Biostratigraphy of the Santa Rosita Formation (Furongian-Lower Ordovician), Cordillera Oriental of Jujuy, Argentina

ZEBALLO, F.J.; ALBANESI, G.L.; ORTEGA, G.

Calgary

Congreso; 3th Internacional Conodont Symposium; 2009

The Santa Rosita Formation crops out in the eastern margin ofthe Cordillera Oriental at the Quebrada de Humahuaca of JujuyProvince, northwestern Argentina, where it is composed of theTilcara, Casa Colourada, Pico de Halcón, Alfarcito, Rupasca,and Humacha members. The formation overlies the middle-upperCambrian Mesón Group and underlies upper Cretaceous-lowerPaleocene strata of the Salta Group or Neogene-Quaternary sediments.The Santa Rosita Formation in this area is a heterolithicsiliciclastic sequence with several transgressive-regressive cyclesof open-marine environments and two fluvio-estuarine depositsincised into underlying rocks. This formation that belongs to thelower Santa Victoria Group is, on average, 900 m thick.The conodonts were recovered from coquinas and calcarenitesinterbedded in the Alfarcito, Rupasca and Humacha members.173 kg processed carbonate samples yielded ca. 9300 conodontelements. The Cordylodus intermedius (Hirsutodontus simplexSubzone), C. lindstromi sensu lato, C. angulatus and Paltodusdeltifer conodont zones were identified. The Tilcara, CasaColourada and Pico de Halcón members are barren of conodonts,despite other sections of the Cordillera Oriental with equivalentunits (e.g., Angosto del Moreno) records the Cordylodus proavusZone. Parabolina frequens argentina (Kayser) is the most frequenttrilobite in anoxic facies of the Casa Colourada Member.The Cordylodus intermedius Zone is represented by the homonymoustaxon together with the guide conodonts Hirsutodontussimplex (Druce and Jones), Albiconus postcostatus Miller, aswell as species of Variabiloconus, Teridontus and Cordylodus.A high amount of proto and paraconodonts, such as Phakelodus,Problematoconites, Prosagittodontus and Westergaardodinawas recovered from these facies as it occurs in other Cambrianbasins with deeper environments. This assemblage is recordedin the lowermost part of the Alfarcito Member associated withthe trilobite Jujuyaspis keideli keideli Kobayashi. This specieswas largely regarded as a marker of the Cambrian-Ordovicianboundary, but the associated conodont fauna indicates a lateCambrian age (Furongian, Stage 10) for its lower record, althoughit is clear that this taxon continues into the Lower Ordovician(lower Tremadocian). C. aff. lindstromi is present in the C. lindstromisensu lato Zone with Teridontus gallicus Serpagli et al.,Variabiloconus datsonensis (Druce and Jones) and Utahconus n.sp. Iapetognathus fluctivagus Nicoll et al. was not recovered in thestudied succession so far, being the Cambrian-Ordovician boundarytentatively traced at the lower-middle part of the AlfarcitoMember, taking into account the most proximate conodont associations.The Cordylodus angulatus Zone is well representedin the upper part of Alfarcito Member including C. angulatusPander, and species of Acanthodus, Drepanodus, Drepanoistodus,Kallidontus, Rossodus, Semiacontiodus, Teridontus, Utahconusand Variabiloconus. Graptolites, such as Rhabdinopora flabelliformisspp. and Adelograptus sp., were found in different sectionsof the middle-upper part of the Alfarcito Member, together withtrilobites of the Kainella sensu lato Zone. The Paltodus deltiferZone partly correlates with the Low Diversity Interval of NorthAmerica; however, an interval with low diversity and high populationas it occurs in Laurentia has still not been identified inthis biozone. Paltodus deltifer pristinus (Viira) characterizes theeponymous subzone in the Rupasca Member, accompanied byspecies of Drepanodus, Drepanoistodus, Kallidontus, Teridontus,Utahconus, Variabiloconus, and Phakelodus. The correspondingtrilobite zone for this member is the Bienvillia tetragonalis Zonealthough its upper part remains not well defined. In the HumachaMember and the Coquena Formation, the uppermost part of thePaltodus deltifer Zone is identified including the conodont Acodusn. sp. and species of Drepanodus, Drepanoistodus, Iapetognathus,Kallidontus, Paltodus, Parapanderodus, Protopanderodus,Tropodus, Utahconus, and Variabiloconus. The Acodus deltatus-Paroistodus proteus Zone is recorded in the Parcha-Incamayoarea, southern Cordillera Oriental; however, the absence of theguide taxon Paroistodus proteus (Lindström) constrains the conodontfauna of the Humacha section to the latter lower biozone.The graptolites “Adelograptus” cf. altus Williams and Stevens,Aorograptus victoriae (Hall) and Ancoragraptus cf. bulmani(Spjeldnaes) are present in this lithological unit, representing theAorograptus victoriae Zone. In the lower Humacha Member andthe upper part of the Coquena Formation, Notopeltis orthometopa(Harrington) is a frequent trilobite that gives name to the zone.The composition of the conodont fauna reveals a particularmixture of Baltic and Laurentian taxa. The association of thesetaxa with endemic species characterizes the South GondwanaProvince of the Cold Domain in the Shallow-Sea Realm. TheColour Alteration Index (CAI) of the conodont elements is about 3, indicating overburden paleotemperatures of 110º-200ºC. Thisvalue remains the same throughout the formation, except for theuppermost Humacha Member, where the conodonts exhibit a CAI2, possibly due to its upper position in the column of the SantaRosita Formation.This contribution presents information of a Ph.D. Thesisunder development by the senior author, which is supported by theANPCyT, CONICET, and the Universidad Nacional de Córdoba,Argentina.