The stratigraphical record of the Argentine Precordillera and its plate-tectonic background

The stratigraphical reeord of the Argentine Preeordillera and its plate-tectonic background

M A R T I N K E L L E R , W E R N E R B U G G I S C H & O L I V E R L E H N E R T

Inst i tut f f ir Geologie und Mineralogie, Universitdit Erlangen, Schlossgarten 5,

D-91054 Erlangen, Germany

Abstract: The stratigraphical record of the Argentine Precordillera from Early Cambrian to Late Devonian times reveals its plate-tectonic history from incipient rifting and the evolution of a marginal platform to the separation from Laurentia, its drift in higher latitudes and amalgamation with Gondwana. This pre-Carboniferous succession can be subdivided into four supersequences bounded by major unconformities and their main features are discussed with respect to plate-tectonic implications. The basal two supersequences which include the carbonate platform deposits are subdivided into 13 third-order sequences, each with a duration of 2-10 Ma. Supersequence A reflects intracratonic rifting, creating a graben system and forming a marginal plateau. In its in higher part a progradational carbonate complex covered the entire platform. Supersequence B shows the development of an aggradational carbonate succession and the evolution of reef ecosystems comparable to those that developed around the margins of the Ouachita embayment. It also shows the demise of the carbonate platform by drowning. Deposits of Supersequence C reflect crustal extension and rifting, which led to the final separation of the Precordillera from mainland Laurentia. Supersequence D reveals the approach, and probably the accretion, of the Argentine Precordillera to Gondwana.

The Argentine Precordillera (AP) is a morpho- structural entity between the main Andean Cordillera to the west and the Sierras Pampea- nas to the east (Fig. 1). It differs from the surrounding geological provinces in exposing a thick Cambro-Ordovician carbonate platform succession. Within this succession, there is a Cambrian trilobite fauna typical of the Early Palaeozoic margins of Laurentia (Borrello 1971; Vaccari 1995). Recent investigations have shown that not only the trilobite fauna is similar to that of Laurentia, but also the Late Cambrian and Early Ordovician conodont faunas (Lehnert 1995). In addition, there is a close similarity between reef ecosystems in the AP and those around the Ouachita embayment along the southern margin of Laurentia (Keller & Flfigel 1996; Keller 1997).

The uniqueness of the carbonate succession, together with the palaeontological data and isotope data from the presumed basement of the AP, led to speculation that the AP was a terrane exotic to South America, derived from Laurentia (Bond et al. 1984; Keppie 1991; Dalla Salda et al. 1992a, b, 1993; Astini et al. 1995; 1996, Dalziel et al. 1994; Dalziel 1997). Although there are models which try to interpret the origin of the AP as being autochthonous (Gonzfilez Bonorino & Gonzf, lez Bonorino 1991) or para- utochthonous (Baldis et al. 1989; Loske 1992), there has been increasing agreement that it is

indeed a Laurentia-derived terrane (see Dalziel 1997 and Dalziel et al. 1996 for discussion). However, models explaining the provenance, the timing of the possible transfer, and the accretion of the AP to Gondwana are highly controversial (Dalla Salda et al. 1992a; Dalziel et al. 1994; Astini et al. 1995, 1996; Keller & Dickerson 1996; Keller 1997). Although many authors believe in Mid-Ordovician accretion, we take the Carboni- ferous deposits of the Paganzo basin, which unconformably rest on the AP and adjacent terranes, as the first unequivocal sign that the AP had finally been accreted to Gondwana. Based on this minimum age of accretion, we will discuss the pre-Carboniferous sediments of the AP and their depositional environment in order to interpret the geotectonic background in which they formed. Many of our conclusions are drawn under the assumption that the AP is indeed a Laurentia-derived terrane.

We will discuss the most important lithostrati- graphical units and make some reference to less important sediments, but will not attempt to give a complete stratigraphy of the AP. Similarly, we will only briefly mention the most important sequence-stratigraphical features of the third- order cycles in the carbonate platform deposits, i.e. those that help in elucidating the plate- tectonic history. For a more detailed sequence- stratigraphical discussion of the carbonates the reader is referred to Keller (1997).

KELLER, M., BUGGISCH, W. & LEHNERT, O. 1998. The stratigraphical record of the Argentine Precordillera and its plate-tectonic background. In: PANK~tJRST, R. J. & RAPELA, C. W. (eds) The Proto-Andean Margin of Gondwana. Geological Society, London, Special Publications, 142, 35-56.

36 M. KELLER E T AL.

Fig. 1. Morpho-structural units of NW Argentina and adjacent areas and components of the Cuyania terrane (1, Eastern Sierras Pampeanas; 2, Sierra de Famatina; 3, Western Sierras Pampeanas; 4, valley of Iglesias-Calingasta-Uspallata).

Regional overview

Geologically, the AP constitutes a high-level fold and thrust belt of mainly Miocene age. The sedimentary succession starts with Lower Cam- brian sediments and, with several gaps, continues into the Triassic. Younger Mesozoic deposits are absent but, with the onset of crustal shortening and the corresponding uplift of the Andes, a thick succession of Tertiary and, locally, Qua- ternary sediments was deposited in terrestrial environments.

The Cambrian to lower Middle Ordovician sediments are predominantly carbonates, whereas in younger deposits terrigenous clastic rocks prevail. The contact with the underlying basement is nowhere exposed, but there is indirect evidence for a metamorphic basement from xenoliths within Tertiary volcanic rocks (Abbruzzi et al. 1993) and from basement clasts in the Ordovician continental margin facies (Keller 1995). Isotope data from the xenoliths show values typical of the Grenville-age belts along the Appalachian margin of Laurentia. Similar

Fig. 2. Distribution of Cambro-Ordovician sediments in the Argentine Precordillera (modified from Keller & Bordonaro 1993) and localities mentioned in the text.

values have been obtained recently from basement rocks in the western Sierras Pampeanas (McDonough et al. 1993; Kay et al. 1996). The recognition of Grenville-type rocks both beneath the AP and in the western Sierras Pampeanas (Sierra Pie de Palo, Fig. 1) led Ramos (1995) to propose the existence of a 'Cuyania' terrane composed of Grenvillian crust and a carbonate platform cover with Laurentian faunas. An additional fragment of the Cuyania terrane has been identified near San Rafael (Bordonaro et al. 1996; Fig. 1), where Lower Ordovician carbonates with a Laurentian fauna are exposed.

PRECORDILLERA STRATIGRAPHY 37

Within the AP proper (Fig. 2), two funda- mentally different tectono-sedimentary environments are recognized (e.g., Baldis et al. 1982; Astini 1992; Keller 1997): a carbonate platform of Cambrian to early Middle Ordovician age overlain by predominantly siliciclastic platform deposits (hereafter referred to as the platform or former platform area), and a western siliciclastic basin which developed mainly during the

Mid- and Late Ordovician as a response to rifting (here referred to as the western basin).

Stratigraphy The pre-Carboniferous sedimentary succession of the AP (Fig. 3) can be subdivided into several major, unconformity-bounded supersequences:

Fig. 3. Lithostratigraphy of pre-Carboniferous rocks in the AP. This chart is a compilation of published information from many different sources.

38 M. KELLER ET AL.

Supersequence A (Late Early Cambrian-Late Cambrian), Supersequence B (Early Ordovician- earliest Mid-Ordovician), Supersequence C (Mid- Ordovician-Late Ordovician), and Superse- quence D (Silurian-Late Devonian).

