N.Jb. Geol. PaLiont. Mh. Stuttgart, Juni 1993

The Cambrian of San Isidro, Mendoza, Argentina: Facies and sedimentology at the platform

slope transition

Von Martin Keller, Erlangen, Osvaldo Bordonaro and Matilde Beresi, Mendoza, Argentina

With 4 figures in the text

KELLER, M, BoRDONARO, 0. & BERES!, M. (1993): The Cambrian of San Isidro, Mendoza, Argentina: Facies and sedimentology at the platform slope transition.­N. ]b. Geol. Palaont. Mh., 1993 (6): 373-383; Stuttgart.

Abstract: In the Argentine Precordillera the Cambrian of San Isidro (province of Mendoza) reflects different sedimentary environments of an outer carbonate platform and the adjacent slope. The Estancia San Martin Fm. is composed of deep-water limestones in its lower part, which upward grade into siliciclastic slope sediments (debris flows, conglomerates). The overlying San Isidro Fm. was deposited on the more internal parts of the platform, where sandstone, sandy .mestones and finally limestones were formed. The shifts of the sedimentary

environments are related to regional fluctuations of relative sea-level.

Zusammenfassung: Das Kambrium von San Isidro (Argentinische Prakordillere, Provinz Mendoza) besteht aus Sedimenten der augeren Karbonatplattform und eines anschliegenden Kontinentalhanges. Die Estancia San Martin Fm. besteht in ihrem unteren Teil aus pelagischen Karbonaten, die nach oben in siliziklastische Hangsedimente iibergehen (Konglomerate, debris-flows). Die hangende San Isidro Fm. wurde auf den mehr intern en Bereichen der Plattform sedimentiert und besteht aus Sandsteinen, karbonatischen Sandsteinen und Kalken. Fur die wechselnden Ablagerungsraume sind regional wirksame Meeresspiegelschwan­kungen verantwortlich.

Resumen: El Cambrico de San Isidro (Precordillera Mendocina, Argentina) esti compuesto por sedimentos de una plataforma carbonatica externa y un talud adyacente. La parte inferior de Ia Formaci6n Estancia San Martin consiste de calizas de Ia plataforma externa, que gradualmente pas an a depositos de un talud siliciclastico (flujos gravitacionales, conglomerados). La sobreyacente Formaci6n San Isidro esta compuesta por areniscas, areniscas calcareas y calizas de un ambiente mas somero (plataforma mas interna). Los cambios en los ambientes de depositaci6n fueron causados por cambios en el nivel del mar regionales.

Introduction

The Cambrian of San Isidro (province of Mendoza; Fig. 1) is part of a vast carbonate shelf, extending over more than 400 km from the province

0028-3630/9311993-0373 $ 2.75

374 M. Keller et a!.

of La Rioja in theN to Mendoza in the S. It represents the southernmost outcrops of the platform and its facies clearly differs from the classical Lower/Middle Cambrian of the provinces of San Juan and La Rioj There, mixed carbonate-siliciclastic sequences form Depositional Grand Cycles, deposited in shallow shelf areas (La Laja Fm.; KELLER &

BoRDONARO, in press). In contrast, the corresponding sediments of San Isidro (furtheron referred to as "Cambrian of San Isidro": Estancia San Martin Fm. and San Isidro Fm.) were always interpreted as slope deposits (BoRELLO 1971) or deep shelf carbonates (PINA et al. 1985; BORDONARO 1985). Detailed sedimentological and facies analysis now reveals a complex pattern of outer shelf and slope sedimentation, possibly triggered by sea level fluctuations during the Lower and Middle Cambrian.

