Int J Earth Sci (Geol Rundsch) (2005) 94: 94–113 DOI 10.1007/s00531-004-0443-2

O R I GI N A L P A P E R

Rosa A. Marquillas Æ Cecilia del Papa Ignacio F. Sabino

Sedimentary aspects and paleoenvironmental evolution of a rift basin: Salta Group (Cretaceous–Paleogene), northwestern Argentina

Received: 23 July 2003 / Accepted: 27 August 2004 / Published online: 21 October 2004 Ó Springer-Verlag 2004

Abstract The rift history of the Salta basin is related to the evolution of the Central Andes and to the activity of the Pacific margin, owing to its geographic location. Sedimentation occurred from the Neocomian to the Paleogene, with deposits reaching up to 5,000 m in thickness. Paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes. Isolated grabens characterized the early synrift stage; differential subsidence provoked distinct environments in the southern and northern subbasins. In the southern subbasins, alluvial-fan, fluvial-fan and lacustrine deposits prevail, whilst in the northern subbasins eolian and fluvial environments dominate. During the Maastrichtian, two major factors controlled the basin fill: the decrease in tectonic subsidence and a relative sealevel rise as recorded in South America. An extensive and shallow Atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the Danian. Until the Middle Eocene, the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods. Paleontological records reflect these climatic changes and show their relationship to the sedimentation regime. Keywords Central Andes Æ Rift basin Æ Cretaceous–Paleogene Æ Salta Group Æ Sedimentary environments

Introduction During the Mesozoic, several extensional basins developed in South America (Uliana et al. 1988; Salfity and R. A. Marquillas (&) Æ C. del Papa Æ I. F. Sabino CONICET and Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 177, Salta, 4400, Argentina E-mail: [email protected]

Zambrano 1990; Hallam 1991; Tankard et al. 1995; Filho et al. 2000). The Salta basin (Early Cretaceous– Middle Paleogene), located in northwestern Argentina (Fig. 1a), is relevant because of its wide extent, excellent preservation (Fig. 1b) and hydrocarbon production (Turic et al. 1987; Marquillas and Salfity 1988; Go´mez Omil et al. 1989; Salfity and Marquillas 1994). The fill of this basin is represented by the Salta Group (Turner 1959). It is composed of three main units, from base to top: the Pirgua Subgroup (Reyes and Salfity 1973), and the Balbuena and Santa Ba´rbara Subgroups (Moreno 1970). The strata of the three subgroups together reach a thickness of 5,000 m. The Salta basin deposits accumulated in seven subbasins: Tres Cruces, Lomas de Olmedo, Meta´n, Alemanı´ a (Reyes 1972; Salfity 1982), El Rey (Salfity 1980), Sey (Schwab 1984) and Brealito (Sabino 2002) (Fig. 1a). The basin fill started after the Araucana orogenic phase of the Andes (Fig. 2) (Stipanicic and Rodrigo 1969), probably in the Neocomian, and ended in the Late Eocene–Early Oligocene (Salfity and Marquillas 1994) with the Incaica phase (Fig. 2) (Steinmann 1930). The basin was affected by the rift-related subalkaline magmatism, which started in the Late Jurassic and evolved into alkaline compositions during the Neocomian (Halpern and Latorre 1973; Turner et al. 1979; Galliski and Viramonte 1988; Viramonte et al. 1999; Cristiani et al. 1999). The sedimentary and volcanic history of the Salta basin was controlled by extensional tectonics (Reyes et al. 1976; Salfity and Marquillas 1986). According to seismostratigraphy, the Lomas de Olmedo subbasin corresponds to a ‘‘typical tectonic graben’’ with three main ‘‘evolutionary stages’’ (Bianucci et al. 1981). Bianucci and Homovc (1982) described the initial stage of the Salta basin (Pirgua basin) as an intracontinental rift. Galliski and Viramonte (1988) compared the Salta basin to a ‘‘foreland rift’’ and classify it as a rift of low volcanicity type. Viramonte et al. (1999) proposed that the Cretaceous plutonic and volcanic rocks from centralsouthwestern South America are related to an intracontinental rift environment.

95 Fig. 1 a Map of the Salta Group showing the main structural highs and subbasins, based on and adapted from Salfity and Marquillas 1994 (and data from Cherroni 1977; Clebsch 1991, and Matthews et al. 1996). Key: 1 Synrift stage basin margin. 2 Early postrift stage basin margin. 3 Late postrift stage basin margin, the grey shade represents positive areas during this stage. 4 PostAraucana to Neocomian marine coastal basin, LO Lomas de Olmedo subbasin, R El Rey subbasin, M Meta´n subbasin, A Alemanı´ a subbasin, B Brealito subbasin, S Sey subbasin, TC Tres Cruces subbasin. b Salta Group (late synrift and early postrift deposits) in the Meta´n subbasin (Juramento River), outcrop thickness of 400 m.

