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

ABSTRACT 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 recording hydrodynamic 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 preservation of 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.