Regional geology and the age of the deposits

The Cambrian of San Isidro is part of the Western Argentine Precor­dillera, which in this area comprises Cambrian to Triassic and Tertiary/ Quaternary sediments. In San Isidro carbonates are restricted to the Cambrian and Ordovician and occur as autochthonous and alloch­thonous units. Autochthonous carbonates include the Estancia San Martin Fm. (ESM) and San Isidro Fm. (SIF) of the Lower/Middlf' Cambrian, the Cerro Pelado Fm. (Upper Cambrian) and the Arenig Sa" Juan Fm. (near Salagasta). The position of the Solitario Fm. (Middle/ Upper Cambrian) and its relation to the other outcrops is uncertain, because of the isolated outcrops of this unit.

Carbonate deposition must have persisted until the Late Ordovician, because Caradoc conodonts were found in limestone boulders of Ashgillian slope sediments (GALLARDO et al. 1988). In the same sequence (Emposada Fm.; Fig. 1) Upper Cambrian limestone blocks occur, which formerly were regarded as an own autochthonous unit (La Cruz Fm.).

In this paper we focus on the deposits of the ESM and SIF. The base of the ESM is always cut by thrusts, so that there is no idea about how sedimentation started in that area. Although repeatedly it was presumed, that sedimentation of the ESM already started during the late Lower Cambrian (e.g. PrNA et al. 1985), all trilobites found during our fieldwork are indicative of the early Middle Cambrian. The ESM is separated from the overlying SIF by an erosional contact, where channelling sandstones rest upon shales. Due to the restricted outcrops, it is not clear whether

Fig. 1. Geology of the area of San Isidro and position of the sections: 1) Quebrada San Isidro section; 2) La Cantera and Quebradita sections; 3) Quebrada La Cruz

and Filo La Cruz sections; 4) Cerro Martillo section.

The Cambrian of San Isidro (Mendoza, Argentina) 375

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376 M. Keller et a!.

this is a low-angle unconformity or just a paraconcordant sequence. Trilo­bites found in th SIF all point to an upper Middle Cambrian age of the deposits. Autochthonous Upper Cambrian sediments are missing in Sar Isidro, the SIF everywhere is overlain by the Ordovician Emposada Fm. Both sequences are separated by a highly irregular erosional surface, which cuts down to different levels, in one place even down to the ESM. Resolution of trilobite stratigraphy is high enough to show the different amounts of sediment removed in sections only few hundred metres apart.

Estancia San Martin Formation

Although a subdivision of this formation into four members has been proposed (PINA et al. 1985), from a sedimentological point of view two major units can be distinguished. The lower one consists of an alter­nation of shales and marls with dark, fine-grained limestones, whereas the upper unit is dominated by siliciclastic sediments. The shale/ limestone ratio of the lower unit varies between 3:1 to 1:3. Due to intensive folding and probably some slumping no real thickness of this lower unit can be given.

The lower part of the ESM

Description: Shales and marls are black, rarely fossiliferous and ofteP fissile. Some contain silt-sized quartz. Limestones are thin- to medium­bedded, dark brown to black and often show an ochreous weathering colour. They are characterized by their dark to black colour and the absence of apparent bioturbation. Bedding is very regular and planar to more undulatory. Erosional features at the base of the beds are absent. The individual beds often show a very fine internal lamination. Mudstones and wackestones predominate. The fauna consists of few agnostid trilobites and sponge spicules, in some cases spiculitic wacke­stones are developed. In many samples small calcitic spherulitic organisms were observed but it is not clear, whether these are radiolarians or calcispheres. Other characteristic types of limestones are calcisiltites, often graded, composed of calcareous and silty components. Small-scale erosional features at the base of the layers are common. Platform-derived bioclasts are trilobites, H)lolites sp., phosphatic brachiopods and undeter­mined microfossils. Pyrite cubes, concentrated in layers are diagenetic features.

Interpretation: The depositional environment is interpreted to be a deep-water setting. This is shown by the dark colour of the sediments, the very regular bedding and the internal lamination, the absence of wave induced structures and the lack of bioturbation, features which together are regarded as distinctive of deep-water limestones (WILSON 1969).

