Journal of Applied Sciences

Year: 2010 | Volume: 10 | Issue: 22 | Page No.: 2754-2772
DOI: 10.3923/jas.2010.2754.2772

Microscale Diagnostic Diagenetic Features in Neoproterozoic and Ordovician Units, Tandilia Basin, Argentina: A Review

Patricia E. Zalba, Martin Morosi, Marcelo Manassero and Maria S. Conconi

Abstract: This review is the result of many years of research on stratigraphical correlation, sedimentology and mineralogy of one of the oldest sedimentary basins of Argentina that experimented numerous diagenetic changes along the evolution of its geological history: Tandilia. Previous and new data are presented and supported with photographs that illustrate different aspects of microscale diagnostic features of diagenesis recorded in weathered crystalline basement rocks and the overlying sedimentary succession represented by the Neoproterozoic Villa Mónica, Olavarría, Cerro Largo and Las Aguilas formations and the Late Ordovician Balcarce Formation. The study gathers optical and scanning electron microscopy data supported by X-ray diffraction analysis. The finding of Microbially Induced Sedimentary Structures (MISS) in siliciclastic and mixed facies helped to unveil hidden biosignatures, fundamental to understanding the origin of life on Earth.

Keywords: texture, Diagenesis, MISS, Neoproterozoic, diagnostic features and Ordovician

INTRODUCTION

Diagenesis comprises a broad spectrum of physical, chemical and biological post-depositional processes by which original sedimentary assemblages and their interstitial pore waters react in an attempt to reach textural and geochemical equilibrium with their environment (Curtis, 1977; Burley et al., 1985). These processes are dynamic and evolve throughout the history of the basin according to local variation in temperature, pressure and chemistry where synsedimentary processes, burial and uplift are main stages and which, in many cases, may be characterized by typical diagnostic features imprinted in the sediments. Nevertheless, in ancient deposits, several superimposed diagenetic processes and also weathering and erosion, may partially or completely erase original fabric characteristics and consequently obliterate diagnostic features. In the case of the oldest Neoproterozoic sedimentary unit of the Tandilia Basin (Fig. 1), the Villa Mónica Formation, these kind of problems difficulted its petrological classification for more than 40 years (Zalba et al., 2010).

The Neoproterozoic Villa Mónica, Olavarría, Cerro Largo and Las Aguilas formations (Fig. 2) were selected to make a review of significant diagnostic diagenetic features detected. Also, few examples of diagenetic processes have been taken from the Ordovician Balcarce Formation at the Chillar area. Yet, some structures are shown for the first time because of recent findings of reliable evidence for the interpretation of their origin.

Textural studies were carried out on clays, well-preserved and weathered stromatolitic dolostones, minor quarzarenites, ooid grainstones and crystalline basement rocks.

On the basis of previous contributions, this review, gives a summary of diagnostic diagenetic features and veiled biosignatures found in siliciclastic and mixed facies.

In the case of the Villa Mónica Formation, for example, rare microbial structures found some years ago at San Manuel area were only interpreted after having compared them with others more complete and better-preserved recently identified in other localities of Tandilia.

When we noticed that some structures in the mixed facies of the Villa Mónica Formation were biogenerated (Zalba et al., 2010) we continued with new petrographical observations in the Neoproterozoic Villa Mónica, Olavarría and Las Aguilas formations, searching for biosignatures and also for more data to understand the controversial origin of minerals like pyrophyllite, alunite and halloysite, which we consider of diagenetic origin (Zalba et al., 2007).

In this contribution we follow the classification of the diagenetic processes through the classical scheme of early diagenesis (eogenesis), burial diagenesis (mesodiagenesis) and uplift-related diagenesis (telodiagenesis) according to the scheme developed at first for limestone diagenesis by Choquette and Pray (1970) and later generalized for clastic diagenesis.

GEOLOGICAL SETTING

The Tandilia Basin, also known as the Sierras de Tandilia or Sierras Septentrionales de Buenos Aires, runs in a NW-SE direction along 300 km in the Province of Buenos Aires, between 36° 30’ to 38° 05’ S latitude and 58° to 62° W longitude, with a maximum width of 60 km in the Tandil area, becoming narrower at both extremes: Olavarría and Mar del Plata (Fig. 1). The sedimentary successions of this basin represent the western end of a bigger basin that extends to Namibia, Africa (Nama Group) and was preserved after the continental break during the Mesozoic (Dalla Salda and Iniguez, 1979).

