MAPA GOLOGICO DEL NOA

AFLORAMIENTOS DE LAS UNIDADES SEDIMENTARIAS CRETACICO-TERCIARIAS DEL NOA

CONFIGURACION DE LA CUENCA DEL GRUPO SALTA

           CONFIGURACION DE LA CUENCA DEL GRUPO SALTA

Isopáquico Subgr. Pirgua

Isopáquico Subgr. Balbuena

Isopáquico Subgr. Santa Bárbara

Configuración Fm Yacoraite

AFLORAMIENTOS DEL GRUPO SALTA EN PURMAMARCA

LAS ACUMULACIONES SINRIFT PREMAASTRICHTIANAS
Los depósitos premaastrichtianos del Subgrupo Pirgua representan la etapa sinrift en la evolución de la cuenca. En el depocentro de Alemanía aflora uno de los perfiles más completos del Subgrupo.

SUBGRUPO PIRGUA
La evolución del Subgrupo Pirgua se divide en dos episodios principales, sea en términos de unidades tectosedimentarias (Gómez Omil et al., 1989) o en base a la distribución regional de sus Formaciones.

El primer episodio de relleno de las fosas comenzó con el Conglomerado La Yesera, muy bien documentado en los depocentros de Alemanía y de Metán.

La Yesera Formation: description

This formation is composed of three main sections defined
in the Alemanía and Brealito subbasins (Fig. 3). The lower La Yesera Formation is composed of conglomerates, and the middle part of the formation is composed of siltstones and sandstones. The lower and middle sections are arranged in a finning-upward sequence.

The upper La Yesera Formation comprises conglomerates similar to those of the base. This upper section represents the base of another finning-upward sequence that continues in the Las Curtiembres Formation. The initial faulting of the basin produced depressions where the La Yesera Formation accumulated (Salfity and Marquillas 1994). The subsidence was remarkable in the Brealito subbasin, where thickness surpasses 2,400 m. This initial sedimentation accumulated conglomerates and, in a lower proportion, sandstones and siltstones (Fig. 3a).

In the southernmost region of the basin, the Alto de Las Salinas Volcanic Complex lavas were produced. The coarse-grained deposits of the lower La Yesera Formation are up to 300 m thick in the Alemanía subbasin and 1,000 m in the Brealito subbasin.
Close to the base, the deposits also show alternation of dirty lithic sandstone with brown-reddish siltstone.
The middle La Yesera Formation is composed of wackes and siltstones; maximum thickness is 350 m in the Alemanía subbasin and 1,300 m in the Brealito subbasin.
The upper La Yesera Formation is almost entirely composed of conglomerates. In the base of this section, the Isonza Basalt (Fig. 2) of probably a Cenomanian age (96±5 to 99±5 Ma, K/Ar whole rock) is interbedded (Valencio et al. 1976).

The Isonza volcanism was associated with the rift margin faults and accumulated several flows of a thickness of up to 40 m. Maximum thickness in this upper section is 300 m in the Alemanía and Brealito subbasins. The contact between the middle and upper La Yesera Formation sections is concordant, with the exception of areas lesser in subsidence, such as
the north of the Alemanía subbasin (near to the Salta-Jujuy high), where the contact is slightly erosive.
In the lower and upper La Yesera Formation, the conglomerates show a dark reddish-brown color, they have scarce interbedded wackes and siltstones. The conglomerates are usually clast-supported without gradation, with clasts that vary from boulders to pebbles.
The conglomerate matrix is poorly sorted, is composed of clay and silt to coarse-grained sand and is lithic or arkosic in petrography (Sabino et al. 1998). These conglomerates are interpreted as debris-flow deposits. The wackes show a brown-reddish color and frequent normal gradation, they are rich in clay and lithic clasts, except in the Brealito subbasin where they are arkosic.
The siltstones are dark brown-reddish in color with slightly marked lamination; they are sometimes massive. The characteristics of these fine-grained rocks are coincident with mud flat deposit. The deposits of the lower and upper La Yesera Formation are associated with
basaltic volcanism.
In the deposits of the middle La Yesera Formation, two main facies associations are observed. The first facies association consists of medium- to fine-grained dirty lithic and arkosic sandstone, with cross or parallel stratification in 0.5 m thick tabular beds and 2–4 m thick lenticular beds. They correspond to sand bar elements of a sandy fluvial environment (Miall 1996). The other facies association is composed of rich clay siltstones and lithic fine-grained wackes in tabular beds of 1 to 2 m thick; this association suggests low energy processes.

