1

Chapter 3 

Sedimentology and paleoenvironment of the Santa Cruz Formation 

 

Sergio D. Matheos and M. Sol Raigemborn 

 

Abstract 

The Santa Cruz Formation in the coastal area of Santa Cruz Province, Austral Patagonia, 

Argentina, is a continental succession characterized by fluvial sediments which are mainly 

fine and tuffaceous in the lower section (Estancia La Costa Member) and are coarser and 

siliciclastic towards the top (Estancia La Angelina Member). The lower member bears a rich 

mammalian association of Santacrucian Age (late Early Miocene). Compositional and 

paleoenvironmental analysis of the lower member, allows the differentiation of three sections: 

lower, middle and upper. The lower section crops out in the northern part of our study area. It 

is a succession of fine primary and reworked tuffs and massive silty sandstones with 

immature paleosols deposited in a relatively low energy fluvial system with vegetated 

floodplains with high sedimentation rates and a volcanic source. The middle section shows an 

evident increase in the coarse facies and paleosols, and a decrease in pyroclastic materials, 

which suggests a relatively higher energy for the system compared to the lower section, with a 

lower sedimentation rate and a more distal volcanic source. The upper section is 

compositionally even coarser and is exclusively siliciclastic in origin, and evinces even more 

energetic conditions of the fluvial system but with a fluctuating flow regimes and places 

where the vegetation was scant. During the deposition of Estancia La Costa Member the 

climate changed from warm with somewhat cooler and/or dryer intervals (lower and middle 

sections) to cool and dry conditions towards the top.  

 



 2

Resumen 

En la zona costera de la provincia de Santa Cruz, Patagonia Austral, Argentina, aflora la 

Formación Santa Cruz (Mioceno temprano) portadora de fauna de mamíferos de Edad 

Santacrucense. Dicha unidad es una sucesión continental de origen fluvial, compuesta 

principalmente por material volcaniclástico fino en su sección inferior (Miembro Estancia La 

Costa) con mayor participación de material silicoclástico hacia la sección superior (Miembro 

Estancia La Angelina). Sobre la base del análisis composicional y paleoambiental 

desarrollado exclusivamente en el miembro inferior, se diferenciaron tres secciones: inferior, 

media y superior. La sección inferior, aflorante en la parte norte del área de estudio, es una 

sucesión compuesta por tobas primarias y retrabajadas y areniscas limosas masivas con 

procedencia de arco volcánico, con frecuentes niveles de paleosuelos inmaduros. Dicha 

sucesión representa un sistema fluvial de energía relativamente baja con llanuras de 

inundación vegetadas, acumulado con una tasa de sedimentación relativamente alta. La 

sección media muestra un incremento en las facies de mayor granulometría, una disminución 

en la participación de material piroclástico y una mayor participación de niveles edafizados, 

indicando una procedencia volcánica más distal y un ambiente con energía relativamente 

mayor que la sección inferior, acumulado bajo una tasa de sedimentación relativamente baja. 

Por último, la sección superior resulta exclusivamente de origen silicoclástico y de facies 

gruesas, lo cual evidencia condiciones aún de mayor energía del sistema fluvial con un 

régimen de flujo fluctuante. Durante la depositación del Miembro Estancia La Costa el clima 

osciló desde cálido, con algunos intervalos fríos y/o secos (sección media e inferior) a 

condiciones más frías y secas hacia la parte superior. 

 

 

 



 3

3.1. Introduction 

Outcrops of the continental Santa Cruz Formation in the coastal area of Santa Cruz Province 

(late Early Miocene), Austral Patagonia, Argentina, present a record of fluvial deposits, which 

are mainly composed of volcaniclastic material, and are very rich in vertebrate fossils 

(Tauber, 1994, 1997; Kay et al., 2008, among others). The Santa Cruz Formation is the most 

widespread and most richly fossiliferous of all Patagonian nonmarine Tertiary formations of 

southern Argentina (Tauber, 1994; Tejedor et al., 2006; Vizcaíno et al., 2006, 2010, 

Tambussi, 2011). For this reason, the unit has been intensely studied since the 1890s from a 

paleontological and biostratigraphical point of view (see references in Vizcaíno, Kay and 

Bargo, this volume), but the sedimentological aspects of the formation have been only briefly 

described (Bown and Fleagle, 1993; Tauber, 1994; Fleagle et al., 1995). 

The deposition of the Santa Cruz Formation in the Austral geologic basin can be 

understood in the context of regional tectonic, geographical and climatic changes (Blisniuk et 

al., 2005, 2006; Barreda and Palazzesi, 2007; Tassone et al., 2008; Ramos and Ghiglione, 

2008; Madden et al., 2010; Quattrochio et al., 2011) and during the time interval (17-15 Ma) 

that experienced a global climatic event named as Mid-Miocene Climatic Optimum (Zachos 

et al., 2001).  

This sedimentological study of the Santa Cruz Formation has the following objectives: 

1) to characterize compositionally the Estancia la Costa Member and to establish the source 

area of its sediments; 2) to present a description of the facies and facies association within the 

formation; 3) to reconstruct the paleoenvironment of deposition; 4) to infer the paleoclimatic 

conditions that prevailed in Austral Patagonia during the mid-Miocene; and 5) to consider the 

regional effects of the volcanism on the sedimentary environment. 

 

3.2. Geological setting and stratigraphy 



 4

The Santa Cruz Formation is part of the fill of the cratonic area of the foreland Austral or 

Magellan Basin in southernmost South America (Russo et al., 1980; Olivero and Malumián, 

2002, 2008; Menichetti et al., 2008). The basin follows a preferential NNW-SSE axis and is 

limited by the Mountain range of the Andes, toward the West, and by the Alto de Río Chico 

to the East (Biddle et al., 1986; Corbella, 2002; Peroni et al., 2002). The latter is the southern 

extension of the Deseado Massif that served as the basement of the Cretaceous sedimentation 

separating to the East the Austral-Magellan Basin from the Malvinas Basin (Rossello et al., 

2008). Our study area comprises the southeastern part of this basin. 

The geological history of the Austral Basin begins in the Triassic with the initiation of 

rifting between South America and Africa (Biddle et al., 1986). The Bahía Laura Group 

consists of volcaniclastic rocks deposited in grabens and half-grabens during the Jurassic 

(Uliana et al., 1985). During the Late Jurassic-Early Cretaceous the margin of the basin was 

closed by the uplift of the western arc (Arbe, 1987) and the deposition of the Springhill 

Formation occurred (Corbella, 2002). Subsequently, from the Early Cretaceous into the 

Miocene, the basin experienced repeated transgressions and regressions of the sea. Towards 

the east the marine Palermo Aike Formation (Campanian-Maastrichtian) was deposited, while 

to the north marine and continental deposition alternated: continental San Julián Formation 

(Late Eocene-Early Oligocene), marine Monte León Formation (Late Oligocene-Early 

Miocene), continental Santa Cruz Formation (late Early Miocene) and finally, the Pliocene 

marine terraces and continental Rodados Patagónicos (Late Pliocene-Quaternary) (Uliana et 

al, 1985; Corbella, 2002; Peroni et al., 2002). The continental deposits of the Santa Cruz 

Formation occurred during an important phase of deformation and uplift of the cordillera in 

the west. Its sedimentation clearly indicates that the regression of the marine deposits of the 

underlying Late Oligocene - Early Miocene Centinela Formation (= 25 de Mayo Formation, 

Cuitiño and Scasso, 2010) in the west was forced by the cordillera uplift (Ramos and 



 5

Ghiglione, 2008). The deposition of the Santa Cruz Formation occurred in a foreland basin 

where sediment supply exceeded the accommodation space. As a result, a prograding 

sequence expanded to the Atlantic coast (Nullo and Combina, 2002). 

 

3.2.1. Distribution of Santa Cruz Formation and study area 

The Santa Cruz Formation is developed over much of the south of Patagonian Argentina both 

in surface exposures and in drill logs, from the Eastern area of the South Patagonian 

Cordillera (Cordillera Patagónica Austral) (Bande et al., 2008; Cuitiño and Scasso, 2010) 

southwards from Lago Buenos Aires to the Río Turbio area, and eastward from Extra-Andean 

Patagonia to the Atlantic Ocean between Golfo de San Jorge and northern Tierra del Fuego 

(Marshall, 1976; Bown and Fleagle, 1993; Tauber, 1997; Malumián, 1999). Maximum 

thickness of these continental deposits is reached along the northern foothills of the 

Patagonian Cordillera with thicknesses up to 1500 m, decreasing to 225 m along the Atlantic 

coast (Riggi, 1979; Riccardi and Rolleri, 1980; Tauber, 1997). 

The outcrops of the Santa Cruz Formation in the study area are arrayed parallel to the 

Atlantic coast of Santa Cruz Province between Parque Nacional Monte León and Río 

Gallegos (see Vizcaíno, Kay and Bargo, this volume, Figs. 1.1 and 1.2). The following 

localities were studied: north to the Río Coyle, Rincón del Buque (= Media Luna; S 50º 39’ 

41’, W 69º 06’ 52’) and Cañadón del Cerro Redondo (S 50º 51’ 25’, W 69º 08’ 03’) (Fig. 