The Tandilia Basin is the first mining area of the Buenos Aires Province, with numerous mining localities. Barker, Sierras Bayas and San Manuel, represent important ones, with limestones and clays as the principal mining commodities. It is a non-continuous succession of hills emerging from the extended plains of the pampa, with heights varying between 50 and 490 m over the sea level. Besides, it is necessary to emphasize the great quality of the clay deposits, which have preserved the necessary physicochemical characteristics for their industrial application.

The sedimentary succession has been studied since the 19^th century from the geological, sedimentological, geochemical, petrological, mineralogical, structural, geochronological, paleontological, environmental and economical points of view. A compilation of the pioneers, the classical and the more recent studies may be consulted in Teruggi and Kilmurray (1980), Zalba et al. (1988, 2007, 2010), Iniguez et al. (1989), Poire and Spalletti (2005).

The Crystalline Basement rocks of the Sierras de Tandilia are covered by apparently tabular sedimentary siliciclastic, biochemical and bioconstructed successions included in different lithostratigraphic units which represent one of the oldest sedimentary deposits known in Argentina, of Neoproterozoic and Early Ordovician ages.

Studies in the Barker locality (Fig. 1) contributed to the understanding and correlation of the different deposits which outcrop between Olavarría and Mar del Plata areas, the two extremes of the Tandilia Basin. From the seventies to the eighties, the first detailed studies were performed at Cuchilla de Las Aguilas and Sierra La Juanita, to the west of Barker (Fig. 1), which permitted the recognition of new lithostratigraphic units: the Las Aguilas Formation (Zalba, 1979); the Cerro Negro Formation (Zalba, 1981), first described for the Olavarría (Sierras Bayas) area (Iniguez and Zalba, 1974) and the creation of the Sierras Bayas and Balcarce formations (Dalla Salda and Iñíguez, 1979). Poire (1987) defined the Cerro Largo Formation at Olavarría, which later was also detected in the Barker area (Fig. 2). The Las Aguilas Formation has been correlated with the Cerro Negro Formation and recently, with the Olavarría Formation (Poire and Spalletti, 2005) but we consider that its stratigraphic position is not definitive yet.

Iniguez et al. (1989); Poire (1993); Andreis et al. (1996) and Poire and Spalletti (2005), are some of the authors who interpreted that different sedimentary cycles, separated by regionally extended unconformities, were created by sea-level changes.

According to Zalba et al. (2007) ‘The sedimentary sequences of Tandilia have probably not exceeded 2-3 km of burial. This statement is based on the comparatively simple authigenic mineralogy of these successions (Iniguez and Zalba, 1974; Iniguez et al., 1989; Zalba, 1982), also revealed on several world examples of onshore basin-margin sequences (Hebridean Basins, Morton, 1987; Yorkshire Basin, Hemingway and Riddler 1982; Dorset Basin, Scotchman, 1991a, b), which, according to these authors, experienced a 2.5 km of burial. Furthermore, the abundance of compaction features like straight-line and suture boundaries and especially the presence of triple junctions in sandstones (Ahmad and Bhat, 2006) observed in the quarzarenites of the Sierras Bayas Group and of the Balcarce Formation (Iniguez et al., 1996) support this, assumption’.

At present there are still many geological problems to solve but, according to recent studies (Zalba et al., 2007; 2010) detailed petrographical work and the interpretation of paragenetic sequences have proved to be of the most importance in the better understanding of many processes which have occurred in these ancient rocks.

DIAGENETIC PROCESSES AND DIAGNOSTIC FEATURES REVEALED BY TEXTURAL ANALYSES

Diagnostic features related to different regimes of diagenesis: syndiagenesis, eodiagenesis, mesodiagenesis (burial) and telodiagenesis were recognized in stromatolitic dolostones, ooid grainstones, shales, marls and quartzarenites of the Tandilia Basin, where continuous diagenetic processes developed throughout its evolution.

Cementation is the process which describes precipitation of authigenic minerals in available pore space causing the lithification of the sediment. The most ubiquitous mineral cement found in most rocks is silica, but there are also many others (carbonates; sulfates; phosphates; ferric, manganese and magnesium oxides; ferric hydroxides, gypsum, etc.), which are widespread in sedimentary deposits.