In the Brealito subbasin (Fig. 1a), siltstones and very fine-grained sandstones, containing some green shale and micritic limestone with chert nodules, were accumulated; these facies have the characteristics of a shallow subaqueous environment.
This formation is poor in fossils. Some remains of undetermined plants and very few undetermined algae and ostracod microfossils were found (Boso et al. 1984) in the thin lacustrine limestone at the top of the middle La Yesera Formation in the Brealito subbasin. Trace fossils are frequent in the sandstone interbedded in the middle section.

Las Curtiembres Formation: description
After the Isonza basaltic volcanic event (Fig. 2), the finegrained sediments of the Las Curtiembres Formation widely accumulated in the basin (with the exception of the Tres Cruces subbasin). The areas of greater subsidence were located near the faults where over 2,000-mthick sediments deposited (Brealito, Alemania, Metan and, probably, Lomas de Olmedo subbasins) (Fig. 1a).
The faults along the rift border were active during the whole synrift stage. The faults in the inner subbasin, which were active during the La Yesera Formation before, became inactive or showed less movement (Bianucci et al. 1981; Cristallini et al. 1998). Similar evolution of the faults has been reported by Gawthorpe and Leeder (2002) in the East African lakes.
The main facies of the Las Curtiembres Formation is dark brown-reddish clayey siltstone, with slight lamination, and with little moulds of halite crystals in some cases (Brealito subbasin). There are thin layers of wacke and lithic fine-grained, light brown-reddish sandstone, sometimes micaceous and some greenish, and also siltstone and claystone with frequent nodules of copper and uranium (Sureda et al. 1984). These facies correspond to shallow subaqueous deposits.
Near the top of this formation in the Alemanía and Metán subbasins, Las Conchas Basalt (Reyes and Salfity 1973) was formed by Campanian pyroclastic flows and lava flows dated 78–76 Ma (Valencio et al. 1976; Reyes et al. 1976; Galliski and Viramonte 1988) (Fig. 3a). This
volcanism occurred in the center of each subbasin, not along their borders as the previous volcanic events.
Green siltstone and some micritic limestone with chert nodules accumulated in the Alemanı´a subbasin. This 15-m-thick deposit contains pipid frogs similar to the Eoxenopoides genus, denominated Saltenia ibanezi Reig 1959 (Baez 1981), and plant remains probably from
Bennetitals (Archangelsky, Ibanez 1960). There are frequent traces such as Palaeophycus and Taenidium (Luis Buatois, personal communications) in the thin layers of fine-grained sandstone that alternate in the pelitic succession.

El segundo episodio de relleno sinrift lo representa la parte superior de la Formación Los Blanquitos, que niveló los depocentros e inclusive se acumuló sobre los altos estructurales
internos de la cuenca (Calete, Cachipunco, Las Víboras y Guachipas).

Los Blanquitos Formation: description
The Los Blanquitos Formation represents a coarsening upward sequence. The base is usually transitional with the siltstones of the Las Curtiembres Formation, while in the middle and upper parts of the formation, the coarse-grained sandstones and the conglomerates are
frequent. The Los Blanquitos Formation thickness is usually more than 700 m (Fig. 3a), but it surpasses 1,500 m towards the central part of the Alemanıa subbasin.
The formation is intensely eroded in the Brealito subbasin (Boso et al. 1984) as a consequence of the inversion of the western area of the Salta basin during the Incaica phase.
During sedimentation of the Los Blanquitos Formation, arkosic and lithic, medium- to coarse-grained and ill-sorted sandstones containing little clay matrix accumulated, as well as fine-grained, pink-orange grayish to brown-reddish conglomerates in layers up to 6 m thick and with normal gradation. Commonly, incipient carbonate paleosols formed with noticeable marks of roots and abundant bioturbation.