3.1a); south of the Río Coyle from north to south, Punta Sur-Cañadón del Indio (S 50º 58’ 

59’, W 69º 09’ 56’), Campo Barranca (S 51º 00’ 36’, W 69º 09’ 15’), Anfiteatro (S 51º 03’ 

12’’, W 69º 08’ 20’’, Estancia La Costa (S 51° 04' 42.6", 69° 08' 02.2"), Cañadón Silva (S 51º 

11’ 03’’, W 69º 05’ 55’’) and Puesto Estancia La Costa (S 51º 11’ 47’,; W 69º 05’ 34’) (Fig. 

3.1b-e), 



 6

This study area has an active sea cliff with numerous slumps, slips and scouring, but 

which subsumes most of the thickness the Santa Cruz Formation with excellent surface 

exposures. In this place the unit shows dips of 1-3º SE (Tauber, 1994, 1997). Lithologically, 

the exposures of the coastal Santa Cruz Formation (225 m-thick on average) are a succession 

of superimposed mudstones, fine to medium sandstones of volcaniclastic origin, and tuffs 

with light colors, containing immature paleosols lay down on a coastal plain incised by sand 

bodies, with some pebbles, representing river channels (Bown and Fleagle, 1993; Tauber, 

1994, 1997). The basal limit of the Santa Cruz Formation is located below the last oyster bed 

(Ostrea orbignyi) (Griffin and Parras, this volume), located 23 m above current sea level at 

Punta Norte (Feruglio, 1938), and towards sea level near Rincón del Buque and Cerro 

Redondo, and buried south of Río Coyle (Fig. 3.1a). The contact between the units in this area 

is considered transitional and conformable (Tauber, 1994; Matheos et al., 2008). The upper 

limit of the Santa Cruz Formation is marked by the marine sediments of the Cape Fairweather 

Formation and/or by the Rodados Patagónicos (Fig. 3.1b and 3.2). 

 

3.2.2. Lithostratigraphy 

The Santa Cruz Formation was formally named near Lago Argentino, located approximately 

300 km west of the coastal area, by Furque and Camacho (1972) and Furque (1973), and 

recently by Cuitiño and Scasso (2010). Furque and Camacho (1972) and Furque (1973) 

distinguished three members (Fig. 3.2). They are, from the oldest to the youngest, the Los Dos 

Mellizos Member (250 m-thick of gray, yellowish and greenish gray clays), the Bon Acord 

Member (150m-thick of conglomeratic sandstones and blue tuffs) and the Los Huelguistas 

Member (95 m-thick of sandstones and conglomerates).  

Tauber (1994, 1997) recognized two members in the coastal outcrops of the Santa 

Cruz Formation south of Río Coyle: a lower very fossiliferous unit, the Estancia La Costa 



 7

Member, with a predominance of pyroclastic deposits with claystones and mudstones, and an 

upper fossil-poor unit, the Estancia La Angelina Member, chiefly composed of claystones, 

mudstones and sandstones (Figs. 3.2, 3.3 and 3.4). The two members are separated by a 

discontinuous surface evident on the sea cliffs from the north of Estancia La Costa to Cabo 

Buen Tiempo. Tauber (1994) and Matheos et al. (2008) states that in the coastal area, the base 

of the Santa Cruz Formation is transitional with the marine Monte León Formation (Griffin 

and Parras, this volume), meanwhile the top of the unit is unconformable overlaid by marine 

deposits of Cape Fairweather Formation or the Rodados Patagónicos (Fig. 3.2). 

Tauber (1994), on the basis of the lithological similarity, suggested that the Estancia 

La Costa Member might correspond to the Bon Acord Member, and the Estancia la Angelina 

Member could be equivalent to the Los Huelguistas Member of Furque (1973) (Fig. 3.2).  

On the base of the sedimentological characteristics of the Estancia La Costa Member 

and the location of its fossiliferous levels, we divided the member informally in three 

sections: lower, middle and upper section (see below and Figs. 3.2 and 3.4). 

 

3.2.3. Biostratigraphy-Biochronology 

The continental Santa Cruz Formation was first recognized by Ameghino (1902) who named 

it “Piso Santacruzeño”, based on its fossil vertebrate content. Later, Ameghino (1906) on the 

basis of its fossil vertebrate content recognized two continental étages within the formation 

santacruzéen: the notohippidéen and the santacruzéen (see Vizcaíno, Kay and Bargo, this 

volume and references therein. The “étage Notohippidien” is typically represented in 

Karaiken (Marshall and Pascual, 1977; Riccardi and Rolleri, 1980) and contains the 

Notohippus fauna. The “étage Santacruzéen” is typically represented in the Eastern region of 

the Santa Cruz Province in the Atlantic coast area (e.g. Marshall and Pascual, 1977), and 

according to Ameghino (1906), this stage is the youngest.  



 8

Mammal faunas from the richly fossiliferous Santa Cruz Formation were the basis for 

recognition of the Santacrucian Land Mammal Age of South America. However, the deposits 

corresponding to the “étage Astrapothericuléen” of Ameghino (1906) that outcrops in the 

northwestern part of Santa Cruz Province, along the banks of the Río Pinturas, were 

correlated with sediments belonging to “Colhuehuapense” (Frenguelli, 1931) and finally they 

were associated with the Santa Cruz Formation (De Barrio, 1984; Marín, 1984; Ramos, 

1999). This assemblage of sediments received a new formal designation, the Pinturas 

Formation (Bown et al., 1988; Bown and Ratcliffe, 1988; Fleagle, 1990). Biostratigraphic 

studies (Bown and Fleagle, 1993) and recent radiometric dates (Fleagle et al., 1995; Fleagle et 

al., this volume) suggested that part of this unit is older than the Santa Cruz Formation at the 

typical coastal localities (Kramarz and Bellosi, 2005). The last authors mentioned that the 

lower and middle sequences of the Pinturas Formation are older than the base of the Santa 

Cruz Formation at Monte León and, consequently, older than the Santacrucian age of 

Simpson (1940). Soria (2001) suggested that all the Miocene fauna from Pinturas Formation 

must be regarded as early Santacrucian in age, whereas those from the Santa Cruz Formation 

are late Santacrucian. A new date for the Pinturense (Upper Fossil Zone of Colhue-Huapi 

Member of Sarmiento Formation), that crops out at Gran Barranca, south of Chubut province, 

gives 18.7 to 19.7 Ma (Ré et al., 2010). 

Tauber (1997) described two Biozones for the coastal Santa Cruz Formation, the 

Protypotherium attenuatum Biozone (the youngest) and the Protypotherium australe Biozone 

(the oldest).  

3.2.4. Geochronology and Chronostratigraphy 

The age of the Santa Cruz Formation has been a subject of debate over a century. Marshall et 

al. (1986) concluded that the Santa Cruz Formation range from 17.6 to 16.1 Ma (K/Ar from 

Monte León and Rincón del Buque localities). Later, Fleagle et al. (1995) dated levels from 



 9

Monte Observación (i.e. Cerro Observatorio at Cañadón de las Vacas) and Monte León 

localities, with Ar/Ar, and yielded an age of 16.5 Ma for the Santa Cruz Formation, and 19.33 

Ma for the top of the Monte León Formation (Fig. 3.2). At the same time, Fleagle et al. 

(1995) dated samples of Pinturas Formation using Ar/Ar and obtained an age of 17.76 Ma for 

the lower sequence of the unit, and 16.5 Ma for the top and middle sequence of the unit (Fig. 

3.2). These data and biochronologic evidence (see above) indicate that the Pinturas Formation 

lies below the levels of Santa Cruz Formation at Cerro Observatorio and that the Monte León 

Formation is older than the Pinturas Formation. Fleagle et al. (1995) indicated that the 

Pinturas Formation is intermediate in age between the Monte León and Santa Cruz 

Formations. Fleagle et al. (1995) proposed that the age of the Santa Cruz Formation at Monte 

Observación (i.e. Cerro Observatorio) and Monte León could be correlated with Chron 5Cn. 

The long section of reversed magnetic polarity above the dated levels would correlate with 

Chron C5Br which ranges from 15.2 to 16.0 Ma. In this volume Perkins et al. provide 

integrated results of the tephra correlations and radiometric ages indicating that the Santa 

Cruz Formation spans the interval ~18 to 16 Ma in the Atlantic coastal plain and ~19 to 14 

Ma in the Andean foothills, with a chronologic overlap between the Pinturas Formation and 

lower part of the Santa Cruz Formation. 

 The radiometric dates mentioned above permit the assignment of the bulk of the 

coastal Santacrucian faunas to the late Early Miocene. At the same time, the beds below the 

Santa Cruz Formation, the Monte León Formation, are consistent with an Early Miocene age 

of the mollusc assemblage from that unit (del Río, 2004). In addition to this, new ages 

obtained by Parras et al. (2008) from the Centinela and San Julián formations, suggest that the 

age of the Monte León Formation along the coast should lie between 24.12 and 19.33 Ma 

(Fig. 3.2). 

