The
Ordovician System in the Famatina Belt: Depositional and Tectonic Evolution
M.
Gabriela MÁNGANO1, Ricardo A. ASTINI2, Luis A. BUATOIS1 and
Federico DÁVILA2
1 Insugeo, CONICET, Casilla de correo 1 (CC), 4000 San Miguel de Tucumán, Argentina.
E-mail: ichnolog@infovia.com.ar2 Cátedra de Estratigrafía y Geología Histórica, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, CONICET, Av. Vélez Sarsfield 299, 5000 Córdoba, Argentina.
E-mail: raastini@com.uncor.eduAbstract.
THE ORDOVICIAN
SYSTEM IN THE FAMATINA
BELT:
DEPOSITIONAL AND TECTONIC
EVOLUTION.
The Ordovician rocks of Famatina exceed 3000 m thick
and include Late Cambrian to Tremadocian carbonates and siliciclastic rocks,
Arenigian to Llanvirnian? volcano-sedimentary deposits, and several Early
Ordovician arc-related intrusives, allowing to reconstruct a relatively complex
continental margin history. Analysis of the Ordovician rocks in the Famatina
Belt provides information on the tectonic setting of the western margin of
Gondwana during the early Paleozoic and the dynamics of peri-Iapetus volcanic
arc-related depositional systems. Although there is agreement on the active
plate margin emplacement of the Famatina Basin, doubts still persist regarding
its more precise tectonic setting. Integration of petrographic and
sedimentologic evidence suggests an arc formed on continental crust, rather than
oceanic crust. While some authors considered the Famatina belt as part of the
western Gondwana margin, recent studies suggest that Famatina may represent an
independent terrane based on paleomagnetic and paleontologic data. Integration
of information from the different areas of the basin sheds light on the
depositional and tectonic evolution of the Famatina Basin. Four evolutionary
stages are recognized: (1) Late Cambrian to early Tremadocian, (2) late
Tremadocian, (3) early Arenigian and (4) middle Arenigian to Llanvirnian? The
first stage represents the onset of sedimentation within the Famatina Basin
after a period of quiescence and subsidence of folded Pampean age basement
rocks. The transition from shelf deposits to blackshales in the early
Tremadocian suggests gradual deepening related to a relative sea-level rise and
consequent drowning. The presence of late Tremadocian silicified sandstones with
volcanogenic detritus in rocks of the second stage represents the first evidence
of volcanism recorded in the Famatina Belt and can be correlated with the onset
of volcanism in the Puna region. The early Arenigian phase is characterized by
relatively deep-marine deposits that accumulated in a series of depocenters
formed by extensional tectonics within an intra-arc basin. Explosive volcanism
is mainly documented by accretionary lapilli in sediment gravity flow and
pyroclastic flow deposits. During the forth stage (middle Arenigian to
Llanvirnian?) sedimentation accumulated in a narrow, highgradient shelf along a
volcanic arc that was close to sea level for most of its history. The volcanic
rocks recorded at the top of the lower Paleozoic succession in the Cerro Morado
Group and Las Planchadas Formation reveal a peak in volcanic activity by the
late Arenigian to probably Llanvirnian. Paleontologic evidence indicates
northsouth diachronism, suggesting facies changes in a relatively complex
architectural mosaic as is usually the case in volcanic arc settings.
Resumen:
EL
SISTEMA
ORDOVÍCICO
EN EL SISTEMADEL
FAMATINA:
EVOLUCIÓN DEPOSITACIONAL
Y TECTÓNICA.
Las rocas ordovícicas
del Famatina superan los 3000 m de espesor e incluyen depósitos siliciclásticos
y carbonáticos del Cámbrico Tardío y Tremadociano, depósitos
volcano-sedimentarios del Arenigiano a Llanvirniano? y numerosos intrusivos de
arco del Ordovícico Temprano, permitiendo reconstruir una compleja historia de
margen continental. El análisis de estas rocas ordovícicas en el Sistema del
Famatina proporciona información sobre el marco tectónico del margen
occidental del Gondwana durante el Paleozoico temprano y la dinámica de
sistemas depositacionales relacionados con arcos volcánicos del peri-Iapetus.
Si bien hay acuerdo sobre el emplazamiento de la cuenca del Famatina en un
margen activo, aún persisten dudas con respecto a su marco tectónico preciso.