The Cambrian of San Isidro (Mendoza, Argentina) 3 77

Typically, such limestones occur near the outer edge of the platform near the transition towards the continental slope (CooK & TAYLOR 1977; CooK 1979; REINHARDT 1979). The carbonate is platform-derived ("peri-plat­form ooze" of ScHLAGER & ]AMES 1978) and was deposited as hemipelagic mud.

The upper part of the ESM

Description: The upper part of the ESM is mostly siliciclastic, shales (in some outcrops slates) constitute more than 90% of the sedimentary column (Fig. 2). There is a marked difference in colour between the lower part of the shale sequence (brown and black) and the upper part, where greenish colours predominate. Black shales are often siliceous or highly aluminiferous. Limestones intercalated into this sequence are of the same lithology as those in the lower part of the ESM. They occur as individual thin beds or as silicified nodules with a diameter of up to 1 m across.

Shales and limestones constitute an autochthonous facies, which is contrasted by the occurrence of channelled conglomerates, debris flow deposits and some clastic limestone intercalations (Fig. 2).

Conglomerates are laterally discontinuous with an erosional base and :onsist of clasts up to 5 em in diameter. Among the clasts there are sand­stones, alum shales and siliceous shales, limestones of different textures and chert. Some of the horizons are graded, the sandy top may be ripple­drift cross-bedded. Similarly, deposits without a conglomeratic base occur as lenticular bodies. These sandstones show a coarse base and a medium to fine grained top with cross-bedding.

Debris flow deposits are up to 40 em thick. Due to the outcrop situation it is not clear whether they are laterally continuous, but is seems that these are large lenticular bodies with an erosional base, wedging out over a distance of about 40 m. Large tabular clasts, sometimes imbricated, float in a sandy matrix. Predominantly, they consist of lime mudstone and wackestone together with calcisiltite. Smaller clasts are composed of quartz, chert and siliceous shales.

Interpretation: Background sedimentation of shales and few intercalated limestones is contrasted by several horizons, which formed as a result of sediment gravity flow. Siliceous shales alternating with alum shales are indicative of a basinal environment (FucHTBAUER 1988). Intercalated calcisiltites and black limestones also point to a deep-water setting.

Among the allochthonous deposits there are typical examples of turbi­dites (graded sandstones and conglomerates; units A to C of the classical BOUMA-sequence). Another type of gravity flow are calcareous

378 M. Keller et al.

turbidity currents, which are preserved as peloidal grainstones and calci­siltites with units C and D of the BOUMA-sequences. A third variety are debris flow deposits. Deposition of these breccias was due to mass flov (NARDIN et al. 1977). The breccias are very similar to sediment gravity flows described by CooK & TAYLOR (1977) and CooK (1979) from Cambro­Ordovician strata in Nevada. There, they are typical of a slope environment, where gravity driven sediment flows cause erosion of upper slope hemipelagic limestones.

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The San Isidro Formation

Description: The SIF is a unit with a great variety of calcareous and siliciclastic rocks (Fig. 3). It rests upon the ESM with an erosional contact. Near the base thick-bedded coarse sandstones, calcareous sand­stones and sandy limestones predominate. Cross-bedding in the silicicla­stics was observed only to a minor degree. Qlartz and feldspar clasts are angular to subrounded, in many samples nearly idiomorphic feldspar crystals were observed. Other components are platform-derived intra­clasts, ooids and oncoids. Only occassionally, the primary internal structure is preserved, most of the rocks are recrystallized, others are micritized. Bioclasts include trilobites and phosphatic bivalves. Uwpard, these sediments grade into packstones and grainstones, still with an high amount of clastic quartz. Components are identical to those of the siliciclastics. With decreasing siliciclastic and platform-derived calca­reous input, wackestones and packstones appear. Many of them are strongly bioturbate, others show nodular textures. The biogenic contents consist of trilobites, sponge spicules and phosphatic valves, other compo­nents are peloids and intraclasts. Towards the top of the formation wackestones and marls are most abundant (Fig. 3). In some layers glauconite was found, either as a replacement of components or as a filling of interclast spaces. The limestones often show a nodular texture, in some horizons trilobites are concentrated to form coquinas.

