Precordillera

Dataciones de xenolitos Rincón Blanco (Precordillera)

En la Sierra de Pie de Palo, inmediatamente el este del Precordillera (imagenes satelitales y figuras), hay exposiciones extensas de basamento metamórfico que indican edades de cristalización de ~1.1 Ga y circones con edades metamórficas de ~1.0 Ga (Ramos et al., 1998). Al sur de Mendoza, en Ponón Trehué (Fig), una sucesión de calizas y rocas siliciclásticas del Ordovicico se apoyan sobre rocas metamórficas con edades circón U-Pb de ~1.2 Ga (Thomas et al., 2000). Principalmente en base a esta evidencia, se considera que el basamento de Cuyania es de edad Grenvilliana. Esto es consistente con el modelo de un origen Laurentiano porque el embayment ("la bahía") de Ouachita de donde se supone que el terrano Cuyania se separó, se ubica dentro de la Provincia Grenville a lo largo del margen del sur de Laurentia.

Una espesa sucesión carbonática cámbrica inferior a medio-ordovícica es particularmente distintiva en Precordillera y única en el Gondwana Occidental (Fig) y forma afloramientos largos, continuos norte- sur en los cinturones plegados y corridos orientales y centrales de la Precordillera y otros mas pequeños en Ponón Trehué y en la provincia de La Pampa. Su caracter poco profundo, con aguas cálidas, su sucesión estratigráfica y la afinidad de sus fósiles bentónicos lo hace similar a los estratos correlativos de Laurentia (Astini et al., 1995; Keller, 1999).

La sucesión típica incluye a las formaciones La Laja, Zonda, La Flecha, La Silla, y San Juan, con un límite Cámbrico-Ordovicico en Formación La Silla (ver Fig). En el Cerro Totora en la parte norte de Precordillera, la sucesión empieza con las fms Cerro Totora y Los Hornos que son facies lateral de la Fm La Laja (Bordonaro, 2003, ver fig.).

Con un intervalo espeso de yeso, limolitas rojas, arenisca, y dolomita, la Fm Cerro Totora se interpreta como sedimentitas continentales (fluviales, bajada y lacustre evaporítico) que se formó cuando Cuyania se separó del embayment de Ouachita en Laurentia (Thomas y Astini, 1999,; Thomas et al., 2001). La Fm Los Hornos Fm, así como La Laja y la sucesión de la Fm San Juan, se interpreta como un margen pasivo (Astini y Thomas, 1999) que se depositó durante la migración de Cuyania desde Laurentia.

Los olistolitos más grandes son bloques de calizas de edad Cámbrico y Ordovícico inferior que son similares o representan facies distal, de agua algo más profunda, de las suceciones de la plataforma Cámbrico a Ordovícica Medio (Fig anterior). Como se describió para el modelo Laurentiano, (Thomas y Astini, 2003 o Keller 1999, que propone una separación en el Ordovícico medio de Cuyania de Laurentia), la Fm Los Sombreros es considerada como depositada en el talud continental a lo largo del borde occidental del microcontinente de Cuyania, junto con los olistolitos generados por desplazamientos de gravedad a lo largo de escarpas de falla o erosión en cañones submarino a lo largo de ese talud o plataforma continental empinada.

La faja occidental de Precordillera están compuestas por estratos del Ordovicico Superior de las Fms Yerba Loca y Alcaparrosa y su equivalentes, que son turbiditas distales y pelitas con intrusivos máficos. Estas unidades, junto con la Fm Los Sombreros, fueron interpretadas por Keller (1999) como un contemporáneo lateral de las sucesiones proximales a distales del margen continental que va desde plataforma continental a facies de turbiditas.

LA EDAD Y AFINIDAD DEL BASAMENTO DE CUYANIA

Edad del basamento de Cuyania: Circones detríticos de las areniscas cámbricas y ordovícicas

Generalidades

La repetida asunción que el basamento de Cuyania es de edad Grenvilliana (Dalziel et al., 1996; Kay et al., 1996; Thomas y Astini, 1996; Dalziel, 1997; Ramos et al., 1998; Sato et al., 2000) es la línea primaria de evidencia que apoyaba la derivación de Cuyania del embayment de Ouachita en Laurentia. No sólo se basó en una evidencia geocronológica limitada, sino que no tiene en cuenta la distribución global del “Grenvilliano". La Provincia Grenville, uno de los orógenos más jóvenes de las fajas en el Escudo del Canadá , tiene su área tipo en el Canadá oriental. Allí hay un mosaico de terrenos geológicos formados por una serie de eventos orogénicos que se extendieron por casi 1 Ga de los cuales entre 1.71-1.60 Ga corresponden a la orogenia del Labrador y desde 1.08 a 0.98 Ga a la orogenia Grenvillian (Tollo et al., 2004).

A lo largo de los 4.500 km entre Labrador y México, la provincia Grenville registra: 1) un periodo temprano (1.3-1.2 Ga) de tectónica acrecionaria llamado orogenia Elzeviriana, 2) un periodo (1.18-1.08 Ga) de AMCG (anortosita-mangerita (granitoides portadores de piroxenos verdes, con composiciones ultramáfica a félsicas ~35-68 wt% SiO2)- charnockita-granito) y 3) un periodo final colisión continente-continente (1.08-0.98 Ga) a lo que varios autores se refieren a como la orogenia Grenvilliana (ej., Gower y Krogh, 2002) o el Pulso Ottawan de la Orogenia Grenvillian (ej., Karlstrom et al., 2001).

Aún mas, ya que varios orogenos colisionales contemperaneos se produjeron en varias paleoplacas durante el ensamble del supercontinente Rodinia, “el término orógeno Grenville se usa para referirse a
todas las áreas afectadas por orogénesis de tipo convergente durante el intervalo 1.3-1.0 Ga” (Tollo et al., 2004, pág. 1), , incluso aquellos cinturones orogénicos que no involucraron colisión con Laurentia (vea Hoffman, 1991,: fig. 1A y figura del supercontinente Rodinia de capítulos anteriores).

Como consecuencia, las rocas con edades de 1.3 a 0.98 Ga son llamadas Grenvilliano, dándose a este término un sentido geocronológico. Para evitar confusión de las diferentes interpretaciones y para evitar la asunción que el término Grenvilliano quiere decir afinidad con Laurentia, se usa el término Mesoproterozoico para las dataciones de 1.6 a 1.0 Ga con sus eventos temprano y tardío (1.6 -1.3 Ga y 1.3-1.0 Ga, respectivamente).

Sólo en Ponón Trehué, cerca de San Rafael (ver Fig) los estratos de la plataforma ordovícica se apoyan sobre rocas del basamento. Areniscas que yacen sobre el basamento y que disponen de circones detríticos provenientes del mismo dan edades 1400-1100 Ma (pico en 1250 Ma) y así puede ser consideradas Grenvilliano en parte.

Relaciones 207Pb/204Pb vs 206Pb/204Pb y 208Pb/204Pb vs 206Pb/204Pb en rocas de Antártida, Precordillera, Islas Malvinas, Cratón de Arequipa, Escudos de Nubia (Africa) y Natal y Provincias Grenvillianas de Norte America y Europa	Relaciones 207Pb/204Pb vs 206Pb/204Pb y 208Pb/204Pb vs 206Pb/204Pb en xenolitos de Precordillera

Sin embargo, Kay et al. (1996) no consideró la posibilidad de que esta signatura podría aparecer en otras rocas Mesoproterozoicas en otra parte en Gondwana. Seguidamente, Wareham et al. (1998) demostró una similitud íntima de isótopos de Pb y Nd, característicos de Laurentia en rocas del Mesoproterozoico en la Antártica Oriental y Occidental, en Natal (SE Africa), y en las Falkland/Malvinas , desafíando la asunción estricta de la afinidad Laurentiana del basamento de la Precordillera. Los defensores de la teoria Laurentiana (Astini y Rapalini, 2003,; Thomas y Astini, 2003,; Thomas et al., 2004) ha fallado en explicar los resultados de Wareham et al. (1998), que pueden ser consistentes con un origen Gondwánico para Cuyania.
Además, las roca del basamento de edad Grenvilliana de la Sierra de Pie de Palo que tienen la misma signatura isotópica de Pb que las rocas del basamento de Precordillera se consideran ahora autóctonas o parautoctonas a Gondwana basándose en los resultados de varias recientes investigaciones (Galindo et al., 2004; Baldo et al., 2005; McClelland et al., 2005; Rapela et al., 2005a).