La
integración de evidencias petrográficas y sedimentológicas sugiere un arco
formado sobre corteza continental, en lugar de oceánica. Mientras algunos
autores consideran al Sistema de Famatina como parte del margen del Gondwana
Occidental, recientes estudios basados en datos paleomagnéticos y paleontológicos
sugieren que Famatina podría representar un terrane independiente. La integración
de información proveniente de las diferentes áreas de la cuenca ayuda en
nuestro entendimiento de la evolución depositacional y tectónica de la cuenca
de Famatina. Se han reconocido cuatro estadíos evolutivos: (1) Cámbrico Tardío
a Tremadociano temprano, (2)
Tremadociano tardío,
(3) Arenigiano temprano y (4) Arenigiano medio a Llanvirniano? El primer estadío
representa el inicio de la sedimentación en la cuenca de Famatina con
posterioridad a un período de quietud y subsidencia de las rocas plegadas de
basamento de edad Pampeana. La transición de depósitos de plataforma a lutitas
negras en el Tremadociano temprano sugiere una profundización gradual
relacionada a un ascenso relativo del nivel del mar y el consecuente ahogo. La
presencia de areniscas silicificadas con detritos volcanigénicos en rocas del
Tremadociano tardío del segundo estadío representa la primera evidencia de
volcanismo registrada en el Sistema de Famatina y puede ser corrrelacionada con
el inicio del volcanismo en la región de Puna. La fase del Arenigiano temprano
está caracterizada por depósitos marinos relativamente profundos que se
acumularon en una serie de depocentros formados por tectónica extensional
dentro de una cuenca de intra-arco. El volcanismo explosivo está principalmente
documentado por la presencia de lapilli acrecional en depósitos de flujos
gravitatorios de sedimentos y por flujos piroclásticos. Durante el cuarto estaído
(Arenigiano medio a ?Llanvirniano) la sedimentación se acumuló en plataformas
angostas de alto gradiente a lo largo de un arco volcánico desarrollado cerca
del nivel del mar durante gran parte de su historia. Las rocas volcánicas
registradas en el tope de la sucesión del Paleozoico inferior en el Grupo Cerro
Morado y la Formación Las Planchadas revela un pico en la actividad volcáncia
durante el Arenigiano tardío a probablemente Llanvirniano. La evidencia
paleontológica indica un diacronismo norte-sur, sugiriendo cambios de facies en
un mosaico arquitectural relativamente complejo, como es habitual en ambientes
de arco volcánico.
Key
words: Famatina Belt. Ordovician. Volcanic arc. Depositional
evolution. Tectonics.
Palabras
clave: Sistema de
Famatina. Ordovícico. Arco volcánico. Evolución depositacional. Tectónica.
Introduction
The
Famatina Belt (Famatina System sensu Petersen and Leanza, 1953: 319) is
located in the La Rioja and Catamarca Provinces, approximately between 27º and
31º S latitude, in the broken foreland of the south Central Andes, northwest
Argentina (Figs. 1, 2). The Famatina Belt embraces several mountain ranges that
at present separate the northern extent of the Argentinean Precordillera to the
west and the Sierras Pampeanas region to the east. A probably Precambrian
low-grade metamorphic basement covered by early and late Paleozoic and Cenozoic
rocks are exposed in the Famatina Range. The Ordovician rocks of Famatina exceed
3000 m thick and include Late Cambrian to Tremadocian carbonates and
siliciclastic rocks, Arenigian to Llanvirnian? volcano-sedimentary deposits, and
several Early Ordovician arc-related intrusives, allowing to reconstruct a
relatively complex continental margin history.
The
presence of Paleozoic rocks in this region has been known since the pioneer work
of German geologists in the late nineteenth and early twentieth centuries, most
notably Kayser (1876), Brackebush (1891), Penck (1920) and Bodenbender (1916).
After a gap with few studies, subsequent work in the late twentieth century
provided additional information on the basin’s paleontological content and the
stratigraphic relations between the different early Paleozoic units (Turner,
1958, 1960, 1964, 1967; Maisonave, 1973; Levy and Nullo, 1973, 1980; Aceñolaza et
al., 1976; Aceñolaza and Durand, 1984; Aceñolaza and Toselli, 1977, 1988).
However, it was not until the nineties that the first detailed sedimentologic,
stratigraphic and systematic paleontologic studies were published, following
renewed interest in the Famatina Basin (Aceñolaza and Rábano, 1990; Mángano
and Buatois, 1990a,b, 1992a,b,c, 1994a,b, 1995, 1996a,b, 1997; Esteban, 1992,
1993, 1994, 1996; Clemens, 1993; Sánchez and Babin, 1993, 1994; Vaccari et
al., 1993; Vaccari and Waisfeld, 1994; Albanesi and Vaccari, 1994;
Benedetto, 1994, 1998, 2003; Tortello and Esteban, 1995, 1997, 1999; Mángano et
al., 1996; Astini and Benedetto, 1996; Toro, 1997, 1999; Toro and Brussa,
1997; Esteban and Gutierrez- Marco, 1997; Esteban and Rigby, 1998; Martino and
Astini, 1998; Astini, 1998, 1999a,b, 2001a,b; Albanesi et al., 1999;
Esteban et al., 1999; Aceñolaza and Gutierrez-Marco, 2000; Astini and Dávila,
2000, 2002; Sánchez, 2001; Conci et al., 2001; Dávila et al.,
2003). Additionally, the nature of the Ordovician igneous rocks and their
geochemistry have been studied by various authors (Toselli et al., 1990,
1993, 1996; Toselli, 1992; Mannheim, 1993a,b; Cisterna and Toselli, 1996;
Mannheim and Miller, 1996; Cisterna, 2001), whereas new isotope and
geochronological data have been recently published by Pankhurst et al. (1998,
2000) and Rapela et al. (1999, 2001), who addressed the nature and
evolution of the igneous suites in the context of the Gondwana active
continental margin.
Several
synthesis have also been published in recent years (Aceñolaza et al.,
1996, Mángano and Buatois, 1996b; Saavedra et al., 1998; Esteban et
al., 1999; Astini, 1999b).
Fieldwork
in the Famatina Basin is complicated by difficulties in accessing most of the
outcrops, rough topography and by the fact that outcrops are commonly
disconnected, preventing the establishment of sound stratigraphic relations.
