Ordovician Basin of Northern Argentina
María Cristina MOYA1
1 CONICET-CIUNSa. Universidad Nacional de Salta, Geología, Buenos Aires 177, 4400 Salta. Argentina.E-mail: firstname.lastname@example.org
BASIN OF NORTHERN
The marine-clastic Ordovician strata of Northern Argentina bears an abundant
benctic and planctic fossil fauna that allows to stablish acceptable
correlations. The Ordovician basin in Northern Argentina was developed in the
margins of the Pampean Craton. These, had a elonged N-S geometry and asymmetric
disposition of the sedimentary environments. A wide stable marine shelf (Chaco
Shelf) was developed during all the Famatinian Cycle (Upper Cambrian-Upper
Devonian) in the areas today occupied by the Cordillera Oriental, Sierras
Subandinas and Llanura Chaqueña. The Puna region was a tectonically unstable
area, whose famatinian history started with clastic and volcano-sedimentary
accumulation (Lower Tremadocian-Middle Arenig) over a shelf afectted by
volcanism (Altiplano Shelf). At the end of the volcanism, the latter records a
strong subsidence with thick accumulation of volcaniclastic turbidites (Upper
Arenig-Caradoc). Silurian-Devonian strata are restricted to the western border
of Puna under Upper Carboniferous red beds. The evolution of the Famatinian
Cycle in the Chaco Shelf was controlled by relative sea-level fluctuations.
These, allowed the development of three second order tectono-eustatic cycles
separated by important unconformities: The Mesonian Cycle (Upper
Cambrian) whose strata covers unconformably the slates and intrusives of the
basement (Upper Proterozoic-Lower Cambrian), are placed below strata of the Victorian
Cycle (late Upper Cambrian-Caradoc) with an erosive unconformity (Iruya
Unconformity). Another erosive unconformity (Ocloya Unconformity) separates the Victorian
Cycle to the Cordilleran Cycle (Upper Ashgill-Upper Devonian). These
cycles include other third or fourth order cycles related to
transgressive-regressive episodes whose sedimentary result is an alternation of
shale and sandstone bodies. Inside the Victorian Cycle are recognized
seven lower order cycles separated among them by sedimentary unconformities of
different magnitude and regional expression. The First Cycle was preceded
by a pronounced eustatic event (LREE) with areal erosion and development of
fluvial system. This cycle start with the FAD of Parabolina (Neoparabolina)
frequens argentina. During its evolution the basin inherit the structural
elements of the Mesoninan Cycle with the sediments accumulating on
tide-dominated shelfs. The Second Cycle starts with the volcanism on the
Puna, and the Chaco Shelf is affected by tectonic events producing the collapse
of certain areas. Within this cycle the Cambrian-Ordovician boundary is recorded
with Jujuyaspis keideli and Rhabdinopora flabelliformis. The Third
Cycle start a generalized flooding of Chaco Shelf pointed by Anisograptus
matanensis. The Fourth Cycle was preceded by subaerial exposures with
the accumulation of fluviomarine and mineralized fan deltas deposits.
Paleontologically is characterized by the FAD of Kainella meridionalis.
The Fifth Cycle represents the ending of the Tremadocian times with the
maximum flooding of Chaco Shelf. During this cycle occurred a tectonic episode
generating important paleogeographic modifications (Tumbaya Phase). The
paleontological elements of the last cycle are different species of Anisograptus,
Adelograptus and Bryograptus. The Sixth Cycle starts during
the Tremadoc-Arenig transition. Along its evolution the different volcanic
episodes are closed, while different species of didymograptids and tetragraptids
characterize its strata. Finally the Seventh Cycle represent a quieter
basinal interval with conodonts and trilobites (Llandeilian- Lower Caradoc).
This cycle rests below ashgill strata with hirnantia fauna of the Zapla
Formation and equivalents, which starts up the Cordilleran Cycle.
