Journal of Applied Sciences

Year: 2010 | Volume: 10 | Issue: 5 | Page No.: 363-379
DOI: 10.3923/jas.2010.363.379

Preservation of Biogenerated Mixed Facies: A Case Study from the Neoproterozoic Villa Mónica Formation, Sierra La Juanita, Tandilia, Argentina

P.E. Zalba, M. Manassero, M.E. Morosi and M.S. Conconi

Abstract: The aim of this contribution was to show through field work and mineralogical microtextural studies a complex history of weathering and diagenesis in the Villa Mónica Formation, the most ancient Neoproterozoic unit of the Tandilia System and to present a proposal of a paragenetic sequence. This unit also shows microbially induced structures described here for the first time. At the Estancia La Siempre Verde, La Placeres and Don Camilo quarries, Sierra La Juanita, near Barker locality, the Villa Mónica Formation is composed of carbonate facies, classically defined for more than 40 years as siliciclastic facies and of reinterpreted mixed facies: carbonate/siliciclastic and heterolithic respectively, both bearing biosignatures. The carbonates are represented by well-preserved columnar head stromatolite boundstones and by laminar microbial mat deposits. Both of them were the host rocks - identified here for the first time - of individual or random aggregates of pyramidal quartz megacrystals and they were later dolomitized, silicified, illitized and hematized. The siliciclastics are composed of quartz grains trapped within both the stromatolites and the microbial mats, of illitic siltstones and claystones and of quartzitic sandstones. Illuviation processes transported cutans to lower horizons. Syndiagenesis involved dolomitization and silicification while burial diagenesis produced pressure-solution effects by overburden and neoformation of minerals: diagenetic illite with rutile needles, among others. Compressive movements from the SW, responsible for basin inversion: telodiagenesis (uplift, fracturing, folding and introduction of meteoric fluids), affected the Villa Mónica Formation with neoformation/transformation of minerals: kaolinite, halloysite and smectite, development of slickensides (stress cutans) and ferriargillans, hydration, dedolomitization and calcification.

Keywords: biogenerated rocks, diagenesis, Neoproterozoic, paragenetic sequence and weathering

INTRODUCTION

There are several previous contributions describing the sedimentary cover of the Tandilia system within the Barker area, Sierra La Juanita (Schauer and Venier, 1967; Cingolani et al., 1985; Manassero, 1986; Iniguez et al., 1989; Allo et al., 1996; Allo, 2001; Allo and Dominguez, 2002; Lopez Escobar et al., 2002; Poire and Spalletti, 2005; Lopez Escobar, 2006; Zalba et al., 2007a; Gomez Peral, 2008). But here, for the first time, mixed carbonate/siliciclastic facies were recognized in microscopic scale in the Villa Mónica Formation, overlying basal quartzite deposits known in the geological literature as the Cuarcitas Inferiores (and which are beyond the scope of this study). Besides, reinterpreted heterolithic facies replaced the previously named Psamopelites defined by Poire and Iniguez (1984) and considered by these authors as part of the overlying Cerro Largo Formation (Poire, 1993).

The Neoproterozoic Villa Mónica Formation is the oldest sedimentary unit of the Tandilia System unconformably overlying the basement rocks (Fig. 1, 2). Originally defined by Poire (1987) in the Sierras Bayas region (stratotype area) as a 52 m thick sedimentary succession and bound by unconformities at the base and top, it is represented, in this region, located 100 km NW from the study area, by two sedimentary facies associations: (1) quartz-arkosic arenites (16 m) at the base (the Cuarcitas Inferiores) and (2) biogenic dolomites (36 m) with columnar or laminar shallow marine stromatolites. Poire (1993) included red, laminated claystone and calcipelite deposits at the top of the unit.

In the Barker study area within the Sierra La Juanita, the most important outcrops of this unit were studied at Estancia La Siempre Verde, La Placeres and Don Camilo quarries, along a NW-SE transect which extends for more than 20 km (Fig. 1). Yet, the deposits of the Villa Mónica Formation at the Sierra La Juanita, Barker area, are much thinner and the exposures are not as good as in the stratotype area.