Based on the sedimentary history, the eruptive episodes and the structural processes, Salfity and Marquillas (1994) describe the tectonic framework and the evolution of the Salta basin. They recognize in the Salta rift basin two principal types of fill: the synrift and the postrift deposits. The first corresponds to the Pirgua Subgroup, when the faults were active and the second is represented by the Balbuena and the Santa Ba´rbara Subgroups, when the thermal subsidence happened. This criterion was adopted in the present contribution. Also, we differentiate an early and a late synrift stage based on the tectonic regime of the basin, and an early and a late postrift stage according to the different patterns of facies

distribution coincident with the major environmental changes of the basin. Previous works Besides the above mentioned contributions, the geology of the Salta basin has been extensively studied from different points of view, mainly during the last decades: stratigraphic point of view (Cazau et al. 1976; Salfity 1979; Salfity and Marquillas 1981; Marquillas 1985; del Papa 1994; Marquillas et al. 1997, 2003), petrological point of view (Carle´ et al. 1989; Galliski et al. 1989;

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Stage South American Mammal Age

b Formation

Diastrophic phase and basin stage

Fig. 2 Stratigraphic chart of the Salta Group consisting of deposits from the Late Neocomian to Late Eocene (time scale from Gradstein and Ogg 1996, South American Land Mammal Ages adapted from Legarreta and Uliana 1994; Marshall et al. 1997; Kay et al. 1999). The gray fields represent hiatuses

Tinguirirican Rupelian

Mustersan

35

SUBGROUP

SERIE

OLIG.

Time (Ma)

NEO. SYSTEM

96

Priabonian

Incaica

Danian

Maastrichtian

SENONIAN

70

80

Final Ranquel

Yacoraite Lecho

v vv v Palmar v v vLargo volcanics

Initial Ranquel

Los Blanquitos v v vv v v v Las Conchas Basalt

Campanian

Las Curtiembres

Santonian

85

Late postrift Mealla Olmedo/Tunal

Early postrift

Itaboraian Tiupampian Selandian

65

75

Maíz Gordo

Late synrift

EOCENE

Casamayoran

Thanetian

Coniacian

?Peruana

v vv v v

90

115

Aptian 120

Barremian NEOCOMIAN

125

130

135

140

Hauterivian

Final Mirano

First synrift cycle

110

Albian

Isonza Basalt v v v vv vv v v

v v vv

Alto de Las Salinas Complex v v v v vv v vvv v

Alkaline granitoids and syenitoids

Initial Mirano

Prerift magmatism

105

C R E TA C E O U S

100

Cenomanian

La Yesera

95

P I R G UA

Turonian

Valanginian

Berriasian

155

Tithonian MALM

JURASSIC

145

150

Kimmeridgian Oxfordian

Second synrift cycle

60

Ypresian ? Riochican

Lumbrera

Early synrift

55

Lutetian

B AL B U E N A

50

PA LEOCENE

45

PA L E O G E N E

40

S A N TA B Á R B A R A

Bartonian

Subalkaline granitoids

Araucana

Risso et al. 1993; Lucansen et al. 1999) and tectonic point of view (Schwab 1985; Grier et al 1991; Salfity et al. 1993; Mon and Salfity 1995; Comı´ nguez and Ramos 1995; Cristallini et al. 1998). Scarce paleontological content of dinosaurs, mammals and other vertebrate and invertebrate groups or flora have been recorded. Most paleontological papers refer to taxonomic aspects (Pascual et al. 1978; Powell 1979; Bonaparte and Powell 1980; Gasparini and Bu¨ffetaut 1980) and seldom to paleoclimatic and environmental aspects (Pascual et al. 1981; Quattrocchio et al. 2000; Quattrocchio and Volkheimer 2000b). Age of the Salta Group The Salta Group and equivalent deposits are widely distributed in the Central Andes region (Fig. 1a). For decades, the Salta Group has been related to the Puca Group in Bolivia (Steinmann 1906; Groeber 1939, 1952; Schlagintweit 1941; Russo and Rodrigo 1965; Reyes 1972; Sempere 1994) and to deposits in the west of Paraguay (Clebsch 1991) and in the north of Chile (Bru¨ggen 1942; Schwab 1973). Similar sedimentary successions to the postrift stage of the Salta Group have also been identified in the north of Chile (Salfity et al. 1985; Matthews et al. 1996, 1997). The age of the Salta Group (Fig. 2) is known by scarce radiometric dating, fossiliferous content and regional correlation. The radiometric data of the synrift basaltic rocks (Bossi and Wampler 1969; Valencio et al. 1976; Reyes et al. 1976; Clebsch 1991) indicate Neocomian to Upper Senonian ages. The late synrift and the early postrift deposits contain dinosaurs from the Senonian (Bonaparte et al. 1977; Powell 1979; Bonaparte and Powell 1980). In the deposits of the early postrift stage, dinosaur tracks (Alonso and Marquillas 1986) and also Danian palynomorphs (Moroni 1984; Quattrocchio et al. 2000) were found . Therefore, the K– T boundary is recorded in these deposits (Fig. 2) (Marquillas et al. 2002, 2003). In the late postrift stage, the ages were obtained from the fossil record of vertebrates. Pascual et al. (1981) assigned them to the Riochican–Casamayoran (South American Land Mammals Ages) (Fig. 2), by comparison with the vertebrates mainly found in the Argentine Patagonia. There is still no radiometric dating available that can guarantee these interpretations. Even though there exist numerous studies, the age of the Salta Group is relatively uncertain. Although it is not our principal aim, we present a discussion on the age of the Pirgua Subgroup that tends to clarify previous discussions on the chronology.

