Juan L. Benedetto

The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A geodynamic model

Paleontologic and stratigraphic evidence points to the early Paleozoic Precordilleran terrane of western Argentina as being the conjugate rift pair of the Appalachians. Stratigraphic similarities of the Cambrian and early Arenig carbonate series and very strong affinities among trilobite, conodont, and brachiopod faunas show their close relationship. The most probable provenance areas are the Appalachian-Ouachita rifted margin and the Newfoundland Appalachians, although the former fits better with geometric and drifting paths suggested by faunal affinities. Increasing Celtic and Baltic brachiopod genera and divergent stratigraphy since the Arenig indicate the drifting of the Precordilleran terrane. Collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed. The collision of the Precordillera with western Gondwana occurred during the mid-Llanvirn to Llandeilo. A magmatic arc related to eastward subduction (present coordinates) was active in the Famatina Range east of the Precordillera. This region of Celtic affinity shows faunal exchange with the Precordilleran terrane since the late Arenig and may represent accreted intra-Iapetus volcanic island-arc complexes. The rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the Precordilleran terrane from periequatorial to peripolar latitudes between the Cambrian and latest Ordovician. The deep ocean to the west of the Precordillera started to close by the Late Ordovician with the eastward drift of the Chilenian terrane. Absence of volcanic or pyroclastic arc-derived rocks in the Precordillera indicate west-dipping subduction. As Chilenia approached the continental margin, a new forebulge was established on the former collided Precordilleran terrane, developing an erosional unconformity in central Precordillera (Talacasto-Tambolar arch). A Gondwanic signature was fully developed by the Middle Silurian when the Malvinokaffric Clarkeia Fauna flourished. Before then, the Late Ordovician glacial record and associated Hirnantia Fauna were the first clear tie to Gondwana. During the Silurian the marginal basin behaved as a foreland, with lithosphere rheology and eustasy governing the sequence stratigraphy. Wrench faulting along its eastern boundary displaced the Precordillera toward the south. Continued shortening during closure with the Chilenian terrane in the mid-Devonian produced thrust loading of the basin and generated a thick graywacke succession. Final accretion of Chilenia (Late Devonian) generated a regional angular unconformity between the lower and upper Paleozoic. New eastward subduction was initiated west of the accreted Chilenian terrane during the Late Permian–Triassic as indicated by the Choiyoi volcanic complex, which presently outcrops in the Frontal Cordillera.

The Allochthony of the Argentine Precordillera Ten Years Later (1993–2003): A New Paleobiogeographic Test of the Microcontinental Model

Abstract Recent and new faunal data from the Cambrian to Silurian rocks of the Precordillera, Famatina and Northwest Argentina basins are used to discriminate between different paleogeographic models, and especially to establish to what extent they are compatible with a previous conclusion that the Precordillera is a Laurentian-derived microcontinent. There is no paleontological evidence to support a para-autochthonous Gondwanan origin of the Precordillera. The strong differences in the Cambrian trilobite faunas and lithologic successions preclude a common origin of the Precordillera terrane, eastern Antarctica and South Africa. Recent discoveries of brachiopods and organisms of the Phylum Agmata strengthened Laurentian affinities during the Cambrian. The latest Cambrian-early Ordovician faunas that inhabited the autochthonous Northwest Argentina basin, including the western Puna volcaniclastic successions, are mostly peri-Gondwanan. The early Ordovician brachiopods, ostracods and trilobites display mixed Laurentian, Baltic and Avalonian biogeographical links supporting a drifting of the Precordillera across the Iapetus Ocean. Increasing Gondwanan elements during the Llanvirn, along with varied geological evidence, indicate that the first stages of collision may have begun at that time, involving a major change in the plate kinematics. The distribution of facies and faunas, basin development, and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the Precordillera terrane with the Gondwana margin. The high level of endemism of Caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike-slip movement of the colliding Precordillera plate with respect to the Gondwana margin. Suggested relationships between facies distribution, geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late Cambrian to Silurian.

