Deciphering the Importance of Mineralogical Changes in the Neoproterozoic Epeiric Seas through the Sedimentary Succession of Tandilia System: A Brief Review

Abstract

The Neoproterozoic (>1160 to ~540 Ma) sedimentary record of the Tandilia System is reorganized into eight depositional sequences based on a detailed review of published sources and new lithological observations. The main compositional attributes compiled from the studied units were used to indicate changes in lithology regarding their origin. Epiclastic sections reveal supply and sources changes through the succession. Basement detritus was dominant during the deposition of the basal sequences turning drastically to a volcanic affinity dominance. The carbonate sections, dominated by intra-basinal components, were deposited in periods of rare or restricted detrital input. The older, described as a cap-dolostone, was related to bio-induced dolomite precipitation under a deglacial to interglacial context. The younger, a carbonate ramp, reveals to have been built by microbial activity adding high levels of oxygen to seawater correlated to a global oxygenation event. Compositional changes recorded in the shallow marine deposits of Tandilia could have been intricately linked to periods of tectonic and paleo-relief configurations, favoring the detrital supply into the basin, followed by relevant episodic biogeochemical changes. This study shows that the basinal-components progression was controlled by paleoclimate and paleoenvironments associated to the extensive interval between the rupture of the Rodinia to Gondwana paleogeographical framework.

1. Introduction

The Neoproterozoic Earth’s biosphere attested a radical transformation of life forms especially at the end of this period. This interval also represents a dynamic spatial–temporal interval dominated by severe climatic changes, influenced locally by tectonic activity that commonly have had an impact in the geochemistry and mineralogy of associated sedimentary rocks that depend on the contemporary sea-water conditions added to weathering and erosion of land-source areas [1]. Mineralogical variations in sedimentary rocks were widely used to evaluate Phanerozoic records, however little is known about these aspects in Neoproterozoic units in general, and particularly in Tandilia System [1,2].

The Cryogenian–Ediacaran sedimentary cover of the Tandilia System (TS) in Argentina unconformably overlies the Paleoproterozoic Río de la Plata Craton (RPC) crystalline basement whose degree of preservation is remarkable compared to other contemporary units [2,3,4,5]. This Neoproterozoic sedimentary succession has been established as a unique example for the clarification of the control factors related to its sedimentation, biostratigraphic content, and geochemical imprint. In addition, given its high degree of primary lithological preservation [3], these units stand out from most of the contemporary units of other regions of the RPC also deposited in the transition between the Rodinia break-up to the Gondwana configuration [3], being considered an essential piece in the cratonic assemblage of the SW Gondwana [6,7,8,9,10,11].

Considering the foregoing δ¹³C data, the negative excursion throughout the upper succession may be supported by their consistent δ¹³C curves [3]. In addition, these curves can be correlated to the global Shuram excursion recorded in coeval successions [12].

The accumulation of these sedimentary units occurred during an interval of the Earth’s history with the most drastic changes linked to redox seawater conditions, relatively rapid tectonic plate movements, and exceptionally massive perturbations in global biogeochemistry happening [13]. Although little is known, the greatest of these disturbances are closely related to geochemical and mineralogical changes, largely associated with the influence of intermittent glacial and greenhouse periods [14].

Regarding the abundance and type of siliciclastic minerals, they are mostly related to changes in source rocks and hydrodynamic sorting during the transport of sediments [15]. In addition, the influence of source rock composition, tectonic activity, variations during sediment transport, and post-depositional changes must be also considered for proper detailed compositional analysis [1] and cited there in.

On the other hand, the mineralogical changes in marine carbonate sequences are particularly related to the seawater geochemical conditions influenced by climate and regulated by a restricted detrital supply [13]. The case of Neoproterozoic carbonate units also shows a relationship between glacial and non-glacial dependance used to interpret cap-carbonates [14].

Largely, the studies of Neoproterozoic units show results without considering the huge mineralogical changes that occurred in that time interval. However, the close relationship of these changes with the environment is the subject of this work. In this work we present a compilation of previous compositional data added to new mineralogical results obtained to complete the information to discuss the relationships between the intrinsic mineralogical components present in each sequence and the controls on their genesis.

With the aim of discussing the intrinsic and extrinsic controls on the mineralogical composition at the time of sediments’ deposition, we integrate previous results with detailed mineralogical information [16,17,18]. It is important to highlight that detailed diagenetic studies were carried out in previous works [16,17,18], so those results will be considered to avoid interference with the results discussed here.

In this sense, the controls in mineral components recorded along all the TS Neoproterozoic units can be linked to paleoclimate, source areas of detritus, paleoenvironments, and seawater oxygenation levels that occurred at this austral South America basin.

2. Geological Framework

The Tandilia System or Northern Buenos Aires Hills is a mountain range (~350 km length and up to ~60 km width, Figure 1A) located in the central-eastern region of the Buenos Aires Province. It has an NW-SE orientation that includes the southernmost sedimentary outcrops of the RPC (Figure 1A [7,8,19,20]).

The TS records an igneous-metamorphic basement, Buenos Aires Complex, composed of tonalitic to granitic gneiss, amphibolite, migmatite, granitic plutons, and scarce schist and marble [8,9,10,11,20]. U-Pb zircon ages related to this complex are of 2.23–2.07 Ga, corresponding to the Paleoproterozoic [21,22]. Moreover, this basement was intruded by calc-alkaline dikes of 2.02 Ga and tholeiitic dikes of 1.58 Ga [22].

In the study area the crystalline basement is covered with a mixed sedimentary succession, both intercalated carbonate and siliciclastic platforms (Figure 1B,C), and with two phosphate-rich levels, compose the entire Neoproterozoic succession (Villa Mónica Formation, Sierras Bayas and La Providencia Groups [4,23,24]). The Sierras Bayas Group is integrated from base to top by Colombo [25], Cerro Largo [23,24], Olavarría [26,27], and Loma Negra formations [28], while La Providencia Group is made up of the Avellaneda, Alicia, and Cerro Negro Formations [4,29]. Most of these units were assigned to depositional sequences by [27] and named Tofolletti (Villa Mónica Fm), Malegni (Cerro Largo Fm), Diamante (Olavarría Fm), Villa Fortabat (Loma Negra Fm), and La Providencia (Cerro Negro Fm).

3. Methodology

The supporting data are based mostly on observations obtained from field and subsoil (drill cores) records in the study area. The sedimentary sections were described from outcrops (quarries) and drill cores from the NE and Central Sierras Bayas Hills (Figure 1B). The integrated succession from field data was collected from Villa Mónica, Piedra Amarilla, Abra Tres Lomas, El Polvorín, and CASA quarries (Figure 1B). Other sections were described in the Central Sierras Bayas Hills, near Olavarría city (La Cabañita drill core—that crossed from the upper Cerro Negro to the crystalline basement—with a total record of 270 mbs; Figure 2). Sections (~250 m thick and 240 m, respectively Figure 2, Figure 3 and Figure 4) were analyzed encompassing the lithologies, mineralogical composition, and depositional indicators (see Section 5).

Mineralogical composition and crystal morphologies of selected components were determined under a scanning electron microscope (FEI Quanta 200 SEM, Hillsboro, OR, USA). For these analyses rock-chips were pretreated—air-dried and coated with Au. The mineral geochemistry was acquired using EDS with a Thermo Scientific Ultra Dry Silicon Drift Detector (Madison, WI, USA). The data processing was performed with a NORAN System 7 X-ray Microanalysis System (accelerating voltage 20 kW, spot size from 20 to 50, marker 5–20 μm).

Thin sections of samples of different lithologies were observed and analyzed with a Nikon Eclipse E-200 (Tokyo, Japan) and Zeiss Axiolab 5 polarized microscopes (Hamburg, Germany) in the Centro de Investigaciones Geológicas, La Plata (CIG-CONICET-UNLP, Argentina). Carbonate lithologies were stained with red S alizarin to discriminate between and within carbonate minerals (calcite, dolomite, aragonite, ankerite, and siderite) [30].