In the case of the studied Neoproterozoic and Ordovician lithostratigraphic units of the Tandilia Basin the main mineral cements are silica in its varieties trydimite, quartz and chalcedony; carbonates (calcite, dolomite); clay minerals (illite-smectite, kaolinite, halloysite); pyrite; hematite; goethite and APS minerals: alunite; svanvergite-Ce-florencite, according to Zalba et al. (2007, 2010). All these cements were formed in different regimes of diagenesis and, sometimes, are found superimposed in the same sample.

Precipitation of calcite from blue-green and red algal and bacterial activity is a well known cementation process and it is considered to be syndiagenetic (Larsen and Chilingar, 1979). It acts as a powerful preservation agent when very tiny and delicate fossil remains are involved. A summary of the main authors who studied this phenomenon can be checked in Noffke (2007).

A related process is recrystallization, which comprises a change in crystal size or shape resulting from thermodynamic instability like reprecipitation of finely grained calcite by coarse grained calcite cement (Hendry et al., 1996) when kinetic barriers are exceeded to allow the reaction to proceed, taking into account that both recrystallization and neomorphism always require the presence of an aqueous medium (Worden and Burley, 2003). This process is different from replacement of a mineral by other which occupies the place of the formerly formed; no matter if it is authigenic of detrital and where dissolution-precipitation process takes place like in carbonate cements replacing detrital quartz (Hesse, 1987). The definition of replacement given by Worden and Burley (2003) is ‘growth of a chemically different authigenic mineral within the body of a pre-existing mineral’.

Recrystallization from microsparite to macrosparite occurs during syndiagenesis, together with dolomitization. It is assumed that dolomitization in the Villa Monica Formation was complete (Zalba et al., 2010) and took place at shallow depth and low temperature (less than 70°C), according to Girard and Deynoux (1991) and Chafetz and Zhang (1998). In Fig. 3a, a delicate domal structure due to microbial activity has been preserved in fresh domal stromatolitic dolostones. Figure 3b depicts a dolomicrite cement recrystallized to dolosparite in well-preserved stromatolitic dolostones. Alizarin-red staining shows that all the micro and macrosparite have been transformed into dolomite. Precipitation of calcite (red) in fractures also cuts the dolomitized stromatolitic rocks and is a telodiagenetic process, separated by millions of years from the dolomitization one.

Figure 3c and d show a completely silicified stromatolitic dolostones. Note well-preserved rhombohedra at the center of the photograph. These euhedral crystals of completely silicified dolomite, suggest that dolomitization preceded precipitation of silica as a postdepositional process and has occurred as an early diagenetic process.

Figure 3e shows trydimite crystals included in quartz and surrounded by chalcedony in fresh stromatolitic dolostones. The silica has precipitated as opal-A due to organic activity and later recrystallized to opal-CT (trydimite was also identified by X-ray diffraction) and then to quartz forming megacrystals up to 20 cm long which precipitated in cavities and voids (Zalba et al., 2010). Chalcedony represents the last pulse of silica precipitation. Figure 3f is a detail of Fig. 3e.

Cellular microbial colonies and crinkled, banded lamination (dark) can be seen in dolomite rhombohedra (in brown) where cavities have been filled with quartz (Fig. 3g). We interpreted (Zalba et al., 2010) that cellular/banded stromatolitic structures and crinkled lamination are the kind of deposits derived from bacteria and blue-green algae activity, according to Schieber (1998). Cathodoluminiscence analysis performed on the previous sample (Fig. 3h) shows dolomite with microbial colonies included in quartz crystals (in orange) and (Fig. 3i) quartz crystal grown in dolomite cavities. The quartz crystals include dolomite rhombohedra, again suggesting that dolomitization preceded silicification processes.

Pyrophyllite has been found in crystalline basement rocks as well as in overlying sedimentary deposits of different Neoproterozoic units (Villa Mónica and Las Aguilas formations).

Hydrothermally formed pyrophyllite has been found in weathered basement rocks (saprock and saprolite) from San Manuel (Zalba and Andreis, 1998) and Sierra La Juanita (Cerro de La Cruz) areas. Thin sections of these rocks are shown in Fig. 4a and b where quartz (dissolved) and kaolinite have reacted to form pyrophyllite. In the system Al₂O₃-SiO₂-H₂O (Evans and Guggenheim, 1998) pyrophyllite is stable over a narrow temperature range (~250 to ~350°C), at 1 and 2 Kbars pressure (Hemley et al., 1980) with the most frequent reaction producing pyrophyllite during prograde metamorphism being:

Pyrophyllite (Fig. 4c) has also been found in clay deposits of the Las Aguilas Formation, Barker area (Zalba, 1979) and as intraclasts (Fig. 4d) in weathered dolomite deposits which have been totally replaced by illite-smectite (Zalba et al., 2010) in the Villa Mónica Formation, San Manuel area (Zalba and Garrido, 1984).