In the Metan subbasin, this formation is composed of thick, massive or lowangle cross-bedded strata of medium- to fine-grained arkosic sandstones (Fig. 3d). In the profiles of this subbasin, the interbedding of bioturbed siltstone is more frequent. In the Alemanıa and Metan subbasins, the sandstones of the Los Blanquitos Formation contain quartz and feldspars of the granitoids that crop out in the west part of the Brealito subbasin (Sabino 2002).
At the top of Los Blanquitos Formation in the southern area of the Alemanıa subbasin, remains of Sauropod dinosaurs Titanosauridae and two teeth of Carnosauria, probably Coelosauria, were found (Bonaparte and Bossi 1967). Later, due to new findings, the remains of titanosaur were assigned to the Laplatasaurus genus. Some bones of another Sauropod of an indefinite family and post-skull remains of a new species of therapod were also found: Unquillosaurus ceibalii sp. nov. (Powell 1979; Bonaparte and Powell 1980).

All their remains were assigned to the Senonian (Powell 1979), thus confirming the age of Los Blanquitos Formation.
The Palmar Largo volcanic rocks (Ma¨ del 1984) lie between the top of Los Blanquitos Formation and the base of the Balbuena Subgroup in the Lomas de Olmedo subbasin. They are dated as having 70±5 Ma in age (K/Ar) (Gomez Omil et al. 1989), which means that the
upper part of the Los Blanquitos Formation must have accumulated in Early Maastrichtian.

LAS ACUMULACIONES POSTRIFT MAASTRICHTIANO-EOCENAS

La ingresión maastrichtiano-paleocena en el norte de la Argentina estuvo vinculada con los cambios globales contemporáneos con el diastrofismo Ranquel. Los dos pulsos de la fase Ranquel señalan, respectivamente, el inicio y la culminación de la inundación epeirogénica del Subgrupo Balbuena, ocurrida en un marco de tranquilidad tectónica.

SUBGRUPO BALBUENA
The Balbuena Subgroup was accumulated during the Maastrichtian to Early Paleocene (Fig. 2); it represents the early postrift stage. The typical section is 400–500 m thick. The lower part is formed of white sandstones (Lecho Formation), and the upper part contains gray limestones (Yacoraite Formation) and dark pelites (Olmedo/Tunal Formations). These deposits cover the Pirgua Subgroup and underlie the Santa Barbara Subgroup.

Lecho Formation: description
The basal deposit of the Balbuena Subgroup is represented by white sandstone of the Lecho Formation; its average thickness is 150 m (Salfity 1980) (Fig. 5a, b). The main facies is of fine- to medium-grained calcareous sandstones thickly stratified to massive, which were
accumulated by medium- to high-energy tractive currents.
Also, clean sandstone containing rounded quartz and high-angle cross strata, and coarse-grained bioturbated calcareous sandstone (quartzose to arkosic) were deposited. These facies are the consequence of wind action and the reworking of the sediments by water currents.
In the Meta´n subbasin (Fig. 1a), the sandy to silty facies contain a Senonian association of tetrapods and birds. The bones of the Sauropods (Saltasaurus loricatus), Coelurosauria (Noasaurus leali) and Carnosauria and from three undetermined orders of continental birds were preserved (Bonaparte and Powell 1980).
Limestone, shale and claystone are present in some sections of the basin, in the lower third of the formation. This could be related to an early flooding event in the basin. In some sections of the Tres Cruces subbasin, this flooding event is represented by limestone, shale and claystone facies. The calcareous microfacies demonstrate the occurrence of low-energy stages (micrite with ostracods and bivalves) alternating with high-energy stages in which grainstone and packstone with oolites, intraclasts and pellets predominated (Marquillas and Salfity
1990).

In other profile sections of the north and northwest parts of the Meta´n subbasin, the base of the formation consists of a decimeter- to meter-scaled assemblage of green, gray and black shales, mudstones and fine-grained sandstones. The pelitic rocks of a reducing environment, although many of them were oxidized later, generally have copper and uranium mineralization; besides, they show a rich content of palynomorphs, which are currently being studied.

The Lecho Formation also shows red facies (sandstone and shale) along the eastern border of the Metan subbasin and red to purple facies in the Alemanıa subbasin. Here, the Lecho Formation (or Quitilipi Formation) consists of mega cross-stratified or massive sandstones, black, green and yellowish-gray shales with subaqueous structures and mud cracks and some levels of limestone and calcareous nodules. The pelitic deposits are laminated and some
sandstones are finely stratified. No fossils were found.