The Villa Mónica Formation was considered to be of Riphean age, (upper Proterozoic) based on stromatolites (Poiré, 1993) and also on Rb-Sr dating of clay deposits associated with dolomites at the Sierras Bayas area, giving an age of 793±32 Ma for the diagenesis of the unit (Cingolani and Bonhomme, 1982). Nevertheless, according to Gaucher (2005) while stromatolite association suggests an early Neoproterozoic age, the similarity of acritarch assemblages preserved throughout the Sierras Bayas Group suggests that there is a short time hiatus between the two basal units of the Sierras Bayas Group: the Villa Mónica Formation and the overlying Cerro Largo Formation, raising the possibility of the Villa Mónica Formation being of Ediacaran age.

Microtextural observations have proved to be crucial in the recognition of biogenic fabrics due to microbial activity. A complex history of weathering and diagenesis has left its imprint in the sediments leading to the present status of the rocks. Where morphology could not be of help in the recognition of biogenic relics, structures certainly were and diagenetic processes (e.g., silicification, hematization) also helped in the preservation of diagnostic remains (cellular structures, rhomboedral pseudomorph crystals of quartz after dolomite, crinkled structures) which, otherwise, would probably have been erased from the sedimentary record.

A paragenetic sequence of events is proposed and the influence of the uplift of the Ventania System, located 150 km to the SE, over the geological evolution of Tandilia is considered on the basis of a recent contribution (Zalba et al., 2007b).

MATERIALS AND METHODS

This study was conducted between 2006 and 2008. Forty-five thin sections were examined using optical microscopy to determine textural and optical properties as well as the mineral paragenesis. Selected samples were tinted with Alizarin red and observed by optical microscopy on uncovered thin sections. A Philips 505 scanning electron microscope was used to examine textural features, morphology and the sequence of mineral formation. X-ray diffraction analyses were carried out on a Philips 3020 Xpert Pro device equipped with an Xcelerator detector, at 40 kV and 20 mA, with Cu Kα radiation and Ni filter. Clay minerals were analyzed on randomly oriented powders for routine analysis (<2 μm size fraction). Determination of interstratified clay minerals (illite-smectite) was achieved using Srodon (1984) method. Quantitative X-ray diffraction analyses were carried out using the Rietveld method (Rietveld, 1969). In the identification of micas on total sample the presence of diffraction lines corresponding to muscovite 2M₁ and the two characteristic lines of the polytype 1M₁ (-112 and 112) in 3.66 and 3.07 Amstrong, respectively were observed. That is why in the refinement by the Rietveld method for the quantification of the samples both muscovite polytypes were considered. When the content of micas did not exceed 40% or when they exhibited poor crystallinity, polytypes were not identified. Crystallographic data of polytype 2M₁ were taken from Liang and Hawthorne (1996) and those of polytype 1M₁ were obtained from Plancon et al. (1985).

INTEGRATED STRATIGRAPHIC SECTION

This contribution integrates the stratigraphic analysis of carbonate (C) and mixed (carbonate/siliciclastic: C/S and heterolithic: H) facies of the Villa Mónica Formation, Sierra La Juanita, with new material collected from three geologic sites. The studied outcrops: Estancia La Siempre Verde, La Placeres and Don Camilo, are located 4.5 km. East, 14 km. East and 20 km SE, respectively from Barker (Fig. 1). The integrated stratigraphic section with all the sedimentary features found in the studied areas is presented in Fig. 3.

From field observations, the C facies are represented by head, brownish, well-preserved columnar stromatolite dolostones. They are well-exposed at the Estancia La Siempre Verde, whereas only the top of these deposits outcrops at La Placeres quarry and, at Don Camilo quarry, are not exposed. These rocks form bioherms (mounds) up to 4 m high (Fig. 4a), containing scarce sandy siliceous trapped sediments and forming cavernous structures. The mounds show distinctive columnar stromatolite structures (Fig. 4b) and are crossed by fractures, filled with silica or with calcite cement. Random aggregates of pyramidal quartz megacrystals, up to 20 cm long and 10 cm wide, are found growing in fenestral cavities in the dolostones (Fig. 4c, d).

The C/S facies show up to 6 m thick banded, brownish to yellowish, weathered dolostones with intercalated siliciclastics, the latter represented by greenish, laminated clay, silt or sand beds (Fig. 5a). The weathered dolostones also bear loose, individual or random aggregates of pyramidal quartz megacrystals, ranging from approximately 10 cm to 1 cm long and 5 cm to few mm wide (Fig. 5b). Complex fracture-network systems, filled with reddish clays showing slickensides, cut the deposits in several directions at different levels (Fig. 5c) and also have spread along sedimentary discontinuities (e.g., between the C/S and the upper H facies).