From field and petrological observations, pyrophyllite in the Las Aguilas Formation, at Barker area and in the Villa Mónica Formation, at San Manuel area, were considered of detrital origin and related to the weathering and erosion of local crystalline basement rocks (Fig. 4a, b). However, the origin of pyrophyllite in the sedimentary deposits has been also attributed to hydrothermal processes (Dristas and Frisicale, 2003), being this subject a long-term matter of discussion.

Pyrophyllite is also found in clasts of silicified basal breccias of the Las Aguilas Formation at Barker area, considered as reworked carbonatic platform sediments. The fabric of these deposits presents abundant silt-to fine sand-sized allochemes (including ooids, oolites of concentric fabric and intraclasts) but also shows wavy-crinckled lamination attributed to microbial processes and similar to fabrics observed in modern and ancient mounds that Riding (2000) has termed agglutinated stromatolites. The deposits are classified as ooid grainstones.

The oolites may show nucleus of quartz, or show no nucleus (Fig. 5a and b). Detrital quartz (center of Fig. 5b) shows syntaxial overgrowths. Compaction has put oolites in contact but they are slightly deformed (Fig. 5a, b). It is clearly observed that pyrophyllite has grown in situ at expense of detrital quartz in fine grained silica cement in the same sample (Fig. 5c). Pyrophillyte has also grown as a selective replacement of oolite nuclei (Fig. 5d). Furthermore, agglutinated mammelar pyrophyllite can be seen in the same deposits (Fig. 5e).

It is not well understood the origin of this pyrophyllite but textural relationship with quartz makes a diagenetic origin a reasonable hypothesis.

Chalcedony cement has replaced all previous carbonate cement within the ooid grainstone. It is possible to recognize two different grain sizes of silica cement-textures (Fig. 5f) which could be the result of selective recrystallization or replacement of rocks of different porosity. Irregular masses of presumably diagenetic pyrophyllite have grown only in the fine grained cement.

In conclusion, calcite precipitation from cyanobacteria, precipitation of opal A opal CT (trydimite) quartz (silicification) and dolomitization are considered textural diagnostic characteristics of syndiagenesis and eodiagenesis. The presence of pyrophyllite in sediments of different Neoproterozoic units (Villa Mónica and Las Aguilas formations) is consistent with a detrital origin. The material was supplied by the erosion and transportation of hydrothermally formed pyrophyllite in the basement rocks of nearby areas (San Manuel and Sierra La Juanita). If also pyrophyllite has formed diagenetically in ooid grainstones is not well understood, but if so, its formation after compaction (burial) and during telodiagenesis is a reasonable option.

1A relatively new concept related to structures found in bioconstructed rocks is the recognition of Microbially Induced Sedimentary Structures-MISS (Noffke et al., 2001), which, according to the original definition of the authors ‘do not arise from chemical processes, but from the biotic-physical interaction of microbial mats with the sedimentary dynamics of aquatic environments’. New defined mixed (carbonate-siliciclastic) facies for the Neoproterozoic Villa Mónica Formation show evidence of biosignatures (Zalba et al., 2010) which are described here as MISS.

Microbial mats are well known from stromatolites in carbonates back to 3.5 Ga (Walter, 1994). Many contributions on stromatolites in Proterozoic basins are applied to carbonate environments (Awramik, 1984; Walter et al., 1992) while there are few references to clastic environments (Schieber, 1998; Noffke et al., 2003; Noffke, 2006, 2009). Siliciclastic stromatolites are scarce compared to their carbonate counterparts in the rock record. The paucity of stromatolites in siliciclastic strata may relate to physical conditions and processes within the depositional environment that inhibit stromatolite growths and preservation (Druschke et al., 2009).