Modelo parautoctono a la luz de los datos del basameno

Las edades Grenvilliana de circones detríticos dominan en muchas areniscas de Ordovícico de Precordillera, cuyas poblaciones se extenden entre 1600-1000 Ma (Finney et al., 2003; Gleason et al., 2007), indicando la presencia de Mesoproterozoico en muchas area del basamento de Cuyania. No obstante, la presencia de basamento Mesoproterozoico en Cuyania, incluyendo aquéllos limitados solo a edades Grenvilliana (1200-950 Ma) podría indicar una afinidad con Gondwana Occidental mas que con Laurentia.

Rocas con edades Grenvilliana componen la faja orogénica Sunsas, ubicada en el margen sudoeste del craton amazónico; ellos componen el cinturon Namaqua -Natal en el margen sur del craton de Kaapvaal, caracterizados por relaciones isotópicas de Pb bajo, y datos geocronológicos de circón recientemente adquiridos (Sims et al., 1998; Casquet et al., 2005; McClelland et al., 2005; Rapela al del et., 2005a, b) indican que la edad Grenvilliana son comunes en ambas Sierras Pampeanas occidentales y orientales de Argentina. Además, areniscas Neoproterozoicas depositado en el Cratón Río del craton de Plata tienen una cantidad significativa de circones detríticos de edad Mesoproterozoic a que incluyen un rango amplio de edades (1600-1000 Ma), encuentradas tambien en areniscas de la Precordillera y que indica la presencia de basamento de esa edad en el craton del Río de La Plata (Finney et al., 2006; Gaucher et al., 2006).

Arcos magmáticos con plutones tonalíticos a trondhjemiticos, la fuente potencial de la arenisca del Olistolito San Isidro, eran comunes en la parte del sur de Gondwana Occidental durante el Neoproterozoico a Cámbrico Temprano (Campos Neto y Figueiredo, 1995, Lira et al., 1997; Rapela et al., 1998; Llambías et al., 1998; Brito Neves et al., 1999; Basei et al., 2000, 2005, Veevers, 2003, Bossi y Gaucher, 2004). Plutonitas que cristalizaron a 1600-1500 Ma son comunes en Gondwana Occidental, particularmente en el sudoeste del craton amazónico (Tassinari y Macambira, 1999) y en el cratón Río de la Plata (Gaucher et al., 2006) and could be the source of the distinctive early Mesoproterozoic zircons (1600-1500 Ma) in the Middle Cambrian and Middle Ordovician sandstones in the Precordillera.

REEVALUACION DEL REGISTRO ESTRATIGRÁFICO DE CUYANIA Significado de la sucesión estratigrafica de la plataforma calcárea del Cambrico-Ordovícico medio La similitud entre las plataformas calcárea de margen pasivo de edad cámbrica-ordovícica media de la parte este de la Precordillera y de los Apalaches en Laurentia (Fig) se cita frecuentemente como ina evidencia de la afinidad Laurentiana fr el terreno Cuyania (Astini et al., 1995; Dalziel et al., 1996; Thomas and Astini, 1996; Astini and Thomas, 1999; Keller, 1999; Thomas et al., 2004). Sin embargo estas similitudes no significan que los estratos de Cuyania se hallaren contiguos a aquellos de Laurentia. Ello requeriría que ellos no hubiesen sido desmembrados por rifting hasta el Ordovícico medio, como fue propuesto por Keller (1999). En adición, la sedimentación en una plataforma calcárea no se inició hasta el Cámbrico tardío en las zonas de Laurentia que rodea la bahía de Ouachita.

En el modelo de microcontinente Laurentiano, el rifting y la separación del terreno Cuyania ocurrió antes del Cámbrico Medio, por lo que las similitudes estratigráfica deben reflejar historias paleolatitud y de ascenso y descenso de nivel eustático que controla las historias de las facies sedimentarias y la acumulación de los estratos en Cuyania, mientras se separó y alejó de de Laurentia. Thomas y Astini (1996) propusieron que Cuyania migró desde el promontorio Alabama en Laurentia, en el Cámbrico Medio; en consecuencia su sucesión estratigráfica cámbrica sería probablemente más similar a la de los Apalaches del Sur. Sin embargo, existen diferencias sustanciales con las rocas clásticas de grano fino que domina gran parte del Cámbrico Medio y Superior de los estratos más australes de los Apalaches (Conasauga Grupo), en contraste con los estratos calcáreos predominantes en la Precordillera (Fig. 6; Resser, 1938; Palmer, 1971; Thomas, 1991).

In the parautochthonous model (Fig), Cuyania would have been at the same paleolatitude as Laurentia, experiencing the same eustatic events, and thus naturally could have been the site of carbonate sedimentation. Although southern West Gondwana was primarily the site of siliciclastic sedimentation during Cambrian and Ordovician time, following a long period of Brasiliano-Pan African orogenesis, carbonate sediments typical of a tropical climate did accumulate locally as recorded by the lowermost Cambrian Cerro Victoria Fm of Uruguay (Sprechmann et al., 2004; Gaucher et al., in press).

It is possible that Cuyania during Cambrian to Mid Ordovician time bore the same relationship to West Gondwana as Florida does today with North America, i.e. as a passive margin, carbonate platform free of siliciclastic sediment and extending into a tropical ocean.

Tectono-sedimentary significance of the Middle- Upper Ordovician stratigraphic record

According to the Laurentian model, the Middle-Upper Ordovician strata of Cuyania record a foreland basin and clastic wedge that formed as Cuyania docked with Gondwana (Astini et al., 1995; Thomas and Astini, 2003). The base of the Gualcamayo Fm, a black shale that overlies the San Juan Fm and records the drowning of the carbonate platform succession, is significantly older in the Guandacol region in the northern part of the Precordillera than at localities farther to the south (Thomas and Astini, 1996, 2003, among others). Thus, the basal contact is shown to be diachronous on correlation charts (Fig. ), becoming gradually younger between localities, and the Gualcamayo Fm is interpreted as representing deposition in a subsiding, migrating peripheral foreland basin.

The coarse-grained, overlying siliciclastic successions (Las Vacas and Trapiche Fms in the north, the La Cantera Fm in the Villicum range to the south) are interpreted as coarser fractions of the clastic wedge, which arose from the collision of the Cuyania microcontinent with the proto- Andean margin of Gondwana and prograded westwards (Thomas and Astini, 2003).
Black shale also can be deposited in pull-apart or stike-slip basins along transform faults, just as has been the case with the organic-rich Monterey Fm of southern California - one of the world’s richest petroleum source rocks (Isaacs and Garrison, 1983).

Separate basins in the California borderland (e.g., Los Angeles, Santa Barbara, and Santa Maria) subsided to bathyal depths at approximately the same time, yet each had its own separate subsidence history, and the age of the Monterey Fm is somewhat different in each basin (Behl, 1999). This example presents a viable alternative explanation for the differences in age of the base of the Gualcamayo Fm in the Precordillera with sections at different localities recording different subsidence histories for different pull-apart basins.

Proponents of the Laurentian microcontinent model have not considered this alternative explanation.
Detailed study and evaluation of the Middle-Upper Ordovician siliciclastic strata overlying the Gualcamayo Fm in various sections in the Eastern Tectofacies of the Precordillera (Fig.) led Astini (1998a, b) and Keller (1999) to conclude that these strata were deposited in extensional basins (Astini et al., 1998a, figs. 7-9; Keller, 1999, fig. 58).

But this was not an original idea, having been proposed by many others, from Borrello (1969) to von Gosen et al. (1995), who have studied these rocks. These basins are characterized by considerable longitudinal and lateral basin asymmetry, including half-grabens, episodic rapid subsidence, abrupt lateral facies changes and local unconformities, greatly variable depositional processes within the same basin (pelagic settling, turbidity flows, mass flows, and gravity flows), marked contrasts in stratigraphy, facies geometry, and unconformities among different basins in the same region (Astini, 1998a, b).

Astini (1998a, 2002) has even used the term “yo-yo” tectonics to refer to the alternating up and down movement of crustal blocks as a result of extension. The Middle- Upper Ordovician stratigraphic successions are typically only 100-200 m thick; only in the Guandacol region does the succession approach 1000 m in thickness. Biostratigraphic correlation of these sections (Fig. ), plotted against an accurate time scale, demonstrates that the Middle-Upper Ordovician succession is dominated by major hiatuses with sedimentation occurring only in pulses of very short duration (Finney et al., 2005a). Sedimentation was episodic within each basin and nonsynchronous between basins.

