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Revista de la Asociación Geológica 65 (1): 215 - 226 (2009) 215

STRUCTURE AND EVOLUTION OF THE AUSTRAL BASIN FOLD-THRUST BELT, SOUTHERN PATAGONIAN

Matías C. GHIGLIONE1,2, Fabián SUAREZ3, Alfredo AMBROSIO1, Gabriela DA POIAN4, Ernesto O. CRISTALLINI2,5, María Fernanda PIZZIO1 and R. Matías REINOSO1

1 Laboratorio de Tectónica Andina, Ciencias Geológicas, C1428EHA, Ciudad Universitaria, Pab. II, Buenos Aires. Email: [email protected] 2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). 3 Consultor, Gaucho Rivero 635 (9000), Comodoro Rivadavia, Chubut. Email: [email protected] 4 Instituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Avenida Bolivia 5150 (4400) Salta Argentina. 5 Laboratorio de Modelado Geológico (LaMoGe), Universidad de Buenos Aires, C1428EHA.

ABSTRACT This study focuses on the evolution of the Southern Patagonian Andes fold-thrust belt and the adjacent non-deformed fore- land of the Austral basin between 49°45' and 52°00'SL. This sector involves mainly Late Cretaceous sequences of the first re- gressive cycle (Lago Viedma Cycle) of the Austral basin foreland stage, and to Paleogene sequences associated with tectonic uplift of its western boundary. From a stratigraphic-sedimentary point of view, a first-order increase in the fill- thickness and depth to the basement exists from north to south including the presence of deeper depositional environments in the same direction. Furthermore, there are strong along-strike variations in width and lateral position of the structural do- mains following the same trend. Based upon previous interpretations, is concluded that the distribution of extensional depo- centers from the early extensional phase of the basin controlled these important sedimentary and structural N-S contrasts. Furthermore, in our presented model, East-west oriented transition zones are interpreted as accommodation zones separa- ting synrift sub-basins.

Keywords: Austral basin, , Cretaceous, Extensional depocenters.

RESUMEN: Estructura y evolución de la faja plegada y corrida de la cuenca Austral, Andes Patagónicos Australes. Este estudio se concentra en la evolución de la faja plegada y corrida de los Andes Patagónicos Australes y del antepaís adyacente no deformado de la cuenca Austral entre los 49°45' y 52°00'SL. Este sector involucra principalmente secuencias cretácicas superiores del primer ciclo regresivo (Ciclo Lago Viedma) de la etapa de antepaís de la cuenca Austral, y secuencias campanianas a paleógenas aso- ciadas con el levantamiento tectónico de su límite occidental. Desde el punto de vista estratigráfico sedimentario, existe un incremento de primer orden de norte a sur en el espesor del relleno y la profundidad del basamento incluyendo la presencia de ambientes deposicionales más profundos en la misma dirección. Se han detectado fuertes variaciones a lo largo del rum- bo en el ancho y la posición lateral de los dominios estructurales en la misma dirección. Siguiendo propuestas previas del sec- tor norte de la zona estudiada, se interpretan estos importantes contrastes sedimentarios y estructurales N-S como controla- dos por los depocentros extensionales y, por lo tanto heredados de rasgos de una fase extensional previa de la cuenca.

Palabras clave: Cuenca Austral, Etapa antepaís, Espesor, Cretácico, Depocentros extensionales.

INTRODUCTION diachronicity occurred through basin. ped oceanic crust. This deepest sector Mid to Late basal sequences was filled by distal turbidites (Fig. 1; Cal- The Austral (Magallanes) basin has been (known as Tobífera, El Quemado, Chon derón et al. 2007). affected by eustatic sea level changes and Aike or Lemaire Formations; Fig. 3), Mechanic subsidence and stretching atte- different deformational regimes since Me- which constitute the lower structural pack- nuated towards the eastern , as sozoic times, resulting a complex tecto- age of the fold-thrust belt, were deposi- shown by the presence of shallower se- nostratigraphic pattern (Figs. 1 and 2). In ted in , in a regional context of li- quences, dominated by continental to he- the sector under study the deformational thospheric thinning and continental rif- mipelagic sequences (Arbe 1989; Manas- regime has changed from a Middle Ju- ting (Wilson 1991, Uliana et al. 1989). sero 1988). The synrift sequences are un- rassic- extensional kinematics Upper Jurassic stretching was maximum comfortably overlie by extended lower to a - Neogene overall com- at the western continental margin, where Cretaceous sag deposits of the arenace- pressional setting, although a significant the Rocas Verdes marginal basin develo- ous Springhill (Zapata- Río Mayer infe- 216 M. C. GHIGLIONE, F. SUAREZ, A. AMBROSIO, G. DA POIAN, E O. CRISTALLINI, M F. PIZZIO AND R. M. REINOSO