Early Ordovician (Arenig) faunal assemblages from western Argentina: biodiversification trends in different geodynamic and palaeogeographic settings

Abstract A survey of early Ordovician faunal assemblages from different geodynamic and palaeogeographic settings from the west of Argentina has been carried out. The distribution and dominance of four fossil groups (rhynchonelliform brachiopods, trilobites, sponges, and bivalves) are analysed and compared in three distinct basins: a passive-margin carbonate platform (Precordillera), a volcanic-arc island platform located at intermediate latitude (Famatina) and a siliciclastic pericratonic epeiric platform placed at intermediate–high latitude (Cordillera Oriental). Scales of analysis range from the level of the patterns exhibited by individual clades to the level of the assemblages along the onshore–offshore gradient. Overall taxonomic diversity (at genus and family levels), alpha or within habitat diversity, and ecospace utilisation were evaluated and contrasted among bathymetric zones and among different basins. On this basis a mosaic of five distinctive type assemblages is characterised: (1) a demosponge–brachiopod type assemblage (Precordillera), (2) a brachiopod–bivalve type assemblage (Famatina), (3) a brachiopod–trilobite type assemblage (Famatina), (4) a trilobite–brachiopod type assemblage (Cordillera Oriental), and (5) a trilobite type assemblage (Cordillera Oriental). Possible large-scale controls in the configuration of these type assemblages are assessed. The structure and distribution of the type assemblages from the west of Argentina are interpreted to be largely controlled by environmental dynamics at each geodynamic setting coupled with the latitudinal position of the basins. Both large-scale factors regulate a number of regional parameters such as sedimentary regime, volcanic activity, oceanic circulation, temperature, etc. In particular temperature appears as a critical control over the food supply and primary productivity as well as seasonality of the resources. According to the evidence analysed in the Argentine basins, the latter variables might have played a key role in the diversification of suspension-feeding organisms that, in turn, promoted the development of the Palaeozoic evolutionary faunas and the Ordovician radiation. Our case study from the west of Argentina further represents a small-scale model of the broad spectrum of the possible regional conditions that promoted the differential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.

Tremadoc (earliest Ordovician) brachiopods from Purmamarca and the Sierra de Mojotoro, Cordillera Oriental of northwestern Argentina

The Late Tremadoc storm-dominated shoreface to inner platform deposits exposed west of the Purmamarca village (Coquena Formation) contain a considerably more diverse brachiopod fauna than previously reported. Coquinite horizons from the lower heterolitic succession have yielded monospecific associations of Nanorthis purmamarcaensis nov. sp. (formerly assigned to N. christianiae KJERULF), which is also reported from the Late Tremadoc rocks of the Cerro San Bernardo area. The fine-grained Upper Member of the Coquena Formation contains a more diverse fauna composed by Nanorthis brachymyaria nov. sp., Astraborthis quebradensis nov. sp. and the new plectorthid genus Lipanorthis (type species L. andinus nov. sp.). A different species of Lipanorthis (L. santalaurae nov. sp.) from the Mid Tremadoc Floresta Formation of the Sierra de Mojotoro is also described.

EARLY ORDOVICIAN (ARENIG) BRACHIOPODS FROM VOLCANICLASTIC ROCKS OF THE FAMATINA RANGE, NORTHWEST ARGENTINA

Abstract This paper constitutes the first monographical study of the rich brachiopod faunas from the Early Ordovician Suri and Molles Formations of the central Famatina Range, which form a nearly continuous, more than 2,000 m thick succession of fossiliferous clastic and volcaniclastic rocks. Conodonts from the brachiopod-rich levels of the upper third of the Suri Formation and Los Molles Formation indicate the upper part of the Oepikodus evae Biozone (mid-Arenig). The systematic study of brachiopod faunas reveals the presence of 22 species belonging to 19 genera, three of which are new. The new genera recognized are the orthid Suriorthis, the hesperonomiid Mollesella, and the rectostrophiid Trigonostrophia. The following 12 new species and subspecies are described and illustrated: the clitambonitoidean Tritoechia mollesensis; the skenidioideans Crossiskenidium? stelzneri and Skenidioides kayseri; the orthoideans Paralenorthis suriensis, Paralenorthis riojanus brevis, Panderina? ambigua, Productorthis angulensis, Hesperonomiella arcuata, and Monorthis transversa; the plectorthoideans Ffynnonia famatinensis and Desmorthis? bifurcata; and the porambonitoidean Rugostrophia protoandina. Associated forms are Tritoechia sp., Pinatotoechia acantha Benedetto, 2001b; Protoskenidioides cf. revelata Williams, 1974; Hesperonomia orientalis Benedetto, 1998a; Paralenorthis riojanus (Levy and Nullo, 1973), Famatinorthis turneri (Levy and Nullo, 1973); and Camerella sp. Brachiopods from the Famatina Range display strong affinities with Welsh and Central Newfoundland, Maine and New Brunswick volcanic assemblages forming with them a statistically well defined Celtic cluster. Faunal evidence suggests that the Famatina volcanic belt continues northward into the western Puna belt.