The H facies, with a maximum thickness of 2 m, consist of a thickening and coarsening upwards, cross-laminated and rippled lenticular quartzites with intercalated greenish clay deposits. (Fig. 6a, b).

A NW-SE normal fault cross the front of the La Siempre Verde quarry and extends towards the La Placeres and Don Camilo quarries. Flexuration, fracturing and local brecciation of the deposits are clearly observed in the front quarries.

RESULTS

Petrography and mineralogy
Carbonate facies (C): The fabric is represented by bimodal rhombohedric carbonate crystals of <340 and <250 μm in size, respectively (Fig. 7a, b). Contacts between grains are planar. Staining with Alizarin red shows that dolomitization was complete and that sparry calcite is present in fractures and in very small cavities (Fig. 7c). The rocks show typical stromatolite lamination (Fig. 7d), which attests to the precipitation of calcite from algae activity. On the basis of Dunham (1962) classification, these rocks are columnar boundstones.

Epitaxial calcite rims on dolomite crystals are observed, being even and uniform in thickness (45 μm) and with lengths of 300 μm. Stained calcite rims (Fig. 7e darker in the photograph) are easily distinguishable from dolomite crystals. Ferric oxides and hydroxides are represented by hematite and abundant goethite, with minor ferric oxides also present in fractures and surrounding dolomite crystals, having precipitated previous to the epitaxial calcite rims.

The dolostones show silicification processes (Fig. 7f). Quartz is present as individual crystals or as random aggregates of megacrystals, which have grown in major cavities, in fractures or as pseudomorphs after rhombohedric dolomite crystals, also surrounded by hematite and/or goethite. Small cavities and fractures are filled with calcite or with quartz. X-ray diffraction analyses show that they consist of 79% replacement dolomite, quartz and illitic material (I+ISII), with <15% of expanded layers (Table 1, sample 00).

Carbonate/siliciclastic facies (C/S): The carbonates of these facies are represented by relic domal or laminar stromatolite dolostones, where rhombohedric dolomite crystals have lost their continuity and have been completely replaced by illitic material (I+ISII), with less than 15% of expanded layers (Table 1). The illitic clay includes rutile needles, which are 20 to 30 μm long, intersecting to form asterisk-shaped units showing a sagenite-like texture (Fig. 8a). Some of the relic rhombohedric dolomite crystals show silicified cellular structures (Fig. 8b) that could be related to microbial activity.

Quartz megacrystals (Fig. 8b) have grown in large cavities (fenestral porosity) and, in some cases, include opal-CT (Fig. 8c) identified as trydimite by X-ray diffraction (Table 1, sample 0).

Cavities are large in comparison with the previous lithofacies and goethite is abundant, surrounding replaced dolomite crystals (Fig. 8b). When cavities increase in size they connect with each other and, occasionally, they are filled with quartz megacrystals.

Typical cutans (Brewer, 1960, 1976) are represented by ferriargillans and clay infillings. Ferriargillans show spherical or ellipsoidal shapes, parallel lamination and are rich in ferric hydroxides (goethite), kaolinite and minor smectite, showing orange to yellowish colors. Orientation of the clay laminae produces extinction when parallel to the polarizers (Fig. 8d). Ferriargillans fill cavities developed among relics of the original rhombohedric dolomite crystals, now replaced by illitic material and disrupt black, hematized microbial mat deposits (MM: Fig. 8d). Two types of clay infillings, which block up cavities, were distinguished: (1) brownish kaolinite (Table 1, sample 20), lacking rutile needles and showing a book texture. They are discernible by their interference color from (2) illitic clay (dark grey) which is composed of a mixture of 1M₁ and 2M₁ polytypes (Table 1), where 2M₁ predominates over 1M₁. Kaolinite, associated with smectite and goethite, fills fractures and micro fractures with different orientations, as have been shown (Fig. 5c) in the field cutting the weathered dolostones at all levels and showing slickensides (stress cutans). The ferriargillans and the original dolomite texture are also cut by micro fractures filled with kaolinite, smectite and goethite. Detrital quartz grains, with overgrowths and undulate extinction, show some orientation within the relic dolostones and also seem to form defined siliciclastic intercalations. The relationships among detrital quartz, ferriargillans, clay infillings, original rhombohedric dolomite texture and fractures are shown in Fig. 8e. In some cases, the ferriargillans have been disrupted and brecciated by the infiltration of kaolinitic clay. In Fig. 8f crinkled dark microbial mat deposits (MM) alternate with illitized carbonate deposits (light color). Flexured micas laminae are clearly observed within the microbial mats.