Yacoraite Formation: description
The Yacoraite Formation, 200 m maximum thickness (Fig. 5a, c), is an excellent marker horizon in the Salta Group, due to its calcareous–dolomitic composition. Its intense yellow of weathering color dominates the gray color of the fresh rocks. Its outstanding topographic relief is most characteristic.
The lowest part of the formation consists mainly of high-energy limestones and calcareous sandstones in well-defined strata of 0.3 m average thickness. They are medium- to coarse-grained oolitic grainstone with spherical sparitic oolites, oolitic packstone, intraclast limestone and light grey calcareous sandstone. White or white-yellowish tuff layers are common. Fossils are scarce, restricted to gastropods, pelecypods and a few miliolid foraminifers, all of them characteristic of restricted marine conditions. The cathodo-luminescence studies of the limestones (Marquillas and Matheos 2000) permitted the detailed definition of the marine cementation events in the early diagenetic processes (high luminescence, high content of Mg and very rich in Mn).
Limestone and fine-grained sandstone, with wave and current ripple lamination sometimes associated with hummocky cross-stratification, are common in the lower and middle part of the formation. In the middle, stratification is thinner, ranging from 10 to 15 cm, which gives a flaggy appearance to the limestones. The predominant facies of the middle part indicate moderate energy. They are represented by fine-grained oolitic grainstone, packstone and wackestone that contain only gastropods. Stromatolitic boundstone is rare, and there are some sandstones. Low-energy facies such as micrite and shale are scarce. Generally, they contain ostracod valves, and fragments of undetermined fish.

Especially in the Meta´n and Alemanı´a subbasins (Fig. 5c), the upper part of the formation is characterized by a recurrent succession of shallowing events. They are represented by fine-grained rocks (black, green and gray shale, calcareous mudstone and dolomicrite), which
alternate with oolitic and intraclastic grainstone, and stromatolitic boundstone. The stromatolites are domal and are up to 90 cm high. There are some layers of gypsum and anhydrite.
Shales usually contain small phosphatized remains of fish. Fish species in the Yacoraite Formation are the Pucapristis branisi, compared with the modern pristidae, which are related to coastal marine environments (Powell 1979) and the Coelodus toncoensis, which would
indicate a similar environment (Benedetto and Sanchez 1972; Cione 1977). Besides, hypocoracoids of Gasteroclupea branisai (Acen˜ olaza 1968; Reyes 1972) and different
siluriforms were found. Even though the siluriforms constitute a very important group of freshwater fish, the Cretaceous and a large number of Paleocene forms were of marine or mixed environments (Cione and Laffite 1980; Cione et al. 1985).

This formation also contains ostracods, foraminifers, pelecypods, gastropods, algae and palynomorphs. The most common ostracod is the Ilyocypris sp. Some of the most frequently mentioned foraminifers are the rotaliids, such as Ligulogavelinella frankei, Orostella turonica and Bilingulogavelinella sp. and forms similar to Discorbis aff. cretacea, Valvulineria infrecuens, V. marianosi and V. allomorphinoides and Miliolinella sp., among the miliolids (Me´ndez and Viviers 1973; Kielbowicz de Stach and Angelozzi 1984). Palynological studies of limestone and shale from Lomas de Olmedo subbasin cores revealed forms resembling Aquilapollenites magnus, Crassitriapertites brasiliensis, Zlivisporis blanensis, Gabonisporis vigourouxii, Psilastephanosporites cf. brasiliensis, numerous polyplicated grains and, in lower numbers, Tricolpites sp., Ulmoideipites sp., aplanospores algae and deflandroid cysts, indicators of brackish conditions (Moroni 1982). Papu and Melendi (1984) mentioned mixed conditions of fresh and brackish water, after the finding of massulae of Azolla cretacea and dinoflagellates. The variable salinity of the water is also recorded by the presence of different charophytes, especially characeae and porocharoideae (Musacchio 1972; Kielbowicz de Stach and Angelozzi 1984) of freshwater. The fossiliferous content is similar to that recorded in deposits of the same age in Bolivia (Gayet et al. 1993).

The deposits of the formation present very well preserved dinosaur footprints (Alonso 1980; Alonso and Marquillas 1986) in outcrops of the Alemanı´a subbasin in the west. There are various morphotypes recorded in calcareous fine- to very fine-grained sandstone of a grayyellowish color, with ripples and desiccation cracks, alternating with green shale. They are ichnites of a Carnosauria (Salfitichnus mentoor) and of two ornithopods, probably hadrosaurids (Taponichnus donottoi, Telosichnus saltensis).