 However, more recently Ar/Ar ages obtained by Blisniuk et al. (2005) of the Santa 



 10

Cruz Formation at the south of Lago Posadas, yielded 22.36 Ma for the base of the unit, and 

14.24 Ma for the upper section (Fig. 3.2). On the basis of these data Ramos and Ghiglione 

(2008) interpreted that the continental foreland basin began at about 23 Ma along the 

foothills, and that progradation had reached the Atlantic coast by about 19 Ma.   

 

3.3. Methodology and Techniques 

3.3.1. Composition 

In order to define the composition of the Estancia La Costa Member of the Santa Cruz 

Formation, to establish the source area and to evaluate its relationship with the paleoclimatic 

context, mudstones, sandstones and tuff samples of different localities were collected to carry 

out diffractometric analyses and petrographic studies. 

A total of 24 samples, including siliciclastic, reworked and primary pyroclastic material 

from Campo Barranca, Anfiteatro and Estancia La Costa localities were analyzed using X-ray 

diffraction (XRD). Soft grinding with a rubber mortar was used to disaggregate the more 

indurated samples, followed by repeated washes in distilled water until deflocculating 

occurred. The <4 μm fraction was separated by gravity settling in suspension, and oriented 

mounts were prepared on glass slides. Clay mineralogy was determined from diffraction 

patterns obtained by using samples that were air dried ethylene glycol solvated and heated to 

550 °C for 2 hours (Brindley and Brown, 1980). Diffractograms were run on X PANanlytical 

model X´Pert PRO diffractometer (Centro de Investigaciones Geológicas, Universidad 

Nacional de La Plata, Argentina), using Cu/Ni radiation and generation settings of 40 KV and 

40 mA. Routine air dried mounts were run between 2 and 32 °2θ at scan speed of 2 °2θ/min. 

Ethylene glycol solvated and heated samples were run from 2 to 27 °2θ and 3 to 15 °2θ, 

respectively, at a scan speed of 2 °2θ /min. Semiquantitative estimates of the clay mineral 

relative concentrations were based on the peak area method (Biscaye, 1965) on glycolated 



 11

samples (17 Å for smectite, 10 Å for illite and 7 Å for chlorite and kaolinite. The relative 

percentages of each clay mineral were determined with the application of empiric factors 

(Moore and Reynolds, 1989). Semiquantification was considered sufficient to define clay 

mineral composition because the presence/absence or dominate/subordination relationships 

clearly allowed establishing significant groups. 

Rocks for petrographic studies included a total of 42 samples with 14 thin-sections of 

very fine to coarse-grained sandstone and 28 thin-sections of reworked and primary tuffs. 

These samples were analyzed with a Nikon Eclipse E-200 petrographic microscope. Criteria 

used to distinguish lithic types, matrix types and other components are those used by 

Dickinson (1970). A qualitative approach was made to characterize the main constituent of 

the rocks, matrix and cement. Both, roundness and sphericity of the clasts were estimated by 

visual comparison chart (Pettijohn et al., 1987).  

 

3.3.2. Facies analysis and paleoenvironmental interpretation 

With the aim of understanding the different sedimentary paleoenvironments and sedimentary 

conditions where the Estancia La Costa Member was deposited, facies and facies associations 

follows Miall (1996) and Bridge (2003) with modification for pyroclastic deposits (Smith, 

1987) were analysed into the outcrops. The shape of the sandy bodies were classified 

following Friend et al. (1979) as tabular bodies (width/thickness ratio > 15), and lenticular or 

lens-shape bodies (ratio < 15). 

 

3.4. Results 

3.4.1. Composition  

Clay mineralogy. Clay minerals identified in the early Estancia La Costa Member of the 

Santa Cruz Formation include smectite (Sm), illite (I), chlorite (Chl) and mixed layer clays 



 12

containing illite/smectite (I/S). The results obtained from the XRD analysis in the <4 μm 

fraction are listed in Table 3.1. The non-clay minerals identified in this fraction, in decreasing 

order of abundance, are quartz and feldspars (plagioclases > K-feldspar), and small amounts 

of amorphous silica (opal), zeolite (clinoptilolite), carbonates and gypsum. In all the 

stratigraphic levels studied, the smectite is the most frequent clay mineral, and is present in 

almost all the analyzed samples, while illite, chlorite, and mixed-layer clays are less frequent.  

On the basis of the presence, type and relative amount of the above mentioned clay 

minerals in the studied samples, three clay mineral assemblages have been defined (Table 

3.1). The S1 assemblage is composed exclusively by Sm, the S2 assemblage is composed by 

Sm>I, I/S and the S3 assemblage is composed by Sm>I, Chl. Stratigraphic distribution and 

proportion of clay minerals and the X-ray diffraction patterns of the <4 μm fraction 

representative of each assemblages are shown in Figure 3.5a.  

1) Smectitic Assemblage (S1): this assemblage is characterized by a total dominance of 

smectite (100%) and is better represented in the lower and middle section of Estancia La 

Costa Member at Campo Barranca, Anfiteatro and Estancia La Costa locality (Table 3.1, Fig. 

3.5a-c).  

2) Smectite-Illite and mixed layer clays bearing Assemblage (S2): the high proportion of 

smectite (90-95%) associated with illite (5%) and mixed layer clays of Illite/Smectite (5%) 

characterize this assemblage. The S2 assemblage dominates the lower section of Estancia La 

Costa Member at Anfiteatro and Estancia La Costa sections (Table 3.1, Fig. 3.5a-c).  

3) Smectite-Illite-Chlorite Assemblage (S3) : this assemblage is the less representative of the 

studied units and is characterized by a lower proportion of smectite (65-80%) compared to the 

other clay assemblages, with small to moderate amounts of illite (15-25%) and/or chlorite (5-

30%). The S3 assemblage has been only identified at Campo Barranca profile in levels 

corresponding to the lower section of the Estancia La Costa Member (Table 3.1, Fig. 3.5a -c). 



 13

Petrography of reworked and primary tuffs. Thin section analysis revealed that pyroclastic 

samples of Estancia La Costa Member are similar in general and classified as vitric tuffs, 

according to the scheme of Pettijohn et al. (1987) (Table 3.2). The samples show moderate to 

poor sorting and a mean size that is coincident with the range of coarse tuff (0.063-2 mm) 

(Fig. 3.6a) and fine tuff (< 0.063) (Fig. 3.6b) sensu Fisher, 1961). The framework grains are 

sub-angular to angular vitroclasts with low sphericity, sub-angular crystaloclasts and sub-

rounded lithoclasts. The vitroclasts are composed by a great proportion of glass shards with 

platy and cuspades types (sensu Fisher and Schmincke, 1984) and pumice fragments with 

flattened or rounded vesicles (Fig. 3.6a). In general glass shards are more abundant than 

pumice fragments, and inside of these, cuspate glass shards and pumice with flattened 

vesicles are the most common. The vitroclasts are fresh and show few signs of diagenetic 

alteration. The crystaloclasts occur as euhedral to subhedral and fresh grains of twinned and 

oscillatory zoned plagioclases, monocrystalline quartz with straight extinction (Fig. 3.6b) and 

k-feldspar. The lithoclasts are composed by frequently unaltered volcanic lithic fragments 

with pilotaxitic, trachytic and felsitic textures. In addition to this, there are accessory minerals 

such as biotite, pyroxenes (hyperstene and augite), amphiboles (hornblende), zircon and 

opaque minerals like magnetite. 

The principal cements are clay and Fe-oxides coatings and calcite in patches. Another 

feature of diagenetic alteration is the distortion of several vitroclasts by compaction. The 

analyzed tuffs were altered only by synsedimentary and pedogenic conditions.  

Some micropaleoedaphic features like aggregates (blocky peds), variety of voids (Fig. 

3.6b), clay-coatings, and concentric and typic Fe-nodules (sensu Bullock et al., 1985), can be 

recognized in certain tuff samples of the lower and middle section of the Estancia La Costa 

Member at Punta Sur-Cañadón del Indio, Cañadón del Cerro Redondo, Rincón del Buque, 

Campo Barranca, Anfiteatro, Estancia La Costa and Puesto Estancia La Costa localities 



 14

(Table 3.2). The characteristics mentioned before and the macroscopic paleoedaphic features 

shown in the outcrop (Table 3.3) can be associated with the development of incipient 

paleosols (Retallack, 2001). However, the presence of blocky peds suggests a better 

developed and more slowly forming paleosol (Retallack, 2001). Tauber (1994) classified the 

first paleosols as probably mollisol and immature, and the second ones as more mature.  

Sandstone petrography. Sandy samples with scant matrix were studied from Rincón del 

Buque, Campo Barranca and Puesto Estancia La Costa localities (Table 3.2). In general the 

samples are moderate to well sorted and the framework grains are rounded to subangular with 

low sphericity (Fig. 3.6c, d). The texture of the sandstones is clast-supported and contacts 

between the grains are tangential or planar (Fig. 3.6c, d). The samples can be classified as 

litharenites (sensu Folk et al., 1970) corresponding to undissected to transitional arc 

provenance and lithic recycled orogen (sensu diagrams of Dickinson et al., 1983). 

