Advances in the Ordovician stratigraphy of Peru: the Floian to ?Hirnantian succession from the Apurímac River valley, Cordillera Oriental

BERNÁRDEZ E.; CHACALTANA, C.; JUAN CARLOS GUTIERREZ MARCO; CARLOROSI JOSEFINA; S. LORENZO

Milano

Congreso; 13th International Congress on the Ordovician System; 2019

International Commission on Stratigraphy

In the northern part of the Central Andean Basin, Ordovician rocks crop out extensively in the CordilleraOriental of Peru and also occur sporadically in some parts of the Peruvian Altiplano. The Ordoviciansuccession in the Cordillera Oriental differs from those in the Altiplano, comprising three sedimentary units,namely (in ascending order) the San José Formation (massive organic-rich shales, up to 3500 m thick), theSandia Formation (quartzose sandstones with interbedded shales, up to 3000 m thick) and the San GabánFormation (glaciomarine diamictites with some sandstones, ca. 160 m thick). Graptolites are abundant inthe oldest formation but being restricted to a short interval in the Middle to basal Upper Ordovician (Maletzet al., 2010), and there is a single occurrence of upper Floian (Lower Ordovician) conodonts in the lowerpart. Biostratigraphic data from the other two formations are much more scarce, having been referred to theSandbian and the Hirnantian through Llandovery intervals, respectively. Field work recently conducted alongthe northeastern margin of the Apurímac River valley, northeast of the city of Ayacucho, led to the discovery ofnew sections in the area mapped by Monge et al. (1998), which were measured and sampled biostratigraphicallyin detail. The San José Formation lies unconformably on a Neoproterozoic metamorphic complex, reaching amaximum thickness of about 700 m. This formation has yielded basal Floian graptolites in its lower ?not mostbasal- part, and these are succeeded by a complete biostratigraphic record documenting the existence of upperFloian, Dapingian, Darriwilian and lower Sandbian strata, which include graptolites, trilobites, brachiopods,molluscs and ostracods, as well as some conodonts and other fossils. The upper part of the San José Formationis unconformably overlain by a thick sandy succession, previously assigned to the Sandia Formation, but hereas the newly defined Kimbiri Formation. It consits of 80 m of massive sandstones with a basal member ofglaciomarine diamictites measuring up to 90 m thick and locally resting on a basal quartzite. The diamictitesalso rarely occur interbedded in the main sandstone member and show some ressemblance to the San GabánFm, that has been reinterpreted as lower Silurian turbidites and gravity flows instead of autochthonous glacialdeposits contemporaneous to the end-Ordovician glaciation. Some fossiliferous pebbles in the Kimbiri Fmcome from deeply eroded areas of the San José Fm.