The Glacial record of the Tandilia System: the stratigraphical significance.

PAZOS, P. J. RAPALINI, A.

Precambrian Glacial Record a tribute to M. Hambrey and D. Harland.

Geological Sociaty of London Memoir 36

Lugar: London; Año: 2011; p. 563 - 569

Current ideas in the geological community emphasize the importance ofdifferentiating diamictite from tillite units in order not to contaminate description withinterpretation when poorly sorted debris is analyzed. In the Tandilia System, centralArgentina, the glacial origin of a very thin succession (10 m thick) representing the Sierra del Volcán Formation has been known for many years (Spalletti & del Valle, 1984), being the first undisputed glacial deposits recorded in the Río de la Plata craton. It is composed of three intervals: diamictite at the base and at the top and pelite and heterolithic levels with undulatory stratification and ripples in the middle. Out-sized clasts previously interpreted as dropstones are present in this interval. The lower diamictite is composed of polimictic clasts, immerzed in a matrix containing abundant kaolinite. The top one exhibits faintly normal grading and is polimictic. A review of the analyzed the interval in a stratigraphical and regional context remarking the lack of a precise age for the unit and a stratigraphic context is discussed. Others possible glacigenic intervals situated at the base of the Cerro Largo Formation and in the Punta Mogotes borehole (unit 4) are also analyzed. The glacigenic origin is discussed but is analyzed, but a rigorous analysis of the stratigraphic position, age, bio and chemostratigraphy information is discussed for the three intervals. Precise correlation between units in the Tandilia System containing diamictites remains speculative. The more likely stratigraphic position for the Sierra del Volcán glacigenic deposits in Late Ediacaran and a direct correlation with the Punta Mogotes borehole (unit 4), is analyzed here and considered feasible. Hypothetical correlation of these with a thin diamictite level, of undetermined origin, at the base of the Ediacaran Cerro Largo Formation is more uncertain as well the glacigenic origin of the last one. However, the glacigenic intervals can be contemporaneous with other Ediacaran glacial deposits of the Río de la Plata craton, according to biostratigraphic schemes, palaeomagnetic data and regional context. Paleomagnetic data obtained so far for the Río de la Plata craton is still scarce and do not permit to place constraints on its palaeogeographic reconstructions for times prior to ca. 600 Ma. However, the available database suggests that the craton stayed at intermediate southern latitudes for the whole time span between 600 and 500 Ma.