LOWER ORDOVICIAN CARBONATE PLATFORM (LA SILLA FORMATION). EASTERN PRECORDILLERA, ARGENTINA.

RAVIOLO, MARIANA M.; PRATT, BRIAN R.; BORDONARO, OSVALDO L.

Québec, Canada.

Congreso; Annual Meeting GAC-MAC-SEG-SGA; 2008

The Early Ordovician La Silla Formation is exposed over some 250 km along the length of the eastern Precordillera in the province of San Juan, Argentina. It is a remarkably uniform stratigraphic unit up to about 350 m thick, overwhelmingly dominated by peloidal grainstone and subordinate ooidal grainstone, with a minor contributions from bioclasts, lime mud, intraclasts, microbial laminites, stromatolites and thrombolites. Dolomite and chert are present; terrigenous material is absent. Our new regional appreciation gives a tripartite subdivision, from base to top the Río del Agua, Río Blanco and Río Salado members, reflecting the relative abundance of dolomite which is most common in the middle member. The array of carbonate rock types can be grouped into eight recurring lithofacies representing sediments that formed solely in a shallow, subtidal marine environment, with no evidence of restricted conditions, hypersalinity or prolonged subaerial exposure. The dominant facies is grainstone composed of peloids and lumps in homogenous, medium to thick, tabular to gently undulating beds. This suggests a shallow platform covered by banks and sand flats forming a low-relief shoal system and influenced by practically constant, gentle wave action. Microbial laminites probably accreted in temporarily protected lagoons. Virtual absence of hummocky cross-stratification and large scours indicates that the platform was affected only by the weakest of storms. Relatively little lime mud was produced. Dolomitic horizons with oscillatory and unidirectional ripple cross-lamination and intraclastic lags overlying scoured surfaces are present sporadically in the Río Blanco Member. These are interpreted to indicate times when shoal-water sedimentation was punctuated by occasional tsunamis whose effects were not erased by the return to background wave-agitated conditions. The concentration of these horizons in the middle member probably records a phase of rift-related tectonic activity in the ocean basin beyond the platform margin to the present-day west and southwest. Superficially, these rocks are similar to other Early Ordovician tropical carbonates, especially from Laurentia. However, La Silla Formation appears to be unique among lower Paleozoic platforms in terms of its remarkably uniform peloidal grainstone composition over a very broad area and through a considerable stratigraphic thickness, and absence of both very high-energy storm-event beds and low-energy tidal flat deposits.