Research was undertaken independently in separate regions and as a result
different sets of data must be integrated. In this chapter we do so by providing
a summary of our present understanding of the depositional and tectonic
evolution of the Famatina Basin. Analysis of the basin is of importance toward
understanding the tectonic evolution of western Argentina during the early
Paleozoic, and toward gathering valuable information on the dynamics of volcanic
arc-related depositional systems.
Stratigraphic
framework and correlations along the Famatina Belt
The
following units represent the lower Paleozoic succession in the Famatina Belt:
Volcancito, Suri, Molles, Portezuelo de Las Minitas, La Alumbrera, El Portillo,
La Escondida, Chuschín and Las Planchadas Formations (Fig. 3). These units are
bracketed between Late Cambrian to probably Llanvirnian rocks and were grouped
on the basis of different criteria. Turner (1964) included the Suri and Molles
Formations within the Famatina Group. Subsequently, Aceñolaza and Toselli
(1981) proposed the more embracing Cachiyuyo Group to include these two
formations together with the underlying Volcancito and Portezuelo de Las Minitas
Formations, and the overlying Cerro Morado and Las Planchadas Formations.
However, priority principles and recognition of unconformities within the lower
Paleozoic succession supports the use of the more restrictive Famatina Group
(Astini, 1998; Astini and Dávila, 2002; Dávila et al., 2003). The
stratigraphic relationship between the Famatina Group and the underlying
formations awaits further study.
The
early Paleozoic volcano-sedimentary succession unconformably overlies folded
low-grade metasedimentary rocks of the Negro Peinado Formation (Turner, 1960;
Toselli, 1978; De Alba, 1979; Astini, 2001a). The lowermost unit is the
Volcancito Formation, composed of about 590 m of mudstone and minor interbedded
sandstone (Turner, 1964), which has been divided into three members (Esteban,
1998; Esteban et al., 1999). The lower and middle members are exposed in
the Volcancito River and Peña Negra area, while the upper member outcrops in
the Bordo Atravesado area (Esteban, 1998). The lower member is 170 m thick and
consists of graded and laminated sandy limestone and marl, commonly with
cross-bedding and microhummocky cross-stratification, calcareous sandstone and
laminated black shale. Major calcareous breccias are well exposed near the base
(Astini and Dávila, 2000) and calcareous trilobite-rich coquinas are present
throughout the sequence (Esteban, 1998; Astini, 2001a,b). It ranges in age from
the Late Cambrian to the early early Tremadocian (Tortello and Esteban, 1999;
Albanesi et al., 1999). This member is interpreted as having been
deposited in a shallow-shelf environment; the carbonate-rich strata indicate
low-latitude warm waters (Astini, 2001a), representing similar environments than
those peripheral to Laurentia (Albanesi et al., 2000). Carbonate breccias
suggest coeval instability. The middle member is at least 260 m thick and is
made up mostly of black shale with minor, thin-bedded interbedded massive
mudstone (Esteban, 1998). The age of this member is constrained by graptolite
biostratigraphy (Esteban and Gutierrez-Marco, 1997; Esteban et al., 1999)
between middle to late early Tremadocian.
Rhabdinoporids
(including Rhabdinopora flabelliformis) and anisograptids are present
throughout this interval. The middle member has been deposited in a relatively
deep-water environment with a well-stratified water column throughout which
fine-grained settling took place in anoxic bottom conditions (Esteban, 1998).
Lack of benthic faunas and trace fossils supports an oxygen-depleted
environment. The Upper Member is 160 m thick and contains massive and laminated
mudstone with a few thin intercalations of massive silty sandstone, sandstone
with microhummocky crossstratification and silicified tuff (Esteban, 1993,
1998). The trilobite fauna suggests a late late Tremadocian to early Arenigian
age (Esteban et al., 1999). Preliminary conodont information seems to
support a late late Tremadocian age (Albanesi, personal communication, in
Esteban et al., 1999).
This
member outcrops in a separate area (Bordo Atravesado) to the south of the Rio
Volcancito region (Fig. 2). The lack of outcrop continuity and the stratigraphic
gap between this member and the middle member of the Volcancito Formation in its
type area suggest that the so-called upper member of the Volcancito Formation is
best regarded as a separate formation, the Bordo Atravesado Formation (Astini,
in press). This unit represents deposition in an outer-shelf environment and
records the onset of Ordovician volcanism (Esteban, 1993, 1998). Similar facies
described in the nearby Chuschín region in the western side of the present
Famatina Range by Mannheim (1993a) were included in the Chuschín Formation and
can be tentatively correlated with the unit exposed in Bordo Atravesado.