Supersequences A and B are composed of a total of 13 sequences (Fig. 4) spanning the Mid-Cambrian to earliest Mid-Ordovician. The duration of each sequence varies between 2Ma and 10Ma. This is within the average duration of third-order cycles (1-10Ma) as originally described by Vail et al. (1977). Some of these sequences are composed of several smaller-scale sequences, which from their size and average duration are intermediate between

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the third-order sequences and the typical fifth- order small-scale cycles with thicknesses vary- ing between 1 m and 5 m. Consequently, these intermediate-scale cycles are regarded as fourth- order cycles.

Supersequence A (Late Early Cambrian-Late Cambrian)

Strata of Supersequence A are exposed in the Guandacol area, in the Sierra Chica de Zonda, and in the Sierra de Villicum. Four formations have been recognized in the carbonate platform deposits of the AP. In addition, deep-water limestones of Middle and Late Cambrian age (Bordonaro & Banchig 1996) are preserved as olistoliths in Ordovician continental margin sediments.

Cerro Totora Formation (c. 340m)

Lithostratigraphy. The Cerro Totora Forma- tion consists of marginal-marine fine-grained sandstones and siltstones in the lower part of the succession. Up section, these grade into evaporites with carbonate interbeds, which in turn are overlain by sandstones, siltstones, and shales alternating with oolitic and bioclastic grainstones. Locally, quartz arenites are present near the top of the formation.

The depositional environment has been interpreted as peritidal marine (Astini & Vaccari 1996). Frequent periods of restricted conditions led to the formation of evaporites in the middle part of the succession. In contrast, the presence of Early Cambrian olenellid trilobites (Vaccari 1994) in the upper part indicates nor- mal marine salinity.

La Laja Formation (>525m)

Lithostratigraphy. The La Laja Formation is the oldest carbonate platform unit preserved in the AP. It displays a varied spectrum of carbonate rocks with intercalated intervals of siliciclastic deposits. The sediments have been grouped into depositional assemblages (Keller & Bordonaro in press) or facies associations (Keller 1997).

The lime mudstone-wackestone-intraclast packstone association is dominated by muddy textures containing well-preserved fossils, among which are trilobites, hyolithids, rare phosphatic brachiopods, and sponge spicules. Some of the mudstones are strongly burrowed. Intraclast packstones are graded and in places show an erosional contact with the underlying


beds. The sediments were deposited below fair- weather wave base in a well-aerated environment. The intraclast packstones represent tempestites, their clast content was derived from shallow shelf areas.

The oncolite-wackestone association is composed of thick, in places cross-bedded, oncolite beds intercalated in wackestones with trilobites, hyolithids, and brachiopods. Erosional features at the base of the oncolites are present locally. The depositional environment of this association is similar to that of the lime mudstone-wackestone- intraclast packstone association. The oncolites are also interpreted as tempestites, although their source area was different. The original site of oncoid formation has not been found.

The oolite-oncolite association is composed of oolitic and/or oncolitic packstones and grainstones. Many of the horizons show cross- bedding, and locally well-developed herringbone cross-stratification has been observed. Similar sedimentary features were found in the packstone-grainstone association, but these rocks are predominantly composed of fossil hash (trilobites, hyolithids, eocrinoids, brachiopods). Det- rital quartz is present is some beds. Sediments of both associations represent winnowed sand bars, barriers, and subtidal shelf sands. The depositional environment was shallow subtidal and lower intertidal where wave and current activities constantly removed the lime mud and led to the formation of herringbone cross-stratification.

The siltstone association is easily recognized in the field by its yellow and brown colours which contrast with the dark grey and black colours of the limestones. Besides siltstones, silty marlstones, marlstones, eocrinoidal or oolitic grainstones are part of this association. Wave ripples and linsen and flaser bedding are the most prominent bedforms. The sandstone- grainstone association is composed of bioclastic, in places oolitic, grainstones and thin beds of quartz arenites or calcareous sandstones. Coquinas of olenellids are common. In both associations, lithology in combination with sedimentary structures indicates a shallow subtidal shelf with a strong tidal influence. However, in three sections thick quartz arenites alternating with black shales are present. Tectonic deformation has obliterated all sedimentary structures; consequently, an interpretation of this succession is difficult.

Sediments of the La Laja Formation represent three depositional systems: a carbonate shoal complex with dominantly grainstones and packstones, an open marine environment with mudstones and wackestones, and a shallow shelf association influenced by terrigenous detritus.

Mudstones and wackestones are interpreted to represent both back-barrier, lagoon deposits and sediments which accumulated seaward of the main high energy zone.

Sequence stratigraphy. Two types of shallowing-upward successions have been observed. There are calcareous successions with a thickness between some tens of metres and more than 100m. The successions start with sediments of the lime mudstone-wackestone-intraclast packstone association. As this association represents the deepest environment present in the La Laja Formation, the basal bounding surface is always a flooding surface. Up section, bioclastic packstones and grainstones are developed. Oolites or oncolites form the top of the carbonate succession. Within the individual successions no small- scale cycles have been observed.

Several of these cycles are stacked to form larger-scale sequences. Four of the latter within the La Laja Formation (Fig. 4) span approximately 10 Ma, hence they have to be regarded as third-order cycles. The most characteristic feature of these cycles are basal units composed of the siltstone association. No dramatic environmental change is indicated by the switch from carbonate production at the top of a carbonate cycle to siliciclastic deposition of the siltstone associations. In contrast, the upper boundary of the terrigenous units is always sharp and records important flooding. Consequently, the surface separating the siltstone units from the overlying carbonate intervals is interpreted as the transgressive surface (Keller 1997) which marks the beginning of the transgressive systems tract.

The transgressive systems tract is relatively thin and is composed mainly of rocks with muddy textures. There is no distinct surface which might be interpreted as a maximum flooding surface. Nevertheless, the change from mud-dominated textures in the lower part of the succession to grain-supported textures in the upper part seems to reflect the transition from the transgressive systems tract to the highstand systems tract. The relatively thick highstand systems tract together with the internal architecture of the entire succession mark a catch-up system (Kendall & Schlager 1981). In such systems, shoal deposition (oolite-oncolite association in the La Laja Formation) is restricted to the late highstand. In most sequences within the La Laja Forma- tion, sediments of the oolite-oncolite association were deposited during this interval related to sea- level stillstand or relative sea-level fall. The position of the siltstone units between late highstand deposits and the transgressive surface at the base of the subsequent carbonate interval

40 M. KELLER ET AL.

indicates that the siltstone units are related to a sea-level lowstand and that their lower boundary is the sequence boundary. No signs of subaerial exposure have been observed along these contacts, nor is there a basinward shift in facies. Consequently, the sequence boundaries are type-2 boundaries, and the siltstone intervals represent the corresponding shelf- margin systems tracts.

The Zonda and La Flecha Formations

Lithostratigraphy. Both formations are composed predominantly of peritidal dolomites. Secondary dolomitization has obliterated most of the primary structures in the Zonda Forma- tion (200-300 m). Where preserved, sedimentary structures are comparable to those of the La Flecha Formation. The sediments of the latter (400-700m) have been described in detail by Keller et al. (1989), Cafias (1995a), and Armella (1989a, b).

Rocks of the mudstone-wackestone association contain a sparse fauna (trilobites, hyolithids) or are bioturbated. The sediments were deposited in shallow subtidal areas, probably under restricted conditions. A few intercalated trilobite packstones represent episodic storms.