Whole-rock and Clay Mineral of samples were determined by XRD on a PANalytical X’Pert PRO diffractometer (CIG) (Worcestershire, UK), using Cu radiation (Kα = 1.5 Å) and a Ni filter, and generation settings of 40 kV and 40 mA. Routine air-dried mounts were run at a scan speed of 2° 2θ/min, from 2 to 32° 2θ. For Clay Mineral patterns three types of samples were analyzed: natural without saturation pre-treatment, glycolated or treated with ethylene glycol-solvated, and heated to 550 °C, and they were run from 2 to 32° 2θ, 2 to 27 °2θ and 3–15° 2θ, respectively. Sample preparation follows the methodology used for [31], and minerals semi-quantification was determined using the intensity of the main peak for each mineral [32,33].

4. Results

From the TSE41 drill-core obtained in La Cabañita Quarry (TSE41 core of 270 mbs; Figure 4), we describe sections that complete the 240 m of core rock and represent a sedimentary section. From this section we obtain samples analyzed encompassing the lithologies, mineralogical composition, and depositional structures added to main sedimentary features (Figure 4). In addition, we indicate the units (sequences) recognized throughout the description in detail of core-rocks.

On the other hand, regarding compositional results obtained from the whole rock and clay-fraction XRD analysis of each sedimentary unit, we estimated the mineral abundances to indicate the ranges summarized in

Table 1. Minerals identified in each sequence expressed in ranges of abundance from the X-ray diffraction results. References: Qz: quartz; FK: alkali feldspar; Pg: plagioclase; Gt: goethite; Hm: hematite; Ca: calcite; D: dolomite; Fap: fluorapatite; I + IS: illite plus illite–smectite mixed layered; C + CS: chlorite and chlorite–smectite mixed layered; K: kaolinite; Sm: smectite.

Unit	Sequence	Whole Rock (Relative Abundance %)									Clay Fraction (%)
		Siliciclastic				Iron Oxide		Carbonate		Phosphate
		Qz	FK	Pg	Clay	Gt	Hm	Ca	D	FAp	I+IS	C+CS	K	Sm
Villa Mónica	Subsequence Ia	26–99	tr–28	1–12	tr–90	nd	tr–8	tr–23	tr–5	nd	80–100	0–11	0–5	nd
	Subsequence Ib	tr–40	tr–2	tr	0–18	0–17	nd	tr–38	36–97	0–20	0–90	0–10	0–5	nd
Colombo	Sequence II	55–100	nd	nd	5–49	0–28	0–32	nd	nd	nd	50–100	nd	0–30	0–50
Cerro Largo	Sequence III	85–100	tr	tr	0–15	0–15	nd	tr	nd	nd	50–100	0–10	0–50	0–30
Olavarría	Sequence IV	30–80	tr	tr	15–45	tr	nd	nd	nd	nd	75–100	0–25	tr	nd
Loma Negra	Sequence V	0–30	nd	nd	0–3	nd	nd	75–100	nd	nd	0–50	0–50	tr	nd
Avellaneda	Sequence VI	35–75	tr–4	tr–7	2–15	nd	nd	35–80	nd	0–60	0–90	0–30	nd	nd
Alicia	Sequence VII	20–65	nd	5–14	25–53	nd	nd	nd	nd	nd	86–96	7–14	nd	nd
Cerro Negro	Sequence VIII	60–72	nd	8–25	12–22	nd	nd	tr	nd	nd	80–95	8–20	nd	nd
Phosphate levels		15–60	nd	nd	0–15	0–20		0–10	0–30	35-65	0-100	0–20	0–10	nd

. These results reflect the abundance variations in the main mineral components recorded in each sequence. In this sense, Subsequence Ia is composed of variable abundances of quartz, feldspar, clays, iron oxides, with minor participation of carbonates. Phosphate is concentrated in beds in the transition from Ia to Ib subsequences. Subsequence Ib is dominated by carbonates (mostly dolomite) and presents variable proportions of aluminosilicates (quartz and illite) and oxides. Sequence II is characterized by variable abundances of quartz, illitic clays, and iron oxides. Sequence III is particularly rich in quartz, and shows moderate to scarce clays, and iron oxides. Sequence IV shows variable proportions of quartz and clays that are associated mostly with changes in grain size. Sequence V is carbonate dominated, mostly composed of calcite. Sequence VI shows a mixed carbonate–siliciclastic composition. Sequences VII and VIII denote similar compositions added to similar abundances with a domain in quartz, plagioclase, and clays (illite and chlorite types) (Table 1).

4.1. Depositional Sedimentary Sequences

Eight sequences of sedimentary infill of the Tandilia Basin are described below.

4.1.1. Sequence I—Villa Mónica Formation (<1160 to >720? Ma)

This succession rests over the crystalline basement through an erosive surface, here named as Buenos Aires Surface, and is covered by the deposits of the Colombo Formation also through an erosive surface, called Piedra Amarilla Surface [2,3]. Sequence I was divided into two informal subsequences, a lower siliciclastic subsequence (Ia) and an upper dolomitic subsequence (Ib). Six lithofacies were recognized, three from the Ia subsequence and three from the Ib subsequence, separated by a surface of maximum regional flooding. Siliciclastic facies are from a shoreface to an offshore transition environment, while the dolomitic facies were deposited in a carbonate platform tidally influenced [6]. The dolostones are plenty of well-preserved stromatolite morphologies [27], interpreted as formed under the influence of post-Sturtian (Cryogenian) meltwaters based on δ¹³C isotopes, microtextures, and ⁸⁷Sr/⁸⁶Sr [6]. The Piedra Amarilla Surface was aged 595 Ma by paleomagnetism data [34].

Subsequence Ia. It is composed of ortho-conglomerate and coarse-grained sandstone, grading to fine-grained sandstone and mudstone. Sandstones of this section constitute the basal siliciclastic sector of the unit (22–28 mts). According to [24], its deposition began in some sectors with a basal, quartz–arkose fine-grained orthoconglomerate, and others directly with sandstones. Above these, there are arkose wackes, sub-arkose sandstones, and finally quartz sandstones, which reflect a progressive increase in the selection and evolution towards more stable littoral conditions. This section culminates with shaly facies with intercalations of thin sandstone levels and phosphate lenses [16,35]. For this section, a maximum thickness of 28 m was recorded in TSE-41 (Figure 3 and Figure 4), which allowed its identification up to the lower contact with the basement. It should be noted that this unit was very poorly exposed in outcrops. Thus, with the subsoil data, it was possible to recognize new compositional elements. Figure 5A–C shows the appearance of some of the facies of the basal sector of the unit both in outcrop and drill cores. Within this section, the laminated pebble mudstone facies is characterized by a texture with two well-marked granulometric modes, one mud and the other pebble, showing mudstone sheets slightly arched as if they had been impacted [6,16]. This same level was identified in the subsoil, where it occurs interbedded with wackes and mudstones recognized in drill cores from the Central hills. The textural bimodality of the rock, as well as the deformational microstructures left by the pebbles in the underlying sheets, allow us to suggest that they could constitute dropstones [36,37]. Thus, this level would be linked to a glacimarine origin [6,16]. In the shale and wacke lithofacies with planar lamination, the presence of phosphatic lenses and concretions (mainly of fluorapatite composition) were linked to an origin from upwelling currents [35] and appear associated with iron concretions, mainly of a goethite composition.

Subsequence Ib. This part of the sequence is characterized by a rich stromatolite dolostone succession (37–42 m thick; Figure 5D–F) [6,16,23]. Its variable thickness was first attributed to the channels developed on the top of the unit [38]. The dolomite rock is formed by stromatolite and non-stromatolite laminated dolostone beds, interbedded with thin claystone beds and ending with marl-mudstone (Figure 5E).

The dolostones of the Subsequence Ib section show a very conspicuous stromatolite assemblage [23,38]. Moreover, analogous morphologies of stromatolites were described in other well-known Tonian–Cryogenian successions [39,40,41,42,43]. The Cryogenian tubestone-dominated dolomites were found and described in VMF, assumed since previous work as typical post-Sturtian morphologies (∼720 and 635 Ma), and that of [16].

