Microscale Diagnostic Diagenetic Features in Neoproterozoic and Ordovician Units, Tandilia Basin, Argentina: A Review
Patricia E. Zalba,
Maria S. Conconi
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.
Received: July 31, 2010;
Accepted: August 19, 2010;
Published: October 11, 2010
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.
||Geologic map of the Tandilia Basin, Buenos Aires, Argentina
(Iniguez et al., 1989). Study localities in
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.
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 19th 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
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
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:
Kaolinite + Quartz Pyrophyllite + Water
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).
||Cementation (a) Dolomicrite with domal structures due to microbial
activity (parallel nicols). (b) Wellpreserved stromatolitic dolostones.
Alizarin red differentially tinted dolomite and calcite (red) showing calcite
only in fractures. La Siempre Verde quarry (parallel nicols). (c) Completely
silicified stromatolitic dolostones where silica pseudomorphs after rhombohedral
crystals of dolomite are clearly seen (parallel nicols). (d) The same sample,
La Placeres quarry (crossed nicols). (e) Quartz crystal (arrow) grown in
cavities of dissolved dolomites with trydimite crystals enclosed (crossed
nicols). (f) Detail of the previous photograph. Trd: trydimite (crossed
nicols). (g) Quartz crystal grown in cavities of stromatolite dolostones.
See white dots which represent silicified microbial colonies. La Siempre
Verde (parallel nicols). (h) Same sample as in (g) observed under cathodoluminiscence.
Quartz (brown), silica (orange). (i) Same sample observed under cathodoluminiscence.
Quartz crystal grown in cavities of stromatolite dolostones which include
dolomite rhombohedra (in brown). Villa Mónica Formation, Sierra La
||Hydrothermal and detrital pyrophyllite (a) Masses of hydrothermally
formed pyrophyllite (Py) also replacing opaque minerals, saprock, San Manuel
(crossed nicols). (b) Hydrothermaly formed pyrophyllite (Py) dissolving
quartz (Qtz), associated with kaolinite ghosts (K), saprock, basement of
Cerro de La Cruz, Sierra La Juanita (crossed nicols). (c) Pyrophyllite intraclast
(center of the photograph) in clay deposits, Villa Mónica Formation,
San Manuel (parallel nicols). (d) SEM. Detrital pyrophyllite in clay deposits
of the Las Aguilas Formation, Cuchilla de Las Aguilas, Barker. Note characteristic
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).
||Ooid grainstone deposits. (a) Oolites with quartz nucleus
(arrow). Slight deformation by compaction (parallel nicols). (b) Same photograph
with crossed nicols. Quartz nucleus (arrow). Detrital quartz (center of
the photograph) with dissolved syntaxial overgrowths in contact with an
oolite and with signals of compaction. (c) Diagenetic pyrophyllite (Py)
growing at expense of detrital quartz (Qtz). Note dissolution of syntaxial
quartz overgrowths (crossed nicols). (d) Pyrophyllite (Py) growing as diagenetic
masses and also as nucleus of oolites (arrows) (parallel nicols). (e) Mammelar
and agglutinated diagenetic pyrophyllite in the same sample (crossed nicols).
(f) Two generations of silica cement in contact. Diagenetic pyrophyllite
grows only in the micro silica cement. Las Aguilas Formation, Cuchilla de
Las Aguilas, Barker (crossed nicols)
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,
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.
||MISS in Carbonate/Siliciclastic facies. (a) Dark, hematized,
wavy crinkly microbial mat laminae (MM) alternating with light parts containing
illitized dolomite with ghost rhombohedral crystals (RC) (parallel nicols).
(b) Part A) Basal, grain-supported silt to sand-sized sediments with graded
structure and minor interlayered microbial mats. Some grains are oriented
with their long axis parallel to bedding plane. Part B) Dark, wavy, crinkly
microbial laminae with detrital grains floating within and all oriented
with their long axis parallel to the bedding plane (crossed nicols). (c)
Dark, ferric, crinkly microbial mat laminae with isolated detrital grains
within the mats oriented parallel to bedding plane (crossed nicols). (d)
Tidal deposits. Quartz grains float in an illitic epimatrix and alternate
with microbial mats (dark part). Detrital quartz grains oriented with their
long axis parallel to bedding planes (parallel nicols). Villa Mónica
Formation, Don Camilo quarry, Sierra La Juanita, Barker
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,
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.
||MISS in Carbonate/Silicicalsic facies (a) Dark, hematized,
crinckly lamination as relics of microbial activity (parallel nicols). (b)
MISS where domes do not superpose upwards, separated by the renewal of continental
input between microbial cycles (parallel nicols). (c) Microbial mats intercalated
in siliciclastic facies. Olavarría Formation, Cruz Pavone quarry,
Sierras Bayas (crossed nicols). (d) Tidal deposits (alternation of clayey
and sandy sediments) with microbial mats (dark, hematized microbial activity
relics) alternating with clastic sediments (crossed nicols). Las Aguilas
Formation, Cuchilla de Las Aguilas, Barker
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).
||Relics of MISS (a) Relics of microbial mats (dark) completely
hematized with an intraclast (center of the photograph) containing ooids.