In the intraclast grainstone-oolite-thrombo- lite association, the intraclasts reflect erosional processes that affected the entire spectrum of rocks present in both formations. Oolite grainstones are present as continuous beds, often with well developed herringbone cross-stratification. However, they also fill erosional channels which in places cut down almost 2m into thrombo- lite mounds. Locally, the oolites onlap individual mounds demonstrating a close relation between mound growth and oolite deposition. The individual thrombolites either grew as isolated mounds or they form laterally linked structures several metres high and hundreds of metres wide. This association records complex sediment build-up interactions in shallow subtidal and lower intertidal areas.

The stromatolite association is composed of different types of stromatolites and accompanying mudstones and grainstones. LLH-stromatolites are most abundant, but SH morphotypes have also been observed. The association formed in environments extending from the shallow subtidal to the lowermost supratidal.

In the microbial laminite-breccia association, thick mudstone intervals, in places with evaporite pseudomorphs, represent storm events during which lime mud was transported from its original site of formation, mainly the shallow

subtidal, onto the supratidal flats. Mudcracks, tepee structures, and pseudomorphs after evaporites testify to prolonged episodes of subaerial exposure, desiccation, and evaporation. The association described here was mainly formed in higher intertidal and supratidal environments. Finally, there are some mudstones with large spheroidal mudlayers and abundant microscopic cracks. These have been interpreted as calcrete horizons (Keller et al. 1989) and were attributed to a terrestrial setting.

Rocks of both formations were deposited in a peritidal environment, many of them under restricted, hypersaline conditions. Small-scale (fifth-order) cycles are a prominent feature in all sections, their internal architecture, however, does not permit a distinction of more proximal from more distal cycles. The only evidence of somewhat 'deeper' environments is present in the Guandacol area, where there is a higher percentage of shallow-subtidal mudstones in the cycles (Cafias 1995a).

Sequence stratigraphy. Sedimentologically, the lower part of the Zonda Formation is a continuation of the uppermost sequence of the La Laja Formation (sequence 5; Fig. 4). This is indicated by the transition from oolites with herringbone cross-stratification (uppermost part of the La Laja Formation) to inter- and supratidal dolostones of the basal Zonda For- mation. These rocks are abruptly overlain by dark subtidal mudstones at the base of another major shallowing-upward succession. The upper boundary of this cycle coincides with the boundary between the Zonda and La Flecha Formations.

The La Flecha Formation is composed of two similar shallowing-upward sequences (sequences 7 and 8; Fig. 4), but there is no well-defined boundary between them. Within the lower cycle the abundance of calcrete horizons increases towards the top but calcretes are absent above the presumed cycle boundary. A reversed pattern is visible in the distribution of thrombolites: they become less abundant towards the top of the cycle, are absent in the uppermost interval, but regain importance at the base of the next cycle.

All three sequence boundaries within the Upper Cambrian strata either show signs of subaerial erosion, coarse detrital quartz, abundant evaporites, or concentrations of calcrete horizons just beneath the main surface. Conse- quently, each of these sequence boundaries has to be regarded as a type-1 sequence boundary (Keller 1997). These sequences were deposited during approximately 10Ma, which qualifies them as third-order sequences.


Middle and Upper Cambrian deep-water limestones and marlstones

These rocks, which recently have been combined under the informal name of the 'La Cruz limestones' (Keller 1997), are mainly present as olistoliths within the Ordovician continental margin facies. Only the outcrop at Cerro Pelado shows these deep-water limestones in a presumably autochthonous position above peritidal dolomites of the La Flecha Formation.

The limestones and marlstones are dark grey and fine grained, and thin- to medium-bedded. Shales have been observed only locally. The fauna consists of agnostid trilobites and sponge spicules. Graded beds, in places with erosional structures at the base, and slumped beds are also present. In general, the components are less than 1 mm in diameter and composed of platform- derived material. The horizons described here are interpreted as distal tempestites, deposited in a background environment of deep-water limestones and marlstones. It is noteworthy that wherever these rocks are exposed as olistoliths within the continental slope facies, there are no indications of mass-flow deposits which might indicate the existence of a Mid- or Late Cam- brian platform margin or slope. In contrast, we believe that the platform gradually passed into deep-water environments without a pronounced slope. Platform configuration might have been a homoclinal ramp (Read 1982, 1985; Burchette & Wright 1992).

Within the most important olistolith, at the Los Tuneles section (Fig. 2), an unconformity is present separating Middle Cambrian deep-water limestones from latest Cambrian or even lowermost Ordovician slope deposits (Keller 1995). These are the oldest slope sediments hitherto known from the AP.

Evolution of Supersequence A

The basal succession of supersequence A consists of redbeds alternating with evaporite layers (Cerro Totora Formation). Astini et al. (1995) interpreted this succession as a rift-related sequence indicating the separation of the AP from Laurentia. In our view, its localized presence and relatively reduced thickness, together with the absence of any indications of rift- related volcanism or ocean floor, all point to intracontinental rifting similar to that of the coeval Birmingham graben of Laurentia (Thomas 1991).

The uppermost horizons of the Cerro Totora Formation are coeval to the basal beds of

the La Laja Formation (El Estero Member). In this part of the succession, both formations show an alternation of sandstones, shales, and grainstones which constitutes the transition to carbonate platform sedimentation. Cafias (1988) described a hardground and an erosional unconformity at the base of the overlying car- �9 bonates in the Guandacol area. This unconformity is matched by a type-1 sequence boundary separating the E1 Estero Member from the Soldano Member of the La Laja Formation. The rocks beneath the sequence boundary are white quartz arenites and black shales of a shallow depositional environment. Above the unconformity and above the sequence boundary, two trilobite zones seem to be absent from the sediments of the AP (Palmer pers. comm.). These data indicate that the corresponding erosional event is approximately equivalent in timing to the Hawke Bay event described from the Appalachian margin of Laurentia (Palmer & James 1980).

The subsequent Cambrian history of the AP is characterized by rapid but decreasing subsidence until the Dresbachian, and a carbonate factory barely able to keep pace with subsidence (catch- up system of Kendall & Schlager 1981). Only during the Late Cambrian is there a keep-up pattern in the sediments. The absence of terrigenous detritus from the Dresbachian onward indicates that the source area had vanished, most probably by onlap of the Upper Cambrian peritidal deposits. Faunal data (Benedetto et al. 1995; Vaccari 1994) indicate that during the Cambrian the AP was faunistically indistinguishable from Laurentia.

Supersequence B (Ibexian-Early Whiterockian)

Supersequence B comprises the Lower Ordovi- cian to basal Middle Ordovician carbonate platform sediments attributed to the La Silla Formation and the San Juan Formation. The most striking feature of this supersequence is the total absence of any terrigenous material other than clay.

La Silla Formation (400 m)

Lithostratigraphy. The wackestone association is composed of mudstones and wackestones with a scarce and monotonous fauna. Abundant hardgrounds and bedding-parallel trace fossils indicate slow and non-continuous mud accumulation, probably under hypersaline conditions.

42 M. KELLER ET AL.

A more favourable environment is indicated by wackestones and packstones with sponges, receptaculitids, and abundant nautiloids and gastropods. In general, sediments of this association were deposited under low-energy, subtidal conditions.

Rocks of the peloidal grainstone association are the most abundant in the La Silla Formation. There are frequent intercalations of intraclast grainstones or intraclast-peloidal grainstones. Most of the intraclasts are of storm origin, the clasts represent the entire spectrum of the microfacies observed in the La Silla Formation.

