Abstract
MISS are primary sedimentary structures formed by interactions between biofilms and microbial mats with physical and chemical dynamics of sediments. Recognizing these structures in the geologic record are difficult because there are no criteria of identification, and sometimes they can be confused with other depositional or erosional physical structures. During the last decades, lots of biological, mineralogical, geochemicals and isotopic studies related to stromatolites and microbial mats were performed in the Lagoa Vermelha. However, the sedimentary structures formed by microbial induction (MISS) have never been the subject of these studies. The aim of this study is the description and characterization of MISS that occur in sediments of the Lagoa Vermelha, and recognize the main biosedimentological factors involved on their genesis. MISS were recognized macroscopically in the field and cores. The main processes recognized are microbial growth (that form laminated leveling structures), bioestabilization (that form desiccation cracks, mat curls, mat chips), metabolism (that form biolaminite, stromatolite, biscuit stromatolite, thrombolytic nodule) and binding, baffling & trapping. The combination of processes can also produce structures (wrinkles, detachment structure, petees). Petrographic analysis supported the identification of binding, baffling & trapping processes and others microscopic microbial features. The results allow a better understanding of the relationship between microbial mats and physical and chemical processes in a lagoon sedimentary environment. The recognition of these structures formed by microbial induction in modern environments helps their recognition in the geologic record of continental and marine successions.
Highlights
MISS are primary sedimentary structures formed by interactions between biofilms and microbial mats with physical and chemical dynamics of sediments
De acordo com Gerdes (2007), as gretas de dessecamento associadas as esteiras microbianas apresentam morfologias distintas daquelas observadas em superfícies não-bioestabilizadas, devido a imperfeições e variações locais na espessura da esteira microbiana
Os estromatólitos biscuit são frequentes na Lagoa Vermelha e ocorrem tanto em superfície quanto em testemunhos
Summary
As MISS (Microbially Induced Sedimentary Structures) são estruturas sedimentares primárias, formadas pela interação de biofilmes e esteiras microbianas com a dinâmica sedimentar física e química, e foram primeiramente reconhecidas por Kalkowsky (1908), que as denominou de “estromatólitos”, e posteriormente por Black (1933), que as classificou como sedimentos “algais” em trabalho realizado na Ilha de Andros, Bahamas. Entretanto, no presente estudo, o conceito de MISS é utilizado em senso mais amplo (Hoffman, 1976; Riding, 2000), abarcando todas as estruturas sedimentares formadas por indução microbiana relacionadas a ambientes de sedimentação siliciclásticos, carbonáticos ou evaporíticos. A Lagoa Vermelha é uma área em sedimentação de grande importância no cenário nacional e internacional devido a vasta ocorrência de estruturas microbianas e estromatólitos que ocorrem em suas margens e interior. O objetivo do presente estudo é a análise e caracterização das MISS que ocorrem nos sedimentos da Lagoa Vermelha, reconhecendo os principais fatores biossedimentológicos envolvidos em sua gênese
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