These are characteristics of strike-slip related basins along major transform faults (Christie-Blick and Biddle, 1985; Nilsen and Sylvester, 1995), and they are completely uncharacteristic of foreland basins (Busby and Ingersoll, 1995; Sinclair, 1997). Olistoliths and boulder conglomerate of the Las Vacas Fm (Astini, 1998a, b) are particularly instructive. This thick, massive unit is part of a stratigraphic succession that includes the underlying shale of the Gualcamayo Fm and the overlying turbiditic siltstone/sandstone that dominates the Trapiche Fm (Astini, 1998b). Thus, this is not a coarsening, thickening upwards succession expected for a foreland basin.

The large rounded boulders of the Las Vacas conglomerate (clasts of meta-sandstone and quartzite and various plutonic and volcanic rocks) indicate erosion from mountainous terrane and transportation by rivers down steep gradients directly to the margins of the depositional basin. Most of the olistoliths are limestone blocks from the San Juan Fm; many are very large (> 1 m), and many are angular with flat surfaces representing bedding and fractures. And, everywhere, even where they are most abundant, the olistoliths are surrounded by rounded boulders of the conglomerate (Figs.).

These olistoliths were deposited with the boulders. It is clear that they were introduced into the stream of boulders only very near the site of deposition; otherwise that would have been pulverized by the boulders. Thus, the depositional basin of the Las Vacas Fm must have been adjacent to steep terrane with rivers that transported large boulders directly to the basin margin, and erosion and incorporation of limestone blocks must have occurred only as the river of boulders entered the depositional basin, the faulted margins of which were underlain by the San Juan Fm. The large, rounded boulders also are abundant in the Upper Member of the Gualcamayo Fm (Fig. ). They were delivered to the basin even during deposition of the shale that, according to the Laurentian microcontinent model, records the early subsidence of a peripheral foreland basin. The Middle to Upper Ordovician strata were not deposited in a foreland basin; they represent a series of pull-apart or strike-slip basins with the Las Vacas having been deposited in a manner similar to that of fan-delta conglomerates in strike-slip basins, such as the Violin Breccia and Castaic Fm in the Ridge Basin of California (Link, 2003) and the San Onofre Breccia in the California continental borderland (Howell et al., 1974).

The Laurentian microcontinent model requires that the Middle-Upper Ordovician strata of the Precordillera be deposited in a foreland basin; thus, many very different attempts have been made to reconcile the clear evidence of strike-slip basins and related extension with the framework of a continent-continent collision. Initially, post-collisional relaxation and rebound were proposed (Astini et al, 1995; Astini, 1998a). More, recently thick-skinned thrusts were proposed to explain generation of the olistoliths of the Las Vacas Fm (Thomas and Astini, 2003; Astini and Davila, 2004), but without reconciling the co-occurrence of extreme compression required for the thrusts with the extensional regime of the adjacent basins in which the olistoliths were deposited, nor explaining the intimate depositional relationship of the olistoliths with the boulder conglomerate, nor explaining the occurrence of Las Vacas boulders in the Gualcamayo Fm. Most recently, Thomas and Astini (2005) proposed a third alternative, namely thin-skinned thrusts to explain the olistoliths. These hypotheses were proposed to explain the Middle-Upper Ordovician stratigraphy that is specific to the Guandacol area, a very small part of the Precordillera, but it is very different from correlative successions in the rest of the Precordillera and in the San Rafael region.
It is readily apparent that the Middle-Upper Ordovician stratigraphy of the Precordillera represents deposition in strike-slip basins. The Laurentian microcontinent model, on the other hand, requires that the stratigraphic record be interpreted in the context of a collisional orogenic setting. Different attempts to do so, however, have not been satisfactory.
The initial, most apparent interpretation of the Middle and Upper Ordovician stratigraphy, namely that it represents strike-slip related extensional basins (Borrello, 1969; von Gosen et al., 1995; Astini, 1998a, b; Keller, 1999; Gleason et al., 2007, and several others), is fully compatible with the parautochthonous model in which these strike-slip basins developed along a transform fault as Cuyania began its migration along the margin of West Gondwana. However, these kind of basins are incompatible with the Laurentian microcontinent model.

Significance of the Middle Ordovician K-bentonites
The occurrence of numerous K-bentonite beds in the upper San Juan Fm and overlying Gualcamayo Fm
of the Precordillera stratigraphic succession (Huff et al., 1998) is regularly cited as evidence for the Laurentian microcontinent model in which Cuyania was approaching and in proximity to the Famatinian magmatic arc in Mid Ordovician time (Astini, 1998a; Astini and Thomas, 1999; Thomas et al., 2002; Thomas and Astini, 2003; Astini and Rapalini, 2003; Fig. 9).

The age of the K-bentonite beds (~ 475-460 Ma) overlaps with that (~ 490-460 Ma) of Famatinian granitoids, and the geochemistry of the K-bentonites is consistent with their origin from subduction-related explosive volcanism (Huff et al., 1998; Fanning et al., 2004).

Thus, it is highly probable that the K-bentonites in the Precordillera succession represent ashes erupted from volcanoes of the Famatinian arc (Baldo et al., 2003; Fanning et al., 2004).

However, contrary to most interpretations, this does not support the Laurentian model in which Cuyania was approaching the Famatinian arc from the west (present coordinates). In recent paleogeographic reconstructions for Early and Mid Ordovician time (Dalziel, 1997, figs. 16; Cocks and Torsvik, 2002, figs. 4 and 5), the Famatinian arc faces to the east and is located between the equator and 30º S latitude. Assuming the same controls and general patterns of atmospheric circulation as those operating today, the prevailing winds in this paleolatitudinal belt, the southeast trade winds, would have blown and thus carried volcanic ashes to the northwest (Ordovician coordinates).

This direction is opposite that necessary to disperse the ashes to the Cuyania terrane if it was approaching from the east (Ordovician coordinates) as required by the Laurentian model
(Figs.).

Instead, the ashes would have been blown to the northwest to that part of Gondwana represented today by southeastern South America and southern Africa, where we place the Cuyania terrane in Mid Ordovician time. Confirmatory evidence for this interpretation is provided by the recognition of altered pyroclastic beds in the Balcarce Fm of the Tandilia region, south of Buenos Aires (Dristas and Frisicale, 1987), which is correlated in part with the Lower Ordovician (Poiré et al., 2003; Rapela et al., 2005c). In citing the evidence from the K-bentonites, proponents of the Laurentian microcontinent model fail to ask the question “Which direction was the wind blowing?” When that question is addressed, the occurrence of Middle Ordovician K-bentonite beds in the Precordillera is found to be consistent with the parautochthonous model. The paleolatitude used here to answer this question may be questioned. Regardless, the question does need to be asked.

REEVALUATION OF CAMBRIAN PALEOMAGNETIC DATA FOR CUYANIA
A paleomagnetically determined Cambrian paleolatitude of Cuyania is often cited as evidence for the origin of Cuyania from the Ouachita embayment of Laurentia. Rapalini and Astini (1998) established a paleomagnetic pole for red beds of the Lower Cambrian Cerro Totora Fm in Cuyania that indicated a paleolatitude of 20o S that was consistent with the paleolatitude of the Ouachita embayment of Laurentia and inconsistent with a Gondwanan location for Cuyania in its present position relative to South America.

However, they did not consider that Cuyania may have occupied a different position within Gondwana. A Cambrian paleolatitude of ~20º S places the Cuyania terrane on the southern margin of West Gondwana (present coordinates) using the Cambrian paleogeographic reconstructions of Hoffman (1991) and Dalziel (1997), and recent refinements in the early Paleozoic apparent polar wander path (APWP) for Gondwana (Meert, 2003; McElhinny et al., 2003) result in a nearequatorial position (~5-20º S) for the present southern tip of South America at ~530 Ma with a narrow Iapetus
Ocean separating it from Laurentia (McElhinny et al., 2003). This is consistent wit the paleogeographic reconstruction of Finney et al., (2005b, fig. 6; Fig. 5 herein), and thus is fully consistent with the parautochthonous model. The paleomagnetic data are consistent with a near equatorial location of Cuyania in the Cambrian.