Scanning electron microscopy shows plates of hexagonal kaolinite and tubes of halloysite developing on the kaolinite surface (Fig. 9a). Kaolinite cutans also fill fractures inclined or parallel to the stromatolitic structure, whereas ferric hydroxides (goethite) coat the fracture walls (Fig. 9b).

The siliciclastics in the C/S facies are represented by silt to clay-sized layers and by minor sand deposits. The fines may show lamination, graded structure and stylolites parallel and inclined to bedding, either filled with clay, sand and/or hematite, the perpendicular stylolites cutting the parallel ones (Fig. 10a).

The quartz sandstones show great compaction, sutured boundaries and concave-convex contacts between grains, undulate extinction, fluid inclusions and dissolution features (Fig. 10b). Deformational features (e.g., folding) are common in these rocks, which show connected cavities filled either with clays or quartz cement. Quartz grains show dissolution effects. Some quartz crystals have also been broken and pulled apart by the introduction of clay, as shown along stylolite surfaces (Fig. 10c). The clay introduced between grains is kaolinite.

As shown by X-ray diffraction, silt to clay-sized beds are composed of predominantly illitic material (I+ISII) with <15% of expanded layers. Quantification of 1M₁ and 2M₁ illite polytypes shows that the 2M₁ polytype predominates over 1M₁ in the total sample (Table 1). Illitic material bears rutile needles (5 to 30 μm long) arranged in a sagenite-like texture (Fig. 11a).

Scanning electron microscopy shows the remains of a face-to-face arrangement of the illite, with certain swirl pattern and dissolution features (Fig. 11b). Cavities and fractures parallel to the lamination, are filled with kaolinite, showing a book texture and is surrounded by ferric hydroxides (goethite). Mica flakes are abundant, oriented parallel to the lamination and flexured. Individual cubic pyrite crystals have been completely hematized and disrupt the lamination.

Heterolithic facies (H): These facies, characterized in the field as sandstone/claystone sediments, show in thin sections textural features, such as crinckly laminae and crinkled surface relief on otherwise smooth bedding planes (Fig. 12a). In the same photograph, black, hematized, crinckly laminae with isolated, detrital grains within represent microbial mats.

Quartz sandy deposits show graded structure and the grains float in an illitic epimatrix (Fig. 12b). Finally, at the top of the H facies (Fig. 12c) compact sandstone layers are composed of well-sorted, rounded quartz grains with secondary overgrowths, fluid inclusions and undulate extinction.

Boundaries between quartz grains are sutured (concave-convex contacts). Euhedral feldspars have been altered to illitic material with <15% of expanded layers. Illitic material, together with quartz grains and hematite also fill irregular cavities and intergranular fractures, bearing rutile needles. Kaolinite tangential clay coatings around quartz grains and kaolinite introduced in fractures are clearly observed in sandstones at the top of the H lithofacies (Fig. 12d) while altered feldspars are surrounded by illite perpendicular coatings (Fig. 12e).

DISCUSSION

According to field evidence, mineralogy and the fabric revealed by petrographic and SEM analysis, successive weathering and diagenetic superimposed events have been recognized in the sediments of the Villa Mónica Formation and a paragenetic sequence is proposed (Table 2).

The well-preserved columnar boundstones (Fig. 3) represent the unaltered bedrock, whereas overlying C/S facies with laminar structures and minor intercalated siliciclastics are attributed to microbial activity systematically interrupted by short-lived detrital cycles and which have been extensively weathered. Introduction of illitic epimatrix in the different facies (quartz grains floating in an illitic matrix) is related either to bioturbation, mass flow and soft-sediment deformation in the fabric of marine sandstones (Walker et al., 1978). In this case, the phenomenon could be linked to soft-sediment deformation and bioturbation produced by microbial activity.

Algal cementation is one of the most important lithification processes in shallow-water limestone genesis. The lime precipitates of blue-green, green and red algae can be considered as more or less of syngenetic origin (Larsen and Chilingar, 1979). We interpreted the well-preserved and weathered dolostones found at the Sierra La Juanita, showing cellular/banded, stromatolitic structures and crinkled lamination, to represent the kind of deposits derived from bacteria and blue-green algae activity, according to Schieber (1998).