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Aim of this study The aim of this paper is to describe the depositional environments of the three subgroups of the Salta Group. It also aims to link the information of the sedimentary environments with the fossil content, and to recognize paleoclimatic changes during the Cretaceous and Paleogene. Stratigraphical and sedimentological information is provided by 27 detailed profiles that were investigated by the authors. They are distributed over several subbasins, except the eastern Lomas de Olmedo subbasin where the Salta Group is buried. The new data obtained by the authors and the paleontological information gathered by third parties are

Fig. 3 a Stratigraphic log of the Pirgua Subgroup in the southern area of the basin (see Fig. 4 for legend). b Lower and middle portions of La Yesera Formation in the Tonco canyon (Alemanı´ a subbasin) (outcrop thickness: 200 m). c Las Curtiembres Formation base overlaid La Yesera Formation in the Tonco valley (Alemanı´ a subbasin) (outcrop thickness: 100 m). d Los Blanquitos Formation in Cabra Corral dam (Meta´n subbasin) (outcrop thickness: 15 m)

used to interpret and reinterpret paleoenvironmental reconstruction. A synthesis is presented on the nine formations that constitute the Salta Group.

Pirgua Subgroup The Pirgua Subgroup (Reyes and Salfity 1973), which represents the Salta Group basin synrift fill, is composed of red beds and volcanic rocks (Fig. 3a). Sedimentation started during the Neocomian (Fig. 2), simultaneous to the Alto de Las Salinas Complex volcanic event (Bossi 1969) providing basalts and rhyolites dated 128–112 Ma (K/Ar, whole rock) in the southernmost area of the basin (Bossi and Wampler 1969). The volcanic rocks at

98

1

Shale, mudstone

2

Siltstone

3

Fine sandstone

4

Medium sandstone

5

Coarse sandstone

6

Limestone, fine conglomerate

7

Basalt, medium to coarse conglomerate Coarse to medium stratification Fine stratification and lamination Cross bedding Trough cross bedding Eolian sand Fine upward Massive Ripples Paleosol - root marks Desiccation marks Stromatolitic boundstone Fine oolite Coarse oolite Calcareous Gypsum Nodules Tuff Lava flow

Fig. 4 Legend of stratigraphic logs

the base of the deposit were dated 126±3.5 Ma (method not indicated) in the Paraguayan portion of the Lomas de Olmedo subbasin (Fig. 1a) (Clebsch 1991). Sedimentation started with the Initial Mirano phase (Fig. 2) defined in the Patagonean Andes (Stipanicic and Rodrigo 1969). The Pirgua Subgroup is composed of sandstones, conglomerates and siltstones at almost all localities. The geometry of the deposits reveals marked tectonic control (Go´mez Omil et al. 1989; Sabino 2002). The thickest successions, over 4,000 m, are contiguous to the normal faults of north–south and northeast–southwest trends. In the Alemanı´ a subbasin (Fig. 1a), maximum thickness rapidly decreases to 350 m, 40 km away toward the northeast. Three main cycles constitute the synrift fill, two in the early synrift stage and one in the late synrift stage: – First synrift cycle (finning upwards): lower and middle La Yesera Formation (Fig. 3a, b), – Second synrift cycle (finning upwards): upper La Yesera Formation and the Las Curtiembres Formation (Fig. 3a, c), – Third synrift cycle (coarsening upwards): Los Blanquitos Formation (Fig. 3a, d).