In the oolite association, most of the sediments show abundant cross bedding. Herringbone cross stratification has been observed only locally. Intraclasts are abundant in some of the beds. Thrombolites and microbial mounds are also part of the association. Many of the mounds are onlapped by ooid sands. The presence of herringbone cross-stratification was taken as evidence of a tidal bar origin of the corresponding oolites. Most of the oolites, however, originated as marine sands deposited following major storm events. Storm reworking of semi-lithified sediment is also responsible for the abundant intraclasts found in some of the oolite beds.

Microbial laminites and mudstones with abundant mudcracks, pseudomorphs after evaporites, and bird's-eye structures are the most important rocks in the microbial boundstone association. The sediments were formed in higher intertidal and supratidal environments. Intercalated flat pebble breccias are the product of desiccation and local reworking of carbonate layers.

Sediments of the La Silla Formation were deposited on a vast platform dominated by shallow subtidal, often restricted environments. In addition, intertidal and rare supratidal areas were present. Although oolites are found throughout the succession, only very few of them can be attributed to shoal systems. The majority of the oolites form sheet-like deposits, typical of redistribution during major storms. Intertidal and supratidal environments are dominated by microbial boundstones and thick storm-induced mudlayers with desiccation cracks and evaporites. Cafias (1995a, b) interpreted the deposits of the La Silla Formation as representing a rimmed shelf. In this model, the oolites are margin-related accumulations. How- ever, there are no indications of a nearby shelf break, nor are the oolites restricted to a single facies belt but are found all across the platform. In the westernmost exposures of the La Silla Formation, intertidal sediments are absent and muddy subtidal rocks prevail. This indicates a ramp like configuration of the platform during

the Lower Ibexian which gently dipped towards the west.

Sequence stratigraphy. Within the La Silla For- mation three major sequences are developed (sequences 9-11; Fig. 4). They start with subtidal limestones, mainly of the wackestone association. Up section, packstones and peloidal grainstones are developed. The lower two sequences are topped by microbial laminites and rare thrombolites, whereas the uppermost sequence shows bird's-eye mudstones and thrombolites in this interval. At the base of the lowermost succession, silty dolomites, large intraclasts and extraclasts testify to exposure and erosion of underlying strata. Consequently, this boundary is a type-1 sequence boundary. The nature of the subsequent boundaries is less obvious. Erosional features are very rare and there is no obvious basinward shift in facies, hence they are interpreted to represent type-2 boundaries (Keller 1997). Based on their average duration of 3 -4Ma, all of these sequences are third- order sequences.

The lowermost and the uppermost successions are relatively thin and contain abundant intertidal deposits. In contrast, in the middle succession, which is relatively thick, subtidal lithologies are most abundant. This tripartite subdivision of the La Silla Formation has a close match in the coeval strata of the northern Appalachian margin. There, this interval, corresponding to the Lower Ibexian, consists of a predominantly subtidal unit sandwiched between peritidal carbonates (Knight et al. 1995). Similarly, the Chepultepec interval of the southern Appala- chians shows a succession of peritidal carbonates, subtidal rocks and, again, peritidal strata (Bova & Read 1987).

San Juan Formation (330 m)

Lithostratigraphy. The most complex facies association is the reef and reef mound association. It consists of biohermal and biostromal accumulations of sponges, receptaculitids, early stromatoporoids, and algae. Non-reef rocks are rudstones, grainstones, wackestones, and in places bird's-eye mudstones. The rocks of this association have been recently described by Cafias & Carrera (1993), Cafias (1995a, b), Cafias & Keller (1993), Keller & Bordonaro (1993), and Keller & Flfigel (1996). The reefs are concentrated in two horizons, one near the base of the formation and one near the top.

The packstone-grainstone association is a lateral equivalent of the reef and reef mound


association. A few isolated biostromes have been found within this association. Packstones and grainstones contain broken and abraded fragments of a varied fauna. Intraclasts are also present, but are only locally abundant. In a few sections cross-bedded, almost monomict pelma- tozoan grainstones have been observed.

The wackestone-intraclast packstone association and the wackestone-oncolite association both represent the background sedimentation (wackestones and a few mudstones) on the carbonate platform during the Late Ibexian and Early Whiterockian. The sediments and their organic content point to quiet subtidal conditions still within the photic zone. The intraclast packstones and the oncolite beds are the results of storm events affecting different source areas. Tempestites are rare in the sections of the Sierra de Villicum and Sierra Chica de Zonda, but they are very abundant in the J~chal area. There, the beds originating from tempestites are thick and often show an erosional base, grading, and cross bedding. Farther towards the west beds tend to become thinner, erosional features disappear, and oncoids and intraclasts become less abundant.

The nodular-wackestone association is dominated by whole-fossil wackestones and few mudstones; tempestites are absent. The association was deposited below storm wave base, but still within the photic zone as indicated by the abundant and diverse fauna. Similarly, the nodular- packstone association records a relatively deep environment; a few graded beds and shallow water biota in these beds, however, indicate some storm activity. Shale partings, thin shale beds, and the total homogeneization of many beds by bioturbation point to reduced sedimentation rates.

The mudstone-wackestone-shale association is characterized by dark colours and platy, thinly bedded rocks. The fauna consists of deep- water trilobites and conodonts. The carbonates are typical deep-water, hemipelagic limestones (Wilson 1969).

An additional association was described by Cafias (1995a, b) from the Guandacol area. There, rocks similar to the hemipelagic limestones described above alternate with fine- to medium-bedded, poorly sorted packstones and grainstones. The sedimentary succession is interpreted as a deep-water setting, but still within the reach of tempestites.

The sediments of the San Juan Formation were deposited on a carbonate ramp dipping gently towards the west. The easternmost outcrops contain the high-energy shoal-water complex which hosts the main reef accumulations.

The upper reef horizon can be traced westward into slightly deeper packstones and grainstones with abundant reef-derived material. A similar pattern holds for all sedimentological units described from the San Juan Formation (Cafias 1995a, b; Keller & Flfigel 1996; Keller 1997). The eastern sections always represent a (slightly) more shallow environment than sections farther to the west. Remarkable, however, is the absence of lagoonal, intertidal, and supratidal deposits, indicating that an important part of the platform is not preserved.

Sequence stratigraphy. The San Juan Forma- tion is composed of two major sequences. The lower one starts above the boundary separating the San Juan Formation from the La Silla Formation. Above this type-2 sequence boundary, several small-scale shallowing-upward cycles form a progradational parasequence set, interpreted as a shelf-margin systems tract. Above the overlying transgressive surface, the lower reef mound horizon formed during a rapid relative rise in sea level. Within this transgressive systems tract, several additional flooding surfaces have been identified. The accompanying deepening finally led to the deposition of a characteristic nodular limestones association deposited on the deep ramp. The most marked deepening is observed in the O. evae conodont zone and was caused by a eustatic sea-level rise of global dimensions (Fortey 1984). The top of this lower nodular limestone association is regarded as the zone of maximum flooding, which marks a change from retrogradation to aggradation and the onset of the highstand systems tract.