Little
is known about the Portezuelo de las Minitas and the La Alumbrera Formations,
which might be assigned to late late Tremadocian? to early Arenigian age (Aceñolaza
et al., 1976; Toro, 1997, 1999; Aceñolaza and Gutierrez-Marco, 2000;
Toro, personal communication, 2002). The Portezuelo de las Minitas Formation
consists of about 1500 m of conglomerate, sandstone and mudstone interbedded
with volcanic rocks (Lavandaio, 1973) and, from the available descriptions, it
may be coeval with the lower part of the Suri Formation. The La Alumbrera
Formation is approximately 120 m thick and consists of evenly laminated
pyrite-rich black shale with scarce thin interbedded silicified sandstone (Toro,
1997). In its upper half it yields a graptolite association from the Tetragraptus
phyllograptoides and T. akzharensis Zones (Toro, 1999). However,
abundant clonograptids and possible adelograptids occur in its lower interval
(Toro, personal communication, 2002), suggesting a stratigraphic position below
the lowest beds of the Suri Formation. The Tremadocian-Arenigian boundary is
probably present within this interval. The thin interbedded silicified
sandstones are possibly tuffs, suggesting a tentative correlation with the unit
exposed at Bordo Atravesado. Its stratigraphic position below Baltograptus
deflexus may also support a correlation with the volcanics located at the
base of the Suri Formation in central and northern Famatina. The La Alumbrera
Formation has been traced into the northern area by Aceñolaza (1978). Although
both La Alumbrera and Portezuelo de las Minitas Formations have been poorly
studied from a sedimentological viewpoint, their paleontological record seems to
indicate a fairly continuous record through the latest Tremadocian to the early
Arenigian. Both units most likely represent deposition in a deepwater setting
according to their graptolite-rich faunal content.
The Suri Formation, lower unit of the Famatina Group, includes approximately 1300 m of interbedded volcaniclastic and volcanic rocks (Mángano and Buatois, 1994a; Astini, 1998). In the Chaschuil area, the base of the Suri Formation is represented by a strongly altered andesite (Mángano and Buatois, 1997); in Sierra de Famatina this formation also occurs above volcanic rocks, referred to as the Cerro Tocino Volcanics (Astini, 1998; Astini and Dávila, 2002) (Fig. 2). The Suri Formation is divided into three stratigraphic members in the Chaschuil sub-basin: Vuelta de Las Tolas, Loma del Kilómetro and Punta Pétrea (Mángano and Buatois 1994a). Detailed stratigraphic sections and facies characterization of these units in the Chaschuil area were provided by Mángano and Buatois (1990a, 1992a, 1994a, b, 1996a,b, 1997). Astini (1998) recognized similar depositional units in the Sierra de
Famatina
area. However, the lowermost unit, consisting of deep-water black shale with
interbedded graded tuff, either is not present in the Chashuil area or
correlates with the lowermost fine-grained deposits of the Vuelta de Las Tolas
Member, representing a lateral facies change. The lower unit of Sierra de
Famatina is of early to middle Arenigian age, based on the presence of
graptolites of the Baltograptus deflexus and the Didymograptellus
bifidus Zones (Toro and Brussa, 1997). The Vuelta de Las Tolas Member, which
reaches a thickness of approximately 600 m, consists of interbedded fine-grained
deposits and volcanic conglomerate, breccia and sandstone. This unit records
deposition on a slope apron flanking the volcanic arc (Mángano and Buatois,
1997). The age of the Vuelta de Las Tolas Member is early Arenigian (Toro and
Brussa, 1997). The Vuelta de Las Tolas Member is succeeded upward by the Loma
del Kilómetro Member, about 600 m thick, which is composed of mudstone,
siltstone and volcaniclastic sandstone. The Loma del Kilómetro Member mostly
records episodic processes related to storms and sediment gravity flows in a
high gradient shelf adjacent to the volcanic arc (Mángano and Buatois, 1996a).
The age of this member is middle Arenigian (Albanesi and Vaccari, 1994; Vaccari
and Waisfeld, 1994). The upper unit of the Suri Formation, the Punta Pétrea
Member, is 50 m thick and consists of volcaniclastic sandstone, conglomerate and
breccia. It records progradation of a volcaniclastic-fan-delta system (Mángano
and Buatois, 1994b). Sedimentary dynamics in the Suri Formation were profoundly
affected by the contemporaneous eruptive activity of the adjacent volcanic arc.
In central Famatina several ignimbrites are represented in the Famatina Group,
being particularly abundant in the upper two members described by Astini (1998,
1999a) within the Suri Formation. The Molles Formation is the upper unit of the
Famatina Group. It consists of 100 m of red clayey sandstone, interbedded
reddish sandy claystone and volcaniclastic sandstone and breccia that
conformably overlie the Suri Formation in the Sierra de Famatina area (Turner,
1964; Astini, 1998). Meter-scale silicified tuffs, volcanic breccias and
volcanogenic sandstones alternate with green packages of muddy siltstones with
abundant Celtic brachiopod associations (Benedetto, 1994, 1998, 2003),
suggestive of a volcano-sedimentary interaction within a shallow marine
intra-Iapetus volcanic-arc setting. Sedimentologic analysis documented the
presence of structures indicative of tidal influence, such as herringbone
cross-stratification and mud drapes in some of the sandy packages (Astini,
1998). Conodont and acritarcs from the upper two members of the Suri Formation
and the Los Molles Formation in central Famatina suggest middle Arenigian or
uppermost Ibexian age (upper part of Oepikodus avae Zone) (Albanesi and
Astini, 2000; Rubinstein and Astini, 2000). Conodonts from the Baltoniodus
navis Zone recorded in the Loma del Kilómetro Member in the north clearly
indicate a younger age (late middle Arenigian) in this zone. Although from a
lithological viewpoint the Los Molles Formation has been correlated with the
Punta Pétrea Member in the Chaschuil area (Astini, 1998, 1999b; Esteban et
al., 1999), the associated shelly faunas indicate that the Punta Petrea
Member is slightly younger, therefore suggesting north-south facies gradients in
a relatively complex architectural mosaic as usually typifies volcanic arc
settings. A correlation between the Morado Group of central Famatina with the
Punta Petrea Member and the Las Planchadas Formation is herein suggested.