Lithofacies, taphonomy, and brachiopod assemblages in the Silurian of western Argentina: A review of Malvinokaffric Realm communities

Abstract Distribution of faunal associations in the Silurian sequences of the Precordilleran and Northwestern Basins of Argentina is integrated with sedimentologic and taphonomic data, demonstrating that marked temporal and spatial changes in the Malvinokaffric brachiopod assemblages are related mainly to the type and intensity of the hydraulic regime of the shelf environment. The vertical arrangement of associations within each basin is controlled by local fluctuations in water depth, whereas the spatial variations are linked to minor environmental changes (substrate type and oxygen content). The basal strata of the Middle-Upper Silurian Los Espejos Formation of the Precordilleran Basin are mudstones of an open-shelf environment. These pass upward into facies of a storm-dominated shelf and shoreface. Four brachiopod assemblages have been recognized in the muddy facies: the Harringtonina Low-Diversity and Australina -Dominated associations characterize distal storm beds; the Harringtonina High-Diversity and Coelospira -Dominated communities occur in the fine-grained rocks of proximal tempestites. Storm shell beds, which dominate the middle-upper part of the sequence, are represented by allochthonous and parautochthonous coquinas. The Clarkeia Association flourished in shallow-water, high-energy, nearshore environments. Silurian rocks in the Northwestern Basin were deposited in an offshore, low-energy, restricted environment, and two main faunal assemblages have been recognized: a more distal Heterorthella Association and a relatively more proximal Ancillotoechia Association.

NEW UPPER CAMBRIAN–TREMADOC RHYNCHONELLIFORMEAN BRACHIOPODS FROM NORTHWESTERN ARGENTINA: EVOLUTIONARY TRENDS AND EARLY DIVERSIFICATION OF PLECTORTHOIDEANS IN THE ANDEAN GONDWANA

Abstract New rhynchonelliformean brachiopods are documented from Upper Cambrian and Tremadoc beds of the Cordillera Oriental and Puna regions, which are representative of the initial radiation of Plectorthoidea on the Andean (South American) shelves of Gondwana. Protorthisina n. gen., so far the oldest known representative of the family Euorthisinidae, and Lesserorthis n. gen. (Euorthisinidae?) are described, as well as the new species Euorthisina? nazarenensis, Kvania mergli, Kvania lariensis, Kvania azulpampensis (with two subspecies), Kvania? primigenia, and Nanorthis calderensis (with two subspecies). Protorthisina, Kvania, and Nanorthis form a stratigraphically continuous series of species displaying well-defined evolutionary trends involving size, shell ornament, and internal features. In the cardinalia, a transition from a V-shaped septalium (typical of Protorthisina) through almost parallel brachiophore plates (typical of Kvania) to the orthoid notothyrial platform of Nanorthis is documented. The species Nanorthis purmamarcaensis Benedetto and “Nanorthis” grandis (Harrington) are considered as the end members of the lineage. The heterochronic (peramorphic) origin of this trend is supported by ontogenetic evidence from species of Kvania and Nanorthis calderensis. The origin of the ‘plectorthoid’ cardinalia of Kvania from a euorthisinid configuration present in the stem group, followed by the appearance of a orthoidlike configuration in Nanorthis, pose an intricate systematic problem. The preliminary phylogenetic analysis of the northwestern basin taxa shows that the Upper Cambrian Protorthisina simplex n. gen. and sp. is the stem group of both the nanorthid and euorthisinid clades. On the basis of the new evidence, the genus Nanorthis is removed from the Orthoidea to the Plectorthoidea, and the diagnosis of the families Euorthisinidae and Nanorthidae are revised.

Braquiópodos del silúrico temprano malvinocáfrico (Formación la Chilca), Precordillera Argentina

Resumen El estudio de los braquiopodos coleccionados en la Formacion la Chilca de la Precordillera Argentina revela la presencia de diez generos pertenecientes a los Dalmanellacea, Strophomenacea, Rhynchonellacea y Retziacea. Se reconocieron doce especies, de las cuales ocho son nuevas, si bien dos de ellas no fueron formalmente definidas. Los horizontes de areniscas portadores de los fosiles estudiados se intercolan o suprayacen capas portadoras de graptolitos de la biozona de Atavograptus atavus, por lo que se los asigna al Llandoveriano temprano (Rhuddaniano). Se analizan brevemente las afinidades biogeograficas entre la fauna de la Formacion la Chilca y otras asociaciones malvinocafricas llandoverianas.