In the Pirgua basin, the coarse-grained deposits are related to less subsidence. They correspond to the beginning of the finning-upward cycles of the early synrift stage and to the end of the coarse-grained cycle of the late synrift stage. During these events, the coarsegrained deposits prograded to the basin center (Blair and Bilodeau 1988). On the other hand, the fine-grained deposits were accumulated during increasing basin subsiding. The fine-grained Las Curtiembres Formation corresponds to the rift climax, when the Las Conchas basaltic volcanism occurred in the center of the basin. In the same way, the accumulation of the Los Blanquitos Formation coarsening upward deposits represents the definitive decrease of the tectonic subsidence during the late synrift stage. Similar synrift fills with two finegrained deposits were described in the Newark, Sudan and West Africa basins (Lambiase 1990). The tectonics mainly controlled the synrift fill, as in other sequences accumulated during greenhouse times (Gawthorpe et al. 1994). The basement of the Pirgua basin consists of Neoproterozoic to Lower Paleozoic granitoids (Brealito and Sey subbasins, Fig. 1a), Neoproterozoic phyllites and schists (Alemanı´ a and Meta´n subbasins), and pelite and sandstone from the Lower and Upper Paleozoic (Meta´n, El Rey, Lomas de Olmedo, Tres Cruces and Sey subbasins) (Salfity 1979; Salfity and Marquillas 1989). The mineralogical composition of the Pirgua Subgroup sediments is directly related to the source area of each subbasin. These sediments texturally and mineralogically are immature and indicate that their distance from their source was short (Sabino 2002). The facies of the Pirgua Subgroup in the Tres Cruces subbasin are not as typical as in other parts of the basin (Sabino 2002). The sediments were mainly of orangereddish, well-selected, medium-grained quartzose sandstones with cross or parallel stratification; their thickness was between 0.5 and 2 m. Some sand layers show highangle cross stratification, frequently 3–5 m, even up to 30 m in thickness. In some sections, lava flows and volcaniclastic deposits are interbedded. The average thickness of the Pirgua Subgroup is about 700 m. The Sey subbasin (Fig. 1a), located in the Argentine Puna, is less known. The basin inversion during the Tertiary provoked intense deformation, and consequently the outcrops are frequently incomplete. Nevertheless, the sedimentary features observed are similar to the Brealito and Alemanı´ a subbasins. La Yesera Formation: description This formation is composed of three main sections defined in the Alemanı´ a 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

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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 Alemanı´ a 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 Alemanı´ a 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 Alemanı´ a 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 Alemanı´ a subbasin (near to the SaltaJujuy 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 lowenergy 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, Alemanı´ a, Meta´n 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 Alemanı´ a and Meta´n 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 (Ba´ez 1981), and plant remains probably from Bennetitals (Archangelsky, Iba´n˜ez 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.

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Los Blanquitos Formation: description The Los Blanquitos Formation represents a coarseningupward 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 Alemanı´ a 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 Meta´n 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 Alemanı´ a and Meta´n 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 Alemanı´ a 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) (Go´mez Omil et al. 1989), which means that the upper part of the Los Blanquitos Formation must have accumulated in Early Maastrichtian.

Paleoenvironmental interpretation of the Pirgua Subgroup The Pirgua Subgroup (Fig. 3a) marks the beginning of sedimentation of the Salta Group. This subgroup corresponds to a tectonically controlled continental deposit that was related to the synrift stage (Late Neocomian– Early Maastrichtian). The initial faulting forms isolated

subbasins that evolved from alluvial-fan environments to lakes in the rift-climax, and to fluvial environments during the late synrift stage. The radiometric ages of the volcanic rocks of the Pirgua Subgroup indicate that synrift stage sediments were accumulated during a lapse of about 60 million years. La Yesera Formation The main facies association of the lower La Yesera Formation (Fig. 3a, b) suggests that the most common environments were debris-flow-dominated alluvial fans and sandy braided rivers; pelite characterizes the mud plains. The lower La Yesera Formation accumulated mainly in alluvial fans. The lava of the Alto de Las Salinas Complex poured in these fans associated with the rift faults. The facies association of these deposits also shows an alternation of mud flats. In the middle La Yesera Formation, the predominance of the wacke and silty facies suggests that the most common sedimentary environment was of mud flats and sandy braided rivers. Brackish to freshwater perennial lakes formed in the western region of the basin (Brealito subbasin) at the end of the first synrift cycle of the Pirgua Subgroup. The second synrift cycle of the La Yesera Formation (Fig. 3a, c) began with the development of wide debrisflow-dominated alluvial fans whose conglomerates are similar to those of the first synrift cycle. These coarse deposits correspond to the upper section of La Yesera Formation where lava flows of the Isonza Basalt alternate. This was simultaneous with the Final Mirano phase (Stipanicic and Rodrigo 1969) defined in the Patagonean Andes. Las Curtiembres Formation After the Isonza volcanic event (Fig. 2), and the deposition of the widespread alluvial fans, some lakes occupied the widest part of the basin except the Tres Cruces subbasin. The lacustrine deposits of the Las Curtiembres Formation are mainly interpreted as being accumulated in a shallow and fresh to brackish water lake. During the Las Conchas event (Figs. 2, 3a), pyroclastic material entered the lakes. Its chemical interaction with the lake water strongly affected the lacustrine environment. This became evident from local higher alkalinity of the lake water as proved by limestone accumulation. Las Conchas volcanism occurred in the central area of the basin during the rift climax. This was unlike previous volcanic events that were initiated through the active faults of the basin producing lava flows interbedded with the alluvial fans (Alto de Las Salinas Complex and Isonza Basalt). The volcanic buildups in the Las Conchas event were located in inner areas of the lakes, which generated frequent phreatomagmatic eruptions (Risso et al. 1993).

