Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy) vol. 125(3): 651-668. November 2019 LINGULIFORM MICROBRACHIOPODS FROM LAS AGUADITAS AND LAS CHACRITAS FORMATIONS (MIDDLE-UPPER ORDOVICIAN) OF ARGENTINE PRECORDILLERA FERNANDO JULIÁN LAVIÉ, FERNANDA SERRA & NICOLÁS ALEXIS FELTES Centro de Investigaciones en Ciencias de la Tierra, Consejo Nacional de Investigaciones Científicas y Técnicas, and Universidad Nacional de Córdoba, Vélez Sarsfield 1611, X5016GCA Córdoba, Argentina. E-mail: [email protected]; [email protected]; [email protected]. To cite this article: Lavié F.J., Serra F. & Feltes N.A. (2019) - Linguliform microbrachiopods from Las Aguaditas and Las Chacritas Formations (Middle-Upper Ordovician) of Argentine Precordillera. Riv. It. Paleont. Strat., 125(3): 651-668. Keywords: Lingulida; Acrotretida; Cuyania; Darriwilian; Sandbian. Abstract. Middle-Upper Ordovician linguliform microbrachiopods are described and illustrated for the first time from the Las Aguaditas Formation at the Los Blanquitos and Mogotes Azules ranges and from the Las Chacritas Formation at the Las Chacritas River section, Central Argentine Precordillera. This systematic study includes nine families, namely Obolidae, Paterulidae, Discinidae, Acrotretidae, Scaphelasmatidae, Torynelasmatidae, Ephippelasma- tidae, Biernatidae and Eoconulidae. This diverse fauna is conformed of specimens corresponding to Lingulops sp., Paterula incognita Mergl, Schizotreta sp., Scaphelasma zharykensis Popov, Torynelasma? sp., Akmolina sp., Ephippelasma sp., Numericoma simplex Holmer, and Eoconulus sp. However, specimens from the order Acrotretida proved to be an impor- tant component of this fauna, enabling the recognition of two new species which are thoroughly described, Conotreta andina n. sp. and Biernatia rhapsody n. sp. INTRODUCTION linguliform and craniiform specimens from the top of San Juan Formation (lower–mid Darriwilian), The study of Ordovician linguliform bra- while Holmer et al. (2016) described two assem- chiopods from Argentina have received much at- blages of micromorphic linguliform brachiopods tention in the last years, most of the material comes from the Ponón Trehué Formation (Darrirwilian) from localities of the Argentine Precordillera, whe- and the Lindero Formation (upper Darriwilian-lo- re brachiopods are frequent fossils. In this terrene, wer Sandbian) in the San Rafael Block (Mendoza the Ordovician System is characterized by extensive Province). More recently, a low-diversity fauna was carbonate platform successions, which are overlain documented at the Las Plantas Formation (lower by siliciclastic deposits with intervening deep-water Sandbian) (Lavié 2018). calcareous sequences (Astini 2003). The aim of the present contribution is to de- Benedetto (2015) described an important scribe the microfauna of linguliform brachiopods Middle Ordovician brachiopod association from from the Darriwilian-Sandbian Las Aguaditas For- the Los Azules Formation, which includes minu- mation and the Darriwilian Las Chacritas Forma- te rhynchonelliforms, obolids, acrotretids and cra- tion in San Juan Province. This systematic study niids. Later, Lavié and Benedetto (2016) reported includes a detailed justification of the generic and specific attributions of the analysed material, pro- viding new data in their geographic and biostrati- Received: March 12, 2019; accepted: July 09, 2019 graphic distribution. 652 Lavié F.J., Serra F. & Feltes N.A. GEOLOGICAL SETTING The Precordillera geological province is lo- cated in western Argentina, between the Frontal Cordillera to the West and the Pampean Ranges to the East, encompassing parts of La Rioja, Mendoza and San Juan provinces (Stelzner 1873; Furque & Cuerda 1979; Baldis et al. 1982). It is characterized by a thick Cambrian – Ordovician succession (ca. 2,200 m) of marine limestones that interdigitate with clastic slope deposits toward the west (Baldis et al. 1982; Astini 1995). Spanning a wide range of depositional environments, from shallow intertidal to marginal shelf and deep ramp settings, the Precordillera is the only lower Palae- ozoic basin in South America having a distincti- ve carbonate platform (Cañas 1999). The Sassi- to, Las Chacritas and Las Aguaditas formations are the only remnants of carbonate deposition in the Precordillera succeeding the continuous Cam- brian – Ordovician carbonate cycle, and the Las Aguaditas Formation is the only unit showing a transitional interval from platform to slope depo- sits of Middle Ordovician age (Astini 1995). In the present contribution we focus on the description of linguliform brachiopods from Fig. 1 - Map showing the geological framework and location of the studied localities in the Argentinian Central Precordillera: (1) the