The next sequence boundary is also well defined by the sudden change from mid-ramp deposits to shallow-water grainstone and packstones. In the La Silla section, a hiatus is presumed to exist between the two facies associations. In addition, the Tremadocian conodont Cordylodus cf. angulatus was found just above the sequence boundary, indicating considerable erosion in the more interior parts of the ramp. Again, the transgressive systems tract hosts the main reef accumulation of the upper reef interval. Several flooding surfaces are present above the reef mounds which indicate successive drowning of the carbonate platform, culminating during the Whiterockian. The drowning succession is characterized by black shales and deep-water limestones, some 10m thick. This succession is overlain by graptolitic black shales. The drowning itself is not a uniform process: drowning of the outer part of the ramp (Guanda- col area) was earlier than in the other areas and was coeval with the upper sequence boundary in


the San Juan Formation south of Jitchal. Another exception is the Las Chacritas section, where some 60 m of spiculitic wackestones rest on the San Juan Formation and form the drowning succession. Finally, in the Las Aguaditas section a carbonate slope and basin developed above the carbonate platform (Keller et al. 1993a). The very different timing of drowning and the different successions themselves are vestiges of the breakdown of the carbonate platform under the onset of crustal extension.

Evolu t ion o f supersequence B

In comparison to supersequence A, which culmi- nated with the deposition of a laterally extensive tidal-flat complex, supersequence B documents the return to more open-marine conditions. Third-order sequences tend to become thinner towards the Whiterockian and the character changes from strongly progradational during the latest Cambrian to aggradational towards the top of the San Juan Formation. The sequence boundaries themselves also show an evolution- ary trend: type-1 boundaries are restricted to the Late Cambrian, whereas type-2 boundaries characterize the Ibexian and Early Whiterock- ian. In the La Silla Formation, the sequences are topped by microbial laminites and inter- to supratidal facies. However, the thickness of these late highstand deposits progressively becomes thinner and in the San Juan Formation no intertidal or supratidal facies are found at the top of the sequences. This overall trend in the evolution of the sequences and the corresponding boundaries is accompanied by the change from mostly dolomitic rocks of the Late Cambrian to exclusively limestones during the Early Whiterockian. The carbonate rocks of supersequence B reflect various attempts at drowning the platform. Although the carbonate factory was prolific well into the Whiterockian, the sum of all external factors was finally too strong to permit further carbonate production.

The Ibexian La Silla Formation shows a rather uniform facies development. Two important events accompanied the deposition of this unit. Near San Rafael (province of Mendoza), Grenvillian basement attributed to the Cuyania terrane (Ramos 1995) was flooded and a carbonate succession similar to the La Silla Formation and basal San Juan Formation was deposited. The cratonic character of this part of the terrane (Fig. 1) is documented by 80m of limestones which correspond to almost 400 m in the AP proper (Bordonaro et al. 1996). The other important event was the incipient evolution of a slope environment to the west of the

platform, although rocks of this depositional setting are only preserved within the Los Tuneles mega-olistolith.

General aspects of supersequences A and B

During the formation of supersequences A and B (Cambrian to basal Middle Ordovician), sedimentation in the AP was controlled by the same factors as those responsible for the evolution of the Appalachian margin. These include a thermally subsiding crust, localized tectonic events (e.g., Hawke Bay event), and the increasing importance of eustasy, which exerted major control on the formation of 13 third-order sequences in the AP (Fig. 4). The corresponding qualitative sea-level curve and its correlation with the curve for the southern Appalachians (Read 1989) is shown in Fig. 5.

In addition, the benthic faunas of the whole of supersequence A and most of supersequence B (Cambrian through Late Ibexian) cannot be distinguished from the faunal succession in Laur- entia (Benedetto et al. 1995; Fig. 6a-c), but are totally different from those of the terranes today adjacent to the AP. The first non-Laurentian fossils are associated with sponge-algal mounds in the lower San Juan Formation (Vaccari 1995). However, the faunal record and the datasets from Laurentia and the AP are not of the same quality. In the AP, there are almost no data on benthic faunas for most of the Early Ibexian. The Laurentian-type conodont succession of the AP (Lehnert 1997; Lehnert et al. 1997) may help to fill this gap.

The compilation of the palaeobiogeographical affinities of benthic macrofossil assemblages of the AP (Benedetto et al. 1995; Fig. 6a) includes data on several groups. These data are not always of uniform quality, because some groups have been recorded only from a short interval and/or are represented only by a few taxa (e.g., sponges: Late Ibexian-Early Whiterock- ian). In addition, there are no data on brachiopods, bryozoans, pelecypods and sponges before the Late Ibexian. Finally, the individual groups may not be of comparable palaeogeographical sensitivity.

Supersequence C (Mid- and Late Ordovician)

Sediments of this supersequence were deposited in two different settings. In the western AP, the developing basin accommodated thick successions of a continental margin and adjacent basin


Fig. 5. Sea-level curve for the Cambro-Ordovician carbonate platform deposits and its correlation with the Appalachians. The Appalachian sea-level curve is modified from Read (1989). A and B show alternative correlation of earliest Ordovician sequence boundaries in the Argentine Precordillera.

environment. In the east, this supersequence developed above the former carbonate platform.

The western basin

The most impressive sediments are those of the Los Sombreros Formation. Black graptolitic shales host various types of mass-flow deposits. Debris-flow deposits, turbidite sandstones and greywackes, mega-breccias, and conglomerates

are volumetrically abundant. However, most spectacular are giant olistoliths, which in places are more than 1 km long and more than 300 m thick. It is in this facies that the Middle and Upper Cambrian deep-water limestone olistoliths are present. Other important components of the mass-flow deposits are clasts of metamorphic basement (Banchig et al. 1990). These clasts are interpreted as having been derived locally (Banchig et al. 1990; Keller 1995). Since in some sections basement clasts are found

46 M. KELLER E T A L .

Fig. 6. (a) Changes in the composition of benthic faunas in the Argentine Precordillera on the genus level (modified from Benedetto et al. 1995). During the Cambrian, the fauna is Laurentian. With the onset of extension during the Whiterockian, the first Avalonian and Baltic elements are observed. Note that an influence of Gondwanan elements cannot be observed before Early Caradoc. In addition, from a faunistic point of view, the Caradoc is the time of maximum isolation indicated by a high percentage of endemic faunas and pandemic genera. (b) The trends in palaeobiogeographical affinities are much better visible on a sketch excluding pandemic faunal elements; (e) includes the aspect that there are no reports on other than Laurentian faunas before the Late Ibexian. Faunal composition contradicts any model which claims for a mid-Ordovician collision of the Argentine Precordillera with Gondwana.

together with clasts of the entire carbonate platform succession, their presence implies the formation of a fault scarp more than 2100m high (the cumulative thickness of the carbonate platform succession). Siliciclastic clasts that might have been derived from a rift-related cover of the basement are only present is the Los Tuneles section, where big blocks of quartz- pebble conglomerates are exposed. These blocks are lithologically comparable with sediments of the Ocoee Group, rift-related deposits of the Appalachian margin (R. Hatcher pers. comm.). The scarceness of rift-related clasts in most sections indicates that the basement of the AP was mostly covered by carbonate platform sediments (Keller 1995).

All mass-flow deposits are proximal, and the giant olistoliths must have been transported by rockfall. In addition, the sediments of the Los Sombreros Formation trace the former margin of the carbonate platform. However, as the siliciclastic succession is younger (latest Llanvirn-Caradoc; see discussion in Keller 1997), there is no sedimentological continuity between the former platform and the newly developed basin. The depositional environment of the Los Sombreros Formation is characterized by local steep scarps which delivered the olistoliths and many of the conglomerates, and a basin area (the continental rise) where these sediments accumulated.