REEVALUATION OF THE PALEOBIOGEOGRAPHIC RECORD OF CUYANIA

Changing affinity of Early and Mid Ordovician benthic and nektobenthic faunas

Most papers supporting the Laurentian microcontinent model (Astini et al., 1995; Thomas and Astini, 1996; Thomas and Astini, 2003; Thomas et al., 2004), or some variation on it that involves a Laurentian origin (Dalziel, 1997; Keller, 1999), invoke as evidence changes in affinities of Early and Mid Ordovician benthic faunas of the Precordillera. Typically, they include figures (e.g., Astini
et al., 1995, fig. 7; Thomas and Astini, 2003, fig. 2; Benedetto et al., 1995, fig. 1; Dalziel, 1997, fig. 7) showing that the percentage of genera of Laurentian affinity in the overall fauna decreases through the Lower and Middle Ordovician succession, while the percentages of endemic and Gondwanan genera increase.

Given the assumption that the larvae of benthic biota, such as brachiopods, can be dispersed across oceanic distances of only 1500 km, the changing composition of benthic faunas has been used, for example, to recognize an isolation stage for Cuyania when it was far removed from both Laurentia and Gondwana during late Early to early Mid Ordovician time, as well as earlier Laurentian and later pre-accretion and Gondwanan stages (Benedetto, 1998). Thomas and Astini (1996, p. 756) went so far as to claim that from 495 Ma to 470 Ma (i.e. during Early to early Mid Ordovician time) “the Precordillera received no faunal migrants from either Laurentia or Gondwana, indicating ~ 1,000 km of open ocean separation from both Laurentia and Gondwana.”

More recently, Thomas et al. (2004) state: “Endemic faunas replaced the Cambrian Laurentian faunas by Early Ordovician time, indicating isolation of the Precordillera after rifting from Laurentia"

Brachiopods
It is necessary to challenge these interpretations of, and statements on, the paleobiogeography of benthic faunas of the Precordillera, which have become so widely repeated in the literature that they have become fundamental assumptions. The use of percentages gives a very different perspective to the data than does the use of actual numbers of genera. The only means to really check this is to use the percentages given for each brachiopod zone in fig. 7 of Astini et al. (1995), Fig. 12 herein, and the full list of genera for each zone found in table 1 of Benedetto (1998).

What one finds is that the percentage of Toquima- Table Head (or Laurentian) genera decreases in successive brachiopod zones through the Lower and lower Middle Ordovician: from 90% in the Archaeorthis zone to 60% in the Huacoella-Niquivilia zones to 65% in the Monorthis zone to 60% in the Ahtiella zone (Fig. 12). Yet, the number of genera varies greatly with the Ahtiella zone having nearly twice as many genera as any of the other zones.

If one uses the percentages and the total number of genera to determine the actual number of Laurentian genera appearing for the first time in each zone, one finds that that number increases dramatically in the Ahtiella zone (Table 1). These data conflict completely with the statement of Thomas and Astini (1996) regarding no faunal migrants reaching Cuyania from Laurentia in the Early to early Mid Ordovician. A large number of Laurentian brachiopod genera, 24 in fact, did reach Cuyania during this time, as well as an appreciable number of Laurentian trilobite genera (namely the genera Holia, Peltabellia, Uromystrum and Ectenonotus reported by Vaccari, 1994, p. 113).

These data also indicate that the number of Laurentian brachiopod genera that dispersed to Cuyania in the early Mid Ordovician is two to three times greater than the number that migrated during the time represented by any zone in the Lower Ordovician.

What does this mean? Obviously, the statement of Thomas et al. (2004) that endemic genera replace Laurentian genera in the Early Ordovician is not accurate given that the number of Laurentian genera actually increased. In addition, the data of Benedetto (1998) demonstrate that dispersal between Laurentia and Cuyania was far easier and more efficient and accomplished by more genera during the time, according to the Laurentian model, that Cuyania was nearing Gondwana and was farthest from Laurentia. In subsequent papers (e.g., Benedetto, 1998; Benedetto et al., 1999) data from all the Lower Ordovician brachiopods zones (Archaeorthis to Monorthis) are grouped together as Arenig and compared to the data from the Ahtiella zone, a single zone taken to represent the Llanvirn. This, of course, presents a very different impression of faunal changes, with the Lower Ordovician faunas appearing to have far greater numbers of Laurentian genera in comparison to the Middle Ordovician faunas.

Regardless, the biogeography of the benthic faunas is compatible with the parautochthonous model. The primary control on the distribution of the Laurentian fauna, as it is for all benthic invertebrates, is water temperature, which, in turn, is related to paleolatitude. In addition, strong westward flowing (early Paleozoic coordinates) equatorial currents readily would have dispersed the larvae from Laurentia and to Cuyania, a carbonate platform located in the tropics on the southern margin of Gondwana

Given these paleolatitudes and current directions, Gondwanan taxa or taxa endemic to Cuyania would not have dispersed to Laurentia, a phenomenon noted and discussed by Benedetto (1998) and Benedetto et al. (1999). Conodonts Proponents of the Laurentian microcontinent model have also invoked the paleobiogeography of latest Cambrian to Mid Ordovician conodonts of Cuyania as supporting evidence. Again, the interpretations of this evidence are challenged. The latest Cambrian Clavohamulus hintzei of the North American Midcontinent conodont faunal province occurs in both Laurentia and Cuyania (Lehnert et al., 1997), and Astini (1998a) and Astini and Rapalini (2003) cite this as evidence to support the Laurentian microcontinent model. However, the Midcontinent faunal province is not restricted to Laurentia; it is representative of shallow warm-water marine environments (Bergström, 1990); and C. hintzei, as well as other Midcontinent province conodonts, also occurs in Australia.

Thus, in the parautochthonous model, the occurrence of C. hintzei can be readily attributed to the low paleolatitude of Cuyania and to larval dispersal by oceanic currents. In upper Lower Ordovician strata of Cuyania, i.e. in the lower part of the San Juan Fm in the Precordillera and within Ponón Trehué Fm in the San Rafael area, conodont faunas of the North Atlantic province replace those of the Midcontinent province at a stratigraphic level corresponding to a global transgression (Lehnert et al., 1998). The North Atlantic conodont faunal province reflects coolwater and is developed in shallow-water platform settings at high paleolatitudes and in deeper-water outer shelf and slope settings at low paleolatitudes.

From the distributions of the Midcontinent and North Atlantic faunal provinces, Lehnert et al. (1998, 1999) concluded that Cuyania was derived from the southern margin of Laurentia at low paleolatitude and that by the late Mid to early Late Ordovician it was located at high paleolatitude, a considerable distance away from Laurentia and near to Gondwana - conclusions that were cited most recently by Astini and Rapalini (2003) as evidence of the Laurentian origin of the Precordillera.

This assertion is here challenged.
According to the Laurentian microcontinent model, Cuyania separated from Laurentia well before accumulation of strata with Midcontinent province conodonts, and although originally conceived as defining separate geographic provinces, the Midcontinent and North Atlantic conodont provinces are now interpreted as reflecting primarily differences in water temperature (Sweet and Bergström, 1984; Bergström, 1990).

Thus, the occurrence of Midcontinent conodonts in Cuyania reflects paleolatitude (expressed in water temperature) and oceanic larval dispersal and not necessarily any direct connection to Laurentia. Besides Cuyania and Laurentia, Midcontinent province conodonts are common in shallow-water sediments that accumulated at low paleolatitude in Australia in the Mid Ordovician (Webby et al., 2000), indicating that larval transport by oceanic currents could readily disperse Midcontinent conodonts from Laurentia to Cuyania on the southern margin of West Gondwana. The replacement of Midcontinent faunas by North Atlantic faunas correlates with a global sea-level rise and can be explained by a change in water temperature attributed to the sea-level rise and possibly also to the movement of Cuyania to higher paleolatitudes associated with the overall long-term migration of Gondwana (Fig. ).

Further complicating paleogeographic interpretations is the statistical analysis by Albanesi and Bergström (2002; in press) that demonstrates that conodont faunas of the Lower to Middle Ordovician successions of the Precordillera and the Marathon region of Texas become more similar upsection with conodont faunas of the lower Middle Ordovician of the Precordillera dominated by Laurentian taxa.