Figure 1: Location of study zone showing main morphostructural units from Austral and neighboring basins. Contours indicate fore- land sediment thickness within the undeformed depocenters in kilome- ters. FTB - fold-thrust belt; RVB - Rocas Verdes Marginal Basin.

rior) and pelitic Palermo Aike (Inocera- proximal facies of these sequences, pro- stable slope and platform of the basin, mus inferior, Pampa Rincón) formations bably representing pulses of accelerated important gas reservoirs are associated (Fig. 3) recognized, respectively, as the uplift and propagation of the deforma- with the fold-thrust belt such as the Tran- main petroleum source and reservoir tional front, are now involved in the fold- quilo () and Glencross (Arg.) gas rock of the Austral Basin ("Sistema Ino- thrust belt, where they are separated by fields (Fig. 2). There also exists a number ceramus inferior - Springhill"; Pedrazzini paraconformities or angular unconformi- of detected resources that have not been and Cagnolatti 2002, Zilli et al. 2002). ties representing long hiatuses (Lago tested properly due to technical limita- Synrift and Sag sequences were overlie by Argentino and Western Austral basin in tions, for example, wells Cerro Palique. Upper Cretaceous regressive sequences figure 3). Their foreland-distal equiva- es-1 (Paso Furh block) and Cancha Ca- of the first foreland-basin stage coming lents, consisting of medium to fine grain rrera.es-1 (Tapi Aike block) (Fig. 2) stres- from the north and west, developing an- sediments, are separated by paraconfor- sing the promising potential of the fold- gular unconformities (Coutand et al. 1999) mities with shorter hiatuses and some of thrust belt. since (Arbe 1989; Kraemer 1998, them represent Upper Cretaceous to Pa- The present work focuses on the Pata- 2003) or times (Fildani et al. leocene reservoirs (Piedra Clavada, Mata gonian Andes fold-thrust belt and the 2003). Amarilla, Campo Boleadoras, María Inés, adjacent non-deformed foreland of the Over the ensuing 100 Myr, contractile Zona Glauconítica; known as the "Siste- Austral basin between 49°45' and 52°00 deformation propagated eastward, gene- ma Inoceramus inferior - Magallanes in- 'SL (Figs. 1 and 4), including the explora- rating at least four orogenic sequences ferior "; Marinelli 1998, Cagnolatti and tory blocks Paso Fuhr, Tapi Aike and Río with an important westward provenance Miller 2002, Zilli et al. 2002, Limeres and Turbio (Fig. 2). The studied sector invol- of sediments from the mountains to- Dellapé 2005). ves mainly Late Cretaceous sequences of wards the undeformed foreland (Arbe Although most of the working petro- the first regressive cycle (Lago Viedma 1989, 2002, Kraemer 2003). The coarser leum and gas systems are located in the Cycle), and Campanian to Paleogene se- Structure and evolution of the austral basin fold-trust belt... 217