Los depósitos ordovícicos en el norte argentino son marino-clásticos y
contienen abundante fauna fósil bentónica y planctónica que permite
establecer correlaciones de aceptable precisión. La cuenca Ordovícica del
norte argentino se desarrolló en las márgenes del Cratón Pampeano. Tuvo una
geometría alargada en sentido N-S y una disposición asimétrica de los ámbitos
de sedimentación. Una plataforma marina amplia y relativamente estable
(Plataforma Chaqueña) se desarrolló durante todo el Ciclo Famatiniano (Cámbrico
Superior-Devónico Superior) en las áreas actualmente ocupadas por la
Cordillera Oriental, Sierras Subandinas y Llanura Chaqueña. En contraposición,
la Puna fue un área tectónicamente inestable, cuya historia famatiniana se
inició con la acumulación de depósitos clásticos y volcano-sedimentarios
(Tremadoc Inferior-Arenig Medio) en una plataforma marina afectada por
vulcanismo (Plataforma Altiplánica). Al culminar el vulcanismo, la Plataforma
Altiplánica registró una marcada subsidencia, cuyo resultado fue la
acumulación de espesas sucesiones de turbiditas
volcaniclásticas (Arenig Superior-Caradoc). Los estratos silúricos y devónicos
se restringen puntualmente al borde occidental de la Puna, donde subyacen a red
beds del Carbonífero Superior. En la Plataforma Chaqueña en cambio, la evolución
del Ciclo Famatiniano estuvo controlada, principalmente, por cambios relativos
del nivel del mar. Esto posibilitó el desarrollo de tres ciclos tectonoeustáticos
de segundo orden, separados por importantes discordancias: El Ciclo Mesoniano (Cámbrico
Superior), cuyos depósitos cubren angularmente a las metamorfitas e intrusivos
del basamento (Proterozoico Superior- Cámbrico Inferior) y subyacen a través
de una discordancia de erosión (Discordancia Iruya) a los depósitos del Ciclo
Victoriano (Cámbrico Superior tardío-Caradoc). Otra discordancia de erosión
(Discordancia Ocloya) separa al Ciclo Victoriano del Ciclo Cordillerano (Ashgill
Superior-Devónico Superior). Estos ciclos a su vez, incluyen otros de tercero y
cuarto orden, los que se corresponden con episodios de transgresión-regresión;
el arreglo estratigráfico resultante es una sucesión alternante de cuerpos de
pelita y de arenisca. Dentro del Ciclo Victoriano se reconocieron siete ciclos
de orden menor, los que están separados por discontinuidades sedimentarias de
distinta magnitud y expresión regional. El Primer Ciclo estuvo precedido
por una descenso eustático pronunciado (LREE), durante el cual ocurrieron
procesos de erosión subaérea y desarrollo de un sistema fluvial. Se inicia con
la aparición de Parabolina (Neoparabolina) frequens argentina. Durante
su evolución la cuenca hereda los elementos estructurales del Ciclo Mesoniano y
los depósitos se acumulan en ámbitos dominados por mareas. El Segundo Ciclo,
con él se inicia el vulcanismo en la Puna. La Plataforma Chaqueña es afectada
por eventos tectónicos que producen el colapso de algunas áreas. En este ciclo
se registraría el límite Cámbrico-Ordovícico, dado que los fósiles más
conspicuos son Jujuyaspis keideli y Rhabdinopora flabelliformis. El
Tercer Ciclo inicia un período de inundación generalizada señalado por la
aparición de Anisograptus matanensis. El Cuarto Ciclo estuvo precedido
por procesos de exposición subaérea que dieron como resultado la acumulación
de depósitos fluvio-marinos y de fan-deltas mineralizados. Paleontológicamente
está caracterizado por la aparición de Kainella meridionalis. El Quinto
Ciclo representa la culminación del tiempo Tremadociano y en él se
registra la máxima inundación de la Plataforma Chaqueña. Durante su evolución
se desarrolla un episodio tectónico que produce importantes modificaciones
paleogeográficas (Fase Tumbaya). El contenido paleontológico de los depósitos
incluye distintas especies de Anisograptus, Adelograptus y Bryograptus.
El Sexto Ciclo se inicia durante la transición Tremadoc-Arenig. Durante
su evolución culminan los episodios volcánicos. Distintas especies de didymográptidos
y tetragráptidos caracterizan a sus estratos. Finalmente, el Séptimo Ciclo corresponde
a un estadio de mayor tranquilidad en la cuenca. Los depósitos contienen
conodontes y trilobites del Llandeiliano y Caradoc Inferior. Este ciclo subyace
a los depósitos ashgillianos portadores de fauna hirnantiana (Formación Zapla
y equivalentes), con los que se inicia el Ciclo Cordillerano.
words: Ordovician. North Argentina. Stratigraphical evolution.