MISS (Fig. 6a) are represented by thin sedimentary units (from microns to few millimeters) composed of two biofabrics: (1) A characteristic dark, ferric, wavy, crinckly microbial mat deposit, with silt to clay-sized detrital grains trapped within (Noffke et al., 1997), interlayered with (2) A micritic, originally carbonate layer (light part of the photograph) with relics of rhomboedral illitic clay subsequent to dolomite enclosed by hematite and where bent micas, signifying detrital origin, are observed. On the one hand, no biogenic structures have remained after carbonate neogenesis in the light part of the unit, presumably because of recrystallization. On the other hand, microbial mat preservation (dark part of the photograph) has been possible due to the precipitation of ferric oxides which did not allow the expansion of diagenetic carbonates, as suggested by Hofmann (1975).

In the siliciclastics of the Carbonate-Siliciclastic facies of the Villa Mónica Formation we can differentiate bioconstructed structures from purely physically deposited siliciclastic beds. In Fig. 6b two different parts can be recognized: Part A) Grain-supported silt to sand-sized sediments interlayered with minor microbial mats with graded structure, showing some grains orientated preferentially with their long axis parallel to the sedimentary surface. Part B) Upwards, the siliciclastic material decreases and a dark, curvy crinckly lamination with detrital material floating within becomes more profuse. These dark deposits represent microbial laminites with great pigmentation (ferric oxides). Within the microbial mats, all the detrital quartz grains are oriented with their long axis parallel to the sedimentary surface. In this framework, these structures are considered part of the MISS, as defined by Noffke et al. (2001). The graded structure could be preserved because the microbial laminae have acted as a paste, occupying, trapping, stabilizing the sedimentary surface and preserving it, according to Noffke et al. (1997). In the upper intertidal and lower subtidal zone of tidal flats, microbial mats stabilized the sedimentary surface (Krumbein, 1994).

The features of microbial mats shown on vertical thin sections (dark colored, intact and wavy, wrinkled laminae), as stated by Noffke (2007), are important indicative characteristics to distinguish similar wrinkle structures but originated by abiogenic processes. While the laminar morphology of the iron-rich laminites may not preclude an abiologic origin, the concentration of elements such as iron in sedimentary laminae is commonly attributed to microbial metabolism (Flugel, 2004).

Figure 6c is another example of MISS where dark, ferric, crinkly microbial mat laminites with isolated detrital grains within the mats oriented parallel to bedding plane are illustrated. Alternating, graded, well-sorted and well-rounded sandy to silty siliciclastic deposits, with some orientation of their long axis parallel to bedding plane are shown.

In Fig. 6d quartz grains float in an illitic epimatrix and alternate with microbial mats (dark part) where detrital quartz grains oriented with their long axis parallel to bedding planes are clearly observed.

Figure 7a, b and c are from the same sedimentary deposits (Olavarría Formation, Cruz Pavone quarry, Sierras Bayas) and exemplify very thin microbial mats developed in siliciclastic (illitic) sediments, also recognized as MISS. The curved, dome, dark surfaces are considered to be bedding planes and the geometry of the domes do not superpose with each other upwards, that it why the same colony could not have grown after a renewal of the sediment supply (Fig. 7b). Other examples of MISS have been recently found in tidal deposits of the Las Aguilas Formation, first described by Zalba (1979) and also in well-preserved and weathered domal stromatolitic dolostones of the Villa Mónica Formation, both at Barker area. These structures are very similar (hematized, dark microbial mats, wrinkle structures, detrital grains with their long axis parallel to bedding planes trapped within the mats, dome structures) to those described previously for the Villa Mónica and Olavarría formations, as can be seen in Fig. 7d. That is why they are considered now as mixed facies and not purely siliciclastic, as they were firstly defined (Zalba, 1979).

The dark, hematized cement showed in Fig. 8a and b has been attributed to microbial mat relics in silicified ooid grainstones, with whole ooids, fractured ooid intraclasts and detrital grains trapped within, interpreted as reworked sediments.

This is the first time that MISS have been identified in different Neoproterozoic units of the Tandilia Basin. The proof that the hematized deposit is microbially induced and not a fracture infilled with hematite, is the textural relationship between the trapped quartz grains and the mat relics (Fig. 8b).

Evidently, a further step on diagenesis caused the fracture of some of the hematized, dark mat relics. The fractures have been infilled with illitic matrix (epimatrix) (Fig. 8c) while in Fig. 8d, microbial mat relics in stromatolitic dolostones are isolated and very difficult to recognize if other diagnostic associated features (e.g., cellular microbial colonies) are not detected or original texture has been totally erased.