The association is complete with numerous oriented but badly preserved footprints of tetrapods, probably herbivore dinosaurs. Stratigraphically, on top of them, tridactylus ichnites of birds (Yacoraitichnus avis) are recorded. Other sections in the same area have numerous well-preserved footprints of Ornitischia (Hadrosaurichnus australis) in limestone in the lower part of the formation. Also, dinosaur footprints are observed in the stromatolitic plain of the Alemanı´a subbasin and coastal deposits of the Meta´n subbasin. The habitat was also favourable for crocodiles such as Dolichochampsa minina (Gasparini and Bu¨ ffetaut 1980).

Tunal and Olmedo Formations: description
The Olmedo Formation (Fig. 2) is a deposit essentially controlled by decantation and evaporation processes.
This formation is composed of black and gray shales, siltstone with salt and gypsum crystals and micritic and dolomicritic limestone. There are also thick accumulations of halite with anhydrite and gypsum.

Evaporites overlying the Yacoraite Formation are known as the‘‘Salino Member’’ in the eastern part of the basin (Lomas de Olmedo subbasin, Fig. 1a). The thickness of the Olmedo Formation outcrops averages up to 60 m, but on the subsurface of the Lomas de Olmedo subbasin (Fig. 1a), thickness varies from 150 to 200 m (Moreno 1970; Carle´ et al. 1989). However, it surpasses 900 m (Moreno 1970) because of a thickening of the Salino Member due to tectonic controls (Gomez Omil et al. 1989; Carle´ et al. 1989), or perhaps due to diapiric
structures. The shales contain pollen.
In a wide region of the Meta´n and Alemanı´a subbasins (Fig. 1a), there are deposits equivalent to the Olmedo Formation, called Tunal Formation (Turner et al. 1979), of 40 m average thickness (Fig. 5a, d). It is made up of gray, green, black and brown-reddish shales and mudstones, gypsiferous fine-grained sandstone and abundant small layers of gypsum, and scarce ochrecolored dolomicrite. Nevertheless, in other sections, the facies of the Tunal Formation have scarce or no sulfates (Novara 2003).

It possesses a rich content of palynomorphs (Quattrocchio et al. 1988, 2000) in which
angiosperms dominate, e.g., Ephedripites sp., Gemmatricolpites subsphaericus, Rhoipites sp., Retitricolporites sp., Pandaniidites texus, Myriophyllumpollenites sp. and Verrustephanoporites cf. Simplex; and Podocarpidites marwickii among the gymnosperms. Besides, it contains algae (Pediastrum sp.), fungi (Dicellaesporites sp., Multicellaesporites sp.) and other palynomorphs
(Mtchedlishvilia saltenia, incertae sedis).

SUBGRUPO SANTA BARBARA
Since the Middle Paleocene (Fig. 2), the subbasins (Fig. 1a) remained active with a very low subsidence rate, which caused the accumulation of three units of regional continuity, the Mealla, Maı´z Gordo and Lumbrera Formations (Moreno 1970) (Fig. 6a). The succession is dominated by red fine-grained sandstone and siltstone and green mudstone. This deposit represents the late postrift stage of the Salta basin.

Mealla Formation: description
The Mealla Formation is the lowermost unit of the Santa Ba´ rbara Subgroup (Fig. 6a). It is characterized by clastic deposits with thickness ranging from 100 to 150 m (del Papa and Salfity 1999). In the Meta´n and Alemanı´a subbasins, it consists of fine- to mediumgrained
sandstone levels with erosive bases, finningupward tendency, lateral accretion structures (LA
macroform of Miall 1985) and current ripples.

Finegrained sediments interbedded with the sandstone succession are integrated by massive, red siltstone, calcareous nodules and very thin beds of fine-grained sand with planar lamination and current ripples (Fig. 6b).
Frequent remains of freshwater turtles (Pelomedusidae) and mammals (Notoungulata, Simpsonotus praecursor sp. nov.) were found in this environment (Pascual et al. 1981). The notoungulates, which had a herbivorous diet, consisting especially of leaves, are significant for paleoenvironmental reconstruction (Pascual et al. 1978).
Toward the east, in the El Rey and Lomas de Olmedo subbasins (Fig. 1a) red massive siltstone, discrete domal stromatolites, and heterolithic facies of green claystone and white sandstone with wavy bedding and wavereworking structures accumulated. This facies association contains palynomorphs, such as Pandanaceae and Palmae (Nypa), Myriophyllumpollenites sp. and Azolla sp., Ulmaceae and Aquifoliaceae and Ephedraceae
(Quattrocchio et al. 1997; Quattrocchio and Volkheimer 2000a). In the Lomas de Olmedo subbasin, the main facies association is composed of gypsum layers interbedded with red siltstone and very fine-grained sandstones (Go´ mez Omil et al. 1989).