Figure 2: Oil and gas blocks of Austral ba- sin in Santa Cruz Province (Argentina) and Chile, showing main morphostructu- ral units and studied exploratory fields. FTB - Fold and thrust belt. quences associated with tectonic uplift study is given, while the relation between The metamorphic basement of the re- (Arbe 1989, 2002, Kraemer and Riccardi compressional faulting and previous ex- gion, located along the pacific archipela- 1996, Malumián et al. 2000). tensional depocenters is discussed with go (Fig. 1), was generated from late Pa- From a stratigraphic-sedimentary point the aim of exploring the presence of pe- leozoic to early times as an ac- of view, the sector includes a first-order troleum-gas systems. cretionary prism developed along the increase in the fill-thickness and depth to Panthalassic (Pacific) margin of Gond- basement from north to south including GEOLOGICAL SETTING wana (Dalziel, 1981, Hervé et al. 1981, the presence of deeper depositional en- Mpodozis and Ramos 1990). The wes- vironments in the same direction. We de- Due to to the fact that is surrounded by tern Basement domain is composed of tected strong along-strike variations in evolving tectonic boundaries (Fig. 1), the the Paleozoic Eastern Andes Metamor- width and lateral position of the structu- Austral basin was affected by extension phic Complex, including polideformed ral domains in the same trend (Fig. 4). and rifting during Gondwana break-up metaturbidites of Late Devonian to Late According to an idea first proposed by (Biddle et al. 1986, Uliana and Biddle Permian age (Calderón et al. 2007). Arbe (1989) and followed by Kraemer 1988, Uliana et al. 1989), and was subse- Initial subsidence began in almost all of and Riccardi (1996) and Kraemer (1998) quently influenced by Andean compres- the Patagonian hydrocarbon-producing for the northern sector of the study sional uplift as a result of the interaction basins during the Triassic-Jurassic conti- zone, these important sedimentary and between the subducting oceanic plates nental rifting of southern South America structural N-S contrasts are interpreted from the Pacific basin (Farallon-Aluk- (Uliana et al. 1989). As a consequence, as being controlled by the distribution of Nazca-Antarctic plates) and the overri- one of the largest known silicic igneous extensional depocenters from the early ding South America plate (Ramos 1989, provinces (LIP) was formed in Patago- extensional phase of the basin. Yrigoyen 1989, Coutand et al. 1999, Di- nia, dominated by rhyolitic ignimbrites, An account of the structural and strati- raison et al. 2000, Somoza and Ghidella which form a bimodal association with graphic features of the region under 2005, Ghiglione and Cristallini 2007). minor mafic and intermediated lavas 218 M. C. GHIGLIONE, F. SUAREZ, A. AMBROSIO, G. DA POIAN, E O. CRISTALLINI, M F. PIZZIO AND R. M. REINOSO

Figure 3: General chronostratigraphic diagram for Austral (Magallanes) basin taken from Ambrosio (2003) after Arbe (1989).

(Pankhurst et al. 1998, 2000). A clear dia- Mukasa and Dalziel 1996). The northern fera Formation in Chile (Thomas 1949). chronism is recognized between the Ear- remnants of the sea floor and bimodal In the Austral basin, these rocks are ab- ly- volcanism of eastern magmatism of the marginal basin are sent on some basement highs, but can re- (Marifil and Chon Aike forma- grouped into the Sarmiento Ophiolite ach over 2000 m in constrained extensio- tions) and the Middle- volca- Complex, located SW from the study zo- nal depocenters (Biddle et al. 1986). The nism of the Andean Cordillera (El Que- ne (Fig. 1) active between 152 and 142 dominant trend of the early Mesozoic mado and Tobífera formations; Fig. 3), Ma (Calderón et al. 2007). rifting was mainly NW to NNW as showing a regular decrease of ages from The Late Jurassic extensional event, shown in the eastern Austral basin by the ENE (187 Ma) to the WSW (144 Ma) which was dominant in the Austral and seismic lines from the undeformed fore- along 650 Km (Pankhurst et al. 1998, , produced several depo- land depocenter (Biddle et al. 1989, Ulia- 2000, Féraud et al. 1999). This progressi- centers controlled by normal faulting (Bid- na et al. 1989). Towards the west, the Late ve stretching culminated towards the dle et al. 1986, Diraison et al. 2000), filled Jurassic rocks form elongated outcrops SSW with the generation of oceanic up by an heterogeneous suite of siliceous with a N to NNE trend that represent an crust in the Rocas Verdes marginal basin volcanic and volcaniclastic rocks called inverted main extensional depocenter along the Pacific margin of the continent Lemaire Formation in Argentina (Cami- (Fig. 4). during Late Jurassic times (Dalziel 1981, nos et al. 1981, Yrigoyen 1989) or Tobí- An uppermost Jurassic-lower Cretaceous Structure and evolution of the austral basin fold-trust belt... 219