The Subsequence Ib is characterized by the combination of lithofacies and stromatolite morphologies, which represent a reef in a tidal carbonate–stromatolitic platform formed under regressive conditions [6]. The concentration of phosphorus (P) at the base of the Subsequence Ib section was considered essential for the cyanobacteria conquer forming metric to 10s of kilometer biostrome and bioherm builds in this carbonate platform [6,33]. This phosphate level was related to upwelling currents in response to cold and anoxic seawater conditions regarding C isotopes and Ce anomalies [33].

4.1.2. Sequence II—Colombo Formation (<720 Ma >600 Ma)

The VMF is bounded at its top by a regional paleokarst-erosional unconformity (Piedra Amarilla Surface, Figure 5E) that could represent up to 100–120 Ma of erosion/non-deposition interval [3,7]. Covering the VMF and underlying the Cerro Largo Fm is a diamictite interval composed of yellowish and whitish mudstones matrix, quartz–illite rich, with lenses of chert breccia and diamictite breccia, quartz sandstone blocks, in amalgamated and deformed lithosomes [25,27].

The Colombo Formation is a 3–13 m thick succession (Figure 5G) that lies discordantly over the VMF. This unit is constituted at its base by red shale and chert breccia followed by matrix supported paraconglomerate with blocks of up to 3 m of diameter interpreted as a diamictite formed by tectonic influence in a continental setting [2,3]. In the basal iron-rich shale levels a paleomagnetic age of ∼600 Ma was suggested for the Sequence II (CF; Figure 5G) [34].

Sequence II is represented by three main lithofacies: massive chert breccia, laminated claystones with iron rich concretional levels, and diamictite matrix supported with blocks of sandstones and dolostones. This unit presents an irregular base filling the paleokarst relief (Figure 5G). The clay mineral association shows a high content of plastic clays (smectite), with variable contents of hematite (Table 1). This association could be attributed to a meteoric and hydrothermal alteration of the original clays showing an irregular distribution, without relationship with the paleotopography [2]. In the Piedra Amarilla Quarry, this claystone also appears, forming intraclasts floating in the matrix of the diamictite lithofacies.

The diamictite shows a variable thickness varying from near 0 to 13 m and is composed of up to 3 m mega-blocks in a mud-sandy matrix with chaotic-massive/folded internal structure. This rock is poorly consolidated and has a supporting matrix texture. The geometry of the diamictite is irregular and constitutes the filling of karst channels and depressions related to the domal stromatolite bioconstructions of the underlying dolomites (Figure 5G). The blocks are subspherical and discoidal, with varied sedimentary composition (dolostone, claystone, wackes, quartz-sandstone, and chert–breccia fragments). The compositional variation of the blocks causes them to show differential behaviors in the face of the effects of compaction. Thus, the sandstone clasts are friable, the massive quartzite clasts are consolidated, and the chert clasts constitute highly consolidated and resistant subspherical fragments. In contrast, the dolostone and claystone clasts appear weathered and ductile deformed.

4.1.3. Sequence III—Cerro Largo Formation (<600 Ma)

These sedimentary beds cover the Colombo diamictite (II) with contrasted bedding over the chaotic appearance of the underlayer unit, then pass transitionally upwards to Olavarría Formation [2]. In this unit (35–55 m), two facies associations are recognized: lower green heterolithic beds (sand and mud) rich in glauconite and quartz (monocrystalline) and upper well-sorted coarse to medium sandstones (quartz sandstone) [2] (Figure 2, Figure 3 and Figure 4). The presence of glauconitic mica in the lower association indicates a stable deep platform environment. Subsequently, the abundance of mud material gradually decreases upwards, going from a muddy–sandy zone to a well sorted sandy zone, passing upwards to a mixed stratification that goes from lentiform to wavy and flaser bedding (Figure 5H–J) [2,27]. The latter indicates a shallowing of the basin.

Quartz sandstones of the upper CLF present different sedimentary structures such as wavy lamination, trough- and planar-cross lamination, tangential cross bedding, trough-cross bedding, hummocky cross-stratification, and herringbone cross-stratification [2] (Figure 5I,J).

4.1.4. Sequence IV—Olavarría Formation (> 590 <600 Ma)

It is transitional with Sequence III and is covered by Sequence IV by means of an unconformity contact. This sequence is composed of a lower member consisting of yellowish clay and silts mixed stratification grading upwards from lenticular to wavy bedding; in sharp contact is an upper member composed of red laminated claystones.

The Olavarría Formation in the Central and Austral hills (Figure 6A–C) is made up of 35 m of yellowish mud/silt and clay in mixed bedding succession. The presence of interbedded iron rich levels, 8 and 20 cm thick, are composed of iron oxides showing concentric layers (Figure 6B). In the San Alfredo and Pura Cal quarries (Figure 6A), a sudden change in color that goes from greenish or dark gray to black and even violet, was classified as dark gray mudstone with planar lamination. This section is 2 and 3 m thick and composed of mudstones with planar and, less frequently, wavy lamination. Its upper contact is net, with a tabular-appearing geometry, although the lower contact is not observed either in outcrops or in quarries (Figure 6A).

4.1.5. Sequence V—Loma Negra Formation (~590–580 Ma)

Above Sequence IV, the Loma Negra Fm (Sequence V) is unconformably deposited. This Sequence is the last in the Sierras Bayas Group [27] (Figure 6C). In the top of the sequence an important karst-erosional unconformity was described and called “Barker Surface” [44,45,46] (Figure 6D) that separates it from the La Providencia Group. It is composed of an association of carbonate lithofacies, with a lower level of reddish carbonate wavy laminated mudstones (Figure 6E) and an upper level of black carbonate mudstones [2,5,27,46] (Figure 6F). The environment varies from an internal to external carbonate ramp in the context of the Clymene epeiric sea [17,47]. C and Sr isotopic data indicated an age of deposition of ~590–580 Ma [5,17].

Sequence V was defined as well-preserved limestone succession (~40 m) deposited under low energy conditions in a shallow carbonate ramp. The depositional conditions represented in this sequence were assumed as turning from a shallow-middle to an outer ramp environmental setting. The calcite microfabric is described as the precipitation of biologically induced micrite (micro-calcite), under a necessary very restricted terrigenous supply. In addition, microscopic descriptions made it possible to indicate the presence of microbially-induced sedimentary structures (MISS) through the whole formation.

4.1.6. Sequence VI—Avellaneda Formation (~570 Ma)

The Avellaneda Formation, the base of La Providencia Group, constitutes the first sedimentary sequence with variable thickness (~3 to ~29 m thick; Figure 7A–C), interpreted as accumulated in a shallow marine environment [3]. This sequence rests upon the regional erosive karst, Barker Surface, at the top of Sierras Bayas Group [45,48] (Figure 6D), marking the return of mixed carbonate/siliciclastic deposition in the basin. The basal section is composed of laminated and massive marls (up to 18 m thick) that are in sharp contact with an upper section of massive red mudstone (up to 9 m thick). The marly section is composed of quartz, calcite, and illite-type clay minerals (Table 1). The red illitic massive claystones at the top present longitudinal scour marks and a few mudcracks, interpreted as deposited under supratidal conditions [4]. Recently, paleomagnetic data from the unit indicated deposition at moderately higher latitudes [47,49] (between 42° and 50° S). The paleomagnetic and C-isotope curve mark a negative anomaly, age-constrained by geobiological and paleomagnetic results, positioning the AVF as deposited near the Shuram excursion at ~570 Ma [50]. Ref. [3,50] These authors analyzed the isolated carbonate phase (primary micrite) for stable C and O isotopes from different sections near La Cabañita Quarry. Considering textural and mineralogical micrite preservation and primary geochemical signal, the isotopic data may reflect seawater conditions. δ¹³C curves show a negative anomaly, age-constrained by geobiological and paleomagnetic results as the incomplete expression of the Shuram excursion according to that suggested by [12]. Hence, the record of this negative anomaly is remarked in the studied sections allowing a very tight correlation.