Norte Argentino. Evolución estratigráfica.
aim of this contribution is to discuss the main sedimentary events recognized in
the marine-clastic Ordovician successions exposed in Puna, Cordillera Oriental
and Sierras Subandinas of northern Argentina (Figs. 1a, 1b).
1. 1a. Map of Geologic Regions. 1b. Map of
Proterozoic-Paleozoic outcrops. 1. Metamorphic basement (Proterozoic-Lower
Cambrian). 2. Upper Cambrian sedimentary rocks. 3. Ordovician sedimentary rocks.
4. Silurian-Devonian sedimentary rocks (shelf deposits). 5. Devonian -
Carboniferous sedimentary rocks (deep deposits). 6. Proterozoic-Middle Cambrian
granites. 7. Ordovician granites. 8. Proterozoic-lower Paleozoic granites. 9.
Upper Paleozoic granites. 1c. Paleogeologic sketch according to the age and
nature of the stratigraphic nucleus. 1. Pan-American Domain. 2. Areas of
uncertain age. 3. Pampean Domain. 4. Famatinian Domain. Famatinian sedimentary
rocks and granites (Famatinian or younger). 5. Ordovician sinsedimentary
volcanism. 6. Granites without differentiated age. 7. Old tectonic front
(previous to the Upper Cambrian). 1d-f. Paleogeographic sketchs of the
ordovician basin. 1. Shallow areas. 2. Area occupied by the old Cambrian basin
3. Emerged to submerged areas. 4. Shallow to submerged areas. 5. Deep areas. 6. Turbidites. 7. Positive areas . 8. Sinsedimentary Volcanism. 9. Intrusives with radiometric age. 10. Storm-dominated shelf. 11. Isolated areas. 12. Lipán Swell. 13. Maximum subsidence axis. 14. Old tectonic front. 15. Erosional western border of the ordovician basin.
about Ordovician deposits in these regions are very abundant, great part of them
were analysed in recent synthesis papers (Aceñolaza et al., 1999;
Bahlburg and Zimmermann, 1999; Coira et al., 1999; Moya, 1999), where the
interested colleague can consult the mentioned complementary bibliography.
Ordovician System subdivision used here corresponds to the accepted British
Series, because all the International Stages of the Ordovician System has not
been fully defined yet. The chronology is the proposed by Cooper (1999).
cratonic areas of South America were attached to Gondwana during the Upper
Proterozoic- Cambrian. The extensional collapse that happened to the orogenic
processes generated, in a diachronic way, intracratonic and pericratonic basins.
These are marine-clastic and evolved in the margins of the South America
general scheme above mentioned is applicable to the Cambrian-Ordovician basins
of North Argentina, which were development in the margins of Pampean Craton
(Pampean Domain, Figs. 1c-1f). The annexation to Gondwana of the Pampean Domain
terranes could have been previous to the one of the basement of the Cordillera
Oriental (Pan-American Domain, Fig. 1c). The Pampean Domain basement is
characterized by plutonic rocks and medium to high-grade metamorphic rocks.
Folded sedimentary rocks, affected by low to very low-grade metamorphism and
intruded by plutonic bodies, compose the Pan-American Domain basement. Two
magmatic-metamorphic episodes could have been recorded in both domains; the
oldest, with ages of 560-540 M.a. (Vendian) and the youngest, of 525-505 M.a.
(Lower to Middle Cambrian) (Becchio et al., 1999).
the Cordillera Oriental, the metamorphic and the intrusive rocks of the basement
underlie in clear angular unconformity (Tilcara Unconformity), deposits of the
Mesón Group (Middle? -Upper Cambrian). The Tilcara Unconformity points out the
end of the sedimentary, metamorphic, tectonic and magmatic events occurred
during the Pampean Cycle.
Mesón Group begins the Famatinian Cycle (Upper Cambrian-Upper Devonian). The
deposits of the Famatinian Cycle are clastic being accumulated in a marine shelf
environment; the deep basin facies are restricted in time (Ordovician) and space
(Famatinian Domain, Fig. 1c).