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The shallow freshwater Las Curtiembres Lake in the Alemanı´ a subbasin evolved in a warm climate. The trace fossils are associated with the shallow littoral facies of the lake.

have been slightly more arid, perhaps due to local topographic conditions. In this subbasin, paleosols are rare and in some sections the column is completely composed of eolian deposits (e.g., in the southwestern area).

Los Blanquitos Formation

Balbuena Subgroup This unit (Figs. 2, 3a, d) was accumulated in the late synrift stage. The coarsening upward basal deposits represent the decrease of the tectonic subsidence, consequently, the basin began to fill (Reyes and Salfity 1973) and the sediment bypassed the topographic barriers. This is evidenced by the burial of the topographic highs between the subbasins and by changes in the sediment composition due to the modification of the source. Unlike the subjacent units where maximum thickness is located at the fault borders, the maximum thickness of the Los Blanquitos Formation is recorded in the central areas of each subbasin, reaching 1,500 m. Although the faults continued to be active (Cristallini et al. 1998), their relative movement was lower than during the accumulation of the previous units. The beginning of the late synrift stage may correspond to a change in the geodynamics of the western border of South America like the Peruana diastrophic phase (Fig. 2). The Peruana phase (Steinmann 1930) is recorded in the Santonian in Peru and related to the southward migration of the Farallon plate (Scheuber et al. 1994). The facies associations suggest that the Los Blanquitos Formation was accumulated by sandy rivers. This habitat was favourable for large vertebrates as Titanosauridae and Carnosauria (Bonaparte and Bossi 1967). The presence of carbonate nodules in the poorly developed paleosols of the formation is related to a semiarid to arid climate (Gile et al. 1966; Milnes 1992) that must have prevailed during the deposition. Pirgua Formation in the Tres Cruces subbasin In the Tres Cruces subbasin, the succession of the Pirgua Subgroup cannot be subdivided into formal lithostratigraphic units (La Yesera, Las Curtiembres and Los Blanquitos Formations), hence the name Pirgua Formation is used. Although thickness is reduced (700 m in average), it represents the entire synrift stage, based on radiometric dating (Rubiolo 1999) and on the stratigraphic relationship to the Balbuena Subgroup. No lakes were formed during the synrift stage in this subbasin. The sedimentary environments that controlled the accumulation were sandy fluvial and eolian. In both environments, orange-reddish sandstone facies prevail. In the fluvial environment, there are also scarce deposits of alluvial conglomerates, which are located at the base of the succession. The absence of lacustrine deposits could have been conditioned by a lower subsidence rate, which did not exceed the clastic supply rate of the Pirgua Subgroup basin. It is assumed that the climate must

The Balbuena Subgroup (Fig. 5) 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 Ba´rbara 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.

102 Fig. 5 a Stratigraphic log of the Balbuena Subgroup (see Fig. 4 for legend). b Eolian facies of the Lecho Formation in the Tres Cruces syncline (Tres Cruces subbasin), outcrop thickness 50 m. c Upper section of the Yacoraite Formation in Juramento River (Meta´n subbasin), outcrop thickness 40 m. d The Tunal Formation in the Corralito River, close to SW Salta-Jujuy high, outcrop thickness: 3 m

The Lecho Formation also shows red facies (sandstone and shale) along the eastern border of the Meta´n subbasin and red to purple facies in the Alemanı´ a 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)

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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

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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).

Paleoenvironmental interpretation of the Balbuena Subgroup The Balbuena Subgroup was accumulated in the initial stage of the postrift (Maastrichtian to Early Paleocene) Salta Group (Fig. 2). This deposit originated by the flooding of the extensive Maastrichtian transgression that affected southern South America. In northwestern Argentina, it overflowed all the previous borders of the synrift basin, except in the southwest, and widely advanced even over structural highs (Salfity and Zambrano 1990; Salfity and Marquillas 1994).

Lecho Formation The depositional environment of the Balbuena Subgroup commenced with fluvial and eolian processes that permitted the accumulation of the Lecho Formation. Distal braided rivers deposited the typical quartzose and arkosic-subarkosic sandstone facies of this formation. With the associated evolution of the dune fields, sorted clean sandstone was deposited. At the same time, the interdune environments accumulated calcareous and bioturbated sands (Go´mez Omil et al. 1989; Galli and Marquillas 1990). Even though the fluvio-eolian white-colored sandy facies dominates the Lecho Formation, a brief but widely distributed flooding event occurred in the initial times of accumulation. The limestones and dark pelites of the lower third of the formation in the Tres Cruces subbasin represent this event. Also, in the Meta´n subbasin, the basal thin pelites represent a lacustrine deposit. In the sandy to pelitic fluvio-lacustrine flood plain, in the south of the Meta´n subbasin, an association of tetrapods lived together. The fluvial facies of the Lecho Formation, linked with the Los Gallos Uplift (along the eastern border of the Meta´n subbasin), is reddish. It is assumed that the accumulation of this facies was influenced by tectonic processes that caused the episodic rise of the Los Gallos Uplift (Salfity et al. 1993), which started to emerge during the deposition of the Lecho Formation. Another remarkable change of lithological facies occurred in the Alemanı´ a subbasin. This deposit was considered as a new lithostratigraphic unit: the Quitilipi Formation (Salfity and Marquillas 1981) due to the marked differ-

