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

MANGO, M.J. ; ORTEGA, G; ALBANESI G.L.

GEOLOGICAL JOURNAL (CHICHESTER)

JOHN WILEY & SONS LTD

Lugar: Londres; Año: 2019

0072-1050

The distribution of conodonts and graptolites from the upper 13 meters of the San JuanFormation and the first meter of the lower member of the Los Azules Formation, LosGatos and Amarilla creeks, Cerro Viejo of Huaco, Central Precordillera from San Juan,Argentina, is studied. The conodonts from the top stratum of the San Juan Formation atthe Los Gatos creek section correspond to the Lenodus variabilis Zone, whereas at theAmarilla creek section they represent the Yangtzeplacognathus crassus Zone, verifyingthe diachronous top surface of the San Juan Formation. In the lower member of the LosAzules Formation, 81 conodont elements are recorded from bedding plane surfaces,where the presence of Paroistodus horridus horridus (Barnes & Poplawski) allows torecognize the Yangtzeplacognathus crassus Zone. On the basis of the graptolite andconodont association, we estimate that the top of the San Juan Formation at Los Gatoscreek corresponds to the Levisograptus dentatus Zone, Levisograptus dentatus Subzone,whereas at the Amarilla creek the Arienigraptus angulatus Subzone of the same zone isrepresented. The presence of the graptolite Arienigraptus angulatus (Mu) in the lowermember of the Los Azules Formation in both sections indicates the Levisograptusdentatus Zone (Arienigraptus angulatus Subzone). The records of graptolites inlimestones of the San Juan Formation, and of abundant conodonts on bedding planesurfaces of the lower member of the Los Azules Formation are important, taking intoaccount the type of facies where they occur. The conodont records in the referred typeof preservation are scarce in the Ordovician, which allows for a more precisebiostratigraphic correlation between conodont and graptolite biostratigraphy.