Las Chacritas and the Las Aguaditas forma- Las Aguaditas Creek, (2) Cordón de Mogotes Azules and (3) tions from three localities in the area surrounding Las Chacritas River sections. Abbreviations: C-P: Carbonif- erous – Permian, Dev.: Devonian; Sil.: Silurian; Ordov.: Or- Jáchal City; namely, the Las Chacritas River sec- dovician; Fm: Formation. tion, the Las Aguaditas Creek and the Mogotes Azules sections (Fig. 1). The suited units are de- scribed below. Juan Formation and the overlying Las Aguaditas Formation is characterized by calcareous-pelitic Las Aguaditas Formation deposits, which are interpreted to have been cau- The Las Aguaditas Formation was formally sed by a flooding event on the platform (Keller et defined by Baldis et al. (1982) at its type locality at al. 1993; Carrera & Astini 1998). The Las Agua- the Los Blanquitos Range. However, in their ori- ditas Formation has been interpreted as being de- ginal stratigraphic description of the unit Baldis posited on structural highs (horsts) and lows (gra- and Blasco (1974) had previously recognized four ben) within the basin (Astini 1995). members; only the lower and middle members of The type section of the Las Aguaditas For- the Las Aguaditas Formation are studied herein. mation, 302 m thick, is located on the eastern From a lithological viewpoint, the formation con- flank of the Los Blanquitos Range in the Central sists of marls, limestones, and reef limestones, Precordillera of San Juan Province, 15 km to the with a characteristic yellowish weathering colour. southwest of Jáchal City (Fig. 1). This formation This unit paraconformably overlies the San Juan is also exposed at the MogotesAzules section, lo- Formation by mean of a regional hardground sur- cated at the northern end of the Mogotes Azules face (Astini 1995) which is easily distinguished range, 20 km southwest of Jáchal city (Fig. 1). In from the Las Aguaditas Formation by its nodular this locality, the unit has a thickness of 55 meters limestones (Baldis et al. 1982). The transitional and is mainly composed of marls and mudstones interval between the uppermost part of the San (Salas 2003). Linguliform microbrachiopods from Middle-Upper Ordovician of Argentine Precordillera 653 Fig. 2 - Stratigraphic columns showing species ranges of the Las Aguaditas Formation in the Las Aguaditas Creek and Mogotes Azules sections and of the Las Chacritas Formation in the Las Chacritas River section. Abbreviation: Darriwil.: Darriwilian. The samples used in this contribution, de- has been referred to as the Las Aguaditas Forma- picted in Figure 2 come from poorly exposed tion by Peralta et al. (1999). laminated mudstones with abundant skeletal The Las Chacritas Formation is a 60 m fragments interbedded with shales and siltstones thick succession characterized by fine-grained si- from the lower member of the Las Aguaditas liciclastic and carbonate sediments deposited on a Formation (43 m thick) (Feltes et al. 2016), and continental shelf setting (Carrera & Astini 1998). also from the upper member of this formation This unit is divided into two members, the lo- characterized by mostly dark grey mudstones wer member consists of tabular, thin to medium (Feltes et al. 2018). bedded fossiliferous strata, dark mudstones and wackestones, while the upper member consists of Las Chacritas Formation wackestones and packstones, suggesting a shal- The Las Chacritas River section is located lowing trend towards the top of the unit (Carrera in the northern part of the La Trampa Range, & Astini 1998; Peralta et al. 1999). According to 50-km south-west of Jáchal City (Fig. 1); where Astini (1995) the Las Chacritas Formation deve- important Middle–Upper Ordovician rocks are loped on structural highs (horsts) that served as exposed. At this locality, the upper Lower and a shallow platform for remnant carbonate depo- lower Middle Ordovician San Juan Formation is sition within the basin. The overlying Las Agua- paraconformably overlain by the Las Chacritas ditas Formation (approximately 10-m thick) is Formation, subject of study in the present con- composed of platy mudstones with a yellowish tribution (Fig. 2). The deeper water limestone alteration, interbedded with black shales (Peralta sequence overlying the Las Chacritas Formation & Baldis 1994). 654 Lavié F.J., Serra F. & Feltes N.A. COMMENTS ON BIOSTRATIGRAPHY Holmer & Popov 2000, 2007). The fossil record of Ordovician conodonts Order Lingulida Waagen, 1885 has been extensively documented in the Precordil- Superfamily Linguloidea Menke, 1828 lera of Argentina, providing a robust database for Family Obolidae King, 1846 a detailed biostratigraphic scheme (e.g. Heredia et Subfamily Elliptoglossinae Popov & Holmer, 1994 al. 2005; Mestre 2010; Albanesi