New conodont records from the Los Sombreros Formation, an Ordovician mélange in the Argentine Precordillera

VOLDMAN G.G.; ALONSO, J.L.; ALBANESI G.L; BANCHIG A.L.; ORTEGA, G; RODRÍGUEZ FERNANDEZ, L.R.; FESTA, A.

Madrid

Simposio; Gondwana 15; 2014

Geological Society of Spain

The Lower Paleozoic slope facies in the Argentine Precordillera, San Juan Province, is wellpreserved in the Los Sombreros Formation, an olistostromic unit that inter-fingers with a Cambro-Ordovician carbonate platform to the east and has ocean-floor sedimentary rocks includingpillow lavas and mafic-ultramafic bodies to the west. It contains blocks as large as hectometrescaleof Lower Cambrian to Lower Ordovician limestones, as well as arkosic sandstones andconglomerates with rounded basement-derived clasts (metamorphic and igneous). Extensionalstructures related to submarine sliding by gravitational collapse give rise to block-in-matrixformations in isolated places. The variable degree of deformation, the scarcity of the faunasand discontinuity of outcrop impede the temporal depositional constraints. Upper Cambrian,Tremadocian, Floian and Darriwilian conodont faunas were recovered at Ancaucha, delTelégrafo Creek, Los Ratones Creek, Ojo de Agua Creek, El Salto Creek and Los Túneles delRío Jáchal. In order to constrain the age of olistostrome formation, we collected 7 conodontsamples (each from 9 kg of digested rock) from a tributary creek close to Río San Juan, betweenPachaco and Los Ratones. A carbonate-cemented sandstone provided 10 poorly preservedconodonts with CAI 6 (~360?550ºC) and abundant fractures and recrystallization. The faunaconsists of Tropodus sp. and Scolopodus sp., suggesting a Floian age. The other conodont sampleswere barren except for a lime-mudstone that yielded over 1100 conodont elements includinga mixed species association with Ansella jemtlandica, Baltoniodus sp., Costiconus ethingtoni, Drepanodusarcuatus, Drepanoistodus bellburnensis, D. costatus, D. forceps, J. serpaglii, Microzarkodina sp., Paltodus?jemtlandicus, Parapaltodus simplicissimus, Parapanderodus nogamii, Paroistodus horridus, P. originalis, Periodonmacrodentatus, Polonodus sp., Protopanderodus gradatus, P. rectus, Pseudooneotodus sp., Spinodus spinatus,Venoistodus venustus along with the index species Oepikodus evae (late Floian) and Eoplacognathuspseudoplanus and Histiodella kristinae of late middle Darriwilian age. These specimens exhibit aCAI 3-4 (~110?200ºC) and are frequently fractured and covered with mineral overgrowths.Since reworked conodont elements behave as heavy-mineral grains, different CAI values mayreflect variable taphonomic/thermal histories. This phenomenon was previously observed inautochthonous and allochthonous conodont elements from the Los Sombreros Formation at LosTúneles del Río Jáchal, where Darriwilian specimens have CAI 3 and Tremadocian elementsCAI 5. The occurrence of reworked conodont elements with high CAI values from rocksassociated with younger conodonts preserved with low thermal alteration in Los SombrerosFormation reflects an Ordovician metamorphic event, which would be consistent with collisionof the Cuyania Terrane with the proto-Andean margin of Gondwana during the Early?MiddleOrdovician.