Calcite derived from microbial activity precipitated as microsparite, which in turn recrystallized to macrosparite. Early diagenetic calcification of microbial deposits promoted the formation of cavities and calcite cement. Some of the voids must have remained open and provided pathways for fluids during later diagenesis (Glumac, 2001).

Trapped sediments between the microbial deposits, represented by detrital sandy or silty quartz, detrital illite (2M₁) and micas, were all derived from weathering and erosion of the granitic basement rocks. Flexuration and orientation of the micas parallel to bedding prove their detrital origin. The 1M₁ polytype is present in minor proportions and is considered to be of burial diagenetic origin.

Staining with Alizarin red shows that all the micro and macrosparite have been transformed into dolomite. The complete process of dolomitization could be interpreted as syngenetic. We assume that dolomitization took place at shallow depth and low temperature (less than 70°C), according to Girard and Deynoux (1991) and Chafetz and Zhang (1998). Dissolution of the dolostones generated fenestral porosity, which developed under appropriate physiochemical conditions and created cavities.

Paragenetic relations between silicification and dolomitization are extremely useful in helping to establish the timing of silicification. Euhedral crystals of dolomite completely silicified in the carbonates suggest that the dolomitization preceded precipitation of silica as a postdepositional process. Silicification has probably occurred as an early diagenetic process. Opal-A, considered of biogenic origin, recrystallized to opal-CT (trydimite), then to quartz megacrystals (which include opal-CT) which developed in the fenestral cavities.

After the silicification process, subaerial exposure occurred. The sediments must have been eroded to a large extent and extensive weathering occurred with the development of pedogenetic features (e.g., clay illuviation).

Dissolution of the boundstones increased and cavities enlarged and connected with each other. Loose, individual or random aggregates of pyramidal quartz megacrystals, remained in situ in the weathered dolostones (C/S facies). Illuviation processes deposited cutans (illitic clay skins: argillans) in the created cavities.

As the original rhombohedral dolomite crystals were dissolved, illuviated illitic clays increased in lower horizons.

The ferric oxides may have derived from the dissolution of micas (biotite) contained in higher eluviated horizons, from the replacement of calcium by iron in the dolostones (Larsen and Chilingar, 1979) and from the activity of Cyanobacteria (blue-green algae) cohabiting on microbial film or mat (Dai et al., 2004).

Detrital clay beds within the weathered dolostones (C/S facies), are represented by major 2M₁ illite and subordinate 1M₁ illite, the latter considered being of diagenetic origin. Some of the clays show no quartz and a face-to-face association of platelets, which were deposited as flocs (card-pack-flocs, Williamson, 1980). They show `swirl patterns´, probably resulting from movements during packing, settling, or soft sediment deformation (Keller, 1978).

Besides, the fines of the overlying heterolithic (H) facies are illitic and bear quartz, which increases upwards. The same illite polytypes are present and considered to be of the same origin as in the previous illitic clays. The upwards increasing trend in quartz content in the (H) facies is consistent with littoral conditions in tidal flats with the development of graded structures due to differential settling from suspension after a flood (Manassero et al., 2007).

During burial diagenetic stages, diverse processes took place in the different lithofacies. Neoformation of minerals is represented by silica overgrowths, development of (ISII) with <15% expanded layers (1M₁ illite) at the expense of detrital feldspars (perpendicular clay coatings) and of 2M₁ illite. Other neoformed minerals are rutile, pyrite and minor anatase. Rutile precipitated from titanium oxide contained in the micas, as in many other Neoproterozoic formations of Tandilia (Iñiguez et al., 1989) and crystallized as needles (resembling a sagenite-like texture) in the illitic clays and in quartz cement, filling cavities or fractures. Rutile needles are also found in quartz precipitated in pores of the quartzites of the (H) facies. Pressure-solution by overburden originated stylolites parallel to bedding and intergranular (e.g., concave-convex grain contacts) dissolution effects developed in the C/S as well as in the H facies.

After deposition of all the other Neoproterozoic units, a compressive event affecting the Tandilia System (Brazilian cycle, 600 Ma) was responsible for regional faulting, fracturing and differential block elevation (horst and graben structure). Peneplanation presumably took place during the Cambrian (Iniguez et al., 1989).