Famatinian Domain understands the Argentinean-Chilean Puna, where the Pampean
Cycle rocks are poorly represented. In this region, the Ordovician deposits does
not expose its base, representing the stratigraphic nucleus. The Famatinian
Domain was a tectonically unstable area affected by volcanism during the Lower
Tremadocian-Middle Arenig (Figs. 1d, 1e); the deposits of
Figure 2. Correlation Chart of the Ordovician of northern Argentina. 1. Concordant stratigraphic relationship. 2. Tectonic relationship, not exposed or unknown. 3. Unconformity. 4. Probable younger age. 5. The identification to GROUP. 6. Idem to Formation. 7. Radiometric age. 8. Granites. 9. Volcanic rocks. 10. Piroclastics rocks. 11. Correlation Time-Lines (see above). 12. Conglomerate. 13. “Diamictite”. 14. Quartz sandstone. 15. Wacke. 16. Sandstone/Shale interbedded. 17. Silstone/Mudstone. 18. Shale. 19. Trace fossils abundant. 20. Sulfides. 21. Calcareous coquinite with Lower Tremadoc trilobites. 22. Concretions. 23. Phosphates (nodules, crusts or lingulid coquinites). 24. Iron ore deposits (hematite, chamosite). 25. Fine-grain Turbidites. 26 Coarse Turbidites. 27. Global Eustatic and/or Regresión Events (in text). Observation: The references 19 to 24 are the best characteristics to identified the differ units. Correlation Time-Lines: LT1. Coquinites with Lower Tremadocian trilobites. LT2. A. murrayi, LT3. T. approximatus. LT4. B. vacillans, A. filiformis, B. deflexus. LT5. D. bifidus/minutus/nitidus. LT6. A. eivionicus, P. minor, X. svalbardensis. LT7, G. acanthus, C. antennarius, U. austrodentatus, A. cucullus. LT8, D. cf. murchisoni, G. hinckii fimbriatus, G. cf. ciliatus. LT9, Dicellograptus sp. LT10, Hoekaspis schlagintweiti. LT11, Dalmanitina subandina, cf. Eohomalonotus, cf. Chattiaspis LT12, Talacastograptus leanzai, Normalograptus aff. normalis, N. rectangularis.
age integrate volcano-sedimentary successions (CVP, Fig. 2), accumulated in
shelf marine environments (Altiplano Shelf, Fig. 1d). A pronounced subsidence
recorded in the Upper Arenig, gave place to the accumulation of thick
volcaniclastic turbidite successions (CTP, Fig. 2; Upper Arenig-Caradoc). In the
Famatinian Domain, the Silurian-Devonian deposits are restricted to the western
border of the Puna, where they underlie discordantly to red beds of Upper
Carboniferous age (Figs. 1b, 2).
more complete Cambrian-Devonian successions crop out in the Cordillera Oriental
and in the Sierras Subandinas (Figs. 1a, 1b); also, they extend toward the east
(out of the limits seen in the figure), in the subsurface of the Chaco Plain.
These regions were part of a wide marine shelf during all the Famatinian Cycle
(the Chaco Shelf, Fig. 1d). An emerged to submerged structure (the Lipán High,
Figs. 1d-f) divided the Chaco Shelf into an eastern and a western area. The
western area was bounded to the west by the Cobres High, effective only during
the Cambrian-Lower Tremadoc (cf. Figs. 1d and 1e-f). The Cobres High is an
extension of the Pampean Craton; their western limit is an old and important
tectonic front (Fig. 1d).