The framework grains are composed in decreasing order of abundance by rock 

fragments, feldspars, and quartz. The lithic fragments are volcanic in origin and have 

pilotaxitic, trachytic and felsitic textures and with fresh aspect (Fig. 3.6c, d). However, 

partially altered volcanic lithics are also present. Sedimentary fragments are identified in 

smaller proportions. The dominant feldspar is fresh plagioclase crystals with oscillatory 

zonation and twinning (Fig. 3.6d), but in a few cases the crystals show signs of dissolution 

(Fig. 3.6c, d) and replacement by clay-minerals or carbonates. K-feldspar is very rare or 

nonexistent. The quartz is generally monocrystalline with straight extinction and inclusion-

free. Policrystalline quartz grains or monocrystalline grains with undulose extinction are 

detected less frequently. Some biotite, glauconite, hornblende, hyperstene and opaque 

minerals are also present.   

The matrix is sparse and is of the orthomatrix and pseudomatrix type. The principal 

cements are clay and Fe-oxides coatings, but in several samples sparry calcite patches (Fig. 



 15

3.6d) and siliceous cement is observed. These diagenetic products suggest that the burial 

conditions were very shallow and coincident with synsedimentary or eodiagenetic conditions 

(Harwood, 1991; Morad et al., 2000; Worden and Burley, 2003).  

Matheos et al. (2008) described similar petrographic and diagenetic characteristics for 

the sandstones and tuffs of the levels that form the transition between Monte León and Santa 

Cruz Formations in the area of the Parque Nacional Monte Léon. 

 

3.4.2. Facies analysis 

According to the lithology, sedimentary structures, thickness and geometry of the sedimentary 

bodies, eight principal lithofacies are identifiable in Estancia La Costa Member: 1) massive 

clast or matrix-supported fine conglomerates (Gm); 2) massive coarse and fine vitric tuffs 

(Tm); 3) massive lithic sandstones (Sm); 4) trough cross-bedded lithic sandstones (St); 5) 

planar cross-bedded lithic sandstones (Sp); 6) horizontal laminated sandstones, in places 

deformed, (Sl); 7) laminated mudrocks (Fl); and 8) massive very fine sandstones, tuff or 

reworked tuff and mudrocks (Fm). The characteristics of facies and a dynamic interpretation 

of each one are summarized in Table 3.3. 

 

3.4.3. Facies associations  

The deposits of Estancia La Costa Member have been grouped in four Facies Associations 

(FA).  

Facies Association 1 (FA-1) is composed of lens- or tabular-shaped gravel-sandstones 

with cross-bedding, fining-upward bodies and slightly erosive-based with intraclasts (facies 

Gm, St, Sp), under and overlain by fine-grained facies (Sm, Sl, Fl, Fm, Tm) (Fig. 3.7a, b and 

c). FA-1 is mainly greenish-grey and brown to yellowish-green. The matrix of the coarser 

facies of this FA is composed of smectite and chlorite (S3 clay mineral assemblage). The 



 16

sandstones (St facies) are rich in lithic clasts (litharenite), smectite, illite and chlorite (S3 clay 

mineral assemblage). In some sandy levels incipient paleosols and bioturbation is recognized. 

FA-1 is especially evident towards the middle and upper section of Estancia La Costa 

Member (Fig. 3.7b, c). This facies association can be interpreted as the infill of fluvial 

channels. The absence of lateral accretion surfaces implies that the channels were laterally 

stable. On the other hand, the absence of internal erosional surfaces, suggests deposition in a 

single channel (Miall, 1996). The presence of fine intraclasts on the base of the bodies is the 

result of the reworking of components of underlying fine sediments (Fm, Fl, Tm facies). 

These channel deposits are present as single bodies enclosed in floodplain facies (Fig. 3.7b 

and c); less frequently channels are multiple units that can crop out at the same stratigraphic 

horizon, suggesting the presence of multiple active channels (Makaske, 2001). The presence 

of pedogenic features and bioturbation in the upper levels of the filling indicates subaereal 

exposure suggesting that the water flow was ephemeral, not permanent. Especially, the 

development of iron-rich concretions in Sp facies indicates mobilization of iron, which 

suggests frequent fluctuation in groundwater levels (Retallack, 2001).  

Facies Association 2 (FA-2) is composed of tabular to lens-shaped, greyish-green 

massive beds (facies Sm), and laminated and/or deformed lithic sandy or silty sandy beds 

(facies Sl), where paleoedaphic features and phytoliths remains are evident (Fig. 3.7b-d). 

Fossil leaves are encountered in medium to fine laminated sandstones in the lower levels at 

Rincón del Buque. Bioturbation is evident in FA-2, especially in the Sl facies (Fig. 3.7d).  

FA-2 assemblages are common in the lower and middle section of Estancia La Costa 

Member. This association is interpreted as deposits generated during periods of overbank 

flooding or sheet flooding and deposited in a proximal floodplain (crevasse or sheet splay 

deposits for tabular bodies and crevasse channels for lenticular bodies sensu Miall, 1996). The 

presence of laminated sandstones (Sl facies) suggests that the streams experienced variations 



 17

in water discharge (Tunbridge, 1984). Following deposition, these bodies were levelled by 

subaerial exposure, bioturbation and pedogenesis. Deformation of soft sediment related to 

liquefaction and fluidization developed during or shortly after deposition (e.g. van Loon and 

Mazumder, 2011 and references therein), and can be interpreted as caused by high-

sedimentation rates and sudden fluctuation in discharge (Owen, 1996). In particular, the 

presence of calcareous concretions (Fig. 3.7f) can be interpreted as an excess of alkaline 

solutes and precipitation due to fluctuations in ground-water table in a well-drained 

environment (Retallack, 2001).  

Facies Association 3 (FA-3) consists of tabular bodies of massive or laminated grey 

and bluish-green very fine lithic sandstones and mudrocks (facies Fm and Fl), in part of 

volcaniclastic origin (vitric tuffs and reworked tuffs), bioturbated, pedogenically modified and 

with desiccation cracks (Fig. 3.7a-c). The clay-mineral composition of FA-3 is characterized 

by the presence of smectite, illite and chlorite (S3 clay mineral assemblage). Vertebrates and 

phytoliths are frequently found in this facies. FA-3 is the dominant association towards the 

middle and top sections of the Estancia La Costa Member. FA-3 is interpreted as material 

settling from suspension in distal setting of the floodplain. The presence of immature 

paleosols and desiccation cracks indicates fluctuating wet to dry surface conditions (Ghazi 

and Mountney, 2009) or desiccation of superficial sediments between flooding events 

(Collinson, 1996). The grey-green colour indicates a perennially water-logged, vegetated 

mudflat setting, indicative of an oxygen-poor environment and a ferrous state of the iron in 

the paleosols levels (Besly and Fielding, 1989). In addition, fine volcaniclastic levels may be 

aeolian deposits (loess) (Tauber, 1994, 1997). 

Facies Association 4 (FA-4) consists of tabular massive bodies composed by Tm 

facies, non-graded, with planar basal surfaces (Fig. 3.7a, b, e). The finest tuffs are greyish-

green in colour and in some occasions show carbonaceous remains; FA-4 also contains coarse 



 18

white tuffs. Compositionally the Tm facies are vitric and rich in smectite (S1 clay mineral 

assemblage) or rich in smectite with small proportions of illite and chlorite (S2 clay mineral 

assemblage). Several beds are weakly pedogenized (Fig. 3.7e) and show bioturbation. Fossil 

vertebrates and the remains of carbonized logs and phytoliths (Brea et al., 2010 and Brea et 

al., this volume) are present in this facies association. FA-4 levels have great lateral 

continuity and constitute guide beds where the principal fossiliferous levels are found. In fine 

Tm facies at Killik Aike Norte, Monte León and Cerro Observatorio localities, Genise and 

Bown (1994) and A. Tauber, personal communication (2010) found burrows of solitary bees 

and nests of scarabeid dung-beetles (see also Krapovickas, this volume). FA-4 is present at 

lower and middle section of the Estancia La Costa Member but is most common in the lower 

section (Fig. 3.7a, b). This facies association was formed by intermittent deposition of tabular 

ash-fall beds in low-gradient, subaereal and subaqueous areas in a poorly drained floodplain 

(Cas and Wright, 1987). After deposition, pyroclastic-fall deposits were slightly modified by 

pedogenesis and bioturbation. The colour of the finest tuff could suggest that these paleosols 

were developed due to slight variation in drainage characteristics across the floodplain. The 

presence of calcareous steinferns of trees (Fleagle et al., 1995), carbonized fossil logs, 

phytoliths and carbonaceous remains attests to the availability of standing water on the 

floodplain (Roberts, 2007) and indicate that this floodplain were vegetated. However, the 

presence of burrows of bees and scarab beetles suggests that at least more open areas were 

developed (Genise and Bown, 1994; Tauber, 1997). 

 

3.5. Implications for paleoenvironmental conditions of the Estancia La 

Costa Member 



 19

Sedimentological and compositional data of the Estancia La Costa Member provide a clearer 

understanding of the paleoenvironmental and paleoclimatic conditions and the nature of the 

source material that persisted during deposition in southern coastal Santa Cruz Province. 

 

3.5.1. Depositional environments  

Depositional processes, facies and facies associations can be used to interpret the 

paleoenvironmental setting for the succession and to reconstruct the depositional history of 

the unit (e.g. Paredes et al., 2007; Roberts, 2007).  

