Bioestratigrafía y provincialismo de conodontes del tramo medio-superior de la Formación San Juan en el cerro Viejo de Huaco, Precordillera, Argentina

Abstract

The present study deals with the conodont biostratigraphy from the middle and upper parts of the San Juan Formation (Lower-Middle Ordovician) exposed at the Los Gatos creek section, west of the cerro Viejo de Huaco, Central Precordillera of San Juan Province. The numerous conodonts recovered, corresponding to 55 species, allow to recognize a series of biozones in the studied section. The biostratigraphic analysis carried out herein allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian. The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section. In general, the levels corresponding to the Baltoniodus triangularis-Tripodus laevis Zone contain more conodonts, than the records of other localities from Precordillera. These specimens are well preserved with a color alteration index (CAI) of 2-2.5, indicating overburden paleotemperatures ranging from 60° and 155° C. The large presence of fragmented conodonts, with recrystallized surfaces and crystal overgrowth, could indicate the effect of distinctive diagenetic fossil processes on the bearer limestone. The analysis of the diversity and abundance of genera and species of conodonts by chronostratigraphic intervals presents a great percentage of cosmopolitan taxa, 18% and 38% in common, at species level, only with the North-American Midcontinent Province (NAMP), and with a small percentage with the North-Atlantic Province (NAP) and the Precordillera, which allows to approximate a greater paleobiogeographic affinity of the conodonts recovered with those of the NAMP than those of the NAP for the entire interval studied. On the other hand, the Precordillera is verified as a province with its own characteristics as identified by several authors.

Conodont and graptolite biostratigraphy of the lower–middle Darriwilian (Middle Ordovician), Cerro Viejo of Huaco, Argentine Precordillera

“…The upper 13 m of the San Juan Formation at the Los Gatos bear graptolite remains identified as dendroid indet., ?dichograptid indet., glossograptid indet., ?Jiangshanites sp., Pseudophyllograptus sp., On the base of previous records of conodonts (Mango & Albanesi, 2018) in the upper part of the San Juan Formation at Los Gatos creek, the P. horridus Subzone of the L. variabilis Zone is recognized, which refers to the lower subzone of the L. dentatus Zone (L. dentatus Subzone of Maletz, 2011). The uppermost strata of the San Juan Formation at the Amarilla creek correspond to the Y. crassus Zone, which can be correlated with the A. angulatus Subzone of the L. dentatus Zone.…”

“…Recently, Mango and Albanesi () studied the conodonts of the upper section of the San Juan Formation in the Los Gatos creek, Cerro Viejo of Huaco, recognizing the Oepikodus evae , Oepikodus intermedius , Tripodus laevis , and L. variabilis zones, although the interspersing Baltoniodus navis and Microzarkodina parva zonal intervals appear barren of conodonts, probably due to facial or environmental bias. In this section, the authors record the conodonts Paroistodus horridus primus Albanesi and L. variabilis from the top of the San Juan Formation, recognizing the Paroistodus horridus Subzone of the L. variabilis Zone for these strata, revealing biostratigraphic differences with other creeks of the Cerro Viejo of Huaco studied in previous works, which proves the diachronous contact of the San Juan Formation with overlying units (Hünicken, ).…”

“…The upper strata include deposits accumulated below the storm wave‐base levels, in an external ramp environment (Sorrentino, Benedetto, & Carrera, ). At the Los Gatos creek section, Cerro Viejo of Huaco, the upper 13 m are made up of wackestones (Mango & Albanesi, ).…”

Ordovician (Floian-lower Darriwilian) conodont biofacies of the San Juan Formation in the Cerro Viejo of Huaco, Argentine Precordillera

“…In the present study, the conodonts recovered from the San Juan Formation present a color alteration index (CAI) varying from 2 to 2.5, which refers to burial paleotemperatures of 60-155°C (Epstein et al, 1977). These values correspond to the Niquivil tectonic thrust, which is the easternmost one of the Central Precordillera (Voldman et al, 2010), as part of an orogenic belt that continues in the Cerro Potrerillo exposure (Albanesi et al, 1998), the Cerro Viejo of Huaco (Ottone et al, 1999;Ortega et al, 2007;Mango and Albanesi, 2018), the Las Chacritas (Serra et al, 2015), and the Cerro La Chilca sections (Serra et al, in press).…”

“…The conodonts from the San Juan Formation exposed in the Cerro Viejo of Huaco are known after the biostratigraphic studies by Ortega (1987), Hünicken and Ortega (1987), Ottone et al (1999), Ortega et al (2007), Mestre et al (2013), and Mango and Albanesi (2018). The detailed conodont biostratigraphy of the San Juan Formation, including the conodont biofacies analysis for this unit, motivated this work after a reference biostratigraphic scheme for the Precordillera published recently (Albanesi and Ortega, 2016).…”