4.1.7. Sequence VII—Alicia Formation (> 560 < 570 Ma)

The 0–205 m thick of the Sequence VII (Figure 7A,D,E) conformably overlies the Sequence VI, with a paraconformable contact, and is constituted by dark shale, massive gray siltstone, and gray heterolithic (silt and mud) beds with lenticular and wavy bedding, (Figure 7E) deposited in suboxic to anoxic offshore/offshore transition settings. The first mention of this unit was made by [4,51], describing a sedimentary package of dark fissile gray shales of more than 150 m thick, only recorded in subsoil (core drill). This succession is notably of finer grain size compared to upper succession and is composed of abundant fine quartz, abundant to moderate clay minerals, and scarce plagioclases. The fine fraction of these rocks is dominated by similar contents of illite and chlorite (Table 1).

4.1.8. Sequence VIII (ex-La Providencia sequence)—Cerro Negro Formation (~560–530 Ma)

Sequence VIII (Cerro Negro Formation) [52] unconformably rests upon Sequence VII and is composed of fine-grained terrigenous sediments forming centimeter-to-decimeter tabular and lenticular beds. These beds are arranged as recurring intercalation of massive and trough cross-lamination fine-grained and clast-supported sandstones, massive mudstones, and heterolithic beds with common contribution of tractive events associated with longitudinal scours, mud pauses, and flute marks (Figure 7A,F,G). The sporadic presence of mud cracks and other subaerial structures (such as raindrop marks) are considered to indicate a deposition in an -inter to sub-tidal setting [4]. The high occurrence of Aspidella plexus as well-preserved discoidal structures was reported in Sequence VIII [53] (Figure 7F´). Based on its correlation with the Ediacaran White Sea assemblage from Australia and Russia Ediacaran successions, the occurrence Aspidella plexus allowed the suggestion of a depositional age around 560–550 Ma for this unit [54,55]. This Ediacaran age is also consistent [56] with the acritarch assemblage of Synsphaeridium sp., Trachysphaeridium sp., and Leiosphaeridia sp. recorded in this sequence [57,58].

The main components of this succession are abundant quartz and plagioclases, and scarce clay minerals composed mainly of illite and in less proportion chlorite.

The exceptional occurrence of Aspidella appears in close association with microbial mats. The high preservation of the discs (Figure 7F) and Intrites-like structures are tied to biotic and abiotic processes related to microbial mats.

The main information about the described sequences is summarized in the following chart (Figure 8).

4.2. Mineralogical Attributes

The main mineralogical attributes present in the Neoproterozoic sedimentary successions of Tandilia Basin are shown below in

Table 2. This shows the main components of study units considering types and origin.

Formation/ Section	Dominant Lithologies	Dominant Component Type	Origin
Lower Villa Mónica	Sandstone, conglomerate, wake	Epiclastic (RF, quartz, feldspars, micas, accessories)	Detrital
Upper Villa Mónica	Dolostone, marls	Intrabasinal (dolomite)	Bioinduced dolomite (MISS)/detrital
Phosphate level VM	Phosphate concretions	Intrabasinal (francolite: upwelling currents)	Apatite bio-volcanic-induced
Colombo	Chert breccias, residual clays, diamictites	Epiclastic (rock fragments), silicification	Detrital/authigenic
Cerro Largo	Heterolithic beds, coarse sandstones (gs)	Epiclastic (quartz, clays) and intrabasinal (glauconite)	Detrital Marine
Loma Negra	limestone	intrabasinal	Bioinduced calcite (MISS)
Avellaneda	Marl (shale)	Mixed epiclastic and intrabasinal d(calcite, quartz, clays, feldspars)	Detrital supply/bioinduced calcite (MISS)
Alicia	Siltstone, claystone, mudstone	Epiclastic (quartz, micas, clays, feldspars)	Detrital
Cerro Negro	Fine sandstone, Siltstone, claystone, mudstone	Epiclastic (quartz, micas, clays, feldspars)	Detrital

and used to indicate the main changes in lithology and dominance of components regarding their origin (Table 2).

4.2.1. Detrital Components

The compositional aspects of the epiclastic/siliciclastic lithologies represented in the sedimentary units of the Neoproterozoic successions of Tandilia Basin are described in detail unit by unit.

Subsequence Ia. This siliciclastic section comprises a textural and compositional wide range that includes fine to very coarse sandstones, interbedded with some fine-grained conglomerate levels that are of a few centimeters thickness (Figure 5B,C). Massive conglomerate levels, represented at the basal most section, are clast-supported, moderately selected, with angular to subrounded shapes. Conglomerate and coarse sandstone are mostly composed of polycrystalline quartz and alkali feldspars (Figure 9A), accompanied by scarce weathered amphibole and biotite, added to chlorite and calcite as secondary products. Whole-rock XRD composition of Subsequence Ia (50 samples) shows contents of minerals that in average are quartz (59%), K-feldspar (16%), clay minerals (14%), and plagioclase (11%). Clay-fraction (<2 µm), in order of abundance, is composed of illite, chlorite, and mixed-layer illite–smectite (IS) (Table 1). These clays are mostly the product of altered feldspars, rock fragments, and other labile components (Figure 10).

The sandstone levels of the middle section (Figure 5B,C) are generally poor to very poorly sorted, although some layers are moderately sorted. Detrital components are composed of moderate to very abundant mono- and polycrystalline quartz, moderate to abundant feldspar (both potassium and plagioclase), scarce rock and chert fragments, frequent mica clasts (both muscovite and biotite), and scarce altered amphiboles and pyroxenes that appear associated with the basal levels of Ia. The matrix content in arenites is usually very low to moderate and generally does not exceed 10%. The protomatrix is partially replaced by orthomatrix and is mainly composed of detrital clay minerals like illite, muscovite, and rare detrital kaolinite. The illite fragments have less than 30 µm in size and muscovite grains appear with larger lamellar shapes (up to 1000 µm) that are frequently deformed (with curved cleavage and fractures). The presence of biotite is scarce, which is partially altered and chloritized. The orthomatrix is characterized by a predominance of recrystallized illite that reaches 25 µm, where intergrowth of chert and small assimilated quartz grains are common in coarse fractions.

Some sandstone levels are well sorted with dominance of rounded to subangular grains of quartz. Cross laminated fine to medium grained sandstone shows intercalation of fine-grained sandy and silty grains composed of quartz and illite. These microfacies show internal centimeter scale gradations into the beds.

Whole-rock XRD composition of the middle section is characterized by high quartz content (70% in average), moderate clay minerals (20% on average), and scarce feldspars (K-feldspar: 5%, and plagioclase: 5% on average) (Figure 10A). Clay-fraction is dominated by illite and IS (Table 1).

In wacke and mudstone levels the matrix (mud phase) is moderate to abundant (15%–70%) and essentially composed of illite. These clays appear completely recrystallized with the development of authigenic illite with intergrowth of chert, or with good preservation of the protomatrix (which is of lower granulometry). Likewise, localized deformation of the matrix in the lower part of the grains and, in some cases, formation of pseudo-stylolites was recognized. Epimatrix is identified sporadically and randomly in response to brittle deformation of the quartz clasts.

Thin sections of mudstone and claystone reveal planar lamination, both of which are composed of illite and scarce quartz grains. At the diamictite level, curved lamination characterizes the pebble bases as response to a syn-depositional deformation. The whole-rock XRD composition of the uppermost levels indicates variable proportions of quartz (64%), clay mineral (30%), feldspar (K-feldspar: 2%, and plagioclase: 2%), and iron oxide (hematite: 2%). Clay fraction shows a predominance of illite and IS mixed-layer clay for this section.

Sequence II. As mentioned before, Sequence II is formed by massive chert–breccia, claystone with iron concretions, and diamictite–clayey matrix with floating blocks of older sedimentary levels. Sequence II shows variable thickness and in response to it fills the karst paleo-relief (Figure 5G). Whole-rock XRD composition shows variable contents of quartz, clays, and iron oxides (Table 1). In the fine fraction the most common clays are illite and illite–smectite mixed layer (Table 1). However, the high smectite (20%–50%) and hematite (10%–20%) content are concentrated in weathered located levels.

Sequence III. The sandstones of the unit are essentially quartzose (Figure 9B,C) and present scarce to absence of pseudomatrix content (deformed glauconite grains), which occurs only in the lower part of the unit. Some sandstone levels present a scarce pseudomatrix (less than 10%), while in the upper section, it is absent.

