Microbial microsequences modulated by storm dynamics in lower shoreface deposits: a case study from the early Ediacaran, Cerro Largo Formation, Argentina

A detailed study of 2-mm-scale microsequences visible in thin-section of cores from the early Ediacaran Cerro Largo Formation (Argentina) reveals recurrent storm–quiescence cycles within a storm-dominated lower shoreface. Each microsequence is characterised by individual intervals stacked upon each other. Each interval shows from base to top a decline in grain size recordinghydrodynamic thresholds. Concomitant grain size decreases, thickness increases and frequency of microbial mat lamination is typical. Petrographic analyses document well-preserved endobenthic and epibenthic mats in such intervals. Each mat type has a distinct stabilisation modus. Raman spectroscopy confirms the presence of disordered carbonaceous matter interpreted as thermally mature microbial fossils. Hydraulic calculations place the fair-weather wave base between intervals II to III (Microsequence I) and I to II (Microsequence II) and the storm wave base within intervals IV to V. Microbial mats increase erosion thresholds by up to 12-fold, thereby ensuring the preservationof microbial lamination and their substrate under conditions that would otherwise favour reworking. This fine-scale record of physical–microbial interactions may constitute a tool to resolve storm frequencies and matground development, especially for mat-rich Ediacaran shoreface settings. Fil: Arrouy, Maria Julia. Comisión de Investigaciones Científicas de la Provincia de Buenos Aires. Instituto de Hidrología de Llanuras "Dr. Eduardo Jorge Usunoff". - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil. Instituto de Hidrología de Llanuras "Dr. Eduardo Jorge Usunoff". - Universidad Nacional del Centro de la Provincia de Buenos Aires. Instituto de Hidrología de Llanuras "Dr. Eduardo Jorge Usunoff"; Argentina Fil: Noffke, Nora. Old Dominium University; Estados Unidos