During early Ordovician, to the SE, subsidence created a new basin, opened to the SE and in which the sediments of the Balcarce Formation were deposited. Tandilia remained as an old topographic height, subsequent to the Ordovician and before the Devonian. Tandilia also represented the NE border of the Claromecó Basin, as suggested by the thinning of all the Palaeozoic deposits of the basin in that direction and the lack of Devonian deposits in the Tandilia mountain range (Lesta and Sylwan, 2005).

During the Permian, uplift of the Ventania System, located 150 km to the SW of Barker, took place (Varela et al., 1985; Von Gosen and Buggisch, 1989). According to Zalba et al. (2007b) simultaneous compression from the SW occurred in Tandilia. Due to basin inversion, a new stage in the geological history of Tandilia began. During this telodiagenetic stage kaolinite formed due to the introduction of oxidized meteoric fluids related to pyrite oxidation, in the Neoproterozoic Las Aguilas Formation (younger than the Villa Mónica Formation and outcropping 7 km to the east of the Estancia La Siempre Verde), where it is also associated with halloysite, together with diaspore and APS minerals. Based on this evidence it is not unrealistic to suppose that regional telodiagenetic processes developed in the Barker area also affected the studied unit.

It is important to underline that kaolinite has been introduced in the sediments because it is associated with minor smectite in ferriargillans and clay infillings related to a criss-crossing network system of fractures cutting the C/S facies and postdating the illuviation of illitic clay infillings (argillans). It represents the redistribution of neoformed clays in the sediments and in the sedimentary discontinuity between the C/S and the H facies. This would explain the mixture of the kaolinite-halloysite assemblage and/or smectite, with illitic clays which, almost certainly, is the same phenomenon that occurred in most studied clay deposits of Tandilia where this mixture of illite with kaolinite-halloysite is found (Zalba et al., 2007b).

Smectite, in variable proportions, is predominately associated with slickensides in reddish clays filling a network system of fractures. The slickensides differ markedly from clay skins, which occur on pedogenetic surfaces resulting from clay translocation. The latter have sharp outer and inner boundaries with distinct extinction patterns and are often finely layered (laminar fabric). The slickensides seem to be aligned preferentially (relative to the stress field) in response to structural deformation (Driese and Foreman, 1992), so they would be of tectonic origin instead of being pedogenetic, which are not oriented and locally form pseudo-anticlines (Driese and Foreman, 1992; Driese et al., 1992; Driese and Mora, 1993; Caudill et al., 1996).

Smectite may derive from the dolostones which contain the Mg and Fe required for its development as a result of neogenesis or transformation from primary minerals (Worden and Burley, 2003). If smectite had formed earlier than the other clay minerals (as in vertisols) it would have prevented the illuviation (translocation) of illitic clay throughout the weathered boundstones. Furthermore, the presence of smectite-kaolinite-halloysite filling fractures suggests a late origin for these minerals. Smectite was one of the latest minerals to form and must have developed during the telodiagenetic stage. This mineral is also present in other Neoproterozoic units of the Tandilia Basin (e.g., Cerro Negro Formation) and was considered to be of late diagenetic origin (Zalba, 1982).

Cubic crystals of pseudomorphous hematite after pyrite could have formed during this telodiagenetic stage due to oxidizing meteoric fluids. Pyrite was presumably formed during early burial diagenesis, as in other Neoproterozoic deposits of Tandilia (Zalba et al., 2007b). Hematite which colored the sediments and is also present as diffusion cutans, needs oxidizing conditions to be formed. Most of the hematite was hydrated to goethite, the latter being a constituent of the ferriargillans. It is present in fractures, in cavities of the weathered boundstones and in the siliciclastics of the C/S facies. Fractures filled with goethite cut the remains of the original boundstones, the ferriargillans and the clay skins in general, demonstrating that goethite post-dated all of them. Petrography shows that goethite grew over smectite and so the first must be younger.

Lateral compressive forces produced pressure-solution effects. Inclined stylolites developed in all the identified facies cutting previous stylolites developed parallel to bedding. They are filled with epimatrix and/or relic ferric oxides (hematite), dissolved and fractured quartz grains and brecciated ferriargillans, which indicate compressive effects.

Folding of the sediments also occurred during telodiagenesis and affected in a different way the lithostratigraphic units because of a different mechanical response of these materials to compressive forces. This folding is clearly appreciated in field and microscopic observations.