ences with the typical facies of the Lecho Formation. The sedimentary setting of the Quitilipi Formation is interpreted to be of wet interdune areas. Yacoraite Formation The environmental conditions of the Maastrichtian postrift (Fig. 2) were substantially modified by the installation of an extensive and shallow carbonate environment, similar to the epeiric sea in the sense of Heckel (1972), where the Yacoraite Formation was accumulated (Marquillas 1985, 1986). The influence of the tides was minor during the deposition of the formation. Local tectonic as well as climatic factors had more influence on the nature of the facies than the variation of the sea level. This is coincident with the characteristics of the epeiric–carbonate platforms that have no modern environmental analogues (Tucker 1990). The lowest part of the Yacoraite Formation was deposited in a littoral shallow marine environment of alkaline, rough clean water (oolitic and intraclastic grainstone facies). During the accumulation of the middle part of the formation, conditions fluctuating between medium- to high-energy prevailed. Flaggy limestones (wackestone, packstone and oolitic grainstone) are typical there. Some sands result from the action of storm swash. In calm water situations, below the wave base level, some micrite and shale were accumulated. During the accumulation of the upper third of the Yacoraite Formation, the depositional regime was characterized by a succession of flooding stages and prograding processes, especially in the Meta´n and Alemanı´ a subbasins (Fig. 1a). This produced records of shallowing-upward sequences of a decimeter-to-meter range, formed mainly by green and black shale and followed by oolitic grainstone and stromatolitic boundstone (Fig. 5c). Frequent episodes of subaerial exposition were marked by mud cracks and intraformational breccia with rib-up clasts, a few mm up to or over 5 cm long. Some stages of drier climate could have favoured the formation of crystals and thin levels of gypsum and/or anhydrite. Also, early dolomitization of the sediment was observed. Lithologic evidence indicates that in the Yacoraite Formation environment the energy was highly variable, as was also the salinity and probably the temperature. For instance, the fossiliferous invertebrate association is restricted and essentially euryhaline. It consists of abundant gastropods, ostracods and pelecypods and of miliolid foraminifers in a minor proportion. The fossil records of these organisms show that small individuals predominated. This was due to critical conditions of the environment owing to the mixture of fresh and saline water and to temperature variations. In the upper part of the Yacoraite Formation in the Alemanı´ a subbasin, there are dinosaur footprints on the

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coastal deposits with fluvial-lacustrine influence. Ichnites are also found in the lower part of the formation, in the Meta´n subbasin. They undoubtedly indicate Cretaceous age. However, palynological studies performed in samples on holes in the Lomas de Olmedo subbasin confirm that the formation finished sedimentation in the Danian (Moroni 1982, 1984). Probably, the same happened in other regions of the basin. This is consistent with the interpretation of the Yacoraite Formation in the context of the transgressions that occurred in the south of South America during the Maastrichtian to Paleocene (Marquillas 1985; Marquillas and Salfity 1988). Tunal/Olmedo Formations In the Paleocene (Fig. 2), the Salta Group basin was dominated by the lacustrine and mud plain systems of the Olmedo Formation whose sediments covered the Yacoraite Formation limestone. In the Lomas de Olmedo subbasin (Fig. 1a), a hypersaline lake was surrounded by an extensive mud plain (Go´mez Omil et al. 1989), where the Olmedo Formation was deposited in predominant anoxic conditions. The shale of the Olmedo Formation contains Paleocene palynomorphs, which are represented by the high content of ulmaceae, tricolpate, tricolporate monocolpate and triporate pollen grains together with Mtchedlishvilia saltenia (Moroni 1982, 1984). During the accumulation of the Tunal Formation (lateral equivalent of the Olmedo Formation) in the Meta´n subbasin, a perennial lake with restricted drainage and saline characteristics developed. Close to the southern border of the Salta-Jujuy high (Fig. 1a), the deposits have scarce evaporitic facies (Novara 2003). The palynomorph association of the Tunal Formation defines the lacustrine environment with marshes, surrounded by forests, confirming a Danian age (Quattrocchio et al. 1988, 2000). In addition, it was determined that the deposit of the Tunal Formation occurred in a humid, warm climate similar to the current climate in the Yungas Province (subtropical jungle in northwestern Argentina) (Quattrocchio et al. 2000). However, there is sedimentary evidence (mainly by evaporitic facies) suggesting that climatic conditions were locally or temporally arid. In places where the Olmedo and Tunal Formations are absent, the Yacoraite Formation is covered by mudstone, red fluvial sandstone and mud plain deposits of the Mealla Formation (Sey and Tres Cruces subbasins and borders of some structural highs) (Marquillas and Salfity 1994).