Maı´z Gordo Formation: description
The Maı´z Gordo Formation overlays the Mealla Formation, the main thickness ranging from 200 to 250 m.
It is characterized by a succession of coarse- to finegrained sandstone in the west of the Alemanı´a subbasin. Beds have erosive bases with coarse-grained sands and pebbles as lag deposits. Trough and tabular cross-bedding, also unidirectional ripples, are the common sedimentary structures. Fine-grained rocks are absent or less thick and are characterized by heterolithic facies and calcareous nodules; in some places, root traces were observed.
According to the main facies association, in the eastern part of the Alemanı´a subbasin and in the El Rey and Lomas de Olmedo subbasins (Fig. 1a), the Maı´z Gordo Formation can be divided into three distinctive sections.
The lower section begins with a thick succession of red massive siltstone with intercalation of centimeterthick fine-grained sandstone displaying parallel lamination (Fig. 6a). Mud cracks, brecciated surfaces and bioturbation are common sedimentary structures.

The middle section is characterized by the occurrence of limestone (Fig. 6c). In the El Rey and Lomas de Olmedo subbasins (Fig. 1a), carbonate facies dominate.
The vertical facies assemblage consists, from base to top, of green laminated mudstone and marls, wackestone, packstone and oolitic grainstone, displaying wavy and lenticular bedding and wave-reworked features Continuous beds of domal stromatolites mark the top.
The green fine-grained portion of the succession is rich in insects like Dermaptera, Orthoptera, Hemiptera, Coleoptera (Cockerell 1925, 1926), Odonata, Palaeomacromiidae
fam. nov. (Petrulevicius et al. 1999), fish like Callichthyidae Corydoras revelatus and Poeciliidae Cyprinodon primulus? (Cockerell 1925, 1926; Bardack 1961; Cione 1978) and palynomorphs. The palynomorph communities are very similar to those in the Mealla Formation. These communities are composed of Pandanaceae and Palmae (Spinizonocolpites sp.) and nonmarine dinoflagellate cysts, also, Azolla sp. and Haloragaceae (Myriophyllumpollenites sp.) (Quattrocchio and del Papa 2000).

This section roughly corresponds to the Thanetian ‘‘Cricotriporites guianesis’’ climatic zone of Quattrocchio and Volkheimer (2000b). In the Alemanı´a subbasin (Fig. 1a), the facies
association consists of fine- to medium-grained sandstones, massive to laminated green siltstones and discrete laminar to low-relief domal stromatolites. Wavy and lenticular bedding, current and wave ripple lamination with less frequent mud cracks are observed.
In the upper section of the Maı´z Gordo Formation, the facies association is composed of green shale, massive mudstone and wave-rippled grainstone with erosive bases and rib up mud clasts. In this level, Pediastrum algae were recognized (Quattrocchio and del Papa 2000).

Lumbrera Formation: description
The uppermost unit of the Santa Ba´ rbara Subgroup is the Lumbrera Formation (400–500 m thick), which unconformably overlays the Maı´z Gordo Formation (Fig. 2). Go´ mez Omil et al. (1989) recognized three sections according to its lithological characteristics .
The lower section is composed of red sandstone and mudstone. It is dominated by medium- to fine-grained sandstone with lateral accretion geometry, with tabular cross-lamination and current ripples. Decimeter-to-meter thick red siltstone and fine-grained sandstone layers
with parallel lamination and current ripples are interbedded, as well as massive red mudstone with calcareous nodules.

The top of this section contains marsupials, ungulates and notoungulates (Pampahippus arenalesi, Bond and Lo´ pez 1993), Crocodylia—Sebecidae (Gasparini 1984) and Squamata—Teiidae (Lumbrerasaurus scagliai sp. nov.) (Donadio 1985).
The middle section of the Lumbrera Formation is known as Faja Verde because of a continuous level of green rocks. The facies association consists of dark green to gray laminated claystone and sheet-like, fine sandstone and stromatolite. Sandstone layers display wavy, flaser-bedding and wave ripple structures.

