Carbonate platform dominated by peloidal sands: Lower Ordovician La Silla Formation of the eastern Precordillera, San Juan, Argentina

Abstract

The Lower Ordovician La Silla Formation of the Precordillera of west-central Argentina is part of the west-facing early Paleozoic carbonate platformal succession that comprises the core of the Cuyania terrane. Up to 350 m thick, it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of 35 km. La Silla Formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert. Copious precipitation of microcrystalline calcite on the seafloor, probably influenced by microbial activity to varying degrees, led to the generation of peloids, ooids and aggregates of these grains, as well as small amounts of lime mud, intraclasts, stromatolites and thrombolites. Rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites; bioturbation is locally present. The array of carbonate rock types is grouped into eight recurring lithofacies, in order of decreasing abundance: (1) peloidal grainstone; (2) laminated dolostone; (3) intraclastic rudstone; (4) microbial laminite; (5) peloidal packstone; (6) ooidal grainstone; (7) thrombolite boundstone; and (8) mudstone. These facies represent sediments that formed solely in a shallow subtidal marine environment, with no evidence of restricted conditions, hypersalinity or subaerial exposure. By far the dominant facies is grainstone composed of well-sorted, fine sand-sized peloids and peloidal aggregates in homogeneous, tabular to gently undulating, medium to thick beds; cross-lamination is scarce. Clusters of sub-metre-sized patch reefs developed sporadically. The shallow platform is envisaged to have been covered by extensive carbonate sand flats and low-relief banks, influenced by frequent, relatively gentle wave action but without experiencing powerful storms or strong tidal currents. The limited amount of lime mud that was generated collected in areally small, shallow depressions. Rare tabular interbeds of intraclastic rudstone record storms. Lenses of intraclastic rudstone and laminated dolostone occur in the middle member, as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep. 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 a broad area, and absence of both high-energy storm-event beds and low-energy tidal flat deposits, show that, given the appropriate combination of climatic, environmental and ecological factors, large portions of some early Paleozoic platforms remained under well agitated conditions within fairweather wave-base without distinct lateral facies differentiation.