Figure 4: Geological map of the study zone from co- llected field data and after SERNAGEOMIN (1982), Kraemer (1996, 2003), Malumián et al. (2000). 220 M. C. GHIGLIONE, F. SUAREZ, A. AMBROSIO, G. DA POIAN, E O. CRISTALLINI, M F. PIZZIO AND R. M. REINOSO

transgressive siliciclastic wedge represen- ponent of right lateral wrenching parallel partially to totally inverted depocenters ted by the Springhill and Pampa Rincón to the Cordillera (Coutand et al. 1999). are located between Argentino and Vied- Formations (Fig. 3; Arbe 1989, 2002, ma lakes, in the western margin of Río Kraemer et al. 2002) follows the Upper GEOLOGY AND STRUCTU- Guanaco (Kraemer 1998) and in Punta Jurassic -related section. These retro- RE OF THE PATAGONIAN Avellaneda (Fig. 4). In this sector there gradational sedimentary sequences took ANDES FOLD-THRUST BELT are westward verging back-thrusts along place while the basin passively subsided the Masters Range uplifting eastward- during the sag phase, characterized by On the basis of mechanical stratigraphy, thinning jurassic synrift deposits, indica- minimal faulting through the Early Cre- the lithostratigrafic units of the belt can ting the total inversion of the westward taceous (Uliana et al. 1989, Ramos 1989). be divided into four main structural pac- boundary of an extensional depocenter The Springhill Formation is a sandstone kages; from bottom to top these are (Kraemer 1998). succession of retrogradational fluvial, (Figs. 3 and 4): (1) Paleozoic metamor- shoreline, and shallow-marine sandsto- phic rocks (Bahía La Lancha and Río Internal fold-thrust belt nes that represents the major hydrocar- Lácteo Formations) intruded by Tertiary The internal fold-thrust belt is composed bon reservoir of the Austral and Malvi- plutonic and volcanic rocks from the of ductile structures, characterized by N- nas basins (Biddle et al. 1986). Patagonian batholith, which constitute S trending thrusts, backthrusts and folds A typical foreland-phase started during the crystalline basement; (2) Upper Juras- (Fig. 4-6). This domain is bounded to the the early Cretaceous, dominated by flexu- sic marine synrift sequences; (3) Lower west by the Upper Jurassic positively in- ral subsidence and cratonward migration Cretaceous platform sequences from the verted and exposed depocenters. The of sedimentary depocenters related to sag stage (Springhill Formation); and (4) structural packages involved are mainly Andean uplift and expansion (Yrigoyen Upper Cretaceous to Neogene continen- Late Cretaceous sequences of the first 1989, Kraemer 2003). Late Cretaceous to tal and marine strata from the foreland regressive cycle associated with tectonic Tertiary units were derived from the basin stage. uplift (Lago Viedma Cycle), composed north and west, show a progressive onlap The structure of the region under study from north to south of the formations geometry from west to east (Biddle et al. can be divided into three tectonomor- Puesto El Moro, Piedra Clavada-Mata 1986, Manassero 1988) and development phic zones, including from west to east Amarilla, Lago Viedma-Puesto El Alamo of synsedimentary structures since Late (after Kraemer et al. 2002; Fig. 4): the and (Fig. 2), belonging to the Cretaceous (Coutand et al. 1999). Hinter- Basement thick-skinned brittle domain, Mata Amarilla subcycle (Arbe 2002). land uplift and erosion caused the depo- the Internal ductile fold-thrust belt and There are strong along-strike variations sition of coarse clastic sediments that in- the External fold-thrust belt. in width of this ductile domain that take dicates the onset of a foreland phase in place in discrete ~E-W transition zones the Austral basin evolution (Biddle et al. Basement domain (transfer faults from Fig. 4). In our study 1986). The age of this first coarse clastic The Basement domain is bounded to the zone, at least three transition zones are infill in Torres del Paine (Wilson 1991) is east by the irregular Upsala thrust with recognized, the two northern zones coin- not older than Turonian (92 ± 1 Ma), on eastern vergence (Fig. 4-6), while its west ciding with Viedma and Argentino lakes, the basis of detrital zircon provenance limits shows predominately a westward while the southern one is located at analysis from the Patagonian Andes vergence (Kraemer 1998). The Late Ju- ~50º55`LS, just north of Sarmiento lake. (Fildani et al. 2003). However, tectonic up- rassic synrift deposits uncomformably These transition zones or transfer faults lift could have started earlier, during late overlie the Paleozoic basement near its are limiting the Río Guanaco, Cordón de Hauter-Barremian, to the northwest of eastern boundary, while all the sequence los Cristales and Torres del Paine sectors our study zone, as shown by an angular is intruded by Jurassic to Neogene calc- (Fig. 4). From north to south, the inter- unconformity between Springhill and pre- alkaline continental margin magmatic arc nal domain get increasingly wider and an viously deformed Aptian sequences in rocks from the Patagonian Batholith overall westward shift of its position take Lago Ghio (Fig. 1, Arbe 1989, 2002). Af- (Calderón et al. 2007). The Late Jurassic place (Fig. 4-6). terward, the offsetting effects of cons- rocks form elongated outcrops with a N A radical change in the Late Cretaceous- tant basement uplift in the hinterland and to NNE trend that represent an inverted early Tertiary sedimentary facies also ta- forward imbrication in the thin-skinned main extensional depocenter, thrusted kes place along the same N-S trend. This fold-thrust belt sustained the progression towards the east over the fold-thrust belt change involves a southward shift from of deformation (Kraemer 2003, Ghiglio- (Kraemer 1998, Pizzio et al. 2008). prograding continental and litoral facies ne and Ramos 2005, Ghiglione y Crista- Some early Cretaceous cover of this de- to marine and deep marine facies that oc- llini 2007). Compressional deformation pocenter is concentrated near the eastern curs at the latitude of Lago Viedma (Río have been associated with a major com- boundary. The best outcrops showing Leona-Cerro Indice; Fig. 4) (Arbe 1989, Structure and evolution of the austral basin fold-trust belt... 221 Structural sections (see figure 4 for location and 6 correlation). Figure 5: 222 M. C. GHIGLIONE, F. SUAREZ, A. AMBROSIO, G. DA POIAN, E O. CRISTALLINI, M F. PIZZIO AND R. M. REINOSO Fault correlation, Fault see figure 4 for location. Figure 6: Structure and evolution of the austral basin fold-trust belt... 223