According to proponents of the Laurentian microcontinent model, these conodont faunas should become more dissimilar upsection as a result of the increasing distance and change in paleolatitudes between Laurentia and Cuyania. From this, it follows that conodont paleobiogeography must be used with caution in paleogeographic reconstructions.
Albanesi and Barnes (2000) invoked the Laurentian microcontinent model as a cause for major environmental perturbations and geographic isolation during deposition of the lower Gualcamayo Fm that provided the opportunity for allopatric speciation in the Paroistodus horridus to P. originalis conodont lineage by means of microevolution of endemic subspecies. In turn, Astini (1998a; Astini and Rapalini, 2003) used this interpretation, which is based on geographic isolation of intermediate subspecies, in support of the argument for the geographic isolation of the Precordillera.

However, given that Middle Ordovician conodont faunas of the Precordillera are diverse, that endemic species are few (and would be expected in diverse faunas), and that most species are globally widespread, the occurrence of two, short-lived endemic subspecies as evidence for drifting and isolation of the Cuyania terrane is surely overstated.

More surprising is the fact that the evolutionary lineage occurs in strata interpreted as foreland basin sediments recording the docking of Cuyania with Gondwana. Thus, on the one hand Astini (Astini and Rapalini, 2003) argues that the conodont lineage represents isolation of Cuyania, while on the other, he interprets the strata containing the conodont lineage to have been deposited during the collision of Cuyania with Gondwana (Astini et al., 1995; Thomas and Astini, 1996; and several others).

Neither Albanesi and Barnes (2000) nor Astini and Rapalini (2003) considered other models for producing the environmental perturbations and geographic isolation that may have influenced the micro-evolutionary event. The parautochthonous model with Cuyania as a carbonate platform on the margin of Gondwana, experiencing extension and subsidence of several pull-apart basins along a transform fault, surely would have.

Conclusion
Taken together, the evidence from the Early to early Mid Ordovician benthic faunas, primarily brachiopods and nektobenthic conodonts, is consistent with the paraautochthonus model but has several significant inconsistencies with the Laurentian microcontinent model, and in some instances these paleobiogeographic data have been misused or misrepresented.

Affinity of benthic shallow-water Cambrian trilobite faunas

The presence of shallow-water trilobites of Laurentian affinity in Cambrian strata in the Precordillera, particularly olenellid trilobites and the problematic fossil Salterella in upper Lower Cambrian strata, is the most compelling evidence for a Laurentian origin of Cuyania.

It is the ultimate argument used by proponents of the Laurentian microcontinent model to negate inconsistencies of their model with any other evidence (e.g., detrital zircon age populations). Nevertheless, Cambrian trilobite paleobiogeography is not inconsistent with the parautochthonous model. In fact, the abundance, diversity, and distribution of trilobites in Cambrian strata of the Precordillera are consistent with Cuyania having been a carbonate platform in tropical waters on the southern margin (present coordinates) of West Gondwana during the Cambrian.

According to the Laurentian microcontinent model (Thomas and Astini, 1996), Cuyania had migrated past the Alabama promontory and was completely separated from Laurentia by Mid Cambrian time and was well into the Iapetus Ocean by Late Cambrian time (Fig. ). The Laurentian model requires, therefore, that Mid and Late Cambrian shallow-water, Laurentian trilobites dispersed across oceanic waters in order to invade Cuyania.

Why couldn’t late Early Cambrian trilobites, largely olenellids, have done the same?

General Distribution

Cambrian trilobites are irregularly distributed through the carbonate stratigraphic successions of the Precordillera (Keller et al., 1994; Vaccari, 1994), which are composed of inner and outer platform carbonate facies with the former occurring in more or less continuous (autochthonous) sections exposed in the eastern and central Precordillera and the latter occurring only in olistoliths within the Los Sombreros and Estancia San Isidro formations and related strata along the western margin of the central Precordillera (Fig. ). Pelagic agnostids, abundant and diverse in outer platform facies, are cosmopolitan, but the benthic polymeroids of both inner and outer platform facies are mainly of Laurentian affinity.

In inner platform facies (the La Laja, Zonda, La Flecha and lower La Silla formations and, in the northernmost Precordillera, the Cerro Totora and Los Hornos formations), trilobites are common and diverse in only a few zonals intervals (Bordonaro, 2003), e.g. the Upper Cambrian Crepicephalus Zone (Fig. ).

Most zones are represented by few species (e.g. 4 species in the Olenellus Zone in the La Laja Fm, but only 1 for that zone in the Cerro Totora Fm; 2 in the Middle Cambrian Bolaspidella Zone; only one in the Upper Cambrian Aphelaspis Zone and two in the Upper Cambrian Saukia Zone).

Some zones (e.g. Cedaria Zone, and Dunderbergia to Prosaukia Zones) are not represented because coeval strata (e.g. the Zonda and Los Hornos formations and part of the the La Flecha Fm) lack trilobites; other zones (particularly Albertella and Plaguira-Poliella) are missing at the lower Middle Cambrian Hawke Bay hiatus (Bordonaro, 2003).

These gaps are similar to, but not identical to, the successions in the Appalachians, where some zones are not represented.

Upper Cambrian
In the Upper Cambrian La Flecha Fm, which bears the richest trilobite fauna of the Precordillera that includes ten Laurentian genera, many of the species are endemic to the Precordillera (35% according to Vaccari, 1994), and some of the genera are represented by species that are not known from the Appalachians, but instead from far removed areas, such as Montana (namely, Crepicephalus brevispinus and Pemphigaspis cagnasi). Other Laurentian genera are represented by few fragmentary specimens of uncertain identification (Coosella by one pygidium; Coosina by one pygidium; Kingstonia – one species by three pygidia and one unidentified species by three cranidia).

The La Flecha Fm, according to Vaccari (1994), which represents the nearly complete Upper Cambrian of the Precordillera, includes three trilobite zones: the Crepicephalus Zone with ten species representing nine genera (at least two of which are based on single fragmentary specimens not identifiable at species level); the Aphelaspis Zone with two species (one unidentified) representing two genera; the Saukia Zone with two species representing two genera. In an un-named unit of the outer platform facies near Mendoza, which according to Heredia (pers. comm., 2005) occurs in olistoliths in the Estancia San Isidro Fm, the lowest Upper Cambrian Cedaria Zone is represented by four or five different genera, only one of which is represented by an identifiable species, and that species is endemic (Bordonaro, 2003). In other olistoliths with outer platform facies, the Crepicephalus Zone is recognized by two identified species and two unidentified species representing four genera; the Elvinia Zone is recognized from four species (two Laurentian and two endemic) representing two genera; the Saukia Zone is based on endemic or unidentifiable species of three non-agnostid genera (Bordonaro, 2003).

In contrast, in the southern Applachians (Resser, 1938), trilobites are found only in the Nolichucky Fm of the lower Upper Cambrian. They represent only the Cedaria and Crepecephalus zones, which include, respectively, 17 species in 11 genera and more than 100 species in 16 genera. Higher trilobite zones are not represented in the southern Appalachians.

Similar large disparities in numbers of genera, relative to the Precordillera, have been reported for the Elvinia and Saukia zones of the central and northern Appalachians and in the more or less complete succession of Upper Cambrian zones of the Llano Uplift of Texas (Lochman-Balk and Wilson, 1958).

Middle Cambrian
Comparison between Middle Cambrian trilobite faunas of the Precordillera and Laurentia is similar to that of the Upper Cambrian with the faunas of the Precordillera being a small sample of those of eastern Laurentia and with several endemic species and genera. These are reviewed in descending stratigraphic order. Thirteen genera occur in the Bolaspidella Zone of eastern North America in primarily cratonic sequences of the Appalachians. Only two genera, both of Laurentian affinity but represented by unidentified species, have been reported from the La Laja Fm (Bordonaro, 2003).

In outer platform facies in olistoliths, this zone is represented by abundant, diverse agnostids and by five genera of polymeroids represented by endemic species (Borrello, 1971). The Bathyuriscus (or Ehmaniella) Zone is represented in the La Laja Fm by four genera with six species, all of which are endemic. In olistoliths, the coeval Oryctocephalus Zone includes five genera that also occur in Laurentia. However, two of these (Tonkinella and Oryctocephalus) occur on paleoplates that were distributed worldwide in tropical and temperate latitudinal belts during the Cambrian (Shergold, 1969; Gozalo et al., 2003). Coeval faunas from the southern Appalachians include 4 to 7 genera with 12 genera reported from the entire eastern United States.