The stratigraphic position and age of the associated volcanic and pyroclastic calc-alkaline rocks have been controversial (cf. Turner, 1967; Maisonave, 1973). The Las Planchadas Formation, which crops out in the northern extreme of the Famatina geologic province, is now thought to fit within the upper part of the succession (Aceñolaza and Toselli, 1977, 1984; Toselli et al., 1990; Mángano and Buatois, 1994a; Cisterna, 2001). In the Sierra de Famatina area, the Molles Formation is uncomformably overlain by the Cerro Morado Group (Astini and Dávila, 2002). This group consists, from base to top, of the El Portillo and La Escondida Formations. The El Portillo Formation (formerly Cerro Morado Formation) is 580 m thick and is composed by acidic volcanics and ignimbrites. The La Escondida Formation is 147 m thick and consists of volcaniclastic sandstone, mudstone, tuff and ignimbrite that accumulated in a shelf affected by contemporaneous volcanism
and
frequent storms, as suggested by the association of tempestites and pyroclastic
flow deposits (Astini and Dávila, 2002). Astini and Dávila (2002) suggested a
link between hundred-meter scale regressive-transgressive cycles within both the
Famatina Group and the Cerro Morado Group and thermal inflation/contraction and
flexural response to loading, a prominent characteristic of volcanicarc
settings. Preliminary conodont data (Albanesi, personal communication, 2002)
suggest a middle to late Arenigian age, but thickness and usual duration of
volcano-tectonic cycles allow tentative assignment to the Llanvirnian (Astini
and Dávila, 2002). Brachiopod assemblages (Benedetto, personal communication,
2002) also support this estimation.
Volcano-sedimentary
successions were intruded during the Ordovician by syntectonic granitoids and
dykes (Cisterna, 1992, 2000; Rapela et al., 1999; Pankhurst et al.,
2000). Isotopic age constraints for intrusive activity along the Famatina belt
have had a considerable advance in recent years (Rapela et al., 1999,
2001; Pankhurst et al., 1998, 2000) allowing to sort out the mingled ages
embraced for years for the Famatina Orogeny, particularly those related to the
Early Ordovician Ocloyic event.
Rapela
et al. (1999,
2001) recently provided high-resolution dating bracketing the age of magmatism
and estimating metamorphic age range associated with the accretion of the
Precordillera terrane to Gondwana. These data allow clear differentiation of a
pre-amalgamation suite previous to 470 Ma, taken as the main intrusive period of
the Famatinian granites (Pankhurst et al., 2000) and a later thermal
metamorphism, largely bracketed between 460-470 Ma. This, in turn, has been
related to the closure of the Famatina basin on the Gondwana margin and
associated to rapid crustal thickening.
The
accretion event has apparently embraced both the western Gondwana margin (with
Pampean ages ca. 540-520 Ma) and the eastern basement sector of the
Precordillera terrane (with Grenville ages ca. 1000-1200 Ma) according to Rapela
et al. (2001) and Baldo et al. (2001). These data are seldom
obscured by a much younger Early Devonian superimposed shortening event recorded
along most of the proto-andean margin (Precordilleranic-Chanic tectonism, see
Astini, 1996), which has been variously interpreted along the western Gondwana
margin. Many of the Late Ordovician and Silurian ages previously assigned to the
Ocloyic event are at present interpreted as reset due to postemplacement events
(Rapela et al., 2001). Subduction related granites regarded as
cordilleran-type plutonism (Rapela et al., 1992) go back to at least ~490
Ma and have been related to a precollisional Andean-type margin.
Tectonic
setting
Igneous
petrology and geochemistry studies indicate that the Famatina belt was formed in
an active plate setting along the early Paleozoic western Gondwana margin (Aceñolaza
and Toselli, 1984, 1986; Toselli et al., 1990; Rapela et al.,
1992; Saavedra et al., 1998; Pankhurst et al., 1998).
Geochemical
studies of plutonic rocks exposed in different zones of the Famatina Range, such
as Sierra de Paimán (Paimán Granitoid), Sierra de Famatina (Cerro Toro
Granitoid), and Sierra de Sañogasta (Nuñorco Granitoid), indicate the roots of
a roughly north-south trending magmatic arc (cf. Toselli et al., 1991,
1993, 1996; Pankhurst et al., 1998; Coira et al., 1999).
Ordovician intrusives are associated with volcanic rocks comprising the Las
Planchadas Formation and Cerro Morado Group.
Plutonic
rocks are cogenetic with volcanics and thus represent the same magmatic event
(Cisterna, 1992; Rapela et al., 1992, 1999). Contrary to what was
previously thought, the recent datings previously mentioned indicate that the
granites predate or, at the most, are coeval with the volcanism, better
explaining their cogenetic relationship. This predominantly calc-alkaline
volcanism has been suggested as the source of K-bentonites (ash-layers) embedded
in the upper section of the platform carbonates and black shales of the
Precordillera terrane to the east (Huff et al., 1998, and references therein).
