Early Ordovician trilobites from the Iruya area (Cordillera Oriental, northwestern Argentina) and their stratigraphic significance

Abstract

The middle part of the Santa Rosita Formation (Tremadocian) is well exposed in the Iruya area, northwestern Argentina. At the Pantipampa and Rodeo Colorado localities, it is composed of shale and sandstone representing a wave-dominated shelf with influence of storm activity. Twenty-two trilobite species are described from these localities. Low-diversity assemblages from the lower part of the succession (Kainella meridionalis Kobayashi, Asaphellus catamarcensis Kobayashi, Leptoplastides marianus [Hoek]) are diagnostic of the early Tremadocian Kainella meridionalis Zone. Trilobites from the middle part of the sequence are much more diverse. Kainella teiichii Vaccari and Waisfeld, Gymnagnostus kobayashii n. sp., Conophrys sp. indet., Asaphellus clarksoni n. sp., A. stenorhachis (Harrington), A. isabelae Meroi Arcerito, Waisfeld and Balseiro, Ogygiocaris? iruyensis n. sp., Niobe (Niobella) inflecta (Harrington and Leanza) n. comb., Metayuepingia riccardii n. sp., Pseudokainella keideli Harrington, Apatokephalus rugosus n. sp., Onychopyge acenolazai n. sp., O. gonzalezae n. sp., Nileus cingolanii n. sp., N. erici n. sp., Leptoplastides marianus, Parabolinella sp. indet., Hapalopleura sp. indet., and Ceratopygidae gen. et sp. indet., occur at different levels of the Kainella teiichii Zone. This biostratigraphic unit includes the oldest records of Nileus Dalman and Ogygiocaris? Angelin; Metayuepingia Liu, Niobe (Niobella) Reed, and Onychopyge Harrington are here first reported from the Tremadocian of southwest Gondwana. Finally, the uppermost part of the succession is characterized by the absence of the genus Kainella Walcott and the occurrence of Bienvillia tetragonalis (Harrington), Asaphellus stenorhachis, Pseudokainella keideli and Leptoplastides sp. indet., which are indicative of the middle Tremadocian Bienvillia tetragonalis Zone. The trilobites described in this paper provide a basis for the refinement of correlations with other Lower Ordovician sections of the Cordillera Oriental. The genera recognized have their closest affinities with faunas from Scandinavia, Great Britain, and China.

Early Ordovician conodonts from the Santa Rosita Formation at Pantipampa, Iruya area, northwestern Argentina

“…Conodonts from Pantipampa come from two levels that are assignable to the uppermost part of the trilobite Kainella meridionalis Zone sensu and the lowest part of the K. teiichii Zone Vaccari et al (2010), respectively (Tortello and Esteban, 2016). In the former level, the trilobite assemblage is composed of Kainella meridionalis Kobayashi, 1935, Leptoplastides marianus (Hoek in Steinmann andHoek, 1912) (=Andesaspis argentinensis Kobayashi, 1935) and Asaphellus catamarcensis Kobayashi, 1935, whereas the upper level contains the index species K. teiichii in association with diverse olenids, asaphids, ceratopygids, richardsonellids, nileids, shumardids, hapalopleurids and agnostoids (Tortello and Esteban, 2016). Other occurrences of conodonts of the C. angulatus Zone associated with Kainella, Leptoplastides marianus and Asaphellus catamarcensis were previously reported from the Incamayo area (Rao and Tortello, 1998;Tortello and Rao, 2000;Albanesi et al, 2008).…”

“…In addition, Tortello and Esteban (2016) recently described a Tremadocian succession from the Pantipampa locality (Iruya area, Salta Province) FIG. 1.…”

“…Location map (A and B) and geologic framework of the Iruya area, northwestern Argentina (C), with the location (asterisk) of the Pantipampa locality (after Figueroa Caprini, 1955;Vilela, 1960;Turner, 1964;Astini, 2003). The Santa Rosita Formation is in ligth gray (Modified from Tortello and Esteban, 2016).…”

“…2. Stratigraphic section of the middle part of the Santa Rosita Formation at Pantipampa (Iruya area, northwestern Argentina) displaying facies succession and provenance of conodont samples (6' and 6*; based on Tortello and Esteban, 2016). Vilela, 1960;Turner and Mon, 1979).…”

“…2 and 3). The Pantipampa section is interpreted as having formed in a wave-dominated shelf with influence of storm activity (Tortello and Esteban, 2016;Esteban et al, 2017). Three sedimentary facies are recognized throughout the succession.…”