Microbial mat formation in bedding planes may support the division of sedimentary units, an important concept supported by Gerdes et al. (1991) and which was taken into account for the detection and understanding of microbial mat relics in the named ferruginous clays (Manassero, 1986), also known as Psamopelites (Poire and Iniguez, 1984) of the Villa Mónica Formation at La Placeres and at San Manuel localities (Fig. 8d and e, respectively). That is why based on field work and petrological studies these facies were recently reclassified as mixed (Zalba et al., 2010). The finding of MISS in these deposits corroborates the presence of biosignatures although diagenesis has almost erased any other signal of organic existence and the sediments are at present mainly composed of detrital illite and pyrophyllite and diagenetic interstratified illite-smectite and kaolinite (Zalba and Andreis, 1998).

MISS are syndiagenetically formed and they have been preserved through silicification and dolomitization (eodiagenesis); mesodiagenesis (e.g., neoformation and transformation of minerals, compaction) and also through telodiagenetic processes (e.g., compression, fracture, introduction of meteoric fluids, mineral transformation, introduction of illitic matrix, etc.).

The presence of iron minerals and dolomite cements suggests microbial mat mineralization. In modern environments photosynthetic cyanobacteria may produce oxygen at the surface of microbial mats but, directly below the oxic surface layer, anaerobic bacteria degrade mat generated organic matter and may create a localized, strongly reducing environment (Gerdes et al., 2000). These environmental conditions favour the precipitation of calcium and ferroan carbonates and pyrite (Schieber, 2007; Schieber and Riciputi, 2004). Such minerals and also oxidized forms of pyrite (hematite and goethite) can be a valuable indicator of the former presence of microbial mats (Noffke et al., 2006; Schieber, 2007). In the case of the examples shown in Fig. 8d and e, hematization during early burial has enabled the preservation of discontinuous microbial mat relics preserved as lonely witnesses of original bedding planes.

Compaction is the process which takes place during burial and involves from ‘simple grain rearrangement during shallow burial as well as the ductile deformation of soft sand grains and intergranular matrix. This is quite different from the process known as chemical compaction, which involves the chemically induced dissolution of grains at intergranular contacts and reprecipitation of the dissolved material on grain surfaces facing open pores’ (Worden and Burley, 2003). When a solid organic or inorganic component present in sediments is dissolved by an aqueous pore solution a cavity or an empty space is created within the host sedimentary deposit (Schmidt and MacDonald, 1979).

Quartz cement can be associated with two different lithofacies: carbonate and siliciclastic. Figure 9a represents quartz cement as syntaxial overgrowths developed in quartzarenites during burial diagenesis, when compaction was not great and large pore space was still available. Figure 9b shows calcite cement filling secondary porosity in dissolved stromatolitic dolostones replacing syntaxial quartz overgrowths, interpreted as a telodiagenetic stage.

Examples of mesodiagenesis can be described in quartzarenites from the Cerro Largo Formation where the presence of triple junctions and sutural, dissolved contacts between grains are interpreted as the effect of deep burial compaction (Fig. 10a). These quartzarenites also show stylolitization. Quartz grains have been disrupted and pulled-apart by the force of the introduction of illitic matrix (arrows in Fig. 10b) in an advanced regime of telodiagenesis. A good example of extensive pressure-solution and sutural contacts between quartz grains due to stylolitization processes is observed in the same deposits (Fig. 10c). Stylolitization has also affected grainstone deposits of the Las Aguilas Formation, where ooids (Fig. 10d) and diagenetically formed pyrophyllite (Fig. 10e) have been fractured and dissolved.

Neomorphism describes the process of replacement and recrystallization of one mineral by a related mineral but involving change in the details of the mineral chemistry, excluding simple pore filling processes (Folk, 1965). The term has widely applied to limestones and dolostones, describing the coarsening of aragonitic micrite into calcite microspar and is equally applicable to sandstones.

This phenomenon commonly preserves textural evidence (ghost fabrics) of the previous phase (Worden and Burley, 2003).

Neomorphism is an important mesodiagenetic process observed in weathered stromatolitic dolostones, where dolomite has been completely replaced by illite-smectite formed during burial diagenesis with rutile needle inclusions in a sagenite-like structure (Zalba et al., 2010). The illite-smectite has replaced almost completely the dolostones where only relics of hematized microbial mat deposits have remained (dark area of the photograph), with some detrital quartz grains trapped within (Fig. 11a). Also, perpendicular clay coatings of kaolinite surrounding a partially altered detrital plagioclase in quartzarenites of the Villa Mónica Formation have been preserved after mesodiagenesis (Fig. 11b).

