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SH RT PAPERS for the SEVENTH INTERNATI NAL SYMPOSIU on the ORDOVICIAN SYSTEM

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BOOK 77

CONTINENTAL SLOPE DEPOSITS IN THE ARGENTINE PRECORDILLERA: SEDIMENTS AND GEOTECTONIC SIGNIFICANCE

Martin Keller Institut fur Geologie und Mineralogie, Universitat Erlangen Schlossgarten 5, D-91056 Erlangen

INTRODUCTION

the south to the surroundings of Jachal in the north. The deposits are known as the Empozada Formation near Mendoza and the Los Sombreros Formation in the remaining areas. However, the simple pattem of platform-slope-basin configuration is complicated at several localities. Near Guandacol (Fig.1), carbonate sedimentation had already ceased during the Arenig and was replaced by graptolite shale deposition. Southwest of Jachal (Fig.1), a local carbonate slope (Las Aguaditas Formation). developed during Middle Ordovician time (Keller and others, 1993a). Further to the south, at Las Chacritas (Fig .1), carbonate sedimentation continued well into the Llanvim before shale sedimentation took over. West of Mendoza at Cerro Pelado (Fig.1), uppermost Cambrian hemipelagic limestones overlie carbonate platform rocks (Heredia, 1990). In the eastem Precordillera, a pre-Silurian unconformity is observed, where erosion cut down to different stratigraphic levels (Lehnert, in press). In several sections, this major event affected even the San Juan Formation. Thus, the middle and late Ordovician history of the Eastem Precordillera is preserved only in isolated sections, which are often difficult to correlate.

The Precordillera of Westem Argentina consists of a thick pile of Lower Cambrian through Tertiary sedimentary deposits. A pecularity of the sedimentary column is the existence of thick Cambro-Ordovician carbonates, which are absent in any of the adjacent tectono-stratigraphic terranes. The presence of these carbonates and their comparison to rocks around the Laurentian passive margin led to the idea that the Precordillera of Westem Argentina might resemble anallochthonous terrane (Ramos and others, 1986). The idea of an allochthonous Precordillera was further developed by Ramos (1988) and culminated in a complex model in which the Precordillera was welded to South America during the Ordovician as part of an Occidentalia terrane (Dalla Salda and others, 1992; Dalziel and others, 1994). The aims of this paper are to document the continental slope deposits with their variety of allochthonous components, to interprete the sedimentary and tectonic regime of deposition, and finally discuss the consistency of the interpretation with an allochthonous origin of the Precordi II era.

THE SLOPE SEDIMENTS AND THE SECTIONS GEOLOGICAL SETTING Shales are the most important constituents of both successions of slope deposits. They are commonly black and graptolite-bearing. Near Mendoza, thick alum-shales are observed. Another autochthonous sediment are dark hemipelagic limestones and marlstones. Shales and hemipelagics form the matrix for classical turbidites, debris flow deposits of different size (megabreccias, olistostromes), conglomerates, and resedimented conglomerates as well as huge slabs of carbonates (olistoliths). An additional important facies are "feldspathic" sandstones, where idiomorphic and hypidiomorphic plagioclases make up to 50% of the whole rock. Size and reworking contradict an authigenic origin. These sandstones have been observed in all sections south of the Rio San Juan. Their association to basic dikes, present in the Los Tuneles and Rio San Juan sections, sug-

In the Precordillera, the Cambro-Ordovician (Fig.!) is characterized by a thick carbonate platform in the east, which was deposited during Early Cambrian through early Middle Ordovician times. Carbonate production was stopped during a regional flooding event, which covered the platform with black shale. From Middle Cambrian times onward an outer shelf with hemipelagic sediments is documented in reworked boulders contained in Ordovician slope deposits of the Central Precordillera. To the west, a thick clastic basinal succession is found paralleling the continental slope. True basinal sediments older than Middle Ordovician have not yet been documented. During the Caradoc "rift-related" pillow-basalts (Kay and others, 1984) extruded into the clastic basin. The slope deposits (Fig .1) extend from Mendoza in