2002). Total sedimentary thickness and connected with the observed changes in domains (Fig. 8C). depth to basement also shows a strong sedimentary facies and thickness in the increase towards the south around this same direction as proposed by Kraemer DISCUSSION AND sector (Figs.1 and 6). (1998). Our data is in agreement with the CONCLUSIONS: EVOLUTION Further north, in the northern sector of possibility that the Late Cretaceous-early OF THE PATAGONIAN the basin, the Internal domain as defined Tertiary sedimentary facies could be re- ANDES FOLD AND THRUST here losses expression, and is replaced by flecting a paleobathymetry-topography BELT AND EXPLORATION a triangle zone where the Paleozoic base- inherited from Late Jurassic extensional OPPORTUNITIES ment interact with Late Jurassic to Ter- times. In this manner, the southward tiary deposits (Ramos 1989). In this nor- shift from prograding continental and li- Evolution of the fold and thrust belt thern sector marine sequences are repla- toral facies to marine and deep marine Our study highlights several key points ced by much thinner equivalent conti- facies that occurs at the latitude of Lago regarding the structural style and tecto- nental deposits of the Río Tarde and Car- Viedma would be reflecting the passage nostratigraphy of the Southern Patago- diel Formations (Ramos 1989). from a sector with minimum stretching nian Andes fold-thrust belt. to a deep basin generated during the ex- 1- Along-strike variations in width of External fold-thrust belt tensional phase (Arbe 1989). The N-S structural domains that take place in dis- The external thin-skinned domain, invol- axial direction from Lago Sofia conglo- crete transition zones seems to be reflec- ving Campanian to Paleogene sequences merates is probably reflecting the same ting the control of positive inverted ex- (Arbe 1989, 2002, Kraemer and Riccardi control as well. tensional depocenters as previously pro- 1996, Ambrosio 2003, Marenssi et al. 2005; These changes wich take place in discre- posed (Arbe 1989; Kraemer 1998). East- Fig. 3), is thrusted with westward vergence te ~E-W transition zones can be inter- West oriented transition zones can be in- forming a frontal monocline, while thin- preted as the result of inverse reactiva- terpreted either as ancient transfer faults skinned folds and thrust with eastward tion of synrift depocenters located bene- or accommodation zones separating syn- vergence are developed to the east (Figs. ath the tertiary structures (Figs. 8 A and rift sub-basins. 4 and 5). The external domain bounds B). East-west oriented transition zones 2- The earlier uplift of the sector located with the Internal domain to the west (Fig. can be interpreted as accommodation zo- to the north of the study zone is one of 4-6), while its eastern limits is the non- nes separating synrift sub-basins (Figs. 8 the most important conditioners of the emergent deformational front (Fig. 1). A and B). While the deepness (Latest evolution of the basin, and unveiling its The external domain also presents strong Jurassic paleobathymetry) of these depo- causes would be important for unders- north-south changes in width (Figs. 1 centers controlled facies arrangement tanding deformational processes and the and 4), that consistently take place and total thickness of Late Cretaceous to present tectono-sedimentary pattern. around the same transfer faults that af- Tertiary sequences, their dimensions The substratum of the Patagonian fold- fect the Internal domain (Figs. 4 and 6). controlled the width and length of futu- thrust belt consists of at least three adja- The structural style of the external do- re compressional structural domains (Fi- cent, approximately N-NE oriented, main is that of a monocline developing a gure 4). These assertions are in agree- elongated extensional depocenters. The- triangle zone, where to ment with the migration of extensional se depocenters show a westward increase Tertiary sequences are thrusted to the depocenters from the ENE to the WSW in the degrees of inversion and exhuma- west over a Late Cretaceous epidermic along 650 Km (Pankhurst et al. 1998, tion: The first, completely inverted and wedge (Figs. 5 and 6). North of Río Tur- Féraud et al. 1999; Fig. 8C). This progres- exposed depocenter is represented by the bio, there is a series of elongated N-S an- sive stretching culminated towards the eastern sector of the Basement domain. ticlines located east from the triangle zo- SSW with the expansion of the Rocas The Inner fold thrust belt comprises the ne (Figs. 4 and 5) where seismic data Verdes marginal basin along the Pacific second inverted depocenter. This depo- shows a strong interaction between faults margin of the continent during Late Ju- centers is less inverted and synrift se- affecting the synrift deposits of Tobífera rassic times (Dalziel 1981, Wilson 1991, quences are not uplift up to superficial le- Formation and shallow anticlines (Figs. 6 Mukasa and Dalziel 1996, Calderón et al., vels, however all of the compressional and 7). 2007; Fig. 8C). structures are dissected. A third depo- In the presented model, the eastern center is interpreted to be beneath the Along strike changes in the fold- boundary of the basement domain re- external fold-thrust belt. This sector is thrust belt presents total inversion and exhumation the northern expression of the Tranqui- Given the above-mentioned observa- of extensional depocenters, while inver- lo-Glencross trend, and therefore has a tions, it was examined if the N-S changes sion and exhumation decreases progres- promising potential for the Mesozoic- in the structural domains are somehow sively towards the Internal and External Tertiary petroleum system. 224 M. C. GHIGLIONE, F. SUAREZ, A. AMBROSIO, G. DA POIAN, E O. CRISTALLINI, M F. PIZZIO AND R. M. REINOSO