Santa Ba´rbara Subgroup 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

106 Fig. 6 a Stratigraphic log of the Santa Ba´rbara Subgroup (see Fig. 4 for legend). b The Mealla Formation broad channel and fine-grained flood plain deposits (road signal is 2 m high). c The Maı´ z Gordo Formation lacustrine rocks, well bedded mudstone and carbonate strata (white levels) (outcrop thickness: 150 m). d The Faja Verde of the Lumbrera Formation lacustrine organic rich shale (Alemanı´ a subbasin). e The Upper Lumbrera Formation ephemeral lake facies in Alemanı´ a subbasin

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

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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 (Fig. 6a). 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).

Paleoenvironmental interpretation of the Santa Ba´rbara Subgroup The occurrence of facies association and their distribution show a circular arrangement of depositional systems. Fluvial environments dominated the external or outer zone and lakes developed toward the center of the basin. The fluvial setting ranges from braided to highsinuosity rivers, and the lake systems range from shallow mud flat to open perennial basins. Mealla Formation In the Mealla Formation (Fig. 6a), fluvial sedimentation took place in the Meta´n and Alemanı´ a subbasins. The presence of channel and lateral accretion bedforms and the thick successions of siltstone are interpreted as deposited in a meandering fluvial system with wide flood plains and paleosols (Fig. 6b). The sandy thin beds with planar bedding and unidirectional ripples interbedded with siltstones suggest sudden and waning flows consistent with crevasse channel and crevasse splay processes. The facies associations of the Mealla Formation in the El Rey and Lomas de Olmedo subbasins suggest deposition in a closed, very shallow lake. In the littoral areas, fine-grained clastic facies dominated while in the central lake evaporitic layers accumulated. Pandanaceae and Palmae (Nypa) dominated the palynological association suggesting shallow brackish water. Myriophyllumpollenites sp. and Azolla sp. are characteristic of calm water. Likewise, the presence of Ulmaceae and Aquifoliaceae reveals subtropical moist forest and montane paleocommunities in the surrounding areas (Quattrocchio et al. 1997). However, pollen grains of Ephedraceae suggest drier periods (Quattrocchio and Volkheimer 2000a). The paleoenvironmental reconstruction of the Mealla Formation suggests an extensive fluvial mud flat setting which was temporally flooded, and perennially active sinuous channels draining the plain. During periods of high discharge, crevassing processes cut the levee deposits and flooded the topographic depression of the plain. The vertebrates, leaf-eating notoungulates and the Ulmaceae pollen reaffirm the presence of forest vegetation. The development of these environments is consistent with the informal ‘‘Rousea patagonica’’ climatic zone dated Selandean (Quattrocchio and Volkheimer 2000b).

Maı´ z Gordo Formation The fluvial environment of the Maı´ z Gordo Formation is recognized in the western margin of the Alemanı´ a subbasin. The presence of a thick succession of sandy beds suggests temporally active channels and extensive

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sand flat deposited in sandy braided rivers. Toward the east, the facies association dominated by green finegrained sediments indicates low-energy sedimentation, in a very shallow lake or pounded mud flats in the sense of Castle (1990). Since the Late Paleocene until possibly the Early Eocene, the basin was gradually flooded. A closed lake with wide littoral zones (ramp type) and brackish, alkaline water was established (del Papa 1999a). Carbonate facies predominate toward the center of the Maı´ z Gordo lake (El Rey and Lomas de Olmedo subbasins, Fig. 1a), while, in the littoral areas, the carbonate facies was restricted to that containing discrete stromatolites (Alemanı´ a subbasin, Fig. 1a). In the inner lake, vertical facies assemblages are shallowing upward depositional sequences, 1–3 m thick, which are regionally extensive enough for a correlation between the subbasins (del Papa 1994). The palynomorph communities dominated by Pandanaceae and Palmae (Spinizonocolpites sp.) and nonmarine dinoflagellate cysts suggest brackish water, while the presence of Azolla sp. and Haloragaceae (Myriophyllumpollenites sp.) denotes periods of freshwater (Quattrocchio and del Papa 2000). During the deposit of the upper part of the Maı´ z Gordo Formation, an extensive transgression of the lake flooded the surrounding areas. Limestone rocks drastically diminished and green shale prevailed in the lake. Among other palynomorphs, Pediastrum algae reveal that the water became fresh. In the alluvial areas, gray paleosols developed, forming a distributed widespread horizon, which indicates poorly drained soils. Turtle remains of Pelomedusidae (Pascual et al. 1981) and Crocodylia Sebecosuchia Bretesuchus bonapartei (Gasparini et al. 1993), a terrestrial predator, were found in the fluvial environment and lake littoral zones. The plates of turtles are common in the filling channel facies, which are broken and accumulated as lag deposits. Probably, in the rainy season, new active channels eroded the alluvial plain, and organism remains were transported as bioclasts by fluvial currents. Lumbrera Formation Sandy beds with lateral-accretion geometry and siltstone with calcareous nodules in the lower section of the Lumbrera Formation record a sandy meandering fluvial system with point-bar structures, levee and thin-bedded alluvial plain settings (Fig. 6a). The presence of crocodiles and turtles in the extensive flood plain deposits is interpreted as characteristic of tropical forest areas (Pascual et al. 1981). During the accumulation of the middle section, the fluvial system was flooded by the Faja Verde lake transgression. The dark green claystone and finegrained, sheet-like sandstone, deposited in a perennial stratified lake (del Papa 1999b; del Papa et al. 2002). In the inner basin, bituminous shale was mainly laid (Fig. 6d). This facies association is indicative of an