The Ordovician Quebrada Grande Formation, Cordón de Lila (Antofagasta Region, northern Chile): stratigraphicand paleogeographic significance

The Cordon de Lila is located immediately to the south of the Salar de Atacama, in northern Chile. The geology of the Cordon de Lila is characterized by extensive outcrops of Early Paleozoic volcanic and sedimentary rocks (Cordon de Lila Igneous and Sedimentary Complex; CISL) that form the ‘Arco magmatico occidental’ (AMO) which is intruded by a multiple suite of Middle Ordovician to Lower Silurian granitoids. In this contribution we report the results of a sedimentological and paleontological study of a recently discovered 1,600 m thick marine Ordovician sedimentary sequence (Quebrada Grande Formation) which unconformably overlies the CISL. The Quebrada Grande Formation comprises three mayor facies associations. In stratigrafic order, these facies associations are: matrix-supported conglomerates (1), interbedded sandstones and siltstones (2) and clast-supported conglomerates (3). Facies association (1) is interpreted as debris flow deposits accumulated in a proximal marine fan-delta setting while the overlying sandstones and siltstones facies (2) represent open-marine platform deposits. Clast-supported conglomerates (3) are the record of high-energy sedimentary episodes during which coarse sediments reached the distal part of the fan-delta. The detrital fraction of the Quebrada Grande Formation is derived from the erosion of the Cambrian?-Lower Ordovician? CISL volcanic arc, the plutonic roots of the arc and its continental basement. The age of this formation is well constrained by the occurrence of brachiopods (Paralenorthis sp., Monorthis transversa Benedetto, and Mollesella? sp.) near the base and graptolites (Tetragraptus sp. and Cryptograptus? sp.) towards the middle of the succession. Both brachiopods and graptolites indicate an Arenigian to early Llanvirnian (Darriwilian) age. The brachiopod assemblage from Cordon de Lila displays affinities with the Famatina basin fauna of western Argentina. The absence of taxa in common between the AMO and the nearby ‘Faja Eruptiva de la Puna Occidental’ (FEPO) of NW Argentina may result from some kind of environmental barrier or some geographic separation from the Gondwana margin. According to the latter hypothesis, the Quebrada Grande succession developed on the allochtonous Arequipa-Antofalla Terrane.

Plectorthoid brachiopods from the Lower Ordovician of north-western Argentina; phylogenetic relationships with Tarfaya Havlíček and the origin of heterorthids

The re-study of species previously attributed to the genus Nanorthis from the Tremadocian of north-western Argentina has led to the recognition of the new genera Gondwanorthis (type species Nanorthis calderensis Benedetto), to which the Iranian species Nanorthis bastamensis Ghobadi Pour, Kebriaee-Zadeh & Popov is also referred, and Lampazarorthis (type species Eoorthis bifurcata Harrington), which includes Nanorthis brachymyaria Benedetto and Lampazarorthis alata sp. nov. New extensive collections from the Cordillera Oriental support reassignment of the upper Tremadocian species Nanorthis purmamarcaensis to the genus Tarfaya. The species Nanorthis carinata Laurie, from Tasmania, is reassigned here to Tarfaya. According to the present taxonomic revision, Nanorthis is not cosmopolitan but is a pan-tropical genus, confined to Laurentia, the Laurentian-derived Precordillera terrane, and probably to Baltica, Siberia and Kazakhstanian terranes, whereas Gondwanorthis and Tarfaya are Gondwanan and peri-Gondwanan endemic taxa. Cladistic analysis of early orthoids, plectorthoids and dalmanelloids, including those recorded from mid to high latitude Gondwana successions, portrays Gondwanorthis and Lampazarorthis as a sister group basal to the tarfayid + heterorthid clade, whereas the South American and North African Incorthis appears as a basal member of this clade. Morphological and stratigraphical evidence provided in this study, along with the results of the cladistic analysis, supports the view that heterorthids could have arisen from the tarfayids by the end of Floian. The enlargement and incipient bilobation of the cardinal process in Tarfaya grandis leaves this species as a potential ancestor of Tissintia, which occurs in Darriwilian beds of the Central Andean Basin (Bolivia), but could have been present in this basin since the late Floian. We speculate that punctae in the Heterorthidae evolved independently from a Tarfaya-like ancestor, and that the transition from an impunctate to a punctate shell may have occurred repeatedly through rhynchonelliform evolution. http://zoobank.org/urn:lsid:zoobank.org:pub:FFE818F6-FA92-4B26-AE76-077617C3D9EA