The depositional paleoenvironment of the Estancia La Costa Member in the study area 

was previously interpreted by Tauber (1994, 1997) as a low energy fluvial system developed 

under warm and humid climatic conditions. On the other hand, he interpreted the upper 

member of the Santa Cruz Formation (Estancia La Angelina Member) as a more high energy 

fluvial system with sheet-like deposits and meandering channels formed under a more arid 

climate. Outcrops of the Santa Cruz Formation northwest of the study area (Lago Posadas) 

were interpreted by Bande et al. (2008) as representing a meandering or anastomosing fluvial 

system. At Lago Argentino, to the west of the study area, Cuitiño and Scasso (2010), interpret 

the lower section of Santa Cruz Formation as a meandering fluvial system. At San Jorge 

Basin, north of the study area, Bellosi (1998) suggested that the Santa Cruz Formation was 

deposited on a vegetated coastal and alluvial floodplain with sheet-like channels changing to 

meandering channels towards the top of the unit. 

We interpret deposition of the Estancia La Costa Member as being dominated by a 

fluvial environment characterized by sheet-flooding, overbank-flooding and ash-fall deposits 

and single laterally stable channels. The preponderance of sheet-flood and overbank deposits 

compared to within-channel deposits, indicates a broad floodplain across which the channel 

was built. The common primary and reworked tuff deposits attest that the setting was 



 20

influenced by ash-fall events. The vertical accretion of the floodplains was by the deposition 

from crevasse-splays and crevasse-channels, sheet-floods and settling from suspension. These 

deposits eventually became desiccated, allowing the formation of subaereal floodplain soils 

(incipient paleosols) with bioturbation. The development of paleosols was influenced by 

fluctuations in the groundwater table. In this sense, the presence of greyish colours, the 

occurrence of calcareous and ferruginous concretions and desiccation cracks suggest slight 

variation in drainage characteristics across the floodplain with fluctuating wet to dry surface 

conditions, even with periods of perennially water-logged and well-drained conditions, or 

desiccation of superficial sediments between flooding events. The presence of phytoliths, 

carbonaceous remains, fossil leaves, logs and steinferns of tress confirms the existence of 

large vegetation indicating the constant availability of water on the floodplains; also see Brea 

et al., this volume.  

However, some paleoenvironmental changes in the Estancia La Costa Member are 

noted. Informally, we consider the member to contain three sections (Fig. 3.4). A lower 

section of this member, outcropping in Rincón del Buque (=Media Luna), Cañadón del Cerro 

Redondo, Punta Sur-Cañadón del Indio and Campo Barranca, had a greater input of 

pyroclastic material and a relatively lower energy for the fluvial system based on the greater 

proportion of tuff beds interbedded with mudrock or silty sandstone beds deposited by ash-

fall events with sheet-flooding and overbank-flooding. In this part of the unit the paleosols are 

less evident than the middle and upper sections, suggesting a higher sedimentation rate and a 

relatively continuous volcanic source due to the pyroclastic composition of the facies. In 

addition to this, the presence of soft-sediment deformational structures in the lower section of 

the unit is consistent with high sedimentation rates and decrease in discharge (Owen, 1996). 

Fossil leaves and logs are exclusively present in this interval and phytoliths remains are 



 21

common. This situation can be linked with paleoenvironments with both arboreal and shrub 

vegetation (interspersed woodlands and grasslands).  

Throughout the middle section of the member (upper outcrops of Rincón del Buque 

and outcrops of Anfiteatro, Estancia La Costa, Cañadón Silva and Puesto Estancia La Costa) 

there is an evident increase in the participation of coarse facies (Tm, Gm, St, Sp), which 

suggests a relatively higher energy for the system. However, towards the upper zone of the 

middle section we again see a gradual decrease in the proportion of sandstones, an increased 

participation of pyroclastic material and an increase in mudrocks. We can interpret this as the 

resumption of a paleoenvironment with lower energy and with more siliciclastic source. More 

developed paleosols and bioturbated beds are frequent in middle and upper parts of the middle 

section. This situation could reflect a lower sedimentation rate compared to the lower section 

of the member, and probably several interruptions in the volcanic source, which favoured 

increased landscape maturity and pedogenesis. The principal fossiliferous levels and the 

phytolith remains are concentrated in the basal and upper parts of the middle section 

(Tauber’s fossil levels FL-1 to FL-4.1 in the former, and FL-5 to FL-7 for the latter). The 

existence of these fossils can be linked with densely vegetated paleoenvironments. 

The base of the upper section of Estancia La Costa Member (outcropping at Puesto 

Estancia La Costa and Monte Tigre-Cañadón Las Totoras) shows more energetic conditions 

with a greater participation of sandstones which are finer towards the middle zone of the 

upper section. The top of the upper section is composed of both sandstones and mudrocks. 

The absence of pyroclastics and phytolith remains suggests respectively that those 

environments only received siliclastic material and that were more openly vegetated 

(grasslands). Paleosols and fossiliferous levels are found in the basal (FL-7.1 and FL-7.2) and 

upper zones in the upper section (FL-8 to FL-10). The most remarkable feature is that the 



 22

paleosols have a clear colour and are marked by the presence of calcareous concretions and 

desiccation cracks consistent with a fluctuating flow regime. 

  

3.5.2. The record of paleoclimatic change 

Clay minerals in sedimentary sequences provide important information on pre- and post-

burial conditions. Pre-burial controls include source area lithology, paleoclimate (chemical 

and physical weathering), depositional environment and topography, among other things (e.g. 

Chamley, 1989; Inglés and Ramos-Guerrero, 1995). Although this information can be altered 

by diagenesis that changes the original clay mineral composition (e.g. Egger et al., 2002), the 

study of the change of detrital clay minerals in sequences that did not undergo intense 

diagenesis becomes a significant tool to understanding the environmental conditions of 

deposition (Net et al., 2002; Raucsik and Varga, 2008; Do Campo et al., 2010). The 

abundance of smectite with limited amounts of illite and chlorite throughout the studied 

sections in the Santa Cruz Formation, together with the low abundance of mixed-layer clays 

and the compositional-diagenetic aspects of sandstones and tuffs, indicate that these deposits 

were not affected by deep-burial diagenesis. This suggests that diagenetic effects (post-burial 

conditions) on the composition of clay mineral assemblages were mimimal, and that the 

assemblages reflect the conditions at deposition.  

Due to the existence of a volcanic source areas and the recurrence of tuffaceous beds, 

the great participation of smectite, together with opal and clinoptilolite registered in the <4 

μm fraction, suggests the partial devitrification and alteration of volcanic ash under relatively 

warm and wet climatic conditions (De Ros et al., 1997; Dingle and Lavelle, 2000). Smectite 

is typically formed with chemical weathering in weakly drained soils under warm and 

seasonal climates with alternating wet and dry conditions (Chamley, 1989; Robert and 

Kennett, 1994; Thiry, 2000). On the other hand, illite, chlorite and mixed layer illite/smectite 



 23

can be developed in immature soils that have undergone little chemical weathering. In 

general, illite and chlorite are indicators of mechanical erosion that affected the parent rocks 

either as a result of cool and dry climatic conditions or as a consequence of a pronounced 

relief (Robert and Kennett, 1994; Thiry, 2000).  

In this context, the three clay-mineral assemblages recognized (S1, S2 and S3) can be 

related mainly to different weathering histories of a similar source material. The presence of 

smectite with illite, chlorite and illite/smectite mixed layers (S2 and S3 assemblages) in the 

lower section of Estancia La Costa Member (Campo Barranca, Anfiteatro and Estancia La 

Costa localities) probably indicates periods with cool or cool and dry climatic conditions 

(Chamley, 1989; Robert and Kennett, 1994; Thiry, 2000). Nevertheless, the presence of 

smectite as sole clay-mineral towards the middle section of Estancia La Costa Member (S1 

assemblage) could indicate a shift to warmer and seasonal climatic conditions (Fig. 3. 5c). In 

addition, the upper levels of the Estancia La Costa Member rich in smectite and illite were 

interpreted by Matheos et al. (2010) as indicating a more persistent cool and dry climate. 

Paleofloristic evolution of the Late Oligocene-Early Miocene of southern Patagonia 

demonstrated that forest elements are gradually replaced by other shrubby and herbaceous 

forms, signaling the beginning of the expansion of xerophytic environments determined by a 

cooling and drying trends (Barreda and Palazzesi, 2007). This is concordant with the 

existence of both closed and open environments for the Santa Cruz Formation (Vizcaíno et 

al., 2010; Tambussi, 2011) and would be comparable with temperate forests and bushland 

with less than 1000 mm of annual rainfall (Vizcaíno et al., 2010).  