evolution of the Famatinian Cycle in the Chaco Shelf was controlled, mainly, by
relative sealevel changes which had, as results, the development of three second
order tectonic-eustatic cycles, limited in base and top by important
Mesonian Cycle (Upper Cambrian). It is developed above the
Tilcara Unconformity and includes two transgression-regression episodes,
represented in the Mesón Group (MG). The first episode corresponds to the
prograding sequence that K1 and K2 integrate; the deposits of the second episode
(K3) are truncated by the Iruya Unconformity (Fig. 2). The MG deposits were
accumulated in tide-dominated paleoenvironments (sub, inter and supratidal
environments) characterized by a high mineralogical and textural maturity. These
indicate that the MG was deposited under conditions of high stability of
basement. The MG basin is restricted to the Cordillera Oriental (Figs. 1b, 1d,
2); its genesis is linked with the extensional processes to which the Pampean
Victorian Cycle (late Upper Cambrian-Caradoc). It is
represented in the Santa Victoria Group (Cordillera Oriental) and in the Tamango
Group (Sierras Subandinas). This cycle is limited in base and top by two
erosional unconformities (Iruya and Ocloya unconformities; Fig. 2). These
unconformities were generated by relative sea level falls, periods where wide
areas of the Chaco Shelf were exposed to subaerial erosion. These events belong
together, respectively, with the Lange Ranch Eustatic Event (LREE, Upper
Cambrian) and with the Hirnantia Regressive Even (HRE, Ashgill) recorded in
other regions of the world. The Iruya Unconformity is only observed in the
Cordillera Oriental, where it separates the Mesón Group of the Santa Victoria
Cordilleran Cycle. (Upper Ashgill-Upper Devonian).
It was defined in Bolivia (fide Suárez Soruco, 2000). In northern
Argentina, the Cordilleran Cycle is better represented in the Sierras Subandinas
(Fig. 1b), where is bounded in base and top by the Ocloya and Chánica
oldest deposits of the Cordilleran Cycle is represented by the Zapla Formation
(Upper Ashgill), that starts the Cinco Picachos Supersecuence (Upper
Ashgill-Lower Devonian). The last one represents the first of the three cycles
of third order that characterize the Silurian-Devonian successions in the
Sierras Subandinas (Vistalli, 1999).
Figure 3. 3a. Location map of the stratigraphic columns. 3b. Paleogeographic sketch of the Ordovician Chaco Shelf: Western area (inclined lines), profiles 1, 2, 3. 3c. 1a, Fluvial conglomerate; 1b, Fluvial-coastal marine conglomerate. 2, Sandstone. 3, Shale. 4, Shale and sandstone. 5, Calcareous mudstone. 6, Gravity flows (beds and blocks). 7, Mudflakes. 8, Fe-Mn nodules. 9, Concretions with calcareous and iron oxide nucleous. 10, Coquinite. 11, Storm succession and storm beds. 12. Bioturbation, trace fossils abundant. 13, Cross-beds. 14, Profiles: La Quesera (1), Pascha (2), Angosto del Moreno (3), Lesser Range (4), Sierra de Mojotoro (5) and Quebrada de Humahuaca area (6). 15, Unconformity.
of the Victorian Cycle
for the Zapla Formation, the rest of the Ordovician deposits accumulated in the
Chaco Shelf belong to the Victorian Cycle, which includes cycles of third and
quarter order, representing transgression-regression episodes. The resulting
succession is composed by alternating sandstone and shale bodies, which were
identified with a wide nomenclature (Fig. 2). The four cycles recognized by Moya
(1999) are represented in the Fig. 2, where it is observed that the first cycle
(Ss1, Sh1, Ss2) is the only one that doesn’t have a time-line of correlation.
In a similar way, the second cycle (Sh2, Ss3, Fig. 2) only has a reference (LT1,
obtained information in the units allowed to define seven stratigraphic
intervals characterized by distinct fossiliferous assemblages, based on which
Moya et al. (this symposium) present a more detailed correlation scheme
(I-VII, Figs. 3, 4). This scheme is valid for the Cambrian- Tremadocian deposits
of the Santa Victoria Group allowing to adjust the regional correlation chart
the stratigraphic arrangement of the Chaco Shelf deposits are notably different
to that of the Puna Ordovician successions (CVP, CTP, Fig. 2), the fossiliferous
assemblages pointed out in the Fig. 2 and those mentioned by Moya et al.