Farther to the west, there are several siliciclastic units deposited in a deep basin. The Porte- zuelo del Tontal Formation contains abundant conglomerates and turbidites (Spalletti et al. 1989) and is interpreted as a deposit of the continental margin. The Don Polo Formation is a succession of proximal to distal turbidites (Fig. 7) and includes some horizons interpreted as contourites (Astini 1991; Keller 1997). The Alcaparrosa Formation consists of graptolitic and aluminiferous black shales. In addition, it contains basaltic pillow lavas (Fig. 7) with MORB-like characteristics (Haller & Ramos 1984; Kay et al. 1984). Most of these sediments are poorly dated, but they all fall in the range from latest Llanvirn to Caradoc. Although the structural relations between these units are not always clear, the spatial distribution of the sediments exhibits an overall fining towards the west (Fig. 7), culminating with basin shales and pillow basalts. West-directed transport has been determined for the proximal deposits and a SW- to S-directed transport for the more distal sediments (Spalletti et al. 1989; Astini 199l).

All sedimentological data indicate that the sedimentary succession of the western basin is a rift-drift related succession marking the


Fig. 7. Tectono-sedimentary environments, evolution of the western basin, and sediment distribution in the Argentine Precordillera following continental break-up. Note that there are places where the rift-onset and the break-up unconformities merge. See text for discussion. (Not to scale)

formation of an Atlantic-type continental margin in the AP during the Mid- and early Late Ordovician. This is also evident from the very high sediment accumulation rates (Fig. 8).

The former platform area

Above the drowning unconformity of the carbonate platform, most of the deposits are graptolitic black shales. Only locally did carbonate sedimentation continue (Las Chacritas and Las Aguaditas sections) and even in these successions there is a clear tendency towards deep-water limestones. In most sections then, there is a hiatus or an erosional unconformity above which mass-flow deposits dominate. The corresponding event affected the platform area during the earliest Llandeilo. In the Don Braulio section, three fining-upward successions were deposited (La Cantera Formation, c. 150m). The basal succession starts with massive conglomerates locally containing large slabs of the underlying black shales; up-section, turbidites are found. The upper two successions start with coarse sandstones and arkoses, which pass into shales. Another hiatus separates the La Cantera Formation from the Don Braulio For- mation, which contains vestiges of the Ashgillian Gondwana glaciation (Peralta & Carter 1990; Buggisch & Astini 1993).

In the Guandacol area, a similar succession is preserved, but with a thickness of more than 1200 m (Trapiche Group). In the basal part of this succession, limestone slabs up to 70 m across are present (Astini 1991). In addition, several high-angle erosional unconformities have been observed in one of these sections (yon Gosen pers. comm.). All these features point to rapid local uplift and the formation of oversteepened fault scarps. The highly varied sediment accumulation rates above the former platform are documented in Fig. 8.

A unique succession is preserved at Rio Sassito where about 40m of mainly Caradoc sediments rest on top of deeply eroded San Juan limestones. The lower part of the succession is composed of about 20 m of siliciclastic rocks, the upper 20m are temperate-water limestones. These limestones apparently formed in a horst position because the adjacent areas are characterized by deep-water siliciclastic material with mass-flow deposits. This outcrops demon- strates that a major period of erosion affected the former platform area after the deposition of the Middle Ordovician drowning succession but prior to the Caradoc.

Everywhere in the former platform area, Ordovician rocks of supersequence C are separated from the overlying supersequence D by a major unconformity. In many places the basal bed of the younger succession is marked

48 M. KELLER ET AL.

Fig. 8. Plot of cumulative sediment thicknesses in different areas and settings in the Argentine Precordillera. The most marked differences between the platform area and the western basin occurred during Mid- and Late Ordovician time, caused by rapid subsidence of the newly forming basin. Inlay shows history of thermal subsidence. This plot includes only data from the carbonate platform and, during the Late Ordovician to Devonian, from the former platform area (from Gonzfilez Bonorino & Gonz/Llez Bonorino 1991).

by a chert-pebble conglomerate. The onlap of the younger succession onto the eroded platform is strongly diachronous (Astini & Maretto 1996). The overlying strata, however, show a rather uniform development.

Evolution of supersequence C

Supersequence C is bounded below and above by two major unconformities. The basal unconformity marks the end of carbonate platform deposition and the onset of a depositional regime which favoured mass-flow sedimentation with highly variable thicknesses; in places, carbonates were deposited. Local unconformities

are common in all successions. The basal unconformity is approximately coeval with the formation of a continental margin and the adjacent western basin. In our interpretation, supersequence C represents major crustal extension related to rifting and the concomitant formation of horst and graben structures, the evolution of a continental rise and an adjacent basin into which MORB-like basalts were extruded. The major fault scarp along which the Los Som- breros Formation was deposited is evidence of the rifting: a scarp more than 2100m high is incompatible with a compressive regime and must be related to large-scale crustal extension. An extensional regime is corroborated by the


pillow basalts and the formation of Caradoc carbonates on an isolated horst. Various studies have attributed individual sedimentary successions to crustal extension (Spalletti et al. 1989; Loske 1992; von Gosen 1992; Keller 1995). Similarly, the pillow basalts were interpreted as having formed in an extensional regime (Haller & Ramos 1984; Kay et al. 1984). Dalla Salda et al. (1992b) were the first to interpret the Middle and Upper Ordovician strata as representing rifting and in particular the separation of the AP from Laurentia. A similar argument has been used by Keller & Dickerson (1996).

Increasing distance from Laurentia is also apparent in the composition of the faunas (Fig. 6). Avalonian, Baltic, and endemic faunas became increasingly abundant during Mid- Ordovician times. Laurentian faunas were able to settle on the disappearing terrane until the Late Ordovician. In contrast, the first Gondwa- nan (Mediterranean) brachiopods did not arrive before the Caradoc, which was the time of maximum faunal isolation (Fig. 6c).

Falvey (1974) proposed a comprehensive model for rifting, continental break-up, and the evolution of passive continental margins. This model includes two major unconformities: a rift- onset unconformity and a break-up unconformity (Fig. 7). Thermal expansion of the crust and concomitant erosion during initial crustal stretching are responsible for the rift-onset unconformity. In a subsequent stage, crustal expansion leads to the formation of horst and graben structures (rift valley stage), whereas the break-up unconformity marks the transition from rifting to drifting. Consequently, the break- up unconformity is almost coeval with the onset of formation of oceanic crust. In the AP, the major unconformity just above the drowning succession, which marks the base of supersequence C, is here interpreted as the rift-onset unconformity (Fig. 7). In the former platform area, the concomitant erosion cut down to highly varied stratigraphical levels in the San Juan Formation (Fig. 9). In the western basin, this unconformity is correlative with the basal strata of the Los Sombreros Formation, which in turn was deposited during the evolution of a continental slope and rise. The rift-valley stage is well expressed on the former platform where highly varied successions of conglomerates, mass-flow sediments, and sandstones were deposited in grabens or half-grabens. Locally, a carbonate slope developed. On corresponding horsts, successions like that at Rio Sassito were laid down. A widespread hiatus is present in the Caradoc, especially on the former platform. This hiatus might have been caused by lateral heat-

flow from the active volcanic centres in the basin. This heat-flow caused uplift and erosion on the platform and the resulting unconformity has to be regarded as the break-up unconformity (Fig. 7). The widespread latest Ashgillian/ Silurian chert-pebble conglomerate at the base of supersequence D marks the break-up unconformity on much of the former platform. However, there are successions (Don Braulio Formation, Empozada Formation sensu Keller 1997) where there are relatively thick Ashgillian sediments preserved just above the break-up unconformity.

