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 FERNÁNDEZ, L.R.; FESTA, A.

Madrid

Congreso; GONDWANA 15; 2014

The Lower Paleozoic slope facies in the Argentine Precordillera, San Juan Province, is well preserved 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 including pillow lavas and mafic-ultramafic bodies to the west. It contains blocks as large as hectometre-scale of Lower Cambrian to Lower Ordovician limestones, as well as arkosic sandstones and conglomerates with rounded basement-derived clasts (metamorphic and igneous). Extensional structures related to submarine sliding by gravitational collapse give rise to block-in-matrix formations in isolated places. The variable degree of deformation, the scarcity of the faunas and discontinuity of outcrop impede the temporal depositional constraints. Upper Cambrian, Tremadocian, Floian and Darriwilian conodont faunas were recovered at Ancaucha, del Telégrafo Creek, Los Ratones Creek, Ojo de Agua Creek, El Salto Creek and Los Túneles del Río Jáchal. In order to constrain the age of olistostrome formation, we collected 7 conodont samples (each from 9 kg of digested rock) from a tributary creek close to Río San Juan, between Pachaco and Los Ratones. A carbonate-cemented sandstone provided 10 poorly preserved conodonts with CAI 6 (~360?550ºC) and abundant fractures and recrystallization. The fauna consists of Tropodus sp. and Scolopodus sp., suggesting a Floian age. The other conodont samples were barren except for a lime-mudstone that yielded over 1100 conodont elements including a mixed species association with Ansella jemtlandica, Baltoniodus sp., Costiconus ethingtoni, Drepanodus arcuatus, Drepanoistodus bellburnensis, D. costatus, D. forceps, J. serpaglii, Microzarkodina sp., Paltodus? jemtlandicus, Parapaltodus simplicissimus, Parapanderodus nogamii, Paroistodus horridus, P. originalis, Periodon macrodentatus, Polonodus sp., Protopanderodus gradatus, P. rectus, Pseudooneotodus sp., Spinodus spinatus, Venoistodus venustus along with the index species Oepikodus evae (late Floian) and Eoplacognathus pseudoplanus and Histiodella kristinae of late middle Darriwilian age. These specimens exhibit a CAI 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 may reflect variable taphonomic/thermal histories. This phenomenon was previously observed in autochthonous and allochthonous conodont elements from the Los Sombreros Formation at Los Túneles del Río Jáchal, where Darriwilian specimens have CAI 3 and Tremadocian elements CAI 5. The occurrence of reworked conodont elements with high CAI values from rocks associated with younger conodonts preserved with low thermal alteration in Los Sombreros Formation reflects an Ordovician metamorphic event, which would be consistent with collision of the Cuyania Terrane with the proto-Andean margin of Gondwana during the Early?Middle Ordovician.