The mats also contain reworked material (clasts and ooids). (b) The same
sample. Microbial mat levels intercalated in ooid grainstone deposits, with
quartz grains and ooids trapped within. Las Aguilas Formation, Cuchilla
de Las Aguilas, Barker. (c) Dark, hematized microbial mat relics in mixed
facies (Carbonate/Siliciclastic). Villa Mónica Formation, La Placeres
quarry, Sierra La Juanita, Barker. (d) Fractured MISS where epimatrix has
infilled cracks in well-preserved dolostones, Villa Mónica Formation,
La Siempre Verde, Sierra La Juanita, Barker. (e) Note the similarity with
Fig. 8d. Isolated and aligned hematized relics of fractured
MISS in weathered stromatolitic dolostones completely replaced by illitic
clays. The alignment is interpreted as representing original bedding planes.
Villa Mónica Formation, San Manuel. All photographs taken with parallel
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
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,
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
||Compaction and cementation. (a) Quartz cement (red arrows)
growing before calcite cement with dissolution of detrital grains. Syntaxial
quartz overgrowth (black arrows). Quartzarenites, Cerro Largo Formation,
La Placeres quarry Sierra La Juanita, Barker (crossed nicols) and (b) Calcite
cement (Cal) filling secondary porosity in dissolved stromatolitic dolostones
and replacing syntaxial quartz overgrowths (red arrow). Villa Monica Formation,
La Siempre Verde, Sierra La Juanita, Barker (crossed nicols)
||Burial diagenesis: compaction and stylolitization. (a) Secondary
overgrowths (red arrows), triple junction (blue arrows) and sutural contacts
(green arrows) between quartz grains (crossed nicols). Quartzarenites, Cerro
Largo Formation, Sierra La Juanita, Barker. (b) The photograph represents
a further step on burial diagenesis where stylolitization developed in quartzarenites
of the Cerro Largo Formation can be seen in two dimensions (arrows). During
a telodiagenetic stage, the force of the introduction of clay matrix has
broken and pulled-apart the quartz grain (parallel nicols). (c) Other example
of stylolitization in the same sample (crossed nicols). (d) Ooid grainstone.
Stylolites (arrow) cutting silicified oolites (parallel nicols). (e) Stylolites
infilled with hematite cutting diagenetic pyrophyllite (arrow) in silicified
ooid grainstones in the same sample (crossed nicols). Las Aguilas Formation,
Cuchilla de Las Aguilas, Barker
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).
||Neomorphism during mesodiagenesis (a) Neoformation of rutile
needles developed in illite-smectite epimatrix replacing stromatolitic dolostones.
La Siempre Verde quarry, Villa Mónica Formation, Sierra La Juanita,
Barker (crossed nicols). (b) Perpendicular coatings around detrital, partially
altered plagioclase. Same rock shown in previous figure (crossed nicols)
||Neomorphism during telodiagenesis. (a) Tangential clay coatings
(arrows) of illitic composition infiltrated (epimatrix) around quartz grains
in quartzarenites, Cerro Largo Formation, Sierra La Juanita, Barker (crossed
nicols). (b) Epitaxial calcite rims on rhombohedral dolomite crystal (arrow).
Sample stained with Alizarin-red (parallel nicols). Villa Mónica
Formation, La Siempre Verde, Sierra La Juanita, Barker
||Neomorphism during telodiagenesis. (a) Slickensides with ferriargilans:
cutans (crossed nicols). (b) Kaolinite cutan (K) in fractures surrounded
by postdating goethite (crossed nicols). Villa Mónica Formation,
La Placeres quarry, Sierra La Juanita, Barker. (c) SEM. Hexagonal, platy
kaolinite transforming to halloysite. (d) SEM. Hexagonal, platy kidney-shaped
kaolinite aggregates growing in fractures. (e) and (f): SEM. Goethite aggregates
growing in fractures and pore space. Villa Monica Formation, San Manuel.
(g) SEM. Pseudocubes of alunite. (h) SEM. Kaolinite, halloysite and alunite.
(I) Backscattered electron image of APS minerals svanbergite-Ce-florencite
(S-Ce-F). (i) High-porosity zones with alunite, halloysite, diaspore and
svanbergite-Ce-florencite. (II) Low-porosity zones rich in pyrophyllite,
kaolinite, micas, svanbergite-Ce-florencite and heavy metals. Photographs
g, h and i correspond to fractures and sedimentary discontinuities. Las
Aguilas Formation, Cuchilla de Las Aguilas, Barker. (j) SEM. Fibrous illite-smectite,
alunite and kaolinite growing in pore space of quartzarenites of the Balcarce
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).
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.
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