Supersequence D

Silurian deposits are widespread between Jfichal in the north and the Rio San Juan in the south. They are absent in the Guandacol area and seem to be absent in the Mendoza area. Two different environments are evident. Along the eastern margin of the AP (Sierra Chica de Zonda and Sierra de Villicum), a deep graben accommodated mass-flow deposits (Rinconada Forma- tion). Clasts and huge olistoliths mirror the underlying stratigraphy and document erosion down to the San Juan limestones. Consequently, the succession of clasts and olistoliths shows an inverse stratigraphy. In addition, there are abundant clasts with lithologies not found in the platform succession (Loske 1992; yon Gosen et al. 1995), mainly acidic to intermediate magmatic rocks. The thickness of the sediments (conservative estimates are on the order of 1000m, von Gosen et al. 1995), abundant slump and slide structures, and the huge olistoliths document rapid subsidence of a basin and uplift of the adjacent shoulders.

The other sedimentary environment is a vast siliciclastic platform with facies spanning the shoreface to areas well below storm wave base (Acefiolaza & Peralta 1986; Peralta 1990; Peralta & Carter 1990; Astini & Maretto 1996). Well into the Devonian, this platform had a clear N-S polarity. Sediments are thickest around Jfichal and get thinner towards the Rio San Juan. In addition, there is an increasing number of hiatuses towards the south, and the time gap represented by the hiatuses increases in the same direction (Benedetto et al. 1992; Herrera 1993; Peralta 1993). The J~chal area was obviously a depocentre, whereas the Rio San Juan area was a structural high.

Furque & Caball6 (1990) described N-S- trending ridges which subdivided the siliciclastic platform during the Silurian. Although this pattern is indeed visible in several transects


from west to east across the platform, the N-S polarity is much more pronounced.

The Devonian sediments document a slow transition from siliciclastic shelf sedimentation to the deposition of turbidites. The Lower Devonian Talacasto Formation is predominantly characterized by shales in its lower part; up-section, sandstones dominate. The overlying Punta Negra Formation consists of flysch-like deposits (Gonzfilez Bonorino 1975) attributed to a fan-delta complex. The contact between the Talacasto Formation and the Punta Negra Formation is strongly diachronous (Herrera 1993), although it is not clear whether this is true diachronism or whether it reflects a major episode of erosion prior to the onset of flysch formation. The loss of section from the top down in a southward direction might favour the erosional interpretation whereas the onset of flysch deposition during the Early Devonian in the Mendoza area might be taken to indicate a northward migration of the fan deltas and hence a diachronous evolution.

Within supersequence D, there is an interest- ing evolution in the heavy mineral spectra (Loske 1992). The Silurian (and Ordovician) sediments are dominated by a low-diversity and low-quantity association of weathering-resistant minerals, such as zircons and tourmalines. In contrast, the Devonian clastic rocks received a high-diversity and high-quantity association, with garnet, zoisite, and apatite, among others. Following Loske (1992) this change mirrors the change in the source rocks from a mature hinterland with unmetamorphosed or slightly metamorphosed rocks, to a hinterland where magmatic and metamorphic rocks were exposed. Zircon ages from the Devonian sediments (1.1 Ga, Loske 1995) show that this magmatic/ metamorphic hinterland might have been the basement of the AP itself (the Cuyania terrane).

during the Wenlockian, the AP becomes faunistically indistinguishable from Gondwana (see Benedetto this volume).

The Devonian sediments indicate a change from platform sedimentation to the evolution of a flysch trough which received detritus from a rising metamorphic or magmatic hinterland. Palaeocurrent data (Gonzfilez Bonorino 1975) suggest northeastern and southeastern source areas which fed two important fan deltas.

Supersequence D also saw the onset of deformation in the AP, marked by a thermal event which affected the western margin of the AP between 425 Ma and 410 Ma (Buggisch et al. 1994). Conodont CAI data (Keller et al. 1993b; Fig. 9) show an increase from 150~ at the eastern margin of the AP to more than 300~ in the continental rise deposits. The concomitant compressional deformation caused west-vergent structures in strata as young as the Devonian Talacasto Formation (von Gosen 1995). West- directed thrusts and imbrications are also present at the eastern margin of the AP (von Gosen 1992; von Gosen et al. 1995). These structures, which are also visible in seismic lines (Comin- guez & Ramos 1990), have a post-Silurian but pre-Late Carboniferous age.

Evolu t ion o f supersequence D

Loske (1992) and von Gosen et al. (1995) attributed the sediments of the Rinconada Formation to a depocentre formed by crustal extension. During the Silurian, this extension obviously also affected the remainder of the platform area. The basins and ridges described by Furque & Caball6 (1990) might reflect the formation of tilt blocks striking N-S. The asymmetry between the Jfichal area and the structural high along the Rio San Juan, however, documents an overall tilting of the shelf in a N-S direction.

Faunistically, the mid- to Late Silurian marks the change towards entirely Gondwanan elements. With the advent of the Clarkeia fauna

Fig. 9. 'Metamorphism' and level of erosion on the carbonate platform due to block faulting. CAI, conodont colour alteration index. Thermal alteration is weak at the eastern border of the Argentine Precordillera and increases towards the west (locations as in Fig. 2).


The plate-tectonic background of sedimentation in the Argentine PrecordUlera

It has become widely accepted that the AP is a Laurentia-derived terrane (Keppie 1991; Astini et al. 1995; Dalziel & Dalla Salda 1996; Dalziel et al. 1996; Keller & Dickerson 1996; Thomas 1996) and that it originated in the Ouachita embayment as originally proposed by Dalla Salda et al. (1992a, b). However, the timing of separation of the AP and the plate-tectonic scenario involved are highly controversial (Dalla Salda 1992a, b; Dalziel et al. 1994; Astini et al. 1995; Keller & Dickerson 1996; Dalziel 1997; Keller 1997).

The rift-drift transition along the Appala- chian margin is supposed to have taken place around the Precambrian-Cambrian boundary (Bond et al. 1984, 1989; Read 1989; Thomas 1996). In contrast, rifting along the Ouachita margin is younger (Thomas 1996) and a passive margin was not established before the Mid- Cambrian. Intracontinental graben systems indi- cative of incipient rifting are an important feature along the Appalachian-Ouchita margin (Thomas 1991). Typical sediments are fine- grained continental redbeds and local evaporites.

Sedimentological evidence from the AP records the following steps.

(1) Crustal thinning during the Early Cam- brian, indicated by the redbeds and evaporites of the Cerro Totora Formation. From the late Bonnia-Olene l lus chron onwards, carbonate sedimentation prevailed.

(2) The occurrence of a widespread hiatus in the AP possibly equivalent to the Hawke Bay regression event and hence confirming its position on Laurentian crust at that time.

(3) During the Glossopleura chron of the Mid-Cambrian, a fully developed carbonate factory became established. We interpret the Early/Mid-Cambrian history of the AP as a record of the formation of an intracontinental rift which separated the AP to some extent from the Ouachita margin.

(4) The newly formed crustal fragment existed as a Laurentian marginal plateau (Lister et al. 1991). Marginal plateaux are large, relatively unstructured crustal fragments, which owe their characteristics to a mid-crustal detachment and the concomitant pull-out of middle and lower crust from beneath the future marginal plateau (Lister et al. 1991). These plateaux are often separated from the main continent by a ramp syncline or a hanging wall basin, which may be relatively deep. In the AP, carbonate sedimentation started on the newly developed marginal plateau, whereas the deep-

water limestones were deposited in the interven- ing hanging-wall basin.