(this symposium), allow to discuss the relationship among the tectonic, eustatic
and sedimentary processes happened in the Ordovician basin of northern
Argentina. The base of the discussion will be the transgression-regression
cycles recognized, whose limits are sedimentary discontinuities of different
magnitude and regional expression. These discontinuities represent relative sea
level falls and they would be equivalent with the events LREE (Lange Range
Eustatic Event), ARE 1
ARE 2 (multiple event sensu Cooper
and Nowlan, 1999; Acerocare Regressive Event), BMEE (Black Mountain Regressive
Event), GARE (Grés Armoricain Regressive Event), VRE (Vallhallfonna Regressive
Event) and HRE (Hirnantia Regressive Event) (Figs. 2, 3, 4). The
paleontological control doesn’t still allow to specify if identified events
here in the Kainella Regressive Event (KRE) and the Notopeltis Regressive
Event (NORE) belong together with the Peltocare Regressive Event (PRE)
and the one with the Ceratopyge Regressive Event (CRE), respectively.
Cycle It is limited by the LREE and ARE 1
events (Stratigraphic Interval I, Figs. 3, 4). This
Cycle includes the deposits of Ss1, Sh1 and the lower member of Ss2. It is
represented in the centralnorthern areas of the Cordillera Oriental (Sections 3,
6; Figs. 3 and 4), in the Sierras Subandinas and Puna Oriental (Fig. 2). The
LREE generated processes of subaerial erosion that removed the deposits of the
MG, even reaching levels of K2. The resulting discontinuity (Iruya Unconformity)
is recorded in the whole basin of the MG, settling fluvial systems over it. The
lowstand systems tract is represented by sandstones and conglomerates of fluvial
braided bars and channels that filled paleovalleys inserted over the MG deposits
and of the basement. These deposits grade vertically and laterally to coastal
facies. The transgressive systems tract begins with a sandy body (Ss1). In this
unit there is the FAD of Parabolina (Neoparabolina) frequens argentina (Kayser).
This sandstones grade to shales (Sh1) of intermediate to distal shelf
environment with conodonts from the Hirsutodondus hirsutus Subzone (Upper
Cambrian). Within Sh1 unit the maximum flooding is recorded, starting the
highstand systems tract. The distal to intermediate shelf facies are
progressively replaced by sandy deposits of bars and subtidal sand sheets (lower
member of Ss2).
deposits of this cycle were accumulated in tide-dominated marine shelf
Figure 4. Eastern area (horizontal lines), profiles 4, 5, 6.
the borders of the MG basin, lying sometimes directly over the basement in the
Sierras Subandinas and in the Puna Oriental (Fig. 2). However, the basin
preserved the same structures of those during the MG (Fig. 1d). In addition,
although there is no direct data, it is probable that the Altiplano Shelf has
begun its evolution during this cycle.
is bounded by the ARE 1 and
events (Stratigraphic Interval II, Figs. 3, 4). It
includes part of Las Vicuñas Formation (CVP, Fig. 2), the middle and upper
members of the Alfarcito and Cardonal Formations in the Humahuaca and Angosto
del Moreno areas and the San José and Caldera Formations in the Sierra de
Mojotoro and Cordón de Lesser (Figs. 3, 4). Although this cycle had a short
duration, its depositional timming is one of the most critical and important in
the evolution of the Ordovician basin. With this cycle it is settled the
volcanic arch in the Altiplano Shelf (Fig. 1d, 2) and the Chaco Shelf becomes a
storm-dominated environment. The areas located in the borders of the Lipán
Swell collapse along normal faults, through which spill out mud and brines
metal-bearing (Pb, Zn, Ag, Fe). The activity of the faults generates frequent
mineralized gravitational flows lying between the deposits (Figs. 3, 4). Dark
gray or black shales and mudstones with autigenic sulfides represent the
transgressive systems tract. In this cycle would be documented the Cambrian-
Ordovician boundary, given that this time interval records the appearance of Rhabdinopora
flabelliformis (Eichwald) that generally accompanies Jujuyaspis keideli Kobayashi
and exceptionally to P. (N.) frequens.
highstand systems tract is represented by fine-grained quartz sandstones with
hummocky cross-stratification deposited in lower to middle shoreface
cycle belongs together with Stratigraphic Interval III (Figs. 3, 4), being
limited by the ARE 2 and BMEE events. It
includes the lower member of Sh2 in the Cordillera Oriental and Sierras
Subandinas; in the Puna Occidental, includes the Tolar Chico Formation and the
upper member of Las Vicuñas Formation (Fig. 2).