Another event attributed to telodiagenesis is the development of kaolinitic epimatrix (tangential clay coatings), clearly connected with fractures, surrounding secondary quartz overgrowths and observed in sandstones of the H facies.

Dedolomitization partially affected the C facies. According to Larsen and Chilingar (1979), dedolomitization can be eogenetic as well as telodiagenetic. In the present case, dedolomitization is recognized by the presence of epitaxial calcite rims on silicified dolomite crystals in the C facies. The rims are even and uniform in thickness. Analysis of the fabric geometry and mineral paragenesis suggest that the rims formed by marginal dedolomitization (Larsen and Chilingar, 1979), attributed to telodiagenetic processes.

Calcification (passive precipitation) also occurred during this telodiagenetic stage. Calcite fills fractures, enlarging them because of its force of crystallization. These fractures cut stylolites, thus postdating them. Calcite also fills large cavities in trapped sediments in the C and C/S facies.

CONCLUSIONS

On the basis of new field and petrographic evidence, three lithofacies overlying the Cuarcitas Inferiores are redefined for the Villa Monica Formation in the study area. (1) Carbonate facies: dolomitized and silicified well preserved stromatolitic boundstones with in situ quartz megacrystals and sandy sediments trapped within, (2) Carbonate/Siliciclastic facies: weathered laminar microbial mat deposits with in situ, loose quartz megacrystals and intercalated siliciclastic episodes and (3) Heterolithic facies: sandstone-claystone beds with microbial mat activity. Lithofacies 2 and 3 are described for the first time at this locality and also for this lithostratigraphic unit. The heterolithic facies substitute the Psamopelites of Poire and Iniguez (1984).

The paragenetic sequence of events evidences that carbonate sediments, with cellular and stromatolitic structures, derived from microbial action and intercalated thin, detrital illitic (2M₁) greenish clay levels (some of them with no quartz) were deposited under tidal conditions. Illitic clays settled down from suspension and were derived from basement rock erosion.

Syngenetic, surficial processes led to recrystallization of micro-spar to macro-spar, dolomitization, at low depth and temperature (lower than 70°C) and dissolution of the boundstones, release of ferric oxides from the dolostones and from microbial mats and titanium oxides from detrital micas. Cavities formed creating secondary porosity where silica precipitated: opal-A (biogenic), which recrystallized into opal-CT (trydimite) and subsequently, into quartz megacrystals (silicification). Subaerial exposure produced erosion and weathering of the boundstones. Leaching and translocation (illuviation) processes were responsible for the deposition of clay skins or cutans (illitic clay infillings) in cavities and fractures.

The heterolithic facies represent a shallowing upward trend in the paleoenvironmental conditions and the last development of microbial activity in this lithostratigraphic unit. That is why the heterolithic facies are considered part of the biogenerated deposits of the Villa Mónica Formation.

After several recognized transgressive-regressive cycles which led to well known resultant unconformities and weathering products, as well as weak epeirogenic movements throughout the geological history of the basin, a major period of basin subsidence occurred, during which the sediments remained sub-horizontal and experienced burial diagenetic processes. In the Villa Monica Formation these processes were represented by (1) Neoformation of minerals: silica overgrowths on quartz grains; development of ISII with <15% expanded layers (1M₁ Illite) at the expense of detrital illite (2M₁) and of feldspars (perpendicular coatings); pyrite and rutile/anatase. (2) Pressure-dissolution during burial led to the development of stylolites parallel to bedding, sutured grain contacts, intragranular and intergranular dissolution, with concave-convex contacts between grains.

As exposed in a previous contribution (Zalba et al., 2007b) after a complex history of compression which involved faulting, differential block movements and peneplanation, a new phase began. Due to the uplift of Ventania (middle Permian) Tandilia experienced compression from the SW which caused a telodiagenetic stage (basin inversion) with the most important processes in the Villa Mónica Formation involving uplift, erosion, fracturing, folding, introduction of oxidizing meteoric fluids and epimatrix (e.g., tangential clay coatings) and neoformation of minerals in fractures (hematite, kaolinite, smectite), hydration (goethite, halloysite), lateral pressure-dissolution (stress cutans, stylolitization, brecciation), dedolomitization and passive calcite precipitation.

ACKNOWLEDGMENTS

This study was supported by grants from the Comisión de Investigaciones Científicas Provincia de Buenos Aires through the Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC). We thank Lic. María Eugenia Rodríguez (CIC-CETMIC) for assistance in the field.

REFERENCES