The Glossopleura Zone is the lowest zone of the Middle Cambrian above the Hawke Bay Hiatus. It is found only in outer platform facies in olistoliths and is represented by five genera, two of which are endemic to the Precordillera.
Of the other three genera, one is known elsewhere only from the northwestern Mexico and is represented in Argentina by an endemic species, and a second is represented in the Precordillera by two endemic species (Borrello, 1971; Bordonaro, 2003). In contrast, the Glossopleura Zone in eastern North America contains more than 15 genera, and, in the southern Appalachians, it is represented by an average of 10-13 species in 8 to 10 genera. The Middle Cambrian trilobites of the La Laja Fm are currently under study by Osvaldo Bordonaro and Brian Pratt (pers. comm., 2005), who are discovering that a higher percentage of the species are Laurentian rather than endemic.

Nevertheless, the Middle Cambrian trilobite faunas of the Precordillera, as with those of the Upper Cambrian, are of lower diversity than those of the Appalachians and include a significant number of endemic taxa. This comparison to faunas of the Appalachians is considered essential for testing the Laurentian microcontinent model because Middle Cambrian strata accumulated only in narrow belts on the eastern and western margins of Laurentia and, according to the model, Cuyania migrated past the southern end of the eastern belt during the Mid Cambrian (Fig.)

Ubicación del Terreno Precordillera en Laurentia	Migración del Terreno Precordillera	Acresión del Terreno Precordillera al margen de Gondwana

Lower Cambrian

Olenellid trilobites inhabited shallow seas in eastern North America and in Cuyania in the late Early Cambrian, and the olenellids of the Precordillera are genera considered to be of “Laurentian” affinity. However, in order to truly evaluate this relationship, one must first appreciate the taxonomic composition of the olenellids and their global distribution through their entire Early Cambrian history.
Olenellids are a large taxonomic group, the Suborder Olenellina (Table 2) within the Order Redlichiida and restricted to the Lower Cambrian, except for one genus. The Order Redlichiida also includes the Suborder Redlichiina with two superfamilies, the superfamily Redlichioidea largely restricted to the Lower Cambrian but with a few genera ranging into the lower Middle Cambrian, and the superfamily Paradoxidoidea that ranges from the upper Lower to the Middle Cambrian.
The olenellids characterize shallow shelf trilobite faunas of Laurentia, Siberia, Gondwana/ Perigondwana (limited to Portugal, Spain, Germany, and Morocco), Avalonia, and Baltica (Fig. ).

Redlichiid trilobites characterize shallow shelf trilobite faunas of Antarctica, Australia, the various paleoplates of China and East Asia, basically East Gondwana and Kazakhstan. Trilobites of both realms overlap in Morocco, southern Europe and Siberia (Pillola, 1990; McKerrow et al., 1992). There also was paleobiogeographic differentiation within the olenellids with most genera restricted to one of five paleoplates: Laurentia, Siberia, Gondwana/Perigondwana, Siberia, Baltica, and Avalonia. Figure below 2 show the distribution of olenellid genera among these paleoplates. The argument for the Laurentian affinity of the Precordillera is based on the restriction to Laurentia of trilobites of the genus Olenellus and closely related genera. However, this assumption must be re-evaluated for the following reasons.

1. Some olenellids occur on more than one paleoplate.
Fallotaspis has for instance been recorded from Laurentia, Siberia, Gondwana, and Avalonia. Kjerulfia is known from Baltica, Avalonia, and Gondwana (Morocco). Holmia occurs in Baltica, Siberia, and Gondwana. Paranevadella and Nevadia occur in Laurentia and Siberia.

2. Much of the paleogeographic differentiation of olenellids reflects geologic time (stratigraphic position). Olenellus and the other closely related genera, naturally grouped together in the Family Olenellidae, evolved and diversified during the late Early Cambrian, a time in which few other olenellids existed elsewhere in the world (Fig. ), having largely gone extinct on the other paleoplates at the end of mid Early Cambrian, probably due to unfavorable environmental conditions. Thus, although genera of the Olenellidae evolved and diversified within Laurentia, their chances of dispersing to other paleoplates were minimal in light of the fact that conditions must not have been favorable on the other paleoplates for whatever reason. The opposite appears to be the case in the Early Cambrian when olenellids of the Family Fallotaspidae were able to evolve and diversity on the Siberian and Gondwana/Perigondwana paleoplates, while conditions were unfavorable for olenellids in Laurentia, Avalonia, and Baltica. One of the earliest olenellids Fallotaspis, however, was able to disperse among Siberia and Gondwana/ Perigondwana and then later to Laurentia and Avalonia (Lieberman, 2002). The mid Early Cambrian was a time of diversification of olenellids on all paleoplates and dispersal between them ( Fig.) . In the mid Early Cambrian, several genera occur on more than one paleoplate (Kjerulfia, Holmia, Nevadia, Paranevadella and possibly Andalusiana and Selindella). In addition, most families of mid Early Cambrian olenellids (Holmiidae, Archaeaspididae, Judomiidae, Nevadiidae) are composed of genera distributed among more than one paleoplate (Table 2).

Thus, when diversity of olenellids was highest and favorable conditions were present on most paleoplates, there was significant dispersal of olenellids between paleoplates.

3. Olenellids in the Precordillera are stratigraphically restricted, correlating with a very short interval in the upper Lower Cambrian – the middle and lower upper Olenellus zone (Fig. ). This correlation is based on the occurrence in the Precordillera of species of four genera/subgenera: Olenellus (Mesolenellus) based on one endemic species, Fremontella based on one endemic species, Bristolia based on specimens unidentifiable at species level, and Arcuolenellus based on one endemic species. The number of available specimens as reported in the literature is very small. The recognition of Fremontella is based on three small fragments of cephalons from the Villicum range (Borrello, 1963) and one small fragment of a cephalon from the Sierra de Zonda (Borrello, 1964), both in the La Laja Fm.

Bordonaro (1986) later described Olenellus (Mesolenellus) from the La Laja Fm from the Sierra de Zonda on the basis of seven listed fragmentary specimens of the species O.(M.) zondaensis and five very fragmentary specimens of an unidentified species. Also, he reported seven very small fragments as an unidentified species of Bristolia. In contrast, Arcuolenellus megafrontalis is based on a large number (~ 30) of very well preserved specimens from the Cerro Totora Fm (Vaccari, 1988), but these are from only a single, thin (~ 20 cm) shale interval at the very top of the formation and nowhere else in the section that is several tens of meters thick. One incomplete glabella from the Los Túneles
olistolith in the Río Jáchal section is referred to Olenellus? (Vaccari and Bordonaro, 1993). From this olistolith, Vaccari and Bordonaro (1993) also described a new, endemic species of the ptychoparid trilobite Sombrerella, which elsewhere occurs only in the Lower Cambrian of Sonora, Mexico.

Finally, Astini et al. (2004) report olenellids, unidentified at family, genus, or species level, occurring with Salterella in the Ancaucha olistolith. This is the total report of olenellids from the Precordillera.

4. Lower Cambrian strata and thus olenellids are absent in that part of Laurentia surrounding the Ouachita embayment. The nearest Lower Cambrian strata with trilobites are to the east in the southernmost Appalachians of Alabama and to the west in Sonora, Mexico, eastern California, and Nevada. In the Appalachians, the oldest trilobites are those of the upper Lower Cambrian, in particular the middle and upper parts of the Olenellus Zone; thus they are coeval with the olenellids of the Precordillera.

They occur from Alabama to western Newfoundland in a vertical facies succession that includes Antietam Quartzite, Shady Dolomite, and the Rome Fm in the southern Appalachians and comparable facies to the north (Resser, 1938; Palmer, 1971). A total of five species of Olenellus and one of Wanneria have been reported from numerous localities in the southern Appalachians. In the western Cordillera of North America, olenellid-bearing strata range from the upper “Fallotaspis” Zone to the top of the Lower Cambrian (Fig. ), and almost all of the Laurentian olenellid genera have been reported from this region with many genera represented by many species (e.g. Palmer and Halley, 1979). Of the “Laurentian” genera in the Precordillera, Fremontella is known only from Alabama; Arcuolenellus is known only from the western Cordillera; Bristolia is known from the southern western Cordillera and Greenland; Olenellus (Mesolenellus) is known from the western Cordillera and Greenland.

5. Given the information in 3 and 4, it follows that the olenellid fauna of the Precordillera is a very small sample, in terms of both diversity and abundance, of the coeval olenellid fauna of Laurentia. In addition, although Cuyania must have been in part adjacent to the southern Appalachians in late Early Cambrian time, according to the Laurentian microcontinent model, the fauna of the Precordillera is endemic at the species level and its genera are more representative of the western Cordillera.