Isotopic studies from pedogenic carbonate contained in outcrops of the Santa Cruz 

Formation at Lago Posadas (northwest of Santa Cruz Province) demonstrated that surface 

uplift of the southern Andes occurred at 16.5 Ma led to a climatic deterioration with a strong 

aridification in the eastern foreland and presumably, strongly increased precipitation rates on 



 24

the windward western side of the mountains (Blisniuk et al., 2005; 2006). These conditions 

were apparently accompanied by a transition from balanced subtropical woodlands and 

grasslands to predominantly grasslands (Blisniuk et al., 2005), corresponding with the lower 

and middle section of the Estancia La Costa Member and with the upper section of this 

member and Estancia La Angelina Member, respectively. In this scenario, the change trend 

towards the youngest zone of the section may reflect that the initial phase of this uplift caused 

an increased supply of moisture to the eastern foreland prior to rain shadow effect became 

much stronger at 15–14 Ma.  

The late Early Miocene clay-mineral composition of coastal Estancia La Costa 

Member indicates some cooling and/or drying alternating with warmer, more humid periods 

registered globally during the Early Miocene (Zachos et al., 2001) (Fig. 3.5c). A shift to 

warmer and seasonal climatic conditions showing at the middle section of the member 

probably will be coincident with part of the global Middle Miocene warmth known as Mid-

Miocene Climatic Optimum (Zachos et al., 2001) that took place between 17 and 15 Ma (Fig. 

3.5c). Tauber (1997) interpreted that this stratigraphic interval, where fossiliferous levels from 

1 to 4 occur, as indicating a wooded environment (woodlands and grasslands) with more 

humid and warmer climatic conditions that the upper interval (fossiliferous levels from 8 to 

11). The return to cooler and dryer conditions towards the upper levels of the member (ca. 16 

Ma) is consistent with the interpretation of Tauber (1997) and Vizcaíno et al. (2006). These 

authors proposed an environment with open vegetation developed in relatively dry conditions 

with marked seasonality, similar to that recorded today in the Chaqueña Biogeographic 

Province, for the upper levels of Estancia La Costa Member and part of the Estancia La 

Angelina Member (ca. 16 to 14 Ma). Moreover, Brea et al. (2010) and Brea et al. (this 

volume) demonstrated that prevail climate during the lower part of Estancia Las Costa 

Member was temperate to warm and subhumid to dry with marked seasonality. In addition, 



 25

ichnoassemblages of the Estancia Las Costa Member defined by Krapovickas (this volume) 

show a general trend from the bottom to the top of the unit from more humid to dryer 

conditions. 

The decreasing temperatures implied by the fossil and isotopic record and clay-

mineral composition may be related to gradual global cooling after ~14.5 Ma (Fig. 3.5c), 

which was associated with rapid Antarctic ice sheet growth, and major biogeographic changes 

on all continents (Zachos et al., 2001). At the same time, the change in clay-mineral 

composition registered at the analyzed section of Estancia La Costa Member would be 

correlated with changes in erosion rates identified by Blisniuk et al. (2005). The presence of 

illite together with smectite towards the upper section of the member (Matheos et al., 2010) 

would be a consequence of an increase of precipitation and erosion rates on the humid 

windward western side of the mountains. Blisniuk et al. (2005) indicated that the increasing 

surface elevation would have blocked more moisture from reaching the leeward eastern side 

of the mountains. 

 

3.5.3. Source areas and volcanic influence 

The compositional analysis of sandstones is a useful tool to characterize the source area of 

sediments (Dickinson and Suczek, 1979; Dickinson et al., 1983). On the other hand, the 

compositional study of volcaniclastic and pyroclastic rocks helps clarify the clast-forming 

processes and establish the character, composition, and setting of the volcanic source (McPhie 

et al., 1993). Nevertheless, diagenesis and weathering are the major factors in altering 

volcanic glass and minerals, and depositional texture destruction (Tucker, 1996).  

The great participation of lithic fragments of volcanic origin, together with the 

preponderance of plagioclase, and the presence of inclusion-free quartz with straight 

extinction in the sandstone samples, indicate that the source-area for these deposits was 



 26

dominated by volcanic rocks (Folk, 1964; Dickinson and Suczek, 1979), as confirmed by the 

abundance of plagioclase crystals with oscillatory zonation (e.g. Spalletti et al., 1993). In this 

context, the presence of volcanic fragments with pilotaxitic and trachytic textures are linked 

with an intermediate volcanic source-rock, and volcanic fragments with felsitic texture with a 

more acidic volcanic source-rock (Dickinson, 1970; Scasso and Limarino, 1997). Both the 

fresh aspect and a bigger size of the volcanic fragments compared with the other components 

of the sandstones suggest that the lithics and the volcanism are contemporaneous (Critelli and 

Ingersoll, 1995). Similarly, the small contribution of quartz and feldspar crystals in the lithic 

fragments confirms active volcanism (Scasso and Limarino, 1997). At the same time, the 

sedimentary lithics and the polycrystalline quartz fragments are derived from deformed 

sedimentary units of the thrust belt (Critelli and Le Pera, 1994). Sources from magmatic arc 

and recycled orogen frequently coexist, especially when the evolution of the orogenic belt 

included active volcanism (Critelli and Ingersoll, 1994). The dominance of vitric fragments 

with glass shards (cuspade and platy-shape) and pumice fragments (flattened- and rounded-

vesicles) in the tuffaceous samples is linked with plinian-like explosive eruptions of felsic to 

intermediate magmas (McPhie et al., 1993; Mazzoni, 1996; Umazano et al., 2009). 

Based on the inferred composition, tectonic setting of source-area (volcanic arc) and 

the characteristic of the volcanism that could have generated the tuffaceous material; the main 

source-area for the sediments of Estancia La Costa Member must be related to the Miocene 

volcanic cycle described by Ardolino et al. (1999). This volcanism was explosive and 

fragmentary (acidic to intermediate in composition) and took place in the Patagonian Andes 

region and Austral Basin during the Early Miocene (Ardolino et al., 1999). However, no 

Miocene volcanic unit crops out in this area and the only evidence of this coeval Miocene 

volcanism is given by the pyroclastic detritus in the clastic sedimentary beds of the Santa 

Cruz Formation. At the same time, the emplacement of subvolcanic bodies (18-16 Ma) and 



 27

the contemporary intrusion of granitic and dioritic rocks in the Andes region (Argentina and 

Chile) were interpreted by Ramos (1999) and Nullo and Otamendi (2002) as the magmatic 

phase related with the uplifting of the Austral Patagonian Andes. In addition, Hatcher (1897, 

1903) described several craters that cross the lower beds of Santa Cruz Formation and 

considered the possible existence and activity of these craters before and during the 

sedimentation of the unit, and proposed that this volcanism could be the source of the 

youngest tuffaceous material of this unit.  

In this geological context, Ramos (1999) and Ramos and Ghiglione (2008) interpreted 

the succession of Santa Cruz Formation as synorogenic deposits that mark the uplifting of the 

Austral Patagonian Andes. Bellosi (1998) mentioned that the paleocurrent orientation in the 

Santa Cruz Formation north of our study area is predominantly eastward, and this is in 

concordance with the uplifting of the Austral Patagonian Andes and foothill areas. 

Considering the presence of subduction processes at the western margin of Patagonia 

during the Early Miocene (Ramos, 1999; Ardolino et al., 1999; Nullo and Otamendi, 2002; 

Ramos and Ghiglione, 2008; among others), contemporaneous with the deposition of the 

Santa Cruz Formation, the tuffaceous material of this unit is likely to represent the distal 

eruptive phase of a coetaneous explosive volcanism generated in an arc magmatic setting. 

This is related to the uplift of the Austral Patagonian Andes, located about 300 km westwards 

of the study area. The ash was probably transported towards the coast by the westerly winds.  

 

3.6. Conclusions 

The coastal Santa Cruz Formation has two members: the Estancia La Costa Member 

(pyroclastic deposits, claystones and mudstones rich in fossils), and the Estancia La Angelina 

Member, (fossil-poor claystones, mudstones and sandstones). On the basis of 



 28

sedimentological features we divided the first member informally into a lower, middle and 

upper section. 

Clay minerals identified in the lower section Estancia La Costa Member of Santa Cruz 

Formation include smectite (Sm), illite (I), chlorite (Chl) and mixed-layer clays containing 

illite/smectite (I/S). The smectite is the most frequent clay mineral, and is present in almost all 

the analyzed samples, while illite, chlorite, and mixed layer clays are less frequent. Three clay 

mineral assemblages have been defined: S1 assemblage composed exclusively by Sm, S2 

assemblage composed by Sm>I, I/S and the S3 assemblage composed by Sm>I, Chl. The 

presence of S2 and S3 assemblages in the lower section of Estancia La Costa Member 

probably results from periods with cool or cool and dry climatic conditions, and this can be 

related with cooling and/or drying alternated with warmer periods. Meanwhile, the upper 

levels of the member are rich in smectite and illite suggesting a more persistent cool and dry 

climate regime. 

The pyroclastic samples of Estancia La Costa Member are vitric tuffs. The sandstones 

are litharenites. The tuffaceous material represents the distal eruptive phase of an explosive 

volcanism generated in an arc magmatic setting and related to the uplift of the Austral 

Patagonian Andes, located about 300 km westwards of the study area. This episode is related 

with the Miocene volcanic cycle. 

