These deposits líe on a crystalline basement named Buenos Aires Complex by Marchese and Di Paola (1974), which is older than 2,000 Ma and it is composed of granitoids, migmatites, ectinites, milonites, anfibolites and basic dykes.  

Many authors have contributed to the knowledge of the Tandilia sedimentary succession (cf. Iñiguez et al., 1989). In this synthesis the framework proposed by Dalla Salda and Iñiguez (1979) and modified by Poiré (1987; 1993) for the Precambrian units is followed. In the same way, the sequencial setting composed of three Riphean sequences, a Vendían-Rephean sequence and a final Ordovician one (Iñiguez etal.,1989) is also accepted.

From the lithostratigraphic point of view (Table 1) the Precambrian sedimentary successions comprise a) the Villa Mónica Formation (Poiré, 1993) and its equivalent Las Aguilas Formation (Zalba, 1978), b) the Cerro Largo Formation (Poiré, 1993), and c) the Loma Negra Formation (Borrello,1966), all of these constituents of the Sierras Bayas Group (Dalla Salda and Iñiguez,1979; Poiré, 1993), and d) the Cerro Negro Formation (Iñigue2 and Zalba, 1974) and its equivalent Las Aguilas Formation (Zalba,1978). The Lower Palaeozoic succession is known as the Balcarce Formation (Dalla Salda and Iñiguez, 1979).

These lithostratigraphic units were grouped by Spalletti et al. (1996) finto five depositional sequences (Table 1): the Tofoletti (I), Malegni (II) and Villa Fortabat (II)) sequences are Riphean, the La Providencia sequence (IV) is Vendían-Riphean and the Batán sequence (V) is Cambrian-Ordovician. On the other hand, Andreis and Zalba (1998) for the Chillar-San Manuel named these sequences as A1, A2, B, C y D, respectively.

Between the crystalline basement and the sedimentary cover, arkosic and quartz-kaolinitic saprolites indícate palaeowethering surfaces (Zalba et al.,1992). A peculiar event is the presente of diamictites between the crystalline basement and the Balcarce Formation reported in the El Volcán Hill (Spalletti and del Valle,1984).

Being all these units unfossiliferous they carry biogenic sedimentary structures (trace fossils and stromatolites) as the only evidente of biocoenosis in the Precambrian and Lower Palaeozoic seas of this region. Precambrian stromatolites are located in the Villa Mónica Formation, where they are arranged in biostromes and bioherms dated between 800 and 900 Ma (Poiré, 1987; 1993). In the Precambrian units, trace fossils are scarce and show a poor ichnodiversity. Palaeophycus isp. and Didymaulichnus isp. have been described in the Cerro Largo Formation (Poiré et al., 1984), while Helminthopsis isp. and probable medusa resting traces have been found in the Loma Negra Formation. Skolithos isp. has recently been registered in the Lower part of the Cerro Negro Formation.  

Upper Precambrian sedimentary rocks  

The Upper Precambrian sedimentary cover of Tandilia in Sierras Bayas (close to Olavarría) is a 167 m thick succession (Sierras Bayas Group) composed of three depositional sequences separated by regional unconformities (Poiré,1987;1993).

The oldest depositional (Tofoletti) sequence (52 m thick) shows two sedimentary facies associations: a) quartz-arkosic arenites to the base and b) dolostones and shales to the top. The first one is composed of shallow marine siliciclastic rocks (conglomerates, quartz and arkosic sandstones, diamictites and shales), and the second is characterised by shallow marine stromatolitic dolostones and shales. This sequence has been dated in 800-900 Ma.

The second depositional (Malegni) sequence (75 m thick) consists of a basal succession composed of chert breccia, fine-stratified glauconitic shales and fine-grained sandstones, followed by cross-bedded quartz arenites which are in turn covered by fine-grained siliciclastic rocks (siltstones and claystones). This sequence represents a shallowing upward succession from sutidal nearshore to intertidal flat deposits. An age between 700-800 Ma has been defined from Rb/Sr dating (Bonhomme and Cingolani, 1980).

The younger depositional (Villa Fortabat) sequence is a 40 m thick unit composed almost exclusively of red and black micritic limestones, originated by suspension fall-out in open marine ramp and lagoonal environments.

On top of the Sierras Bayas Group a regional unconformity is recognised (Barrio el al, 1991). This surface has been related with a sea-level drop, meteoric dissolution-of the carboriatic sediments, and consequent development of a karstic surface on which residual clays and brecciated chert accumulated.

The Vendian Cerro Negro Formation (La Providencia Depositional Sequence) appears on top of the above described unconformity. It is a more than 100 m thick unit characterised by claystones and heterolithic fine-grained sandstone -claystone interbeds, mainly formed in upper to lower intertidal flats.  

Lower Paleozoic sedimentary rocks  

The Lower Palaeozoic siliciclastic succession is known as Balcarce Formation (Batán Depositional Sequence). This unit has been studied by del Valle (1987a). It is overlaying the crystalline basement or the former sedimentary units (Cerro Negro Formation, Las Agudas Formation, Sierra del Volcán Diamictites and Punta Mogotes Formation).

