New records of Silurian graptolites from Cerro La Silla, Precordillera Argentina

TORO, B.A.; HALPERN, K.; CARRERA, M.G.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

CCT-CONICET, Mendoza

Several stratigraphic sections bearing Silurian graptolites crop out at the Central Precordillera Argentina. Their records restricted to a few levels from La Chilca and Los Espejos formations (Tucunuco Group), became a useful tool for dating this sequence as ranging through the Ordovician-Silurian boundary to Ludlovian age. This study deals with new records from the Cerro La Silla section, located nearby Jáchal city in San Juan Province. It was considered traditionally a key section to study Ordovician calcareous rocks, but recent fieldwork allowed noticing an overlaying siliciclastic unit composed by siltstones and fine sandstones reaching 10 m thick, which can be assigned to La Chilca Formation. Glyptograptus persculptus (Upper Ordovician) and Atavograptus atavus (lower Silurian) biozones were firstly recognized across the lower 130 m of the Cerro del Fuerte section. Later on, other assemblages including G. persculptus, Pseuclimacograptus robustus, Climacograptus angustus and C. cf. medius; Climacograptus acceptus, G. maderi? and Raphidograptus sp.; and Lagarograptus praeacinaces, Talacastograptus leanzai and Clinoclimacograptus sp. indicated the presence of the Glyptograptus persculptus, the probable Parakidograptus acuminatus, and the possible Atavograptus atavus biozones, respectively, in the lower 3.65 m of La Chilca Formation at Talacasto section. The studied deposits at Cerro La Silla bear a well preserved brachiopod assemblage and scarce graptolites, flattened or filled with pyrite preserving partial relief. Slender specimens (reaching 1 mm of maximum width), exhibiting everted apertural theca, gently ondulating median septum and longer tubaria than previously described at the Cerro del Fuerte section (up to 20 mm) could be compared with Pseudoclimacograptus (Clinoclimacograptus) cingolani. This endemic taxon, present in all studied levels, is associated with strong specimens of Diplograptus sp. in the lowermost fossiliferous strata, and together with C. innotatus brasiliensis, C. normalis and C. rectangularis in the uppermost association. The stratigraphic range of the last two mentioned species expand through the P. acuminatus and A. atavus biozones at Talacasto section, but their association with C. innotatus brasiliensis and P. (Clinoclimacograptus) cingolani, previously described from the early Llandoverian strata at the Cerro del Fuerte section, constrain the studied assemblage to the Atavograptus atavus Biozone. As a result a similar (Silurian) age could be suggested for the accompanying Heterorthella brachiopod assemblage developed a few meters above, as well as the correlation of the yielding graptolite deposits with the lower levels of La Chilca Formation exposed at Cerro La Chilca, Cerro del Fuerte and Talacasto sections, in which the Atavograptus atavus Biozone was previously recognized.