Tangential illitic clay coatings (cutans) disposed as detrital, mechanically infiltrated clays (Fig. 12a) around syntaxial quartz overgrowths in quarzarenites are a typical example of telodiagenesis in the Cerro Largo Formation (Zalba et al., 2010). An example of dedolomitization in the carbonate facies of the Villa Mónica Formation (Zalba et al., 2010) was interpreted as a telodiagenetic process taking into account the fabric geometry and mineral paragenesis which suggest that the rim (Fig. 12b) was formed by marginal dedolomitization (cf. Larsen and Chilingar, 1979).

Ferriargillans (typical cutans, Brewer, 1960, 1976) are found in kaolinitic clays along with smectite and goethite filling fractures which cut weathered stromatolitic dolostones. Further compaction processes caused the development of slickensides (stress cutans). Orientation of the clay laminae in the ferriargilans produces extinction when parallel to the polarizers (Fig. 13a). Kaolinite cutans fill fractures inclined or parallel to the stromatolitic structure, whereas postdating ferric hydroxides (goethite) coat the fracture walls (Fig. 13b).

Tubes of halloysite formed at expense of platy, hexagonal kaolinite (Fig. 13c), kidney-shaped kaolinite aggregates (Fig. 13d) and goethite aggregates growing in fractures and cavities (Fig. 13e and f) of Carbonate/Siliciclastic deposits of the Villa Mónica Formation at San Manuel locality are observed by Scanning Electron Microscopy (SEM). All these phase minerals have been interpreted as formed during a telodiagenetic stage according to Zalba et al. (2010).

The presence of alunite as pseudocubic crystals (Fig. 13g) associated with kaolinite, halloysite (Fig. 13h) and also diaspore and goethite, have been reported by Zalba (1982) and Zalba et al. (1988) and interpreted as formed by the introduction of meteoric fluids through sedimentary discontinuities of clay and heterolithic deposits of the Las Aguilas Formation (Zalba et al., 2007). Moreover, SEM images obtained by the authors in backscattering mode and microprobe analyses allowed the identification of disseminated crystals of Aluminum Phosphate Sulfate (APS) in a clay matrix (Fig. 13i) which fall in the compositional field of a solid solution between svanbergite (SrAl₃(PO₄,SO₄)(OH)₆) and Ce-florencite (CeAl₃(PO₄)₂(OH)₆), two APS minerals of the beudantite and the crandallite groups, respectively, according to Gaboreau et al. (2005). These SEM images indicate that APS minerals have preferentially crystallized in the more porous sediment.

Most of the time, it is not possible to determine the absolute time of formation of diagenetic minerals. In some cases, we can define the order of formation of the minerals present in the host sediments (paragenetic sequence). Observation of the phenomenon must be followed by interpretation, which is related to the knowledge of the consequences which brings to the fabric a determined process (e.g. compaction, authigenesis, etc.). In the case of the origin of APS minerals, associated with kaolinite, halloysite, diaspore and goethite in the Neoproterozoic Las Aguilas Formation, K-Ar data of alunite provided an age of 254±7 Ma (middle Permian) which sets boundary on the age of the diagenetic processes that led to the formation of all these paragenetic minerals (Zalba et al., 2007).

Recently, SEM observations made on the quarzarenites of the Balcarce Formation (Early Ordovician) sampled at Chillar locality (Fig. 1) proved that also these paragenetic minerals are present in porous spaces and cavities of these rocks (Fig. 13j).

All these telodiagenetic features described for different sedimentary Neoproterozoic and Ordovician units all along the Tandilia Basin show a striking coincidence in time with the deformation of the Ventania Basin, located 150 km SW of the study area. The time of formation of alunite is coherent with the Permian age proposed by Von Gosen and Buggisch (1989) and Varela et al. (1985) for the main deformation and folding stage of the Ventania System, relating it with the uplift, erosion and telodiagenetic stage that occurred in the Tandilia Basin (Zalba et al., 2007).

ACKNOWLEDGMENTS

The authors would like to thank the Comisión de Investigaciones Científicas Provincia de Buenos Aires and the Centro de Tecnología de Recursos Minerales y Cerámica for financial support. We are grateful to all our collaborators of previous pieces of research on the subject on which we based this review.

REFERENCES