However, this posses a new relationship which certainly influences the suggested tectonic setting. In Sierra de Fiambalá (western Sierras Pampeanas), the presence of Lower Ordovician deep crustal arc
rocks
suggests that the arc extended eastward into the western Sierras Pampeanas (cf.
Grissom et al., 1991). Outcrops of this lower Paleozoic peri-Gondwanic
magmatic arc extend for about 1200 km, including the northern Puna intrusives,
volcanics and volcaniclastics of the so-called “Faja Eruptiva de la Puna
Occidental” (Coira et al., 1982; Aceñolaza and Toselli, 1984;
Breitkreuz et al., 1989; Bahlburg, 1990). More or less coeval igneous
rocks exposed in Sierra de Ancasti and Sierra de Quilmes (eastern Sierras
Pampeanas) point to a back-arc setting for this eastern region (Rapela et al.,
1990; Quenardelle and Ramos, 1999, Rapela, 2000), whereas the Famatina Belt
would represent the main arc.
While
some authors considered the Famatina belt as part of the western Gondwana margin
(e.g. Toselli et al., 1996; Pankhurst et al., 1998), recent
studies have proposed that Famatina may represent an independent terrane
(Quenardelle and Ramos, 1999; Ramos, 1999, 2000). This alternative is supported
by paleomagnetic data (Conti et al., 1996, Rapalini et al., 1999)
from the Las Planchadas Formation, the brachiopod fauna with strong Celtic,
intra-Iapetus affinities (Benedetto, 1998), and by the recent finding of
low-latitude calcareous algae in the Volcancito Formation, which casts some
doubt on the authochthony of the region by the latest Cambrian-earliest
Tremadocian (Astini, 2000, 2001a). A much lower latitude position is suggested
considering the predominance of carbonates in a non-volcanic setting. Recent
isotope data indicate, however, that by the Early Ordovician (bracketed between
~490-470 ma; Rapela et al., 1999, 2001) large batholithic masses emplaced
in a thickened continental crust with coeval trondhjemite-tonalite-granodiorite
(TTG), metaluminous I-type and highly peraluminous S-type granites (Pankhurst et
al., 2000). Moreover, Sr- and Ndisotopic data and trace element analysis
suggest that apart from minor TTG plutons of astenospheric origin, the rest of
the magmas were largely derived from melting of a thickened Proterozoic
crustlithospheric crust section. The S-type granites in turn show a strongly
similar isotopic and inherited zircon pattern derived from Cambrian supracrustal
metasedimentary rocks deposited during the Pampean cycle and apparently derived
from them by anatexis, suggesting crustal recycling on an active continental
margin along western Gondwana (Pankhurst et al., 2000, Rapela et al.,
2001).
Although
there is agreement on the active plate margin emplacement of the Famatina Basin,
doubts still persist regarding its more precise tectonic setting. Earlier
studies suggested that the Famatina Range represents a volcanic island arc (Aceñolaza
and Toselli, 1984, 1988). Subsequently, Mannheim (1993b) further developed this
model and interpreted the Famatina basalts as island arc tholeiites. However, Mángano
and Buatois (1996b, 1997) related the arc tholeiites to an extensional regime
within an intra-arc formed in continental crust, rather than oceanic crust as
previously thought. As noted by Quenardelle and Ramos (1999) and Rapela (2000),
the predominance of granodiorites and granites also indicates that the arc was
developed in continental crust.
Most
authors have assumed that it represents a backarc basin (e.g. Mannheim, 1993a;
Clemens, 1993; Toselli et al., 1996). However, because of its extended
temporal development, filling and deformation of the basin should be seen as a
continuum probably involving progressive fore-arc, intra-arc and back-arc stages
(e.g. Saavedra et al., 1998). The distinction of forearc, intra-arc and
bakarc positions is not always clear in ancient settings due to subsequent
deformation and intrusions related to collision-accretion procesess along the
active margin. However, intra-arc basins are located on the arc platform and
they closely record the evolution of the arc. Proximal volcanic deposits
interbedded with dominantly volcaniclastic deposits as those recorded in the
Suri Formation and Cerro Morado Group, are critical to the recognition of an
intra-arc emplacement.
Intra-arc
models for the volcanic setting of the Suri Formation were favored by Mángano
and Buatois (1996b, 1997) who indicated an extensional arc-setting stage (see
also Quenardelle and Ramos, 1999; Ramos, 2000). Within this framework, the
coincidence of a thick Arenigian sedimentary succession and volcanic-plutonic
arc rocks in the Famatina Basin may indicate an extensional or transtensional
arc setting (Mángano and Buatois, 1996b, 1997). The suggestion of an
extensional regime associated with the Famatina volcanic arc is consistent with
evidence presented from the Cordón de Lila in the northern Chilean Puna by Damm
et al. (1991). These authors documented Ordovician volcanic rocks
interbedded with marine sediments associated with crustal extension in a
segmented horst and graben-type setting.
Depositional
evolution
Integration
of information from the different areas of the basin sheds light on the
depositional and tectonic evolution of the Famatina Basin. Four evolutionary
stages are recognized here and their main features summarized below:
Late
Cambrian - early Tremadocian
This
stage is recorded by the lower and middle Members of the Volcancito Formation.
