Microbrachiopods from the Las Aguaditas and Las Chacritas Formations (Middle Ordovician), Precordillera terrane of western Argentina: a preliminary taxonomic analysis

FERNANDO LAVIÉ; FERNANDA SERRA; NICOLÁS A. FELTES

Congreso; 8th INTERNATIONAL BRACHIOPOD CONGRESS; 2018

Linguliform brachiopods from Ordovician strata of theArgentine Precordillera have received little attention in past years.Recently, however, there has been growing interest in the knowledgeof their taxonomy and stratigraphic ranges. Benedetto (2015)described a Middle Ordovician lingulate brachiopod assemblagefrom the black shales and marls of the Los Azules Formation.Lavié and Benedetto (2016 a, b) described the linguliforms andcraniiforms from the Darriwilian Athiella argentina brachiopodZone of the San Juan Formation, and Lavié et al. (2017) listedtwo assemblages of microbrachiopods from the top of the SanJuan Formation at Cerro La Chilca. On the other hand, Holmeret al. (2016) described a fauna composed of lingulids, siphonotretids,and acrotretids from the San Rafael Block (Lindero andPonón Trehué Formations). Here we describe for the first timeassemblages of lingulate microbrachiopods recovered from theLas Aguaditas and Las Chacritas Formations cropping out in theJáchal area of the San Juan Province.The Las Aguaditas Formation crops out on the eastern flankof the Los Blanquitos Range, in the Central Precordillera whereit rests conformably on the San Juan Formation limestones. Thisunit includes shales, marls, limestones, and reef limestones depositedin varied deep-water environments (Astini, 1995). It consistsof a 43 m-thick lower member characterized by mixed calcareouspeliticdeposits of early to middle Darriwilian age (Feltes et al.2016), reflecting a deepening of the platform caused by a floodingevent (Keller et al., 1993; Carrera and Astini, 1998). Recently,Feltes (2017) recognized in the Las Aguaditas Formation a 138m-thick middle member and a120 m-thick upper member ofSandbian age. In the northern part of the La Trampa Range atthe Las Chacritas River section, the middle Darriwilian LasChacritas Formation is paraconformably overlain by the lowerSandbian Las Aguaditas Formation (Carrera and Astini, 1998).The Las Chacritas Formation is a 60 m-thick succession madeup of fine-grained siliciclastic and carbonate sediments depositedon a continental shelf setting. According to Astini (1995), it wasdeposited on structural highs within the basin, which served asa reservoir of carbonate remnants. The overlying Las AguaditasFormation represents further drowning and cessation of carbonatedeposition on the platform. From a biostratigraphic viewpoint,the Yangtzeplacognathus crassus Zone was recognized in thebase of the Las Aguaditas and the Las Chacritas formations intheir respective type sections, the Eoplacognathus pseudoplanusZone in the upper part of the lower member of the Las AguaditasFormation (Las Aguaditas Creek section) and in the middle partof the Las Chacritas Formation (Las Chacritas river section), theEoplacognathus suecicus Zone in the upper part of the latter unit,and the Pygodus anserinus and Nemagraptus gracilis zones inthe upper part of the Las Aguaditas Formation in both study areas(Serra et al., 2015; Feltes et al., 2016).This preliminary taxonomic study allows us to group thespecimens in two high-diversity associations of Darriwilian age;however, few valves were recovered in the upper member of theLas Aguaditas Formation (Sandbian age). The two Darriwilianassociations have five taxa in common, mainly dominated bya new species of the genus Conotreta Walcott, and include theobolid Elliptoglossa Cooper and the acrotretids Eocunulus Cooper,Numericoma Popov and Scaphelasma Cooper. Fragments ofOrbiculoidea D?Orbigny and valves of Opsiconidion Ludvigsenappear only in the Las Chacritas Formation, and MyotretaGorjansky, Paterula Barrande and Spondylotreta Cooper areconfined to the Las Aguaditas Formation. Both associations areclosely comparable to the contemporaneous faunas from PononTrehué and Lindero Formations (Holmer et al., 2016), and alsoshow affinities with the Antelope Valley Limestone in Laurentiaas well as the assemblages from the Kunda and Azeri RegionalStages in Baltica. The specimens recovered from the SandbianLas Aguaditas Formation consist of rare valves assigned withdoubts to Lingulops Hall and Mendozotreta Holmer et al., whichdo not allow us to set out their biogeographic affinities.