Carboniferous postglacial brachiopod faunas in the southwestern Gondwana margin

CISTERNA, G.A.; STERREN, A.F.

Nanjing

Congreso; 7 th International Brachiopod Congress; 2015

The glaciomarine sediments related to the Late Paleozoic Ice Age (LPIA), have an excellent stratigraphic record in Argentina, particular those associated to the Late Carboniferous glacial episode identified along the southwestern South American basins (López Gamundí, 1997). The deglaciation processes connected to this event produced an important sea level rise and consequently an extended postglacial transgression from Bolivia (Tarija basin) to Argentina (Precordillera and Patagonia).The aim of this contribution is mainly a biostratigraphy update of the carboniferous brachiopod faunas that occur in the earliest postglacial interval (late Serpukhovian-Bashkirian) in Precordillera and its regional correlation with those equivalent in the nearby basins (Fig. 1A). Two faunal assemblages with significant taxonomic, taphonomic and paleoecological differences, appear associated to the earliest postglacial transgression in the Carboniferous diamictite-bearing sequences of the Calingasta-Uspallata basin in Precordillera (Fig. 1B): the Levipustula fauna, widely studied because its biostratigraphic, paleocologic and paleobiogeographic implications (Cisterna and Sterren, 2010), and a less known assemblage from the El Paso Formation. The Levipustula fauna is composed by bryozoans, bivalves and brachiopods, with less abundant gastropods and crinoids. Brachiopods that integrate this fauna are dominated by spiriferids (Costuloplica leoncitensis, Kitakamithyris booralensis, Kitakamithyris immensa, Kitakamithyris sp., Torynifer tigrensis, Spiriferellina octoplicata, Septosyringothryis keideli), accompanied by the ubiquitous productid Levipustula levis and the terebratulid Beecheria sp. This fauna has been particularly studied from the postglacial shales of the Hoyada Verde Formation in the core of Barreal Hill, where it occurs either as shell beds of 1-5 cm thick or as nests. Brachiopod communities herein recognized would have been developed in an open shelf with moderate bottom currents; paleocological variations identified appear to be controlled by substrate types and food supply fluctuations during the postglacial transgression. The fauna described from the glaciomarine succession of the El Paso Formation occurs a few kilometers from the Hoyada Verde Formation outcrops, in the southernmost part of the Barreal Hill. The fossil assemblage appears very scattered in thick mudstone packages with calcareous concretions to the upper part of this unit. The brachiopod faunas recognized, that integrate an assemblage composed of bivalves, gastropods, conularids, nautiloids and corals, are dominated by productids (Aseptella aff. patriciae, Tuberculatella peregrina, Productellidae indet., Linoproductidae indet.), accompanied by chonetids (Micraphelia indianae, Chonetoidea indet.), orthids (Rhipidomella sp.), inarticulates (lingulids and Orbiculoidea sp.), terebratulids (Beecheria patagonica) and athyrids indet. Analyzed under of the sequence stratigraphy viewpoint, these invertebrate faunas would be housed in the glacial submarine retreat (transgressive) facies association (López Gamundí, 1997) and it consists of open marine, fine-grained sediments with or without ice-rafted debris in mudstones and claystones. This study suggests that the El Paso Formation can be considered equivalent to the Hoyada Verde Formation (López Gamundí and Martínez, 2003). However, compositional and paleoecological features of their faunal assemblages are different. The brachiopod association Aseptella-Tuberculatella/Rhipidomella-Micraphelia from the El Paso Formation exhibits a particular biostratigraphic correlation value: Aseptella-Tuberculatella has been also identified in the Calingasta-Uspallata basin from the glacial interval of the Ciénaga Larga del Tontal Formation, and Rhipidomella-Micraphelia has been recognized in the earliest postglacial transgression of the Agua del Jagüel Formation. The age of Aseptella-Tuberculatella/Rhipidomella-Micraphelia, has been recently defined as late Serpukhovian-Bashkirian by the palynological data associated, i.e. Raistrickia densa-Convolutispora muriornata (DM) Palynozone (Cisterna et al., 2013; Vergel et al., in press). In the Tarija Basin from Sub Andean Bolivian area, the brachiopod Levipustula levis Maxwell was described from the postglacial sequences of Taiguatí Formation (Rocha-Campos et al., 1977); the Bolivian species has been considered comparable with western Argentinian specimens and the Australian type material of Levipustula levis (Taboada and Shi, 2011). In Patagonia the two faunal assemblages related to the carboniferous glacial episode integrate the Lanipustula biozone (late Serpukhovian-Bashkirian?, Pagani and Taboada, 2010) and the Tuberculatella biozone assigned to Kasimovian-early Asselian interval (Taboada, 2008), mainly recognized in the glaciomarine intervals with diamictite and mudstones of the Pampa de Tepuel Formation (Tepuel-Genoa Basin). Considering the database available (Pagani and Taboada, 2010), the Lanipustula fauna would be particularly dominated by bivalves and bryozoans, accompanied by gastropods, echinoderms, ostracods and cnidarians with scarce cephalopods and trilobites. Brachiopods are mainly represented by the key species Lanipustula patagoniensis and Lanipustula kletsi, although a diversified spiriferid fauna has been also cited. The brachiopod assemblage of the Tuberculatella biozone, mainly composed by bivalves, brachiopods, gastropods and cephalopods, is characterized by Tuberculatella? laevicaudata, Amosia sueroi and Verchojania archboldi. The species Aseptella patriciae and Beecheria patagonica have been also included in this biozone (Simanauskas and Sabattini, 1997) but their stratigraphic location is not clear.In conclusion, two postglacial faunas associated to the diamictite bearing sequences of the late Serpukhovian-Bashkirian interval are recognized: the widely extended Levipustula fauna identified in the Argentine Precordillera and Bolivia, and the Aseptella-Tuberculatella/Rhipidomella-Micraphelia association from Precordillera. Strong faunal contrasts between western Argentina and Patagonia have been proposed because of the paleogeographical position of Patagonia, considered to be a terrane accreted to Gondwana in the Late Paleozoic (Ramos, 2008). However, the Lanipustula fauna dominated by bryozoans and bivalves with diversified spiriferid brachiopods appears to be comparable with the Levipustula fauna from western Argentina. Also, the brachiopod assemblage from Tuberculatella biozone would exhibit some common elements (Tuberculatella, Aseptella, Beecheria) with the Aseptella-Tuberculatella/Rhipidomella-Micraphelia association, hence it could have also an interbasinal correlation value. The significant compositional differences observed between this assemblage and the relatively equivalent Levipustula fauna, should be explained by paleoecological controls. The present review offers an appropriated scenario to future statistics paleoecological analyzes, with the purpose of to understand the evolution of the Carboniferous postglacial faunas in Gondwana.