transgressive systems tract rests with a clear ravinement surface on the top of
the previous cycle deposits; the maximum flooding coincides with the appearance
of Anisograptus matanensis Ruedemann. The high systems tract is
represented by brown, pink and purple, generally bioturbated, sandstones and
mudstones, which were deposited in subtidal and intertidal environments.
cycle belongs together with the Stratigraphic Interval IV (Figs. 3, 4), being
bounded by the BMEE and KRE events. The latter includes the lower member of Sh2
in the Cordillera Oriental an Subandean Range. The relative sea level fall is
assigned to the BMEE, being pronounced. In the areas next to the basin borders
(Angosto La Quesera, Section 1, Fig. 3), subaerial erosion removed deposits of
previous cycles and even great part of Ss2. This unit underlie with a clear
erosional unconformity quick discharge fluvial-coastal deposits. represented by
coarse to coarser conglomerate.
the flanks of the Lipán High, fan deltas were developed with mineralized debris
and mud flows (Fig. 4). A pelitic wedge is developed in the more distal parts of
the shelf. These fine-grain deposits represent the transgression systems tract
that is covered successively by sandy and sandy-calcareous shoreface
tempestites. This cycle records the appearance of Kainella meridionalis Kobayashi;
which is present in all the deposits of this cycle; even, in the matrix of the
Fifth Cycle is bounded by the KRE and NORE events, including the Stratigraphic
Intervals V and VI (Figs. 3, 4). The deposits of this cycle are well represented
eastwards of Lipán Swell.
this structure, the Tumbaya Unconformity truncates great part of these deposits
(Figs. 2, 3). During the deposition timing of this cycle, the Tremadocian basin
reached its maximum extension.
transgression systems tract is represented by clayey and muddy shales
accumulated in distal outer-shelf environments. These deposits contain abundant
graptolites of Lower and Upper Tremadoc (Anisograptus spp., Adelograptus
spp. Bryograptus spp.). The maximum flooding is reached within the
levels with Adelograptus, starting the settling of the highstand system
tract, represented by shales of middle to proximal offshore grading quickly to
sandy tempestites of inner shelf (shoreface).
deposits include abundant coquina levels with Notopeltis orthometopa (Harrington)
and Ceratopyge forficuloides Harrington and Leanza; for this reason, in
the Fig. 2 it is drawed the accumulation of these deposits during the CRE.
Nevertheless, focussing on this data (Section 5, Fig. 4), is clear that the
stratigraphic position of the CRE assigned by other authors (to the Araneograptus
murrayi Zone) is younger that NORE, which is here considered the sequence
Lipán Swell, deposits of the highstand systems tract were suppressed by the
Tumbaya Unconformity (Upper Tremadoc). The tectonic processes during this time
caused important paleogeographics modifications (cf. Figs. 1d, 1e): The Chaco
Shelf tilted to the west, the maximum subsidence axis located to eastwards the
Lipán Swell (Fig. 1d), was displaced to the eastern border of the Famatinian
Domain (Fig. 1e). The Cobres High collapses being affected by magmatic events
(Eruptive Belt of the Puna Oriental; FEP, Fig. 2); while the Lower Tremadoc
shallow environment deposits (Matancilla, Taique and Potrerillo Formations) were
covered by turbidites (Coquena Formation). The collapse of Cobres High during
the Upper Tremadoc had a greater magnitude that the one affecting the flanks of
Lipán High at the beginning of the Tremadoc. If the emplacement model of the
mineral deposits is similar in both areas, it is probable that in the old Cobres
High, they were housed in the basal Lower Arenig deposits and/or underlain it.
cycle includes Sh3 and Ss4 in the Cordillera Oriental and Sierras Subandinas and
its temporarily equivalent units in Puna (upper CVP and lower CTP) (Fig. 2).
