CIG   05423
CENTRO DE INVESTIGACIONES GEOLOGICAS
Unidad Ejecutora - UE
artículos
Título:
Preservation of Biogenerated Mixed Facies: A Case Study from the Neoproterozoic Villa Monica Formation, Sierra de la Juanita, Tandilia, Argentina
Autor/es:
ZALBA, P. MANASSERO, M., MOROSI, M AND CONCONI, S.
Revista:
Journal of Applied Sciences
Editorial:
Asian Network Science -Science Alert
Referencias:
Lugar: NY-London-Asia; Año: 2010 vol. 10 p. 363 - 379
Resumen:
The Villa Mónica Formation is the most ancient Neoproterozoic unit of the Tandilia System, and it is composed by carbonatic (C), carbonatic/siliciclastic (C/S) and heterolithic (H) facies, bearing biosignatures, at the Estancia La Siempre Verde, La Placeres, and Don Camilo quarries, in the Sierra La Juanita, near Barker locality. They have been classically defined as siliciclastic facies for more than 40 years and now they are redescribed as mixed facies: The carbonates are represented by well-preserved columnar head stromatolite boundstones and by microbial mat deposits which have been dolomitized, silicified, illitized, and hematized, and bear `in situ´ grown individual or random aggregates of pyramidal quartz megacrystals, being their host rock also identified for the first time. The siliciclastic are composed of quartz grains trapped within both the stromatolites and the microbial mats, of illitic siltstones and claystones, and of quartzitic sandstones. Field work and microtextural studies revealed a complex history of weathering and diagenesis which, nevertheless, could not completely erase biosignatures and permitted the recognition of biogenerated rocks. Illuviation processes transported cutans to lower horizons. Syndiagenesis involved dolomitization and silicification, while burial diagenesis produced pressure-solution effects by overburden, and neoformation of minerals: growing of diagenetic illite with rutile needles, among others. Compressive movements from the SW, responsible for basin inversion: telodiagenesis (uplift, fracturing, folding, and introduction of meteoric fluids), affected the Villa Mónica Formation with neoformation/transformation of minerals: kaolinite, halloysite and smectite, development of slickensides (stress cutans) and ferriargillans, hydration, dedolomitization and calcification. A paragenetic sequence is proposed. Key words: biogenerated Neoproerozoic rocks, carbonatic, mixed facies, weathering, diagenesis, paragenetic sequence