The stage represents the onset of sedimentation within the Famatina Basin after
a period of quiescence and subsidence of folded Pampean age basement rocks. The
sedimentary record spans for approximately 5 m.y. through the
Cambrian-Ordovician boundary with carbonate sedimentation taken as indicative of
low-latitude warm waters. Recognized low-latitude fossil algae (Nuia-Girvanella
associations) are unique to western Gondwana, which is usually positioned at
higher latitudes. Megabreccias embedded in this interval dominated by
shallow-marine storm-dominated features indicate a tectonically unstable
setting. The transition to black-shales in the early Tremadocian (~487 Ma)
suggests gradual deepening related to relative sea-level rise and consequent
drowning. Pelagic organisms (graptolites and phylocarids) and lack of bottom
communities indicate a progressively more restricted depocenter with settling of
fines and pelagic faunas through a well-stratified water column with anoxia
developed near the bottom. This thick black-shale package can alternatively
represent restricted outer-shelf depocenters of foredeep environments. The early
Tremadocian transgressive event undoubtedly has interbasinal significance and
can be traced into the Northwest Argentina Basin, where the Cambrian-Ordovician
transition is associated with a major rise in sea level. No evidence of
concomitant volcanic activity has been detected yet in this unit, but plutonic
ages in the Famatina Belt indicate an active Cordilleran-type magmatism along
this region by the early Tremadocian.
Late
Tremadocian
This
stage is represented by the so-called upper member of the Volcancito Formation,
which is better regarded a separate formation. As outlined above this unit
outrops in a separate region and its stratigraphic relationship with the other
Ordovician units of Famatina is rather unclear. This unit records deposition on
a relatively deep shelf under oxygen-poor conditions (Esteban, 1992). Silicified
sandstone beds have volcanic detritus and suggest deposition under the influence
of coeval volcanism (Esteban, 1993). This represents the first evidence of
volcanism recorded in the Famatina Belt (~ 482 Ma) and can be correlated with
the onset of volcanism in the Puna region (Moya et al., 1993; Koukharsky et
al., 1996). In Central Famatina the Cerro Tocino volcanics probably record
initial andesite volcanic flows along the arc. Volcanic rocks also occur at the
base of the section in the northern Chaschuil region. Tentatively this volcanics
are assigned to the Late Tremadocian although there is no real constrain other
than the fact that they underlie the Suri Formation. The lowermost interval of
the La Alumbrera Formation might also be correlated with these volcanics as
previously suggested. At a regional scale, fine-grained deposits that
characterizes this stage may also correlate in part with outer-shelf deposits of
the Santa Rosita/Saladillo-Parcha Formations in the Cordillera Oriental.
Early
Arenigian
The
early Arenigian phase is represented by the upper part of the La Alumbrera
Formation, the Vuelta de Las Tolas Member and the lowermost black shale member
of the Suri Formation in central Famatina. The Portezuelo de Las Minitas is
included in this phase also, but the lack of detailed studies and uncertainty
with respect to their relation with the Suri Formation prevent further analysis
at this time. This stage records approximately 6 m.y., encompassing the complete
early to early middle Arenigian, but it may include the latermost Tremadocian.
Facies analysis by Mángano and Buatois (1997) suggests that the Vuelta de Las
Tolas Member records sedimentation on a slope apron formed within an intra-arc
basin on a flooded continental arc. Concomitant explosive volcanism is mainly
documented by accretionary lapilli in pyroclastic-laden deposits associated with
slope turbidite channels (Mángano and Buatois, 1997).
During
this stage, tectonic subsidence linked to extension led to the formation of a
series of depocenters. This allows explaining the lateral facies changes and
diachronism observed along the Famatina belt. Subsidence provided high
accommodation potential, but prevailed over sediment supply, precluding filling
of the tectonic depressions. Trace-fossil distribution in the Vuelta de Las
Tolas Member seems to have been controlled by oxygen fluctuations induced by
turbidity currents within an oxygen-depleted setting, highly suggestive of
limited deep-water circulation in topographically restricted sub-basins (Mángano
et al., 1996). Deposition of unbioturbated, thick, uniform mudstone
packages probably record ponded sedimentation in the confined setting of
isolated depocenters within the arc (Mángano and Buatois, 1997).
These
slope deposits are poorly organized and reflect the strong imprint of allogenic
processes.
Small-scale
thinning- and fining-upward successions appear to be restricted to the fill of
individual channel systems in Northern Famatina (Fig. 3). Small-scale
coarsening- and thickening-upward cycles, which otherwise would reflect
progradation of depositional lobes, have not been detected. As a whole, the
Vuelta de las Tolas Member succession displays a crude overall fining-upward
pattern that may reflect migration of the arc from the depositional site,
decrease in volcanic activity, and/or decrease in extensional faulting. In any
case, the presence of wave-reworked beds in the uppermost part of the succession
suggests an overall shallowing trend and indicates the position of the
slopeshelf transition. This regressive trend was accentuated during accumulation
of the shallow marine deposits of the overlying Loma del Kilómetro Member. In
Central Famatina, the succession shows gradual transition to more oxygenated
environments and an overall coarsening-upward pattern (Astini, in press).