6. The enigmatic fossil Salterella together with unidentified (or unidentifiable) olenellids in the Ancaucha olistolith is offered as convincing evidence of the Laurentian origin of Cuyania (Astini et al., 2004). As with genera of the Olenellidae, Salterella is known only from Laurentia, where it is widespread and occurs with Olenellus and associated genera. However, Salterella is most abundant and widespread in Laurentia in carbonate rocks, such as the Shady Dolomite of the southern Applachians and correlative carbonates to the north, that contain not only Olenellus but also archaeocyathids and the olenellid Wanneria (Fritz and Yochelson, 1988; Resser, 1938; Palmer, 1971). Therefore, it is notable that Wanneria and archaeocyathids do not occur in the Precordillera in coeval carbonate rocks in the La Laja Fm and in skeletal limestones with Salterella in the Ancaucha olistolith.

Given the reasons listed above, the location of Cuyania within southern Laurentia in the Early Cambrian is questioned. If it was, its outer margin would have been more-or-less continuous with Lower Cambrian strata of the southern Appalachians. Why are the olenellids of the Precordillera not more abundant and diverse? Why are the species endemic? Why are there more genera from the western Cordillera? Why are there no archaeocyathids and Wanneria with Salterella?

On the other hand, the Cambrian trilobite evidence is compatible with the parautochthonous model. On the southern margin of West Gondwana, Cuyania in the Early Cambrian would have been a carbonate platform in the tropics, west of Laurentia and directly within the path of westward flowing equatorial currents. Almost all of West Gondwana was mountainous terrain covered with siliciclastic sediment following the Brasiliano/Pan-African orogenies.

Given the complete absence of trilobite bearing Cambrian strata in all of West Gondwana except Morocco, far removed at high southern paleolatitude, the only way in which trilobite faunas could have populated Cuyania would have been by larval dispersal from Laurentia, and that larvae likely would have included “Laurentian” olenellids as well as Salterella. Several genera of olenellids were able to migrate across narrow oceans and between paleoplates in early Early and mid Early Cambrian time.

Thus, there is no reason to assume that some genera could not do so again in late Early Cambrian time.
Per Ahlberg (pers. comm., 2005), who has worked and published extensively on olenellids, supports this in a written communication quoted as follows: “Endemism is so pronounced among the olenellid trilobites and it is obvious that they could not disperse across wide oceans. Some genera, however, seem to occur on more than one paleocontinent. Fallotaspis has for instance been recorded from both Siberia and Gondwana, Kjerulfia is known from Baltica and Gondwana (Morocco), and Holmia occurs in Baltica and Siberia.

Thus, it appears likely that their larvae could disperse across narrow oceans, provided that the environment was roughly the same on both sides of the ocean.Your model with dispersal of Olenellus and Olenellus-like trilobites from Laurentia westwards across a narrow ocean to the Precordillera terrane seems not unlikely.”

It is significant that before the terrane hypothesis, trilobite experts (Palmer, 1972; Ross, 1975; Bordonaro and Banchig, 1995) attributed the paleobiogeography of Cambrian trilobites, including the occurrence of Laurentian trilobites in the Precordillera, to latitudinal or climatic belts and to dispersal by ocean currents. While some of these experts later invoked the terrane hypothesis, i.e. the Laurentian microcontinent model, it was not because they claimed that the alternative - dispersal by oceanic currents - was deficient.

Finally, Benedetto (2003) states that the inner-shelf Olenellid trilobite Realm in the Precordillera
cannot be explained by patterns of oceanic circulation, due to the fact that dispersion of benthic organisms across oceanic basins can only be achieved, at most, by a few eurytopic genera but never by the fauna as a whole. Of the eleven genera and subgenera of olenellids within the middle and lower upper parts of the Olenellus zone of Laurentia, only four occur in the Precordillera, generally in very low abundance and represented by endemic species. Although Salterella occurs in the Precordillera, the olenellid Wanneria and archaeocyathids that commonly occur with Salterella in Laurentia (Fritz and Yochelson, 1988) are absent.

Thus, the fauna of the Precordillera is a very small sample of the “Laurentian” fauna; a situation that, according to Benedetto (2003), is consistent with dispersal by oceanic circulation.

Conclusion
The distribution of trilobites in Middle and Upper Cambrian strata of the Precordillera follows the pattern established in the upper Lower Cambrian with the degree of affinities between faunas of the Precordillera and Laurentia varying with intensity of oceanic circulation, evolution of trilobite faunas and their dispersal abilities, evolution of facies within the carbonate platform of Cuyania, changes in sea level, and changes in the distance between and the relative paleolatitudes of Cuyania and Laurentia.
An objection to the parautochthonous model is the difference in trilobite faunas between Cuyania and East Gondwana, particularly Antarctica. Given that Antarctica was to the west of Cuyania in the Cambrian and that equatorial currents flow to the west, it is highly unlikely that redlichiid trilobites of East Gondwana would have dispersed eastwards to Cuyania.

Whether or not olenellid trilobites dispersed westward to Antarctica is difficult to determine because of their brief existence in Cuyania and the very limited and incomplete Cambrian fossil record for Antarctica (Palmer and Gatehouse, 1972; Palmer and Rowell, 1995). Some non-olenellid, benthic trilobites of Laurentian affinity did disperse to East Gondwana in the Early Cambrian, namely Bonnia and Kootenia, and the lower Upper Cambrian Aphelaspis Zone of Laurentia is recognized in the Minaret Fm of Antarctica by the occurrence of Laurentian species (Shergold and Webers, 1992).
At other times there may have been barriers to westward circulation and dispersal of “Laurentian” trilobites from Cuyania to Antarctica. Accordingly, the Cambrian trilobite record is compatible with the parautochthonous model for Cuyania, but there are some difficulties in reconciling it with the Laurentian microcontinent model because of the very limited abundance and diversity of Cambrian trilobites in the Precordillera relative to Laurentia.

Juxtaposition in the Mid Ordovician of Atlantic and Pacific province graptolite faunas by the Laurentian microcontinent model Although the paleobiogeography of benthic faunas is cited widely in econstructing the paleogeography of the Cuyania, the paleobiogeography of lanktonic graptolites is largely ignored. Is this because of an assumption that “shallow-water faunas provide an excellent biogeographic signal, whereas marginal faunas provide evidence of open ocean circulation but are not so diagnostic biogeographically” (Dalziel et al., 1996), or is it because the affinity of Mid Ordovician graptolite faunas in Cuyania are not easily reconciled with the Laurentian microcontinent model?

Graptolites are common and faunas are diverse in Middle and Upper Ordovician strata of the Precordillera (Maletz and Ortega, 1995). They represent the Pacific faunal province that was of global extent at low paleolatitude (Finney and Chen, 1990). In contrast, graptolites of the Atlantic faunal province, also of global extent but at high paleolatitude, occur in Lower and Middle Ordovician strata of the Famatinian belt and the Cordillera Oriental.

The sharp contrast between the two regions in faunas of the Mid Ordovician when provincialism was
at its greatest was noted by Finney and Chen (1990), Maletz and Ortega (1995) and Mitchell et al. (1997), among others. In the Laurentian microcontinent model, the Pacific province and Atlantic province faunas would have inhabited adjacent, but contiguous, areas of marine waters along the Gondwanan margin. Dalziel (1997) tried to attribute this juxtaposition to separation by oceanic currents, but in light of models of graptolite paleoecology (Finney and Berry, 1997), in particular the ability of graptolites to disperse globally, this distinction of globally distributed faunas in close geographic proximity could not have been maintained, a conclusion also favored by Maletz and Ortega (1995) and Mitchell et al. (1997). On the other hand, these graptolite distributions are compatible with, and readily explained by, the parautochthonous model. Through the Mid Ordovician, Cuyania was at relatively low paleolatitude within the realm of the Pacific graptolite faunal province, far removed from the Famatinian belt and the Cordillera Oriental, which were at substantially higher southern paleolatitude within the realm of the Atlantic province.

DISCUSSION: THE PARAUTOCHTHONOUS MODEL RECONSIDERED

All the evidence described above, including the occurrence of olenellid trilobites, is compatible with the
parautochthonous model (Aceñolaza et al., 2002; Finney et al., 2005b). Questions have been raised regarding two aspects of the model proposed by Finney et al. (2003, fig. 3; 2005b, fig. 6), namely the Cambrian location of Cuyania and its subsequent migration to its present position outboard of the Famatina belt. The Finney et al. (2005b) reconstruction (Fig. 5) is a schematic diagram based on the reconstruction of Gondwana by Hoffman (1991).