Interpretation: The sudden input of siliciclastics mixed with carbonates, showing platform derived components marks a pronounced shift in the depositional environment towards a more proximal part of the platform. This is corroborated by the types of limestones deposited and especially by the trilobite faunas, which change from pelagic pandemic forms to endemic platform types (BoRDONARO 1989; 1992). As wave-induced structures are absent, the sedimentary environment must have been below storm weather wave base. The occurrence of large amounts of glauconite points to sometimes reduced sedimentation rates (FucHT­BAUER 1988).

Discussion

The sedimentary sequence of San Isidro is time-equivalent to the La Laja Fm. in the province of San Juan (Fig. 4). For the latter a series of sea-level oscillations was recently documented (KELLER & BoRDONARO, in press), which led to the formation of several regressive sequences. Changes of relative sea-level can also be documented for the Cambrian of San Isidro.

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There, sedimentation starts with an alternation of shales and hemipe­lagic limestones, deposited on the outer platform, which upward evolve into a clastic slope sequence (ESM). This indicates a moderate relative rise of sea-level. The corresponding unit in San Juan (Soldano Member) also is characterized by a rising sea-level, but this rise is outpaced by carbonate production. When sea-level rise comes to an end or slightly begins to fall, the whole shelf is covered with fine-grained siliciclastics (KELLER & BoRDONARO, in press). A corresponding unit in the province of Mendoza might be the lower part of the SIF, which also marks a relative sea-level fall with respect to the ESM (coarse-grained clastics above an erosional unconformity). Decreasing clastic input and the reestab­lishment of a carbonate producing system indicate the next relative sea-level rise. This rise yet cannot be correlated to the sedimentary sequence in the province of San Juan, because nothing is known about the hiatus between the ESM and the SIF. Therefore, it is difficult to say, whether the SIF correlates with the Rivadavia Member or the Juan Pobre Member of the La Laja Fm., although a comparison of all faunal elements indicates, that the SIF probably is equivalent to the Rivadavia Member.

Conclusions

The Cambrian of San Isidro in the province of Mendoza, western Argentine Precordillera, reveals a complex sedimentation pattern of

382 M. Keller et a!.

carbonates and siliciclastics. The Estancia San Martin Formation was deposited on the outer platform and the adjacent slope, the San Isidro Formation on the more internal part of the platform, but still below storm weather wave base. Sedimentation was triggered by changes of relative sea-level. The resultant sea-level curve for the Middle Cambrian shows a moderate sea-level rise (ESM) followed by a sudden drop and a renewed rise. A similar pattern is observed in the corresponding platform sequence of San Juan. This indicates that these changes were effective at least on a basin-wide scale. Whether in the future these changes may be correlated to similar events on a larger scale (global?), for the moment must remain open.

Acknowledgement

We would like to thank W. BuGGISCH and an unknown referee for critical reading of the manuscript and many suggestions, which helped to improve the original version. Financial support for this study was given by the CONICET, DFG (BU 312-17 /1) and the Volkswagenstiftung (I/63074), which is gratefully acknowledged.

References

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- (1989): Biogeografia y evoluci6n Gondwanica durante el Paleozoico Inferior en America Latina.- Carrel. Geol., 5: 109-130, Univ. San Miguel de Tucuman; Argentina.

- (1992): El Cambrico de Sudamerica. - [In:] GuTIERREZ-MARco, ]. C.; SAAVEDRA,]. & RABANO, I. (eds.): Paleozoico Inferior de Ibero-America, 69-84; Pub!. Univ. Extremadura.

BoRELLO, A. V. (1971): The Cambrian of South America.- [In:] HoLLAND, C. H. (ed.): Cambrian of the New World, 385-437; London/New York (Wiley).