In
terms of facies and depositional setting, the middle Arenigian succession of the
Aguada de la Perdiz Formation in Puna is similar to the Vuelta de Las Tolas
Member (cf. Breitkreuz et al. 1989; Bahlburg 1991; Bahlburg and
Breitkreuz 1991). However, general facies trends from the Vuelta de Las Tolas
Member suggest progressive shallowing, whereas the Aguada de la Perdiz Formation
deepens upwards, which probably reflect the imprint of local tectonics rather
than global sea level changes. Volcano-tectonic activity is clearly indicated by
the episodic influx of coarser-grained, volcanogenic and juvenile pyroclastic
detritus derived from the flanks of the arc, mainly via gravityflow processes (Mángano
and Buatois, 1997). These units can be correlated with the lower section of the
Acoite Formation in the Cordillera Oriental.
Middle
Arenigian to Llanvirnian?
The
middle Arenigian to Llanvirnian? phase is represented by the Loma del Kilómetro
and Punta Pétrea Members of the Suri Formation and by the partially coeval
Molles Formation as well.
The
volcanic episode represented by the Cerro Morado Group and the Las Planchadas
Formation is included in this phase as well. The Loma del Kilómetro Member
records sedimentation in a stormand mass flow-dominated high-gradient shelf
adjacent to the volcanic arc (Mángano and Buatois, 1996a). Volcano-tectonic
activity was the important control on shelf morphology, whereas relative
sea-level change influenced sedimentation. The storm stratal patterns with rapid
vertical and lateral facies shifts and the envisaged sand dispersal mechanism
altogether suggest a narrow, geographically restricted, relatively high-gradient
shelf (Mángano and Buatois, 1992b, 1996a). In active margin settings, the slope
of the shelf is largely controlled by volcanic arc growth; as the arc grows, the
slope steepens and a high-gradient shelf may be formed, fringing the volcano (Mángano
and Buatois, 1996a). Resedimentation of volcanic material was a key process in
this shallow-water system.
Volcaniclastic
detritus and andesitic volcanics on the arc flanks were eroded, transported
basinward and redeposited by gravity flows. Slump and storm wave liquefaction
may have triggered unidirectional flows that deposited a large volume of
detritus on the lower shoreface and offshore (Mángano and Buatois, 1996a).
Biostratinomic, paleoecologic, and ichnologic evidence support this
paleoenvironmental interpretation and provide independent evidence for the
dominance of episodic sedimentation in an arc-related shallow marine setting (Mángano
and Buatois, 1992c, 1995, 1996a).
The
lower part of the Loma del Kilómetro Member is thought to record mud blanketing
during highstand and volcanic quiescence. Progradation of the inner shelf and
lower shoreface deposits of the middle interval represents an abrupt basinward
shoreline migration during a forced regression. The upper part of the Loma del
Kilómetro Member records a transgression with no evidence of contemporaneous
volcanism. In the Chaschuil area, a subsequent regressive pulse was recorded by
the Punta Pétrea Member, which reflects the progradation of a volcaniclastic
fan delta (Mángano and Buatois, 1994a). Fan-delta progradation is coincident
with a remarkable peak in volcanic activity. In the Sierra de Famatina area,
tidal sedimentation is recorded in the overlying Molles Formation (Astini,
1998).
Transgressive-regressive
cycles most likely induced by volcano-tectonic activity seem to be particularly
common in both the upper part of the Famatina Group and in the Cerro Morado
Group. These cycles are interpreted as a product of the flexural and thermal
response during active volcanism and intervals of quiescence, where
shallow-marine environments interact with important volcaniclastic input (Astini
and Dávila, 2002). This episodic basin dynamics influenced sea-level
fluctuations and exerted the cyclical pattern recognized throughout most of the
Ordovician succession in the Famatina Basin with patterns that can be
interpreted as forced regressions and abrupt transgressions. In this context,
the non-topographically controlled intra-Ordovician angular unconformity
described between the Early Ordovician Famatina Group and the Cerro Morado Group
in central Famatina (Astini and Dávila, 2002; Dávila et al., 2003) can
be considered a unique evidence of major folding in the Famatina belt, most
likely related to the Ocloyic orogeny. Depositional evolution suggests that
during middle Arenigian to probably Llanvirnian overall sedimentation is
represented by shallow-water facies assemblages punctuated in the upper part of
the succession by subaerial exposure episodes and that the volcanic arc was
close to sea level for most of its history (Mángano and Buatois, 1992a, 1994a;
Astini and Dávila, 2002).
The
volcanic rocks recorded at the top of the lower Paleozoic succession in the
Cerro Morado Group and Las Planchadas Formation reveal a peak in volcanic
activity by the late Arenigian to probably Llanvirnian. This is consistent with
shallow-level igneous activity along the Famatina belt, because most recent
plutonic ages (Rapela et al., 1999, 2001) show active magmatism since the
early Tremadocian. Important crustal contamination of the Cordilleran-type
plutonism favors an upperplate position in an active Andean-type margin. Such a
setting may have been true until collision of the Precordillera terrane and
development of the Middle Ordovician Ocloyic orogeny as supported by various
independent lines of evidence.
Acknowledgements. We express our sincere thanks to Andy Rindsberg who read the manuscript and offered valuable suggestions. We acknowledge the various institutions who provided funds to carry work in Famatina: Consejo Nacional de Ciencia y Tecnología, Consejo de Investigaciones Científicas y Tecnológicas de Córdoba, Secretaría de Ciencia y Técnica de las Universidades Nacionales de Córdoba y Tucumán and Sigma Xi.
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Recibido:
25 de Febrero de 2003
Aceptado: 4 de Marzo de 2003