The Balcarce Formation (100 m. thick) is composed of white quartz sandstones and granule sandstones with subordinated levels of mudstones (kaolinitic-rich clays) and quartz conglomerates. The geometry of the sandstone beds is sheet-like; most sedimentary bodies are bounded by convex-upward surfaces, though some wide channel-like features are also present. Planar and tangential cross-stratifications are the dominant structures within sandstone bedsets, and large-scale sigmoidal bodies are frequent in most sections. Sheet-like and lenticular sandstone-mudstone interbeds are commonly intercalated among sandstone storeys. Trace fossils are abundant at the top surface of the sandstone member in sandstone-mudstone interbeds. The quarries all around Batán and Chapadmalal towns allow to depict the stratigraphic architecture of the Balcarce Formation. Based on their contrasting geometry; two main groups of layers can be defined in this siliciclastic succession: one group is characterised by a sub-horizontal stacking pattern (aggradational geometry) and the other shows very well developed depositional clinoforms (progradational geometry). The observatiónal facies of the Balcarce Formation as well as the inferred transport mechanisms and type of deposits are usted on Table 2.

Diagnostic criteria to recognise tidal processes in fair weather and storm events have been summarised on Table 3. Tidal processes are inferred from the features of cross-bedded sandstone facies (bars) and heterolithic (wavy and lenticular) facies (swales). Large to medium scale laterally persistent bodies of cross-bedded sandstones, exhibit rhythmic lateral variations in the thickness of foresets and in clay content due to spring and neap tide alternation. Clay drapes covering foresets and other sedimentation surfaces, herringbone cross-bedding, opposite palaeocurrent trends in successive sedimentary bodies and reactivation surfaces also suggest tidal deposition. The migration and accretion of bidimensional sand bars seem to be controlled by highly asymmetrical time-velocity tidal currents. Subordinated, high-energy storm episodes are suggested by hummocky cross-bedded sandstones, sheet conglomerates armouring previous tidal sand bodies, and heavy mineral concentrations in the wavy sandstone laminae of heterolithic facies.

An epicontinental shallow marine open shelf is inferred for the Cambrian-Early Ordovician in the Tandilia basin. Most sedimentary facies were developed in the nearshore and inner shelf environments of a fide-dominated and storm influenced platform.

Trace fossils  

On the account referentes about the Precambrian trace fossils of Tandilia are scarce. Palaeophycus isp. and Didymaulichnus isp. have been descxibed in the Cerro Largo Formation by Poiré etal. (1984), while Helminthopsis isp. and probable medusa resting traces in the Loma Negra Foxmation. Skolithos isp. have recently been registered in the lower part of the Cerro Negro Formation.

On the other hand, the rich variety and the abundante of trace fossils in the Lower Palaeozoic Balcarce Formation have been mentioned by many authors: The first findings have been made by Hauthal (1896) and Nágera (1919, 1926), but it was Borrello (1966) who studied and described a wide trace fossil collection taken from different localities of Tandilia. Aceñolaza (1978) revised the subject a gave a modernised focus to the study of trace fossils from Tandilia, together with other Lower Palaeozoic ichnofossils of Argentina. Later records are due to Alfaro (1981) in the La Numancia-Licenciado Matienzo area, Regalia and Herrera (1981) in the San Manuel arca, Zalba et al. (1982) in the Las Aguilas Hill, Cingolani et al. (1985) in the Corral Hill, del Valle (1987a y b) in different localities from the Balcarce-Mar del Plata area, Poiré and del Valle (1994,1996) in the coastal outcrops of Mar del Plata, and Poiré (1998) in the Chillar area.

The Balcarce Formation shows a great quantity of trace fossfs and a much higher ichnodiversity. After revising the already published material thoroughly and taking into account the recent discoveries made by the authors, the following up-dated list of trace fossils is presented: Ancorichnus ancorichnus, Arthrophycus alleghanensis, Arthrophycus isp., Bergaueria isp., Cochlichnus isp., Conostichuis isp., Cruziana furcifera, Cruziana isp., Daedalus labeckei, Didymaulichnus lyelli, Didymaulichnus isp., Diplichnites isp., Diplocraterion isp., Herradurichnur scagliai, ?Monocreterion isp., Monomorphichnus isp., Palaeophycus alternatus, Palaeophycus tubularis, Palaeophycus isp., Phycodes aff. pedum, Phycodes isp., Plagiogmus isp., Planolites isp., Rusophycus isp., Scolicia isp. and Teichichnus isp. (Fig. 2).  

The age of Balcarce Formation  

The precise age of the Balcarce Formation is difficult to determine. Through radiometric dating (600Ma) and the presence of acritarchs the underlying Cerro Negro Formation has been dated in the Vendian (Cingolani et al., 1991). The upper limit of the Balcarce Formation is sustained by an intrusive diabase body dated around 450 and 498 Ma (Rapela et al., 1974). Consequently the unfossiliferous Balcarce Formation would be assigned to the lapse Cambrian-Ordovician. During the seventies and eighties Cruziana has been considered a useful biostratigraphy indicator (cf Seilacher,1970; Crimes,1975). Based on this concepts, the presence of Cruziana furcifera has been one of the most substantial elements to accept an Arenigian age for the Balcarce Formation. Nevertheless, recent contribution by Marwood and Pemberton (1990) seriously questioned the validity of Cruziana in biostratigraphy. On the other hand the appearance of Plagiogmus isp. would strongly indicate a Cambrian age (Glaessner,1969; Crimes,1975).

According to the available stratigraphic and geochronologic information the Balcarce Formation can be either considered Cambrian and/or Ordovician. Future and more detailed ichnological studies and their comparison with other unfossiliferous quarzites rich in trace fossils from this Gondwana region, as the Lower Palaeozoic successions from South Africa and Malvinas Islands, could result to establish a more accurate age of such a peculiar unit of the Tandilia System

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