The detrital components are monocrystalline quartz and less common polycrystalline quartz. Occasionally, chert fragments and argillitic intraclasts (glauconite grains) have been observed. The sandstones of the middle section are moderate to well-sorted, distinguishing fine- to medium-grain sectors (150 and 400 µm) and others are coarse to very coarse (500 µm and 2 mm). In the upper section, under a microscope the clasts are almost exclusively rounded to subrounded, composed of monocrystalline quartz, well sorted (200 µm to 600 µm in size).

The compositional analysis of the sandstones reveals different microfacies, which correspond to the middle and upper portions of the sequence, respectively. In the lower section sandstones are quartzose to sublithic, in agreement with the average composition suggested previously of [2]. There is a notable predominance of monocrystalline quartz over the other components. Among lithic grains, polycrystalline quartz can be identified along with preserved and deformed round glauconite grains that form a pseudomatrix (Figure 9C).

The quartzose sandstones of the middle section are characterized by their scarce matrix (<10%) composed of illite (average size of 5 µm) and sheets of muscovite (up to 10 µm), both of detrital origin (protomatrix). Recrystallized illite and interstitial chert (orthomatrix) are also described. Occasionally, a pseudomatrix composed of very angular fractured quartz grains with variable sizes between 30 and 70 µm is recognized.

The upper section is made up of quartzose sandstones (Table 1, Figure 9C) composed almost entirely of monocrystalline quartz, and eventually, a very few grains of polycrystalline quartz, chert, or muddy clasts can be observed. In this microfacies the matrix is absent to very scarce (<1%) composed of clay minerals with recrystallization (orthomatrix) and intergrowth with chert and iron oxides.

As shown in Table 1, quartz is the main component in the whole rock, whereas illite constitutes the most abundant component (50%–90%) in the clay fraction of this unit. Kaolinite appears as a product forming the inner glauconite grains. This clay is relatively abundant in some levels (30%–50%) of the basal part of the formation where levels rich in greenish sandstones are concentrated (Figure 9G).

Sequence IV. In this formation, it has not been possible to define the modal composition because the granulometry is, in general, very fine, predominantly mud texture. However, various features could be identified under a microscope as well as the compositional characteristics obtained from fine fractions by X-ray diffraction. In the mudstone and sandy mudstone levels of this unit (Figure 9D; Table 1), the clay/silt fraction is very abundant (>60%) and differentiated as massive and clayey levels from other laminates with clays enveloping the sand clasts, recognizing two main types: one very fine (clayey) and massive and another laminated with arching of the clay minerals rounded the sand clasts. In the basal heterolithic facies, the yellowish mudstones (Figure 6B) are composed of very fine quartzose siltstones, with ferruginous cement and tangential crossed microlamination. The clasts are subangular monocrystalline quartz, with grain sizes that vary between 30 and 200 µm (fine silt to fine sand) with average around 50 µm (coarse silt). The clayey matrix is scarce to moderate (10%–20%) and illitic in composition, consisting of a partially recrystallized protomatrix to an orthomatrix.

Towards the top of the unit, a tabular level of around 8 m of reddish claystones (Figure 7A,C) is observed in sharp contact with the lower section, presenting detrital components of fine silt-sized quartz and predominantly illite clays. A level of intraformational conglomerate was identified with the presence of two types of clasts: clayey and chert intraclasts, both with sub-prismatic to subrounded or rounded shapes, oriented parallel to the stratification and to a lesser extent vertically. This section is poorly sorted and characterized by floating fabric.

Clay minerals such as illite and IS interlayers characterize this unit (Table 1). The illite in the fine fraction was associated with a diagenetic origin, while the illite–smectite interlayers have a high percentage of expansive layers (>70%) so the diagenesis that these levels would have reached would correspond to a moderate mesogenetic field [2].

Sequence VII. Fine- to very fine-grained muddy rocks of this unit are composed mostly of clay minerals (Figure 9E). Silty and sandy clasts are very scarce, mainly composed of quartz and biotite as the most common mica concentrated in laminar planes, in lower proportions opaque pyrites form micro-layers. Other cubic pyrite crystals appear disseminated in these shales. Cements are poorly represented and of siliceous or calcitic composition. Microscopic analysis shows dominance of detrital components composed of monocrystalline quartz, biotite, Na-plagioclase, illite and chlorite, pyrite, hematite, and very rare rock fragments. The dominant component in silts and sandstones is monocrystalline quartz that appears as subangular to subrounded with uniform extinction. The grain size of the coarser levels ranges from 250 to 150 μm showing parallel to undulous lamination (Figure 9E). Plagioclase is relatively abundant and forms subhedral and subangular grains with the typical polysynthetic twins of albite. The Michel Levy method indicated a composition from oligoclase to andesine. Biotite grains are brown and green, with perfect cleavage with angular and elongated shapes of 200 and 100 μm long. Rock fragments are of 200–100 μm with subrounded to rounded shapes and with undetermined composition. Regarding microstructures, petrography parallel laminations (Figure 9E) and convoluted laminations.

XRD results indicate abundant quartz, moderate to scarce plagioclase, moderate clays (with local variations in contents), very scarce to scarce pyrite, very scarce alkali feldspars, and sporadic calcite (Table 1). Illite is the primary clay mineral (80% on average) followed by chlorite (15% on average), very scarce illite–smectite mixed-layers (1%–5%), and sporadic chlorite–smectite mixed-layer clay (≤1%) (Table 1).

Under SEM, illite–smectite clays formed small flakes intergrown with illite, while irregular and finer curled flakes of chlorite–smectite mixed-layers appear intergrown with chlorite (Figure 9H). Illite–smectite clays formed equidimensional irregular lath associated muscovite deformed sheets (Figure 9H).

Sequence VIII. The detrital components identified in the fine sandstones of this unit are monocrystalline quartz, biotite, chlorite, few plagioclases, and to a lesser extent lithoclasts (Figure 9F). A tight and compact microfabric is observed, evidenced by concave–convex edges between grains. Monocrystalline quartz varying from subangular to subrounded is the predominant mineral in sandstones. The extinction of quartz crystals is generally uniform; however, some are wavy extinction related. Other grains present engulfment on their edges, although it is not ruled partial dissolution. Plagioclase is an important component in several sandstone beds; however, it is observed in contents commonly lower than 10% in the whole rock.

The dominant clay minerals are illite and chlorite, and micas are of both types of muscovite and biotite. Generally, the micaceous sheets are deformed and found between the lamination planes or less frequently forming the matrix. Lithic fragments are very scarce (<5%), and the grain size is fine to very fine sand. Rock fragments are rounded to subrounded, with dark colors between gray and brown. Inside the fragment a dense framework of plagioclase tablets was observed in a completely oxidized paste, probably of glass, constituting volcanic rock fragments. The opaque minerals do not exceed 5% of the rock, in general they are in some micro-laminae and not along all the profiles. Pyrites have cubic to pseudo-cubic shapes with defined edges, and to a lesser extent subrounded and frequently present an oxidation rim.

XRD results indicate similar composition to Sequence VII with abundant quartz, moderate to scarce plagioclase, scarce to moderate clays very scarce to scarce pyrite, and sporadic calcite (Table 1; Figure 10I). Illite as the main clay (80% on average) followed by chlorite (average 15%), very scarce illite–smectite mixed-layers, and scarce to moderate chlorite–smectite mixed-layer clay (Table 1).

SEM analysis allows to observe the detail of main lithoclastic components composed of illite-clays, micas, chlorite, and quartz of the rocks are observed. The phyllosilicates (most abundant) are organized in parallel laminae and show typical detrital morphologies (Figure 9I).

4.2.2. Bio-induced Components

Phosphate levels. Compositional analysis makes it possible to distinguish three phosphate levels in succession, two in the VMF and one in the CNF. These levels are composed of micro- and nanocrystals of francolite (amorphous fluorapatite) as the main mineralogical phase of the concretions, lenses, and reworked clasts [35]. Some sections show spherulitic microstructure (20–30 μm in diameter, Figure 10A) surrounded by chert cement and associated with irregular microfractures filled by hematite and goethite. Phosphate concretions show mono- and polycrystalline quartz grains (200 μm to 1 mm) floating in francolite cement [35]. Phosphate clasts are well rounded and compositionally are common at the base of Subsequence Ib where present dolomite cementation is associated with iron oxides.