Dark gray shale constitutes a decimeter thick, homogeneous succession containing between 1 and 9% of organic matter, punctuated by very thin beds of coarse-grained siltstone
(Fig. 6d). Diverse palynomorphs: Notopollenites sp., Liquidambarpollenites cf. Brandonensis orest, Pediastrum and Botryococcus algae (Quattrocchio 1978; del Papa et al. 2002) and the fish Lepidosiren paradoxa (Ferna´ ndez et al. 1973) were identified in this succession.
The upper section of the Lumbrera Formation (Fig. 6a) is 300 m thick but, in some places, it is less thick due to the erosive unconformity that limits the top.
It is composed of red massive siltstone and mudstone. Minor fine-grained sandstone beds with parallel lamination and wave-ripple structures (Fig. 6e), sporadic
gypsum/anhydrite nodules forming continuous levels, mud cracks and vertical burrowing characterize this section. Mammal remains of Eomophippus sp. were found in this level (Mule´ and Powell 1998).

Conclusions
For almost 100 million years (Neocomian to Eocene), different sedimentary environments succeeded each other in the northwestern region of Argentina occupied by the Salta Group basin, as summarized in Fig. 7.

The environmental changes that took place through five recognized evolutionary stages (three synrift stages and two postrift stages) are exemplified in four subbasins.
The distribution of the sedimentary environments demonstrates how the tectonic regimes conditioned the geomorphology and the fill of the synrift stage.

The synrift stage comprises three sedimentary cycles. Two of them are successive finning-upward cycles that correspond to the early synrift stage; they represent two cycles with increasing subsidence rate. The third cycle is a coarsening-upward sequence; it represents the late synrift and corresponds to the decreasing subsidence rate of the basin. The beginning of the first and the second synrift cycle each correlates with the initial Mirano and
final Mirano phases (Fig. 2). Effusion of lava accompanies the beginning of both cycles (Alto de las Salinas and Isonza volcanic events). During the second synrift cycle, the climax of the rift accompanied by the volcanism in the center of the basin occurred (Las Conchas event). In the Salta basin, the start of the late 108 synrift stage may correspond to the Peruana diastrophic
phase (Fig. 2). The end of the late synrift stage and the beginning of the postrift stage are marked by thermal subsidence of the basin.

Debris-flow dominated alluvial fans and scarce basaltic flows characterize the start of the synrift fill cycles (lower and upper sections of the La Yesera Formation; Fig. 3a). The increasing subsidence rate during the accumulation of the middle section of the La Yesera
Formation and during the Las Curtiembres Formation (Fig. 3a) (synrift climax) led to the establishment of permanent lakes, which must have increased the humidity in the region. The final decrease in the synrift stage subsidence rate allowed communication of the
subbasins; as a consequence, sandy rivers dominate the depositional setting (Los Blanquitos Formation;

In the Tres Cruces subbasin, no lakes formed due to the lower subsidence rate as compared to the other subbasins. Common eolian deposits confirm drier local conditions. In this subbasin, the ‘‘Pirgua Formation’’ sandstones represent the three lithostratigraphical units that were identified in the southern region.
The postrift fill occurred in a framework of a relative tectonic quiescence. The beginning of the postrift stage (Maastrichtian to Danian) was marked by low topography and warm climate dominated by shallow marine carbonate sedimentation (Yacoraite Formation; Fig. 5a), characterized by the production of abundant oolite, and the development of the stromatolitic plains.
The scarce variety of species and the small size of the organisms are explained by the general stress conditions to which they were subjected due to the variations of the environmental parameters. The dinosaur record is coincident with littoral positions with a marked continental
influence. The deposition of limestones was preceded by the accumulation of fluvial-eolian white sands of the Lecho Formation (Fig. 5a); both facies are
sometimes interbedded.

A drier climate in the Danian led to a general regression. Saline to hypersaline lacustrine
systems and extensive mud plains developed (Salino Member, Olmedo Formation; Fig. 2). Brackish, freshwater and swampy lakes also evolved (Tunal Formation; Fig. 5a).
In the late postrift stage (Danian to Late? Eocene), the distribution of the sedimentary environments suggests plains surrounded by low mountains and forest areas. The fluvial dynamics of each style is in close relationship to the flood plain preservation potential and the remains of the (Maastrichtian to Danian) was marked by low topography and warm climate dominated by shallow marine carbonate sedimentation (Yacoraite Formation; Fig. 5a), characterized by the production of abundant oolite, and the development of the stromatolitic plains.