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gests that the plagioclases are reworked from extrusives. This is corroborated by zircon analysis of the sandstones (Laske, 1992), which points to a volcanic origin. On the western slope, there are relatively few clasts among the carbonates, which can be attributed to the Cambro-Ordovician platform sediments. Instead, many of the boulders and slabs in the Los Sombreros Formation represent a Middle Cambrian outer platform/slope environment with predominantly hemipelagic dark, fine-grained carbonates. Almost all of these boulders yielded Middle Cambrian trilobites (e.g., Bordonaro, 1990). Platform-derived material is mainly found in carbonate or mixed turbidites or in conglomerates. In the Los Tuneles section, the largest carbonate slab anywhere observed in the Precordillera is found. It is more than one kilometer long and shows a stratigraphic section of about 300m. In this block about 250m of Middle Cambrian limestones are present, conformably overlain by a succession of breccias (clast size up to 2 em) and turbiditic carbonate grains tones. This latter sequence, which belongs to the reworked block, is a typical slope deposit. Few badly preserved conodonts indicate a Late Cambrian/Early Ordovician age. Hence a hiatus seems to be documented between proven Middle Cambrian and the Late Cambrian/Early Ordovician, which itself was redeposited onto the Middle Ordovician slope. Another important boulder in this section consists of Lower Cambrian trilobite coquinas alternating with silty shales. In the Rio San Juan section the most impressive feature is a block of Middle Cambrian limestones, with a measured section of 47 m. It is part of a thick pelitic unit with abundant olistoliths. Among the clasts are green metaquartzites (up to 60 em across!) with a well developed cleavage, and gneisses as well as slightly metamorphic carbonates. The Ojos de Agua section is characterized by a variety of conglomerates and carbonate breccia bodies. At the base huge boulders (up to 4 m across) are exposed, which consist entirely of conglomerates. Among the clasts there is a variety of sedimentary and basement rocks similar to those at the Los Ratones locality. Additionally, garnet-bearing rocks have been described (Banchig and others, 1990). The section at San Isidro shows stratigraphically the greatest variety of clasts, although metamorphic boulders have not been observed. Middle Cambrian outer platform limestones only recently have been documented to be of allochthonous origin (Bordonaro and others, 1993). In addition, there are upper Cambrian hemipelagics, Early Ordovician shale boulders with a good graptolite fauna and finally carbonate clasts

Argentina

N

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carbonate platform rocks

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~

t/

slope sedime

~

basin clastics

1 2 3 4 5 6 7 8 9 = 10 = 11 =

Guandacol Los Tuneles Las Aguaditas Las Chacritas Rio San Juan Ojos de Agua Rio Sassito Don Braulio Quebrada de Saito Cerro Pelado San Isidro

IIIII Mendoza

Figure 1: Distribution of Cambro-Ordovician sediments in the Precordillera and localities mentioned in the text which yielded Caradoc conodonts (Heredia and others, 1990). THE AGE OF THE DEPOSITS Hitherto, it was assumed that a Cambro-Ordovician slope existed in the Precordillera (e.g.Cingolani and others, 1989). However, Bordonaro (1990) discussed

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the possibility that all occurrences of these Middle Cambrian limestones in the Los Sombreros Formation, which heretofore were thought to be autochthonous slope sediments, have to be regarded as allochthonous blocks. The same holds true for the Empozada Formation, which recently has been proved to be entirely of Middle and Upper Ordovician age (Bordonaro and others, 1993). All clasts older than Arenig/Llanvirn are now regarded as allochthonous, but younger blocks are also present. They include black shale clasts with an Arenig graptolite fauna. Hence, biostratigraphic data, where available, seem to indicate that sedimentation of the Los Sombreros and Empozada Formations started near the Arenig/Llanvirn boundary. This is documented for the Jachal area (Ortega and others, 1991), for the Mendoza area (Bordonaro and others, 1993) and for the type section (Cuerda and others, 1986).

PROVENANCE OF THE CLASTS In the western slope facies, two groups of clasts are conspicuous in the olistostromes: metamorphic clasts and huge clasts and slabs of outer platform/slope hemipelagic limestones. Noteworthy is the scarceness of platform carbonates and magmatic boulders. Neither transport across the platform area from the east is likely (complete section at Don Braulio; no bypass channels present on top of platform), nor a source area in the west (general fining of the sediments towards the west; paleo-current data (Spaletti and others, 1989)). The most likely assumption is that the inajority of the clasts are of local origin. This certainly holds for the huge Middle Cambrian olistoliths, which cannot have been transported for long distances, nor is there any other likely transportation mechanism than gravitation. This assumption also implies that during Middle Ordovician times the basement must have been locally exposed and served as a source area for the big boulders found in the Rio San Juan section.

PROBLEMS The presence of Middle Cambrian outer platform olistoliths and basement boulders in the slope facies, both of which seem to be of local origin, has several severe implications for the geotectonic history of the Precordillera. Near the Ordovician slope, a relatively thin