Phosphate concretions were also described at the base of CNF which are composed of submicroscopic francolite (as the beds recorded in VMF), in this case associated with chert and residual clays. Very fine subhedral to anhedral fluorapatite crystals (<1 µm; Figure 11E) were observed from SEM-EDS. EDS analyses confirm a mixture of phosphate and carbonate characteristic of the francolite, associated with chert and other scarce silicates in very low abundance. Phosphate concretions show associated quartz with variable abundance (moderate to abundant), and traces of feldspar and clay minerals. Calcite cements are of variable abundance from absent to very abundant. From XRD analysis fluorapatite was the main mineral phase of these concretions (Figure 11C, Table 1). Clay fraction shows a preponderance of illite (70%−90%), scarce to absence of illite–smectite mixed layer (0%−10%), traces to scarce chlorite (tr−10%) and absent to scarce chlorite–smectite mixed layer (0%−15%).

Subsequence Ib. In this subsequence, dolomite is the dominant mineral (80%–95%) of dolostones accompanied by very low proportions of clays, siliceous cements, and iron oxides. Dolomite crystals show a range of microfabrics with different degrees of recrystallization, varying from preserved to destructive microcrystalline fabrics. These microfabrics were organized into six main types of dolomite mosaics (Types I–VI) and then used to indicate their sin-depositional to diagenetic evolution [3,16,59,60,61].

Whole-rock XRD compositional data of dolostones show a dominance of dolomite (74%), followed by quartz (16%), clay minerals (6%), and calcite (4%) on average. Fluorapatite was recognized in some samples from the base of the section, in proportions varying between 5% to 21% (Figure 10B). Clay-fraction (<2 µm) is characterized by similar abundances of illite and mixed-layer IS.

Petrographically stratiform stromatolites consist of an alternation of millimetric non-planar micritic, and light planar dolomicrosparite inner laminae. This microstructure is characteristic in the middle part of Subsequence Ib.

The microtexture of dolomitic marls of the upper Subsequence Ib shows a mixture of dolomicrosparite, iron oxides, and illite-type clays. In addition, ooids are observed completely replaced by chalcedony and chert [3]. Dolomite is the predominant mineral (66%), followed by quartz (13%), clay minerals (8%), calcite (10%) and goethite (3%). Illite and mixed layer IS occurred in similar proportions. Kaolinite occurs occasionally in concentrations <2% (Table 1).

Sequence V. The Loma Negra Formation was characterized by its very homogeneous composition made of fine-grained limestones with a high degree of preservation. Two peculiar facies associations were described in this sequence. The basal section, 8 m thick, is composed of reddish or greenish micritic mudstones and mostly composed of micritic calcite with 8% to 12% of detrital quartz, feldspars, and clay mineral grains. This shows wavy or ripple cross-lamination, hummocky crossbedding, and troughs. The upper section, of 25 to 33 m thickness, is composed of laminated black mudstones with high calcite contents (~90% in average), very scarce terrigenous quartz and feldspars (<5%), and very scarce organic matter content.

These limestones are mainly formed by low magnesium calcite microcrystals (4 to 15 μm; Figure 11D) accompanied by scarce sparite filling fractures and voids. Detrital silty grains of clays (chlorite and illite), angular quartz, subhedral feldspars, and sporadic zircon were also observed. The calcite microcrystals constitute mosaics with typical xenotopic texture. Near the Barker surface, the upper contact with Sequence VIII shows a widespread siliceous cementation as replacement of the carbonate phases. The fracturing of the rock and diagenesis favor the precipitation of cements (sparry calcite, microquartz) and authigenesis (pyrite, opal, iron oxides, clays) processes depending on the fluid circulation. Microscopic stylolite planes were differentiated commonly as the result of chemical compaction during the early burial. The high degree of depositional microbial activity was indicated based on the preservation of the organic matter and algal lamination [1,18].

Sequence VI. This succession can be divided into two sedimentary facies successions. The basal section is dominated by massive reddish and green laminated marls which impart a characteristic striped appearance. The upper section of this sequence is formed by massive to laminated very fine-grained siliciclastic red shales. The marly section is composed of intercalated microcalcite with terrigenous-organic (a mixture of quartz, feldspars with clays, oxides, and organic matter) laminae. These marly rocks show sporadic sparry-calcite veins (Figure 11C). Under SEM, microcrystals of calcite can be observed interbedded with detrital layers formed by detrital quartz, clays, organic matter, and iron oxides (Figure 11F).

XRD results indicate the dominance of calcite, accompanied by abundant to moderate quartz, and scarce to moderate clay minerals (illite, and mixed-layer illite–smectite, Table 1). Generally, calcite crystals are primary micrite (<4 μm in size, Figure 10C), with scarce recrystallization to microspar (Figure 10H). Marls and calcite rich mudstones can be both massive and with penetrative irregular horizontal lamination (Figure 11C,F). The marly laminated microfacies are characterized by their alternation of light and dark laminae mentioned before (Figure 11C). While dispersed detrital grains can be found in the basal section (mostly marls and mudstones), terrigenous deposits (rich in quartz, micaceous minerals, and minor feldspar grains) dominate the upper 6 to 8 m of the upper section [12].

5. Discussions

5.1. Detrital Components: Origin and Evolution

The epiclastic sections represented in the Tandilia Basin (Villa Mónica, Sierras Bayas, and La Providencia Groups) are highlighted because of its preservation character. In this sense, some interpretations about its sediment provenance and the presence of dubious glacial deposits are needed to be considered. The drastic change during the filling of the basin from modest depositional areas to a widespread basin related to an active continental margin throughout the Neoproterozoic was reported in [62,63]. The basal Subsequence Ia composed of immature detritus of alkaline composition towards the top shows the incorporation of best sorted and reworked material that reflects its local detritus source. Over Subsequence Ia, the cap-dolostone of Subsequence Ib was installed once the clastic supply was disrupted.

In previous geochronological studies [63], sandstones showed a dominant concentration of Paleoproterozoic detrital zircons, accompanied by other populations of Archean to lowermost Paleoproterozoic and Mesoproterozoic ages [4]. Archean detrital zircons of this succession indicate a source of detritus from the Nico Perez Terrane (Uruguay), which is closer to Tandilia up to the present [64]. In addition, the lack of Neoproterozoic detrital zircons confirms the deposition in a cratonic margin, probably in an epeiric sea deepening to the east and south [4].

The composition of the sandstones of Subsequence Ib vary from felspathic to quarzitic within a few meters, where quartz was more abundant towards the top of this member than feldspars. The proportion of monocrystalline quartz also increased relative to polycrystalline quartz. Polycrystalline quartz is an abundant component of these rocks that can be considered a stable component when it comes from the cratonic igneous basement rocks as is in this case [62,63,64,65,66]. Opaque minerals such as hematite were identified as accessory minerals, along with rutile and rounded and fractured detrital zircons.

5.1.1. Compositional Variations during Rodinia Breakup—Sequence I

The recorded compositional changes in Subsequence Ib can be due to variations in detrital supply (recycling, mineralogy, and source to sink distance), where the paleoenvironmental setting was controlled by the sea level changes, with depreciable diagenetic modifications [2,62].

XRD whole rock data indicate variations in the vertical mineralogical trends in Subsequence Ib section showing higher K-feldspar contents in its base, abundant quartz, and very scarce clays that were related to energetic currents in a swash/surf zone of an upper shoreface setting. Throughout the top, the succession becomes enriched in detrital quartz, deposited probably in a lower shoreface to offshore transition setting [3]. The clay fraction is dominated by detrital illite, accompanied by authigenic IS and chlorite [2]. This mineralogical composition was associated with a direct supply from the igneous–metamorphic basement [2,62], whereas the enrichment in detrital quartz towards the top was indicated as due to the incorporation of reworked and recycled detritus from the underlying beds. The increase detrital clay supply to the top of Subsequence Ia was related to the migration to more quiescent zones with mud decantation (lower shoreface to offshore transition zone). On the other hand, phosphate levels recorded in the transition from Subsequence Ia to Ib were related to upwelling carrying P rich fine-grained sediments from the deep and non-oxygenated shelf zone (cf. Ce anomaly data [35]).