CooK, H. E. (1979): Ancient continental slope sequences and their value in under­standing modern slope environments.- [In:] DoYLE, L. J. & PILKEY, 0. H. (eds.): Geology of continental slopes, Soc. Econ. Paleont. Mineral., Spec. Pub!., 27: 287-305; Tulsa, Okla.

CooK, H. E. & MuLLINS, H. T. (1983): Basin Margin Environment.- [In:] ScHOLLE, P. A.; BEBOUT, D. G. & MooRE, C. H. (eds.): Carbonate Depositional Environments, Am. Ass. Petrol. Geol., Mem. 33: 539-617; Tulsa, Okla.

CooK, H. E. & TAYLOR, M. E. (1977): Comparison of continental slope and shelf environments in the Upper Cambrian and lowest Ordovician of Nevada.­[In:] CooK, H. E. & ENos, P. (eds.): Deep-water carbonate environments, Soc. Econ. Paleont. Mineral., Spec. Pub!., 25: 51-81; Tulsa, Okla.

FDcHTBAUER, H. (1988): Sedimente und Sedimentgesteine.- 1141 pp.; Stuttgart (Schweizerbart).

GALLARDo, G.; HEREDIA, S. & MALDONADO, A. (1988): Depositos carbonaticos al6ctonos, Miembro Superior de la Formaci6n Empozada, Ordovicico Superior de la Precordillera de Mendoza, Argentina. - 5. Congr. Geol. Chilena, Aetas, 1: 37-53; Santiago de Chile.

M. Keller et a!., The Cambrian of San Isidro 383

KELLER, M. & BoRDONARO, 0. (in press): Cyclic carbonate platform evolution controlled by major sea level changes: The Lower/Middle Cambrian La Laja Formation, Western Argentine Precordillera. - J. South Am. Earth Sci.; Oxford.

NARDIN, T. R.; HEINAR, F.J.; GoRSLINE, D. S. & EDWARDS, B. D. (1979): A review of mass movement processes, sediment and acoustic characteristics, and contrasts in slope and base-of slope systems versus canyon - fan - basin floor systems.- In: DoYLE, L. J. & PrLKEY, 0. H. (eds.): Geology of conti­nental slopes, Soc. Econ. Paleont. Mineral., Spec. Pub!., 27: 61-73; Tulsa, Okla.

PrNA, L.; BALDIS, B. A. & BoRDONARO, 0. (1985): Formaci6n Estancia San Martin (nov. nom.) del Cambrico Inferior-Cambrico Media en Ia Comarca de San Isidro, Mendoza. - 1. J. Geol. Precordillera, Aetas, 1: 7-11; San Juan, Argentina.

REINHARDT, J. (1977): Cambrian off-shelf sedimentation, Central Appalachians.­In: Cook, H. E. & Enos, P. (eds.): Deep-water carbonate environments, Soc. Econ. Paleont. Mineral., Spec. Pub!., 25: 83-112; Tulsa, Okla.

ScHLAGER, W. &}AMES, N. P. (1978): Low-magnesian calcite limestones forming at the deep-see floor, Tongue of the Ocean, Bahamas. -Sedimentology, 25: 675-702; Oxford.

WILSON, J. L. (1969): Microfacies and sedimentary structures in "deeper water" lime mudstones.- [In:] FRIEDMAN, G. M. (ed.): Depositional environments in carbonate rocks, Soc. Econ. Paleont. Mineral., Spec. Pub!., 14: 4-19; Tulsa, Okla.

2ingang des revidierten Manuskriptes bei der Schriftleitung in Munster am 21. September 1992.

Anschriften der Autoren:

Dr. MARTIN KELLER, Institut fiir Geologie unci Mineralogie der Universitat, Schloggarten 5, D-8520 Erlangen. OsvALDO BoRDONARO unci MATILDE BERESI, Centro Regional de Investigaciones Cientificas y Tecnicas, C.C. 131,

5500 Mendoza, Argentina.