The lower section of the Estancia La Costa Member is dominated by pyroclastic 

material deposited in a relatively low energy fluvial system with a high sedimentation rate; 

the environments were vegetated. The middle section of this member shows an evident 

increase in coarse facies which suggests a relatively higher energy of the system compared to 

the lower section, but with a lower sedimentation rate. The upper section of the member 

evinces even more energetic conditions of the fluvial system but with a fluctuating flow 

regimes and places where the vegetation was more open. 



 29

During the deposition of Estancia La Costa Member the climate was seasonal and 

changed from warm with some cooler and/or dryer intervals (lower and middle section) to 

cool and dry conditions towards the top. 

The climate and the source of volcaniclastic material played important roles in the 

evolution of this system. Based on the clay-mineral assemblages and paleoedaphic features, 

we conclude that during the deposition of the Estancia La Costa Member the climate was 

seasonal and warm trending towards cooling and/or drying stages towards the top. The 

volcanic deposits in the Estancia La Costa Member reflect distal pyroclastic events related to 

the uplift of the Austral Patagonian Andes. Uplift of Patagonian Andes created an effective 

barrier to the prevailing winds, producing one of the most severe rain shadows in the world. 

 

3.7. Acknowledgments  

The authors thank Sergio Vizcaíno, Richard Kay and María Susana Bargo for their invitation 

to participate in this book. Our special thanks to S. Vizcaíno and his group of collaborators for 

the logistical support and to S. Vizcaíno, S. Bargo, A. Tauber. J. C. Fernicola, N. Muñoz, V.  

Krapovickas, and J. Vizcaíno for their help in the fieldwork. This is a contribution to the 

UNLP N549 and PIP-CONICET Nº 114-200801-00066 projects to SDM, and NSF grants 

0851272 and  0824546 to Richard F. Kay. 

 

3.8. References  

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crétacés de Patagonie. Boletín de la Academia Nacional de Ciencias, 17, 5-70. 

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Patagonie avec un parallelé entre leurs faunes mammalogiques et celles de l´ancien 

continent. Anales del Museo Nacional de Buenos Aires, 8, 1-568. 



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 Arbe, H. A. (1987). El Cretácico de la Cuenca Austral. Boletín de Informaciones Petroleras, 

9, 91-110.  

Ardolino, A, Franchi, M., Remesal, M. and Salani, F. (1999). El vulcanismo en la Patagonia 

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Minería de la Nación, Servicio Geológico Minero Argentino e Instituto de Geología y 

Recursos Minerales, 18, 579-612. 

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Barreda, V. and Palazzesi, L. (2007). Patagonian Vegetation Turnovers during the Paleogene-

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Bellosi, E. (1998). Depósitos progradantes de la Formación Santa Cruz, Mioceno de la 

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111.  

Besly, B. M. and Fielding, C. R. (1989). Palaeosols in Westphalian coal-bearing and red-bed 

sequences, central and northern England. Palaeogeography, Palaeoclimatology and 

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Biddle, K. T., Uliana, M. A., Mitchum Jr., R. M., Fitzgerald, M. G. and Wright, R. C. (1986). 

The stratigraphic and structural evolution of the central and eastern Magallanes Basin, 

southern South America. In Foreland Basins, eds. Allen, P. A. and Homewood, P , 

Special Publication International Association of Sedimentologists 8, 41-61.  

Biscaye, P. (1965). Mineralogy and sedimentation of recent deep-sea clay in the Atlantic 

Ocean and adjacent seas and oceans. Geological Society of American Bulletin, 76, 803-

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Figure captions  

Figure 3.1. a- Photograph showing the lower and middle section of Estancia La Costa 

Member at Punta Sur of Rincón del Buque locality. White arrows indicate the first 

conspicuous tuff level of the unit, close to the last oyster bank that outcrops (black arrow). b- 

View of the lower section of Estancia La Costa Member at Punta Sur-Cañadón del Indio 

locality. Arrows indicate the first three more conspicuous tuff levels of the unit. The person in 

the bottom is working at the beds that contain the fossil logs (detail in picture c). The dashed 

line marks the contact between Santa Cruz Formation and the Rodados Patagónicos. d- 

Outcrops of the lower and middle section of Estancia La Costa Member at Campo Barranca 

locality. Arrows are indicating the same tuffaceous levels that in picture b. e- General view of 



 44

the middle and upper section of Estancia La Costa Member and the lower levels of Estancia 

La Angelina Member at Puesto Estancia La Costa locality. The contact between both 

members is indicated by the dashed line. 

Figure 3.2. Proposed lithostratigraphic correlation of units exposed in the west, northwest and 

east part of the Santa Cruz Province. The different ages of the stratigraphic units are 

according to the authors mentioned in the chart, to Blisniuk et al. (2006) and to Parras et al. 

(2008). 

Figure 3.3. Representative sedimentary profile of coastal Santa Cruz Formation modified 

from Tauber (1994). 

Figure 3.4. Representative sedimentary profile of Estancia La Costa Member of Santa Cruz 

Formation (modified from Tauber, 1994 and Krapovickas et al., 2008) with the three sections 

in which the member is divided in this paper. Information on the left corresponds to the 

position of the fossiliferous levels (FL) at Rincón del Buque-Media Luna (RB-ML), Cañadón 

del Cerro Redondo (CCR), Punta Sur-Cañadón del Indio (PS-CI), Campo Barranca (CB), 

Anfiteatro (ANF), Estancia La Costa (ELC), Cañadón Silva (CS), Puesto Estancia La Costa 

(PLC) and Monte Tigre-Cañadón Las Totoras (MT-CT) localities. Lithological references are 

in Figure 3.  

Figure 3.5. a- Representative X-ray diffraction patterns (A: air dried; E: ethylene-glycol 

solvated; H: 550°C-heated) of the each clay-mineral assemblage recognized in the < 4 μm 

fraction in the lower and middle section of the Estancia La Costa Member. Sm: smectite; C: 

chlorite; I: illite; IS: illite-smectite mixed layer. b- Triangular sketch showing the distribution 

of clay minerals and the clay-mineral proportion in the samples analyzed. c- Neogene global 

ocean temperature curve after Zachos et al. (2001) showing the possibly geochronological 

position (see figure 2) of the Monte León and Santa Cruz formations, the two members of the 

former unit and the three sections of the Estancia La Costa Member, in relation with the 



 45

global climatic events. ELC Mb.: Estancia La Costa Member; ELA Mb.: Estancia La 

Angelina Member; l: lower section; m: middle section; u: upper section.  

Figure 3.6. Microphotographs of the Estancia La Costa Member rocks. a- (4X, parallel 

nicols): general view of coarse vitric tuff (Campo Barranca locality) showing cuspate glass 

shards (GS) and pumice with flattened vesicles (PF). b- (4X, parallel nicols): view of a fine 

vitric tuff with micropaleoedaphic features (voids), a quartz fragment (Q) and fine glass 

shards (GS) (Campo Barranca locality). c- (4X, parallel nicols), and d- (4X, crossed nicols): 

general view of a lithic sandstone (Puesto Estancia La Costa locality) showing different 

volcanic lithic fragments, plagioclase fragments (P) and sparry calcite cement (CC).  

Figure 3.7. Selected photographs of facies and facies associations of Estancia La Costa 

Member. a- Tabular bodies of FA-2, 3 and 4, composed of facies Sm, Sl, Fm, Fl and Tm, and 

a tabular sandstone body of FA-1 integrated by Gt, St and Sp facies. White arrows point the 

Tm facies and the dotted line indicates the upper contact of the unit with the quaternary 

deposits. This picture is representative of the lowest section of the unit (Punta Norte of 

Rincón del Buque locality). b- Outcrops of the lower and middle section of the unit at Punta 

Sur-Cañadón del Indio locality. The lower section is characterized by tabular bodies of 

massive and laminated silty sandstones (Sm, Sl facies) interbedded with massive or laminated 

pyroclastic and siliciclastic mudrocks (Fm, Fl facies) (FA-2 and FA-3).  The middle section is 

integrated by tabular bodies (Tm, Fm a facies) of FA-3 and 4 that enclose a lenticular coarse 

sand body (Gt, St facies) of FA-1. The dashed line indicates the upper contact of the unit with 

the quaternary deposits. c- Photograph of the middle and upper section of the member at 

Puesto Estancia La Costa locality. Note that the composition is essentially siliciclastic and the 

arrangement of FA-1 with FA-2 and 3. The dashed line indicates the upper contact of the unit 

with the quaternary deposits. d- Bioturbated and laminated fine sandy beds (Sl facies) with 

evident soft-deformation towards the top (Cañadón del Cerro Redondo). e- Detail of fossil 



 46

root traces and Fe-nodules in an edaphized reworked tuff (Tm facies) at Punta Norte of 

Rincón del Buque Locality. f- Calcareous concretion in a paleoedaphized Fl facies at Campo 

Barranca locality.  



 47

Table 3.1. Clay mineral composition from the Estancia La Costa Member in the  

fraction < 4 µm. 