anoxic bottom in which a sporadic underflow current occurred providing temporal oxygenation. Among the palynomorphs, the presence of pollen grains Notopollenites sp. and Liquidambarpollenites cf. Brandonensis is indicative of a tropical forest. The occurrence of Pediastrum and Botryococcus algae is associated with lake water fluctuation. Thus, in periods of lake-level fall, the Botryococcus became the dominant form and in periods of lake-level rise, the Pediastrum was the main alga (del Papa et al. 2002). In littoral areas, the presence of heterolithic facies, discrete stromatolite, fine-grained sand and mud cracks suggests a margin of low-energy sedimentation. In this setting, the fish Lepidosiren paradoxa was found, indicating pounded areas with progressive or temporal desiccation (Ferna´ndez et al. 1973). The sedimentary facies association of the upper section of the Lumbrera Formation, massive siltstones, evaporitic layers and sheet-like sandstones, document deposition in a closed shallow saline lake. Mud cracks and burrowing suggest frequent periods of desiccation or very shallow water. Some authors (Zachos et al. 1991) claim that global cooling and an increase in drier conditions occurred during the Late Eocene up to the Oligocene. The evolution of the Lumbrera lakes records climatic change from a humid to a drier period. Similar conditions were interpreted in many others basins in Argentina, like the Malargu¨e Group basin (Malumia´n et al. 1998), the Colorado basin (Quattrocchio and Guerstein 1988), and the Sarmiento Formation basin (Mazzoni 1979).

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

Alemanía subbasin

Littoral freshInner freshwater lake water lake Meandering river Sublittoral Sandy braided Littoral brackishbrackishriver to brackish alkaline lake alkaline lake mud-flat Brackish-saline lake Meandering river Brackish lake Hypersaline lake Lake-Swamp Carbonate shallow sea

Thanetian

Itaboraian Selandian Danian

River-lake

Maastrichtian

Late synrift Coniacian

90

Turonian

95

110

Brackish lake Albian

115 Aptian

125 130

NEOCOMIAN

120

Eolic-river Meandering river

Brackish lake

Alluvial-fan

First synrift cycle

105

Eolic-river-lake Braided to meandering river

?

Cenomanian

C R E TA C E O U S

100

Braided river

Campanian

Santonian

85

(Eroded)

Second synrift cycle

SENONIAN

80

Saline shallow lake

Mud-flat to sandy braided river

Alluvial-fan to braided river

Alluvial-fan Mud-flat to sandy braided river

Alluvial-fan

(Subsurface data not available)

? Riochican

Saline mud-flat Late postrift

Lutetian

70 75

Lomas de Olmedo subbasin

(Eroded)

Early postrift

65

EOCENE

PA L E O G E N E

60

Metán subbasin

Rupelian

Ypresian

PALEOCENE

55

Brealito subbasin

Bartonian

40

50

Basin stage

Mustersan

35

45

Stage South American Mammal Age

Casamayoran

30

SERIE

Time (Ma)

OLIG.

Fig. 7 Simplified temporal and spatial distribution of the main sedimentary environments of the Salta Group (see Fig. 1 for location of the subbasins)

NEO. SYSTEM

109

Alluvial-fan to braided river

?

Barremian

? Hauterivian

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; Fig. 3a). 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

110

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. Acknowledgements This study was supported by the grants PIPCONICET 0883 (RM), PEI 6091(CdP), PICT-ANPCyT 12419 (RM) and 12492 (IS) of Argentina; also by grants CIUNSa (Universidad Nacional de Salta, Argentina) 1220 (RM) and 1281 (IS). We acknowledge Dr. Jose´ A. Salfity’s and Dr. Wolfgang Volkheimer’s constant support and valuable advice. We are grateful to the reviewers, Prof. Andreas Scha¨fer and Prof. Heinrich Bahlburg, for their constructive comments on the manuscript.

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