in acoastal environment. The hydraulic energy was measured by water level sensorsrecording flooding events that inundate the colonized tidal flat. Changingsurface morphologies were monitored after storms, revealing the importanceof biological processes in the preservation of ripples. A mud drape over rippleswas observed several days after the undulated surface formation, knownas a sinoidal sedimentary structure, which is a thin biofilm covering the ripples,caused by the presence of a microbial mat. Because bedforms are essentialpredictors of palaeoenvironmental reconstruction, interpretation in thegeological record should take into consideration the important effect that colonizedsediments have on the preservation of ripples. A geobiological modelexplains the flaser sedimentation, common in depositional coastal environments,suggesting that the hydrodynamic conditions may not be directlyreflected by the grain size at the time of deposition. The study reveals that flasersedimentation involves an interaction with benthic organisms, reflectedby the sequence of microbial mats with sand ripple marks. A detailed descriptionof heterolithic sequences shows that the presence of microbial activitycan drive ripple preservation. This suggests that hydraulic interpretation ofthe sedimentary record based only on physical processes might be erroneous.