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 Sierradel 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 iscomposed of three intervals: diamictite at the base and at the top and pelite andheterolithic levels with undulatory stratification and ripples in the middle. Out-sizedclasts previously interpreted as dropstones are present in this interval. The lowerdiamictite is composed of polimictic clasts, immerzed in a matrix containing abundantkaolinite. The top one exhibits faintly normal grading and is polimictic. A review of theanalyzed the interval in a stratigraphical and regional context remarking the lack of aprecise age for the unit and a stratigraphic context is discussed. Others possibleglacigenic intervals situated at the base of the Cerro Largo Formation and in the PuntaMogotes borehole (unit 4) are also analyzed. The glacigenic origin is discussed but isanalyzed, but a rigorous analysis of the stratigraphic position, age, bio andchemostratigraphy information is discussed for the three intervals. Precise correlationbetween units in the Tandilia System containing diamictites remains speculative. Themore likely stratigraphic position for the Sierra del Volcán glacigenic deposits in LateEdiacaran and a direct correlation with the Punta Mogotes borehole (unit 4), is analyzedhere and considered feasible. Hypothetical correlation of these with a thin diamictitelevel, of undetermined origin, at the base of the Ediacaran Cerro Largo Formation is moreuncertain as well the glacigenic origin of the last one. However, the glacigenic intervalscan be contemporaneous with other Ediacaran glacial deposits of the Río de la Platacraton, 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 notpermit to place constraints on its palaeogeographic reconstructions for times prior to ca.600 Ma. However, the available database suggests that the craton stayed at intermediatesouthern latitudes for the whole time span between 600 and 500 Ma.