LA SILLA CARBONATE PLATFORM (EARLY ORDOVICIAN), PRECORDILLERA, WESTERN ARGENTINA: BORING IS THE NEW INTERESTING

La Silla Formation is part of the early Palaeozoic tropical carbonate platform succession that comprises the core of the allochthonous Cuyania terrane. Up to 360 m thick, the unit is exposed for 250 km along depositional strike and over a palinspastically unrestored distance across strike of about 35 km. It is an extraordinarily monotonous, pure limestone. Precipitation of microcrystalline calcite, probably influenced by microbial activity, led to copious quantities of peloids and aggregates of clotted micrite, locally with ooids, yet little appreciable lime mud. Scarce bioclasts are limited to abraded gastropods and trilobite sclerites. The overwhelmingly dominant facies is well-sorted peloidal grainstone in homogeneous, tabular, medium to thick beds. These shallow-marine sediments show no evidence of restricted conditions, hypersalinity, subaerial exposure, major storms, tidal currents, bathymetric changes, or eustatic sea-level fluctuations. The shallow platform is envisaged to have been covered by extensive peloidal sand flats and low-relief banks. The setting was probably microtidal and may not have been affected by strong trade winds. It was washed by frequent, relatively gentle wave action in a tranquil climatic region. In the middle member, however, sporadic, sharply downcut scour surfaces up to 20 cm deep are mantled by anomalous lenses of intraclastic rudstone overlain by wave-ripple cross-laminated dolomitic grainstone. Rather than tempestites, these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift-faulting seaward of the platform margin. The remarkably uniform peloidal grainstone composition over such a broad area shows that, given the appropriate combination of environmental and ecological factors, large portions of some early Palaeozoic platforms were dominated by grainy sediment and remained under well circulated conditions within fairweather wave-base, without distinct lateral facies differentiation, major reefal development, significant lime mud production, or tidal-flat aggradation. La Silla Formation is strikingly different than its stratigraphic neighbors, just as in other early Paleozoic passive margin successions, and the causes of their contrasting facies architecture have yet to be explained.