The scarce variety of species and the small size of the organisms are explained by the general stress conditions to which they were subjected due to the variations of the environmental parameters. The dinosaur record is coincident with littoral positions with a marked continental
influence. The deposition of limestones was preceded by the accumulation of fluvial-eolian white sands of the Lecho Formation (Fig. 5a); both facies are sometimes interbedded. A drier climate in the Danian led to a general regression. Saline to hypersaline lacustrine systems and extensive mud plains developed (Salino Member, Olmedo Formation; Fig. 2). Brackish, freshwater and swampy lakes also evolved (Tunal Formation; Fig. 5a).

In the late postrift stage (Danian to Late? Eocene), the distribution of the sedimentary environments suggests plains surrounded by low mountains and forest areas. The fluvial dynamics of each style is in close relationship to the flood plain preservation potential and the remains of the organisms that lived on it. In braided rivers, high channel mobility provoked continuous cannibalization of the overbank deposits. It resulted in a low record of the endemic
fauna (Maı´z Gordo Formation).

Likewise, in high sinuosity systems, permanent and slowly migrating channels favoured the preservation of fine-grained flood plains. These settings are rich in fossil remains that record
the contemporaneous fauna, like turtles and crocodiles (Mealla and Lumbrera Formations; Fig. 6a). This situation is exemplary in the different fluvial systems that were formed in the late postrift stage. The fluvial depositional systems associated with fossil records indicate that the
geography was dominated by sand plains and mud flats with extensive pastures, temporally flooded in warm climate but with marked, alternating dry and rainy seasons.
The herbivorous and leafy diets of the vertebrates together with the presence of pollen and paleosols confirm these conditions.

A succession of lakes was formed in the basin center that evolved from shallow saline (Mealla
Formation; Fig. 6a), shallow brackish-alkaline (Maı´z Gordo Formation; Fig. 6a), perennial freshwater (Faja Verde, middle part of the Lumbrera Formation; Fig. 6d) and clastic-saline lakes (Upper Lumbrera Formation; Fig. 6a). The development of one lake or the other was
regulated by the alternating humid and drier climate periods. The highest humidity conditions are recorded in the Faja Verde of the Lumbrera Formation because the deepest, freshwater perennial lake formed there. The sudden desiccation of this lake, evidenced by brecciated
surfaces and mud cracks, records the beginning of a dry period in the Upper Eocene that continued up to the Oligocene.

Los  mapas y esquemas utilizados en esta página han sido tomados/adaptados de la bibliografía correspondiente, entre la cual se incluyen los siguientes trabajos, que se recomiendan para la lectura:

Marquillas, R.A., del Papa, C. and Sabino, I.F. 2005. Sedimentary aspects and paleoenvironmental evolution of a rift basin: Salta Group (Cretaceous–Paleogene), northwestern Argentina. Int J Earth Sci (Geol Rundsch), 94: 94–113

Sánchez, M.C. y Marquillas, R.A. 2010. Facies y ambientes del Grupo Salta (Cretácico-Paleógeno) en Tumbaya, Quebrada de Humahuaca, provincia de Jujuy. Revista de la Asociación Geológica Argentina 67 (3): 383 - 391.

Mingramm A., A. Russo, A. Pozzo y L. Cazau, 1979. Sierras Subandinas. Segundo Simposio de Geología Regional Argentina. Academia Nacional de Ciencias. Córdoba, I: 95-137.

Mon R., 1979. Esquema Tectónico de los Andes del Norte Argentino. A.G.A., revista XXXIV (1): 53-60.

Salfity J.A., R. Omarini, B. Baldis y W.J. Gutierrez, 1975. Consideraciones sobre la evolución geológica del Precámbrico y Paleozoico del Norte Argentino. Actas Segundo Congreso Iberoamericano de Geología Económica. IV: 341-361.

Turner J.C.M. y V. Méndez, 1979. Puna. Segundo Simposio de Geología Regional Argentina. Academia Nacional de Ciencias. Córdoba, I: 13-56

Turner J.C.M. y R. Mon, 1979. Cordillera Oriental. Segundo Simposio de Geología Regional Argentina. Academia Nacional de Ciencias. Córdoba, I: 57-94.