Sm K I Ch LOCALITY SAMPLE 
RELATIVE 

PERCENTAGE 

I/S 
CLAY 

MINERAL 
ASSEMBLAGE 

FACIES FA 

CB-2 100 0 0 0 0 S1 Tm FA-4 

CB-1 70 0 0 30 0 S3 Gm FA-1 

CB-O 100 0 0 0 0 S1 Tm 

CB-T-2 100 0 0 0 0 S1 Tm 

CB-T-3-4 100 0 0 0 0 S1 Tm 
FA-4 

CB-22 70 0 20 10 0 S3 St FA-1 

CB-21 80 0 15 5 0 S3 Fm 

Campo Barranca 

CB-19 65 0 25 10 0 S3 Fm 
FA-3 

ANF-3 100 0 0 0 0 S1 Tm 

ANF-2 100 0 0 0 0 S1 Tm 

ANF-1 100 0 0 0 0 S1 Tm 

MR-5 95 0 0 0 5 S2 Tm 

MR-6  95 0 0 0 5 S2 Tm 

MR-7 100 0 0 0 0 S1 Tm 

MR-8 95 0 0 0 5 S2 Tm 

Anfiteatro 

MR-9 100 0 0 0 0 S1 Tm 

FA-4 

MR-1 90 0 5 0 5 S2 Tm 

MR-2 90 0 5 0 5 S2 Tm 

MR-3 100 0 0 0 0 S1 Tm 
Estancia La Costa 

MR-4 100 0 0 0 0 S1 Tm 

FA-4 

 
Note: Sm: smectite; K: kaolinite; I: illite; Chl: chlorite; I/S: mixed layer illite-smectite. S1: Smectitic 

Assemblage; S2: Smectite-Illite and mixed layer clays bearing Assemblage; S3: Smectite-Illite-Chlorite 

Assemblage. Gm: massive clast or matrix-supported fine conglomerates; Tm: massive coarse and fine vitric 

tuffs; Sm: massive lithic sandstones; St: trough cross-bedded lithic sandstones; Sp: planar cross-bedded lithic 

sandstones; Sl: horizontal laminated sandstones, in places deformed; Fl: laminated mudrocks; Fm: massive very 

fine sandstones, tuff or reworked tuff and mudrocks. FA-1: Fluvial channel facies association; FA-2: Proximal 

floodplain facies association; FA-3: Distal floodplain facies association; FA-4: Pyroclastic-fall deposits facies 

association. 

 

 



 48

Table 3.2. Classification of sandy and tuffaceous samples of Estancia La Costa Member. 

LOCALITY SAMPLE CLASSIFICATION FACIES FACIES ASSOCIATION 

CÑ-1 medium vitric reworked  tuff Tm 
CÑ-2 medium vitric tuff Tm 
CÑ-3 medium vitric tuff with paleoedaphic features Tm 

CI-1 fine vitric reworked tuff with paleoedaphic 
features Tm 

Punta Sur-
Cañadón del 
Indio 

CI-2 fine vitric tuff with paleoedaphic features Tm 

FA-4: Pyroclastic-fall deposits 

CCR-1 fine vitric tuff with paleoedaphic features Tm FA-4: Pyroclastic-fall deposits 
CCR-3 very fine vitric tuff Fm FA-3: Distal floodplain Cañadón del 

Cerro Redondo 
CCR-10 medium vitric tuff Tm FA-4: Pyroclastic-fall deposits 
PRB-ML-1 medium vitric tuff Tm 
PRB-10 medium vitric tuff with paleoedaphic features Tm 

FA-4: Pyroclastic-fall deposits 

PRB-11 very fine vitric tuff Fm FA-3: Distal floodplain 
PRB-12 fine to medium litharenite St FA-1: Fluvial channel 
PRB-13 medium vitric tuff Tm 

Rincón del 
Buque (Media 
Luna and Punta 
Norte)  

PRB-14 medium vitric tuff Tm 
FA-4: Pyroclastic-fall deposits 

MR-9 medium vitric tuff Tm 
CB-2 fine vitric reworked tuff  Tm 
CB-3-4 medium vitric tuff Tm 

FA-4: Pyroclastic-fall deposits 

CB-CM coarse litharenite St 
CB-24 medium litharenite St 

FA-1: Fluvial channel 

CB-23 fine vitric tuff with paleoedaphic features Tm FA-4: Pyroclastic-fall deposits 
CB-22 medium litharenite St 
CB-20 medium litharenite St 

FA-1: Fluvial channel 

Campo Barranca 

CB-19-b very fine litharenite with paleoedaphic features Fm FA-3: Distal floodplain 

MR-8 fine to medium vitric tuff Tm 
MR-7 medium vitric tuff Tm 

MR-6 medium vitric tuff Tm 

MR-5 medium vitric tuff Tm 

ANF-4 medium vitric tuff Tm 

ANF-3 medium vitric tuff Tm 

Anfiteatro 

ANF-2 fine vitric tuff with paleoedaphic features Tm 

FA-4: Pyroclastic-fall deposits 

MR-2 fine vitric reworked tuff Tm Estancia  
La Costa MR-1 medium vitric tuff with paleoedaphic features Tm 

FA-4: Pyroclastic-fall deposits 

PPLC-4 medium vitric tuff with paleoedaphic features Tm FA-4: Pyroclastic-fall deposits 

PPLC-2 fine litharenite Sm FA-2: Proximal floodplain 
(overbank and sheet-flood) 

Puesto Estancia 
La Costa 

PPLC-OB medium litharenite St FA-1: Fluvial channel 

 

Note: Abbreviations as in table 3.1



 49

Table 3.3. Lithofacies identified in the Estancia La Costa Member. 

FACIES 
CODE 

LITHOLOGY 
AND 

COMPOSITION 

SEDIMENTARY 
STRUCTURES 

THICKNESS 
AND 

BOUNDARIES

FOSSIL 
CONTENT 

PALEOEDAPHIC 
FEATURES/  

BIOTURBATION/ 
OTHERS  

INTERPRETATION

Gm 
(massive  

conglomerates) 

Clast or matrix-
supported fine 

grained 
conglomerates 
with intraclasts 

Massive 

0,2-0,3 m. 
Erosive, 

concave upward 
lower boundary 

Frogs - 

Lag deposits, small 
cores of longitudinal 

bars. Moderate to high 
flow regime 

Tm 
(massive tuffs 
and reworked 

tuffs) 

Coarse to fine 
vitric tuffs and 
reworked vitric 

tuffs 

Massive to poor 
lamination 

0,2-1,5 m. Sharp 
boundaries 

Vertebrates, 
logs and 

phytoliths 

Paleoedaphic 
features, 

bioturbation and 
carbonaceous 

remains 

Ash-fall events. 
Pedogenic processes 

Sm 
(massive 

sandstones and 
silty 

sandstones) 

Medium lithic 
sandstones and 
silty sandstones 

Massive 

< 2,5 m. Sharp 
or slightly 

concave upward 
lower boundary 

and sharp or 
transitional 

upper boundary 

Phytoliths 
Paleoedaphic 

features (calcareous 
concretions) 

Stream-flow during 
high discharge 
conditions with 

subaereal exposure 
and pedogenic 
modification  

St 
(trough cross-

stratified 
sandstones) 

Fine to very 
coarse lithic 
sandstones 

Trough cross-
bedding, fining-

upward trend 

< 3,5 m. Sharp 
or slightly 

concave upward 
lower boundary 

and sharp or 
transitional 

upper boundary 

Vertebrates 
including 

frogs  

Intraclasts in the 
bases 

Subaqueous sandy 3-
D dunes. Lower flow 

regime 

Sp 
(planar 

stratified 
sandstones) 

Fine to medium 
lithic sandstones 

Planar cross-
bedding, fining-

upward trend 

0,1-3 m. 
Slightly concave 

upward lower 
boundary or 

sharp 
boundaries 

- 
Paleoedaphic 

features (iron-rich 
concretions) 

Deposits of sandy 2-D 
bars. Transitional to 
lower flow regime. 
Subaereal exposure 

and pedogenic 
modification 

Sl 
(laminated 
sandstones) 

Fine to medium 
sandstones 

Horizontal 
lamination (in 

places deformed) 

< 1 m. Sharp 
boundaries or 
transitional 

lower boundary 

Leaves 
Bioturbation. 
Carbonaceous 

remains 

Upper stage plane-bed 
flow (synsedimentary 

deformation). 
Subareal exposure 
with bioturbation 

Fl 
(laminated 
very fine 

sandstones and 
mudrocks) 

 

Very fine 
sandstones and 

mudrocks 

Horizontal 
lamination 

< 1 m. Sharp 
boundaries  - 

Bioturbation, 
paleoedaphic 

features (calcareous 
concretions and root 

traces). 
Carbonaceous 

remains 

Settling from 
suspension with 

subaereal exposure, 
pedogenetic processes 

and bioturbation  

Fm 
(massive very 

fine 
sandstones, 

tuffs, reworked 
tuffs and 

mudrocks) 

Very fine 
sandstones, tuff or 
reworked tuff, and 

mudrocks 

Massive  
< 1 m. Sharp or 

transitional 
boundaries 

Vertebrates 
and 

phytoliths  

Bioturbation, 
paleoedaphic 

features (calcareous 
concretions, root 

traces, blocky peds) 
and dessication 

cracks 

Settling from 
suspension with 

subaereal exposure, 
pedogenetic processes 

and bioturbation. 
Aeolian deposition of 

pyroclastic origin 



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