The controversial stratigraphy of the glacial deposits in the Tandilia System, Argentina.

PAZOS, PABLO; RAPALINI, AUGUSTO E.

Geological society Memoir No.36 "The Geological Record of Neoproterozoic Glaciations"

The Geological Society

Lugar: Bath; Año: 2011; p. 565 - 570

Chapter Summary Current ideas in the geological community emphasize the importance of differentiating diamictites from tillites in order not to contaminate description with interpretation when poorly sorted debris are analysed. (e.g. Eyles & Jannuczack, 2004). In the Tandilia System, central Argentina, 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: diamictites at the base and to the top and pelites and heterolithic levels with undulatory stratification and ripples at the middle. Out -sized clasts previously interpreted as dropstones are present in this interval. The lower diamictite is composed of polimictic clasts, inmersed in a matrix containing abundant kaolinite. The top one exhibits faintly normal grading and is also polimictic. Recently, Pazos et al. (2008) analysed the interval in a stratigraphical and regional context remarking the lack of a precise age for the unit and reviewing recent sedimentological information. The glacial related origin of this succession is indisputable, but a rigorous discussion about the stratigraphic position and correlation with any of the world wide well constrained global glacial events or even its relations with other Precambrian units in the Tandilia System remain speculative.  The possible correlation of the Sierra del Volcán Formation with diamictite levels of the Punta Mogotes Formation, defined from borehole information, is analysed and considered feasible. Hypothetical correlation of these with a thin diamictite level, of undetermined origin, at the base of the Ediacaran Cerro Largo Formation (Poiré & Spalletti, 2005, Gomez Peral et al., 2007) is more uncertain. However, they () may be contemporareous with other Ediacaran glacial records of the Rìo de la Plata craton. Paleomagnetic data obtained so far for the Rio de la Plata craton is still scarce and do not permit to place constraints on its paleogeogrpahic reconstructions for times prior to ca. 600 Ma. However, the avaliable database suggests that the craton stayed at intermediate southern latitudes for the whole time span between 600 and 500 Ma.