In Subsequence Ib, the presence of grain rounding, and resistant minerals increased at the expense of labile minerals. Considering diagenetic, petrographic, and sedimentological data, the upper unconformity, Piedra Amarilla Surface, was assumed as evidence of a long dissolution and erosion time [2,3].

5.1.2. Compositional Variations during Gondwana Configuration (Sequences II, III, IV)

Overlying Sequence I, Sequence II (Colombo Formation) may represent a continental depositional environment strongly influenced by tectonics. The diamictite of this sequence (Figure 2 and Figure 4) is characterized by its mud–sandy matrix with chaotic-massive/folded internal structure containing blocks of rock fragments from the underlying levels (mostly from the Villa Mónica Formation, but other sedimentary rocks unknown in the area). This unit was tentatively thought of as a glacial deposit; however, its origin is still a matter of study. The compiled information allows the proposition that diamictite should have been formed by mass-transported sediments influenced by steep slopes related to tectonic deformation in a continental environment. Its accumulation probably occurs during a period of an important sea level drop probably ascribed as a postglacial isostatic rebound considering also the spatial–temporal interval represented by the basal discordance of Sequence II (~100–120 Ma, [2,3,19]). The varied composition of blocks in the diamictite results in differential responses under syn- and post-depositional effects. Sandstone blocks that are friable and commonly fractured were well-consolidated. Chert–breccia clasts show a high degree of cementation and are observed as resistant rock fragments. Dolomitic and clayey rock fragments were conspicuously weathered and plastically deformed (Figure 5G). Although still poorly known, these sediments were likely to have been mass transported and influenced by tectonic deformation in a continental environment during significant sea level drop resulting from postglacial isostatic rebound. In addition, its deformation could be related to the interval between the Rodinia breakup until the Gondwana configuration (Figure 12).

Overlaying the diamictites are the heterolytic beds and sandstones of Sequence III (Cerro Largo Formation), that are composed of reworked grains from Subsequence Ia, containing mainly felsic granitic and metamorphic rounded and well sorted grains with evident basement affinity. In addition, this unit contains variable amounts of glauconite (rich in Fe-kaolinite, Figure 8). The upper section of III, entirely formed by quartz-sandstones, shows a transitional increment to mudstones-rich and the heterolithic beds passing gradually to Sequence IV (Olavarría Formation).

5.1.3. Compositional Variations in SW-Gondwana (Sequences VII and VIII)

Sequence V (Loma Negra Formation) is discordantly overlayed by the Ediacaran successions of La Providencia Group. In the latter, detrital material analyzed from the upper section was related to a continental arc related to mafic and felsic sources. The lack of typical volcanic materials in the underlying beds could indicate that most of the La Providencia rocks were deposited associated with an active continental edge probably related to a retro-arc or retro-arc foreland basin context [62]. In addition, in Sequence VII subtle changes in the chemistry and mineralogy between the two facies successions were recently interpreted as a change from anoxic to suboxic seawater conditions [51]. The Cerro Negro Formation, Sequence VIII, is thought to be the first known stratigraphic record of the hidden passive margins from Gondwana, corresponding to an epiclastic shallow-marine ramp developed in the southern part of this supercontinent during the Ediacaran–Cambrian transition [64]. Geochronological data based on mineralogical, geochemical, and geochronology of detrital zircons, coupled with robust stratigraphic and sedimentologic background, support that this unit keeps more tectono-sedimentary affinity with Paleozoic sedimentary rocks of the Mar del Plata area (SE of Tandilia System) than the Neoproterozoic Sierras Bayas Group (NW Tandilia System, Figure 1). In addition, the absence of any metamorphism and deformation related to the Brazilian cycle and shallow marine deposition strongly suggest that the paleogeography and physiography of the original southernmost continental platform of Gondwana were similar in the latest Ediacaran and in the Paleozoic [64] (Figure 12).

5.2. Icehouse/Greenhouse Influence in the Composition

The Snowball Earth envisaged multiple synchronous global glaciations that subsisted for millions of years during the Neoproterozoic era leading to the frozen earth. Recent palaeomagnetic evidence [68] suggested that ice sheets may have reached the tropic resulting from runaway ice-albedo feedback due to the unusual large land masses prevalence in middle and low latitudes. The association of global successions with “warm” lithologies (carbonate platforms) was interpreted as a rapid transition between icehouse and greenhouse conditions. However, it was demonstrated that cap-dolostones are the common product of cold Cryogenian seas [3,16] and cited therein.

As shown by our study, the Snowball Earth hypothesis [69] cannot be entirely supported by the sedimentary Neoproterozoic record, which has wave- and tidal-influenced deposits that would not be possible in a fully ice-covered sea [70,71,72,73]. A critical examination of sedimentological evidence, stratigraphic data, and climatic simulation suggests that regardless of the severity of glaciations some seas essentially persisted ice-free which was effective for the evolution of early life forms as well as the continuation of the global cycle of carbon and hydrologic circulation [68].

In Subsequence Ib (Figure 1) a distal glaciomarine deposition was recorded with dominated fine-grained sediments and less common coarse-grained facies such as massive diamictite or pebbly sandstone. Laminated mudstone with sporadic dropstones-bearing siltstone/mudstone was interpreted as accumulated in a postglacial meltwater-induced distal glacimarine setting [3,73]. The basal orthoconglomerate of Subsequence Ib subsequence was related to high energy currents in a swash zone with clastic supply direct from the crystalline basement erosion [3]. These kind of sediments were commonly found in zones associated with severe cold climates such as those recorded in the Arctic [74,75,76]. In addition, the pebbly (probably dropstones) mudstone bed recognized in the basal Ib subsequence, was indicated as a glaciomarine origin [2,3,16]. The high roundness in conglomerates indicates a long time/distance of transport for gravels before their accumulation or they were part of pre-existing sedimentary rocks [74].

5.3. Phosphorus Strategy in MISS-Forming Cyanobacteria

The low phosphate added to low atmospheric oxygen abundances of pre-Cryogenian seas were used to explain the maintenance of low-nutrient oceans during that period. In addition, these conditions could promote the removal of phosphate together with the precipitation of iron minerals in the old ecosystems [77]. Phosphates of Subsequence Ib showed Ce anomalies related to reducing conditions during precipitation [35]. In addition, glaciations may have perturbed the positive P–Fe-feedback, overturning a new state into the ecosystem. Subglacial weathering increments the seawater phosphorus when carbon dioxide levels show drastic variations in transporting a surplus of nutrients into the seas [77]. Later, this was used to explain the negative δ13C excursions recorded in the phosphate–dolomite transitional section of the Subsequence Ib [16]. This transitional section was characterized by its abundance of authigenic francolite throughout the first stromatolite cycle. The rise in phosphorous in these levels can explain the success of cyanobacteria in the construction of meters to tens of kilometers of biostromes which probably was influenced by the Sturtian glaciation ending.

Dolomite preceded phosphogenesis and was formed directly by communities of microbes in a mixed-fresh and seawater setting [16]. Other dolomite–phosphate stromatolite associations from the Paleoproterozoic [78] and Neoproterozoic were interpreted similarly. The alternation of phosphate-rich and dolomite-rich beds from the transitional pass between Subsequences Ia and Ib were interpreted as related to relative sea level changes [79]. The transition from the epiclastic to the cap-carbonate, was assumed as part of an unconsolidated clayey substrate with phosphate lenses after being reworked to generate an intrabasinal conglomerate (Figure 5D). In addition, the change in redox conditions from a reducing to oxygenating environment can be explained by significant seawater circulation [16]. This important distinguishable change in conditions implies a gradual shift from the terrigenous to stromatolite dolostone platform affected by marked sea level changes [3].