Located on the southern (present coordinates) margin of West Gondwana in the Cambrian, Cuyania would have been part of, or adjacent to, Mesoproterozoic and Neoproterozoic orogenic belts composing and bordering the Río de la Plata and Kalahari cratons - a location consistent with detrital zircon and paleobiogeographic evidence. Considering that Hoffman’s reconstruction may not portray accurately this continental margin, Cuyania is shown in a general location and as an oversized terrane to increase its visibility in Fig. 5. The constraints imposed by Hoffman’s reconstruction also require that Cuyania be shown migrating along a curved path from Cambrian to Devonian time and rotating in the process. However, given the proposal that it migrated along a transform fault, it is more likely that it followed a relatively straight path and did not rotate.

The width of the Iapetus Ocean is of considerable importance to the Laurentian microcontinent model and to the Finney et al. (2005b) version of the parautochthonous model (Fig. 5). The great width in the Laurentian microcontinent model (at least 2,000 kms) is based on interpretations of paleogeographic data that include an isolation stage for Cuyania, resulting from it being separated from both Laurentia and Gondwana by at least 1,000 kms in Early Ordovician time (Thomas and Astini, 1996). Here, the representation and interpretation of the paleobiogeographic data are challenged, and it is emonstrated that Laurentian faunal elements continued to migrate to Cuyania during the Early Ordovician and even
in the Mid and Late Ordovician. Thus, the parautochthonous model includes a relatively narrow Iapetus Ocean.
Evolving paleogeographic affinities of the benthic and pelagic faunas of Cuyania are attributed to its migration across paleolatitude initially during Cambrian to Mid Ordovician time as a part of Gondwana as the supercontinent itself migrated southwards (Fig. ) and from Mid- Ordovician time onwards by further relative southward migration along a transform fault(s).

The parautochthonous model is a hypothesis that appears to be most consistent with all available evidence. As with any hypothesis, there are still many aspects of the model to be further tested and checked and new evidence to be obtained and incorporated. No doubt, it will evolve further. Nevertheless, it has generated exciting, creative new research, new discoveries, and new ideas that would not have been possible if one is constrained by the Laurentian microcontinent model.

For example, Peralta (2005a, b) has realized and subsequently confirmed, that many of the large olistoliths composed of siliciclastic strata in the Los Sombreros and Rinconada formations were derived from the La Chilca, Los Espejos, Talacasto and Punta Negra formations of Silurian and early Devonian age, indicating that various parts of the Los Sombreros and Rinconada formations were deposited during early Devonian time. Peralta has interpreted the olistostromes to represent extensional pull-apart basins along major strike-slip faults. In addition, Peralta and Heredia (2005) have discovered that the Upper Ordovician Empozada Fm at San Isidro is overlain stratigraphically by a thick succession of shale and siltstone with olistoliths of shale that have Devonian land plant fossils, as well as abundant, huge olistoliths of Empozada Fm. The Empozada Fm stratigraphically overlies the Estancia San Isidro Fm that includes huge olistoliths composed largely of Middle and Upper Cambrian carbonate strata.

According to the Laurentian microcontinent model (Thomas and Astini, 2003), deposition of the carbonate olistoliths, those of both the Estancia San Isidro and Los Sombreros formations, occurred on the western continental slope and rise of the Cuyania microcontinent (Fig. ). However, the stratigraphic succession at San Isidro (Estancia San Isidro and Empozada formations) more likely accumulated in a strike-slip extensional basin because of 1) its great variety of sediment types representing a diversity of shallow and deep water depositional processes and 2) the Empozada
conglomerate bed composed of huge (1 m), rounded granitic boulders representing subaerial exposure and fluvial transport to the basin.

Now Peralta and Heredia (2005) have discovered that this extensional setting, possibly driven by movement along major strike-slip faults and operating from Mid to Late Ordovician time, was also active in Devonian time. Taking into account Peralta’s discoveries and re-interpretations of the Los Sombreros and Rinconada formations, it is apparent that huge strike-slip basins are present in the Precordillera, some of the most amazing examples in the world, and that they operated at least from Mid Ordovician to Devonian time.

If constrained by the Laurentian microcontinent model, they would not have been recognized.
Similarly, other discoveries have been made in the Cuyania terrane and the neighboring western Pampeanas ranges that are not clearly consistent with the Laurentian microcontinent model and perhaps also justify consideration of new models. These include: 1) The evidence of Rapela et al. (2005a), Galindo et al., (2004), and Baldo et al., (2005) that “the Western Sierras Pampeanas, which are considered part of the exotic Argentine Precordillera terrane of allegedly Laurentian derivation, could be autochthonous or parautochthonous to Gondwana.”
2) The suggestion that rocks between the Precordillera and the Famatina belt and previously considered part of Cuyania and of Laurentian origin are, instead, “a distinct crustal fragment of Gondwana or peri-Gondwana affinity…” (McClelland et al., 2005).
3) The description of Devonian age shear zones along the eastern margin of Cuyania (Roeske et al., 2005).
4) The discovery of Grenvillian age basement and overlying Neoproterozoic meta-sedimentary rocks in the Western Sierras Pampeanas (Casquet et al., 2005). It was obvious at the Gondwana 12 Conference in November 2005 (Pankhurst and Veiga, 2005) that there is much exciting research taking place on the rocks of the Cuyania terrane and the Sierras Pampeanas. It is a time for creative, innovative thinking and exciting new discoveries. This research should not be constrained in its starting assumptions, its methodological approach, or its data interpretation by any one model, whether it is the Laurentian microcontinent model or the parautochthonous model.

CONCLUSIONS
Although substantial, varied geological evidence is considered by many geologists to support the Laurentian microcontinent model of Thomas and Astini (1996), recently acquired U-Pb geochronology of detrital zircons from Cambrian and Ordovician sandstones in the Precordillera cannot be reasonably explained by a Laurentian affinity. Instead, these data are most consistent with a Gondwanan provenance and the parautochthonous model of Aceñolaza et al. (2002). Accordingly, it is incumbent that the many lines of evidence cited in support of the Laurentian model be re-examined critically for their consistency with the parautochthonous model. This assessment has revealed that:

1) potential basement rocks of Cuyania of Neoproterozoic to Early Cambrian age and Early Mesoproterozoic age are characteristic of Gondwana, rather than Laurentia;

2) Pb isotopic ratios of Grenvillian-age basement rocks are not only similar to those of Grenvillian base ment in Laurentia but also to those in other areas of West Gondwana;

3) the similarity of the Cambrian-Ordovician carbonate platform succession of Cuyania to that of Laurentia reflects similar paleolatitude and eustatic histories and not a direct connection;

4) the Middle-Upper Ordovician siliciclastic successions of Cuyania do not represent a peripheral foreland basin, but instead were deposited in strike-slip basins in a transform fault zone;

5) Middle Ordovician K-bentonites do not indicate that Cuyania was approaching the Famatina magmatic arc from the west (modern coordinates), but instead that it was located to the southeast;

6) the paleomagnetically determined Cambrian paleolatitude of Cuyania is consistent not only with the location of the Ouachita embayment of Laurentia but also with the southern margin of West Gondwana;

7) brachiopod and conodont faunas in lower Middle Ordovician strata of the Precordillera have many more genera in common with Laurentia than those in Lower Ordovician strata;

8) Cambrian trilobites faunas of Cuyania are of very limited abundance and diversity in comparison to correlative faunas of southeastern Laurentia; many species are endemic to Cuyania; olenellid trilobites considered to be restricted to Laurentia probably had the ability to disperse between paleoplates with similar environments; and

9) Mid Ordovician graptolites of the Precordillera on the one hand and of the Famatinian belt and Cordillera Oriental on the other belong to different oceanic provinces and likely did not live in close proximity.
Accordingly, the evidence used to support the Laurentian microcontinent model is found to be more consistent with the parautochthnous model, in which the Cuyania terrane was located on the southern margin of West Gondwana until the Mid Ordovician when it began migrating along a transform fault to its present position outboard of the Famatina magmatic belt in Devonian time.

arriba

BLOQUE DE SAN RAFAEL, LAS MATRAS Y CHADI LEUVU

Afloramientos del basamento Grenvilliano y Calizas en Mendoza	Geología de detalle del sector de Ponón Trehue	Datación Fm Cerro La Ventana

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