(5) During the Late Dresbachian, much earlier than in other parts of Laurentia, siliciclastic input from a crystalline source area had ceased, testifying to flooding of the plateau.

(6) From the Dresbachian to the Early White- rock�9 sedimentation on the carbonate platform proper was almost exclusively controlled by eustasy. The marginal-plateau model helps to explain the absence of margin-derived mega- breccias and other mass-flow deposits. The gradient between the platform and the slowly subsiding basin to the west was not great enough to permit gravity-driven processes to operate on a large scale.

Supersequence A records the initial intracontinental rifting, the formation of the marginal plateau accommodating the carbonate platform, and the evolution of a hanging wall basin in which deep-water limestones were deposited (Fig. 10). Sedimentology and faunal associations cannot be distinguished from mainland Laur- entia. In its higher part, the supersequence documents the loss of a source for terrigenous input and the evolution of a progradational carbonate complex which covered the entire platform.

The Early and earliest Mid-Ordovician saw the following events.

my

360

370 ~ _ _

2 380 >

390 I~

400

410

,20 g - ~

m i d

440 _ _

45O _~

470 "~ _ _

4Bo (~

500 _ _

510 g T ~ "C"

530 (.~

540

~o{~ P u n t a Neg ra F l ysch

L > i -

TaLacasto 0J platform ~O

D ~ xxxx•215 ~--

thermal overprint O,. "s Villicurn

chertpebbleconglQmerate �9 Q �9 graben I � 9

,~ ~ V V V ~ V V ~ r i f t i ng ~ pillow "basalts

rift onset u ncon f o r m ~

I l l l I

~ II I I i I : : i i i i

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Fig. 10. Tectonosedimentary evolution of the Argentine Precordillera as discussed in this paper.


(7) Successive return to an aggradational carbonate system which escaped drowning several times. The corresponding increase in relative sea level is probably a combination of litho- spheric cooling of the marginal plateau and of eustasy, as many of the Laurentian sea-level events are recognized in the AP (Keller 1997).

(8) The carbonate platform was drowned during the Whiterockian. Sea-level rise was an important factor, but facies successions in combination with sequence stratigraphy demon- strate that there was also a tectonic component reflecting crustal extension. Supersequence B as a whole reflects the demise of the carbonate platform until its complete drowning. During deposition of supersequences A and B, the history of thermal subsidence and of sediment accumulation (Figs 8 & 10) is characteristic of passive-margin evolution.

The Mid- and Late Ordovician deposits mirror a climax of tectonic activity.

(9) The onset of crustal extension occurred during the earliest Mid-Ordovician.

(10) The climax of rifting occurred during the late Mid- and the earliest Late Ordovician. It resulted in major block movements in the former carbonate platform area and the evolution of an Atlantic-type continental margin with fault scarps locally more than 2100 m high. The pillow basalts in the western basin are interpreted as remnants of the adjacent ocean floor.

(11) During the Ashgillian, after its separation from Laurentia, the AP occupied a palaeogeographical position in higher latitudes within the reach of icebergs, and hence ice-rafted sediments. According to Brenchley & Newall (1984), icebergs may have drifted as far north as 30~ during the Late Ordovician glaciation. Consequently, ice-rafted sediments alone (such as the diamictites of the Don Braulio Forma- tion) cannot be taken as evidence the accretion of the AP to Gondwana.

Faunal data and the change of the palaeogeographical affinities of the benthic associations display strikingly the timing of separation and the subsequent drift towards isolation of the AP (Fig. 6a). This is even more evident if pandemic faunal elements are excluded from the diagrams (Fig. 6b-c).

Supersequence C reveals the continental break-up and the final separation of the AP from Laurentia and the complete opening of the Ouachita basin (see Dickerson & Keller this volume). The supersequence is bounded at the base by the rift-onset unconformity and at the top by the break-up unconformity (Fig. 10). The contrasting sediment accumulation rates

in the former platform area and the newly developing basin are documented in Fig. 8.

The Silurian and Devonian history include the following.

(12) The drift to high latitudes and the approach to Gondwana. This is shown by the Mid-Late Silurian Clarkeia fauna, because this fauna is only present in a few areas of high- latitude Gondwana (Cocks & Fortey 1990).

(13) There is a marked change from siliciclastic platform deposition to flysch sedimentation during the Devonian.

(14) Heavy-mineral populations in the Lower Palaeozoic strata indicate that a predominantly sedimentary hinterland was recycled prior to the (Mid-) Devonian. Subsequently, however, a source area was present which was composed of granitoids and metamorphic rocks (Loske 1992; Kury 1993; Astini & Maretto 1996). This change of source rock exposure is here interpreted as an effect of the final accretion of the AP to Gondwana.

(15) The Late Silurian-Early Devonian metamorphism and the corresponding deformation in the western part of the AP indicate the onset of structural deformation (Fig. 10). They are interpreted as reflecting closure of the western ocean and collision with the Chilenia terrane (Ramos et al. 1984, 1986), whereas concomitant structures in the east may be related to the accretion of the AP to Gondwana (von Gosen 1992). This scenario is supported by the magma evolution and igneous activity in the Sierras Pampeanas indicating continuous subduction into the Silur- ian, changing gradually towards collision during Late Silurian-Devonian times (Rapela et al. 1992). It seems logical that the separation of the AP from Laurentia and the formation of oceanic crust must have been accompanied by subduction of the opposite side of the terrane. This in turn may have been subduction beneath the Sierras Pampeanas, causing the corresponding magmatism there (see Saavedra et al. this volume and Pankhurst et al. this volume).

(16) The accretion of the AP to Gondwana was terminated prior to the deposition of the Late Carboniferous molasse sediments. Super- sequence D is an expression of the final approach and the subsequent accretion of the AP to Gondwana, but also of the approach of Chilenia in the west.

The Early Palaeozoic history of the AP resulted from the complex plate interactions between Laurentia, western Gondwana, and the Chilenia terrane (Ramos et al. 1984, 1986; Astini et al. 1996). We are convinced with some certainty that sedimentological evidence and the faunal data from the AP exclude models

P R E C O R D I L L E R A S T R A T I G R A P H Y 53

favouring Mid-Ordovic ian collision of the AP terrane (Astini et al. 1995), of the Cuyania terrane (Ramos 1995), or of the Occidental ia terrane (Dalla Salda et al. 1992a, b) with the eastern Sierras Pampeanas . A scenario as pre- sented by Astini et al. (1995, 1996) and especially its t iming is also unlikely. Based on our sedimentological and faunistic data and con- sidering the structural evidence (von Gosen 1992, 1995; von Gosen et al. 1995; Cominguez & Ramos 1990), we favour a model in which the A P was separated f rom Laurent ia dur ing the Ordovician and af terwards slowly approached G o n d w a n a dur ing the Silurian and Devonian .

This paper is an output of a large project of the University of Erlangen on the evolution of the AP. We thank W. von Gosen and S. Krumm for many helpful discussions and support. We also thank all our Argentinian and non-Argentinian colleagues and friends, who participated in one form or another, for their co-operation and hospitality. The support of project funds to all three of us from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. This paper benefited from reviews by S. Flint (Liver- pool) and an anonymous reviewer.

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The stratigraphical record of the Argentine Precordillera and its plate-tectonic background

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