Cambrian–Ordovician conodonts from slump deposits of the Argentine Precordillera: new insights into its passive margin development

Abstract

The Los Sombreros Formation represents the western continental margin slope deposits of the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont samplings along with sedimentological and structural analysis carried out in the Los Sombreros Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone samples from this succession include conodonts referable to the pandemicHirsutodontus simplexSubzone of theCordylodus intermediusZone (upper Furongian, Cambrian) and from theMacerodus dianaeZone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera.

New conodont records of the Los Sombreros Formation (Cambrian–Ordovician) from the Western Precordillera, Argentina: biostratigraphic and palaeoenvironmental implications

“…Considering the complex structure of the Los Sombreros Formation and the scarcity of biostratigraphic data, the age of the olistostromal unit is controversial, having been attributed to the Ordovician (e.g. Benedetto & Vaccari, 1992;Banchig & Bordonaro, 1994;Keller, 1999;Voldman et al 2009;Voldman et al 2016) or Devonian systems (e.g. Peralta, 2005;Peralta & Heredia, 2005).…”

“…However, this was based on a single sample without accurate stratigraphic location. Recently, Voldman et al (2016) reported species of the genus Cordylodus from the basal part of the Los Sombreros Formation cropping out in the eastern flank of the La Invernada Range. At regional scale, the C. proavus zone was identified in Gondwanan basins of NW Argentina, namely, in the Lampazar (Cajas Range of the Cordillera Oriental of Jujuy (Rao, 1999; later revised by Albanesi et al 2015)), El Moreno (Moya et al 2003) and Alfacito (Zeballo et al 2008) localities of the Cordillera Oriental, and in the Volcancito Formation of the Famatina Range, La Rioja Province (Albanesi et al 2005).…”

“…In the Eastern Precordillera of San Juan, Sarmiento et al (1995) and Mestre & Heredia (2013) recognized the L. variabilis Zone in the highest part of the San Juan Formation, exposed at the Don Braulio creek, on the western flank of the Villicum Range. Also, Sarmiento et al (1988) recovered an association of characteristic species from the upper part of the L. variabilis Zone in the lower strata of the Rinconada Formation in the Eastern Precordillera of San Juan Province, whose genesis is partly related to that of the Los Sombreros Formation with the intervening Ocloyic Orogeny (Voldman et al 2009(Voldman et al , 2016.…”

“…report, 1984), the Invernada Range and the Yerba Loca Range, with the northernmost outcrops being located in the Ancaucha River area (eastern flank of the latter range; Albanesi et al 1995). At regional scale, the thickness of the formation is estimated between 500 and 600 m. Voldman et al (2016) interpret that most of the facies in the formation are indicative of sedimentation by gravity flows (e.g. Lowe, 1982;Mutti, 1999).…”

“…They form breccias, conglomerates and olistostromal intervals, with the blocks dispersed chaotically or arranged in stratified intervals. According to their palaeontological content, these exotic blocks were sourced from lower Cambrian to Lower Ordovician formations building the shallow-water platform located to the east (Cuerda et al 1983;Banchig et al 1990;Bordonaro, 2003;Alonso et al 2008;Voldman et al 2016). The formation crops out discontinuously along a N-S belt of c. 200 km in the Western Precordillera (Benedetto & Vaccari, 1992;Albanesi et al 1995).…”