Middle Ordovician conodonts and graptolites at Los Cauquenes range, Central Precordillera of San Juan, Argentina

VOLDMAN G.G.; ORTEGA, G; ALBANESI G.L

Mendoza

Congreso; 3th International Conodont Symposium; 2013

Pander Society

THE Ordovician System is superbly represented in the Precordillera of western Argentina, at the Andean foothills. During the Middle Ordovician, an important paleogeographical rearrangement of depocenters and source areas took place associated with the demise of the Eopaleozoic carbonate platform of the Precordillera. This critical interval is recorded in the basin through widespread deposition of black shales over the fossiliferous limestones of the San Juan Formation, punctuated by local deposition of olistostromes, debris flows, conglomerates, and turbidites (e.g., Astini et al., 1995; Keller, 1999). Fossil dating of the lowest black shales and equivalent units indicates a diachronous deposition from north to south in the Precordillera basin (Hünicken, 1985; Albanesi et al., 1998). The occurrence of Ordovician clastic rocks in the Jáchal area is known since the study of Borrello and Gareca (1951), who recorded Nemagraptus gracilis (J. Hall) in the Ordovician upper shaly succession that is exposed in the western flank of the Cerro Viejo, a prominent mountain situated midway between Huaco and Jáchal city. Harrington (in Harrington and Leanza, 1957) distinguished two formations in the lower and upper parts of this succession, the Cerro Viejo Shales and Los Azules Shales, respectively. Furque (1979) referred all the Ordovician clastic sequence to the Los Azules Formation, equivalent to the Gualcamayo Formation elsewhere. Later, Ortega (1987) refined the biostratigraphy of the Los Azules Formation by determining a lower member, made up of dark claystones and siltstones interbedded with K-bentonites (lower Darriwilian), a middle member that consists of black shales and siltstones (upper Darriwilian), and an upper member of yellowish calcareous siltstones and calcarenites (upper Sandbian). A hiatus covering the highest Darriwilian and the lowest Sandbian is recorded between the middle and upper members. The Los Azules Formation is arranged in small isolated outcrops aligned in narrow elongated belts with meridian orientation. The best exposures of the Los Azules Formation are located at the foothill of the Cerro Viejo, between 30º 11? 40" and 30º 15? 30" S Latitude, and 68º 34?30" and 68º 35?20" W Longitude, uplifted by the Niquivil Thrust (e.g., Ortega et al., 2007). It also crops out along western thrusts in the less studied successions of the La Chilca, Mogotes Azules, and the Aguada de Los Azulejitos on the northernmost Talacasto Range. The purpose of this paper is to describe for the first time a restricted stratigraphic exposure that bear a significant conodont and graptolite fauna of the upper section of the San Juan Formation and the overlying the Los Azules Formation (Fig. 1), which is currently under study. The stratigraphic section is located at the Los Cauquenes Range, a parallel thrust belt to the Cerro Viejo de Huaco thrust. The latter thrust belt is 4 km to the east, where the Los Azules Creek, the type section of the homonymous formation, is located.