5.4. Bio-induced Precipitation of Carbonates

5.4.1. Cap-Dolostone—Subsequence Ib

The compositional transition from Subsequences Ia to Ib related to detrital contribution (quartz and clayey well rounded intraclasts), and phosphate supply (authigenic francolite) were described in the first stromatolite cycle; dolomite beds are much more frequent and purer (without terrigenous components) and with lack of cement [3]. Towards the top of Subsequence Ib, clays are increasingly common, and marl and shale beds indicate the increased detrital supply related to a supratidal setting. Calcite cementation and dedolomitization processes are relatively common at the upper karst surface contact, accompanied by iron rich cement giving a typical reddish and purple coloration [3]. Similarly, siliceous cementation is widespread near the karstic surface and was associated with strong weathering added to intense silicification [2].

In Subsequence Ib, terrigenous supply was necessarily constrained by sporadic clay inputs when suspension and decantation occur in subtidal to supratidal settings. Increased detrital input is reflected in the upper part of the sequence related to claystone and marl beds, probably explained by a shallower setting under deglacial meltwater with suspended fine material.

The shift from negative to positive δ¹³C excursions in Subsequence Ib and low Sr content were indicators of a drastic cold to warm climate change and support the interpretation of this section as a cap-carbonate [16]. The intraclastic conglomerate at the base of this cap-dolostone was interpreted as formed during a sea-level drop added to interaction with currents promoted by deglacial water agitation [80,81]. In addition, global deglacial cap-carbonates were associated with the reinstallation of microbial mat ground-dominated systems [81].

The dolomitic platform of Subsequence Ib, represented by the widespread stromatolite builds, was therefore related to deglacial to interglacial conditions with increasing biodiversity reflecting a shift to warmer climatic and oxygenation conditions [3]. A main clue was the explanation of the dolomicrite and dolosparite origin of the cap-dolostone, considered mostly formed by primary dolomite under peculiar environmental conditions [16]. According to many authors, the nature of Precambrian dolomicrite implies that deglacial dolomite precipitated possibly in the water column as a powder [82,83,84]. The well-known dolomite saturation of the Cryogenian seas was mentioned for different units worldwide, related to rapid changes during or immediately following a glaciation [69]. This formation possibly implies a short 103–104-year time frame during lithospheric relaxation consistent with the Snowball-Earth Hypothesis [84,85,86,87].

5.4.2. Preglacial Carbonate Ramp—Sequence V

The extensive marine deposition of limestones of Sequence V (Loma Negra Formation) was referred to as the connection of interior seas in cratonic basins, known as the Clymene Ocean [88]. This context may provide ideal conditions for the microbialites forming most of the Ediacaran carbonate successions related to shallow seawater settings. The paleogeographic context was considered to explain the widely positive δ¹³C record commonly related to carbonate successions of ~580 Ma around the world [17,89,90,91,92]. The positive δ¹³C values suggest high rates of photosynthetic microscopic algae and cyanobacteria productivity in those seas [93].

Microbial communities emerged in the Proterozoic oceans and have dominated seas for most of the Earth’s history, inhabiting different environments, and playing a key role in releasing oxygen [92,93,94,95]. The observed oxygenic photosynthesis in modern microbial mats [96,97,98,99] served as a model to explain Neoproterozoic sediments connected with oxygenation events. Microbial mats conform organic-rich layers, and their extracellular mineral particles can trap and bind sediments in the interface between water and sediment [94,95]. In carbonate, bio-induced, depositional settings, microbial mats commonly construct stromatolites. These marine bio-induced carbonate successions can reflect the seawater geochemistry independently of their age [46,87,100,101,102]; redox-sensitive trace elements were used as indicators of global seawater oxygenation events (OOEs). In a dominant anoxic global Ediacaran Ocean [51,100,102], these authors recognize three relevant oceanic oxygenation events (OOEs): the first at, ca., 635 Ma, the middle at, ca., 580 Ma, and the late, ca., 560 Ma [100,101].

Ref. [18] These authors recently provided evidence to explain the causal relationship between algal activity and water oxygenation rise. This rise in oxygen was also related to geochemical indicators added to the proposal of its intrinsic relation with photosynthetic bacterial activity during the Middle Ediacaran Oceanic Oxygenation Event (MEOOE) documented in Sequence V [46].

Bubble-like gas escape microstructures were associated with microbial mats activity recovered in lower and middle parts of Sequence V (Loma Negra Formation). These abundant microstructures were assumed to be formed by the gas trapping beneath the mats. The bubble-like morphologies were compared with oxygen bubbles produced in the laboratory by a modern cyanobacteria-microbial mat during photosynthetic oxygen liberation. These characteristic hemispherical forms can be related to the increase in gas pressure, displacing grains and organic matter [18]. The rise in O₂ related to the MEOOE of Sequence V was thought to be the result of the intense photosynthetic oxygen production by these microbes [18].

5.4.3. Supporting Shuram Bio-Induced Carbonates (Sequence VI)

In the lower and middle sections of Avellaneda Fm, both laminated and massive marls were interpreted as the product of deposition of combined fine carbonate–epiclastic material (detrital and bio-induced components), deposited in a shallow marine setting. Massive structure of marls can be explained by the sinmixis and/or flocculation of clays before their decantation [4].

A dual polarity component was used to reconstruct the magnetostratigraphy basal section of Sequence VI (Avellaneda Formation) [47] in which directional results showed the dominant occurrence of reverse polarity magneto-zones intercalated with a normal magneto-zone. In addition, short intervals of normal polarity were observed in the upper section of which the origin is still under debate [47].

The recently obtained δ¹³C data show a highlighted negative excursion throughout the marly section of this unit, achieving a very tight correlation in five sections supported by their consistent δ¹³C curves [3,12]. In addition, these curves were correlated to the global Shuram excursion, that was also related to a reverse polarity chron. From the correlation of this excursion in coeval successions, they show strong similarity in δ¹³C record and consistency in their polarity patterns. Chemostratigraphic correlation of Sequence V pointed to a minimum depositional age of ca. 570 Ma (Shuram age). Finally, results indicate that up to the present, only a few centimeters of continuous magnetostratigraphic sequences could constrain the geomagnetic Ediacaran field behavior, and the latest studies could have contributed to such a significant goal [12,48].

6. Conclusions

The Neoproterozoic depositional history of the sedimentary succession of the Tandilia System begins during the Tonian?–Cryogenian interval with the deposition of Subsequence Ia, epiclastic sandstone, conglomerate, and wacke deposited in a shoreface and composed mostly of detritus from the crystalline basement turning through more recycled arenites upwards. Over this, Subsequence Ib, a carbonate platform was formed mostly by bio-induced dolomite precipitation during a deglacial to interglacial period.

Piedra Amarilla Surface (PAS), the huge unconformity over Sequence I, separates it from the Ediacaran sequences (II–V), consisting of intermittent intervals of epiclastic or carbonate deposition. The accumulation in the basin was discontinued, or eroded, for a prolonged period (>120 Ma), up to the Ediacaran. And it could be assumed that during this time the breakup of Rodinia evolved to the Gondwana configuration leaving the PAS as its scar.

Sequence II indicates a time of mass transport of terrestrial sediments in an active tectonic setting. The shallow marine sandstones of Sequence III (Cerro Largo Formation) were composed of reworked and mature grains easily linked to a basement affinity. This sequence transitionally passed to heterolithic and muddy beds of Sequence IV that reveals a similar source affinity.

Sequence V was essentially built by microbial-induced micrite related to high levels of oxygen in seawater linked to the middle Ediacaran Global Oxygenation Event. On top of these limestones the Barker Surface (BKS) limited the succession from the next units, which also marks a drop in the basement detrital supply. Resting on BKS, sequences VI–VIII reveal a drastic detrital source change started with deposits of Sequence VI—marls and mudstones—followed by Sequence VII—fissile mudstone and siltstone beds deposited in an offshore environment. Sequence VIII unconformably terminates the succession, featuring fine-grained arenites and heterolithic beds. These three sequences show a dominance of clastic grains of volcanic affinity of which the study is the aim of future work.

The last sequence of the Ediacaran succession (VIII), the epiclastic fine grained rocks, was recently related to the transition to the lower Cambrian period, and therefore probably deposited after the SW-Gondwana amalgamation.

Finally, this study supports and changes our understanding of the influence of controls such as detrital supply, climate changes, bio-induced precipitation, over sediment composition during the Neoproterozoic interval represented in the Tandilia Basin.