Eastwards of Lipán Swell this cycle starts from the Aorograptus victoriae Zone
(Interval VII, section 5, Fig. 4). Westwards the Lipán Swell, the oldest levels
over the Tumbaya Unconformity correspond to distal outer-shelf deposits with Tetragraptus
phyllograptoides and T. approximatus. Towards the northwest and west,
they couple with fine-grain volcaniclastic turbidites slightly older (Tolillar
Formation) with A. murrayi. It is understood that these last ones
correspond to the shelf-margin wedge systems tract, previous to the
transgression systems tract represented by Sh3 and their equivalent units in the
Puna Oriental (Chiquero Formation).
is probable that the Altiplano Shelf had a similar evolution, although up to
day, the base of the Aguada de la Perdíz Formation is not known. The high
systems tract is developed starting from the late Lower Arenig. Shales and
sandstones with Didymograptellus bifidus (Middle Arenig), accumulated in
a storm-dominated proximal shelf (lower to middle shoreface), prograding over
the old muddy shelf. D. bifidus is also present in the uppermost part of
the volcano-sedimentary successions (CVP, Chiquero and Aguada de la Perdíz
Formations, Fig. 2); allowing a Middle Arenig age for the culmination of the
volcanic episodes of the Puna. When the volcanic activity decreased, the old
Altiplano Shelf overdeepened rapidly, the maximum subsidence axis is displaced
again towards the west (Fig. 1f). This situation gave place to the beggining of
the deposition of thick successions of sandy and sandy-conglomeraditic
volcaniclastic turbidites of the CTP (Coquena Formation, Fig. 2).
the end of volcanism in the Puna, the Chaco Shelf turns again into a
tide-dominated area; with a general shallowing trend started on this region. The
coastal progradation process ends with sandstones and bioturbated mudstones
accumulated in subtidal and intertidal environments, with crust and phosphatic
nodules (Ss4). (Fig. 1f, 2).
cycle includes Sh 4 and Ss5 in Cordillera Oriental and Sierras Subandinas and
the upper CTP in the Puna (Fig. 2). In the Cordillera Oriental, Sh4 contains
conodonts of the lower Llandeilian- Caradoc (Albanesi and Moya, 2002). The
relative emersion of the Lipán High starting from the Middle Arenig isolated
the eastern area of the Chaco Shelf, where was installed a tide-dominated shelf
only influenced by relative sea-level changes. The Sh4 deposits correspond to
mudstones and fine dark wackes, without visible stratification, frequently
bioturbated; scarce and thin banks of dark coquinoideous limestone (assigned to
sporadic storms) interbedded with clastic-sedimentites. These deposits contain
well preserved shelly-fauna being rich in organic matter and phosphates. The
early mentioned characteristics, together with the absence of current
sedimentary structures, denote restricted circulation and slow sedimentation
conditions. The coastal progradation is also gradual; the deposits (Ss5)
correspond to bars and subtidal sand waves complexes. Westwards Lipán High, the
conditions were less restricted and the Sh4 interbedded bigger quantity of
tempestites. Likewise, its relationship with the Ss5 is signed by a clear
the Chaco Shelf, strata referred to the Victorian Cycle discordantly underlain
(Ocloya Unconformity) the Zapla Formation (Upper Ashgill). In most of cases, the
Zapla Formation is represented by pebbly mudstones with hirnantian fossils.
These deposits are assigned to mass flows accumulated in coastal and submarines
fans. In the basin borders, the Zapla Formation begins with conglomerates and
sandstones accumulated in bars and channels of a braided fluvial system. The
Ashgillian deposits of the last mentioned unit represents the lowstand systems
tract that starts the Cinco Picachos Supersequence (Late Ashgill-Early
Devonian). This sequence represents the first of the three cycles of third order
that integrate the Cordilleran Cycle.
traditionally accepted concept is that the Ocloya Unconformity point out the end
of an orogeny (Oclóyic Phase) that would have folded the Cambrian and
Ordovician (pre-Ashgillian) deposits. None of the authors who support this
hypothesis indicates any section or area in Northern Argentina, where the
ashgillian deposits overlie in angular unconformity the Caradocian deposits.
only folded deposits that underlie the ashgillian diamictites are those of the
Lower Tremadocian (Las Vicuñas Formation, Puna Occidental, Fig. 2). These means
that the deformation of Las Vicuñas Formation could have been previous,
specially if it is taken into account that during the Upper Tremadocian a
tectonic event (Tumbaya Phase) produced important paleogeographics modifications
(cf. Figs. 1d and 1f).
Acknowledgements. To the Prof. Dr. Florencio Gilberto Aceñolaza, for his gentle invitation to be part of this work.
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