Figure 7: Seismic section of external fold-thrust belt, north of Río Turbio. See Fig. 4 for location.

Figure 8: a and b) Sketches (modified from Barredo et al. 2007) showing rifting patterns of extensional depocenters separated by strike-slip or transfer faults as proposed for the studied zone (see text for discussion). Referen-ces are shown to illustrate the extensional stage of the hypothetical Lago Argentino transfer fault (a and b). c) Pa-leogeographic reconstruction of southern South America illustrating troughs and sags, after Uliana et al. (1988). Half-grabens defining the main Jurassic tectonic structure of the region controlled the Cretaceous bathimetry and sedimentation, and the Tertiary structural geometry (see discussion in the text).

Exploration Opportunities sin, there also exists an important poten- cently bid new exploratory blocks in the The Austral or Magallanes basin is one of tial for the presence of oil or gas reser- region. the less explored basins of Argentina and voirs in the deep basin and fold and The best precedent related to the fold- Chile. Although most of the proved pe- thrust belt (Fig. 2) that remains in the thrust belt found in Argentina is associa- troleum and gas systems are located in frontier of exploration. To revert this si- ted with Tertiary gas reservoirs from the the stable slope and platform of the ba- tuation, Argentina and Chile have re- Magallanes formation in the Glencross Structure and evolution of the austral basin fold-trust belt... 225

field (Fig. 2) operated by Petrobrás Ener- Chico x1 well from Glencross field (Fig. rios de la Formación Magallanes. En Schiuma, gía. The Tranquilo field (Fig. 2), discove- 2), which encourages us to continue Hinterwimmer y Vergani (eds.): Rocas Reser- red in 1960, is the most promising exam- studying the source rock distribution and vorio de las Cuencas Productivas de la Ar- ple from Chile, where Tertiary clastic se- maturity. gentina: 91-117. quences (Loreto and Leña Dura Forma- Calderón, M., Fildani, A., Hervé, F., Fanning, C. tions) are gas productive reservoir in ACKNOWLEDGMENTS M., Weislogel, A. and Cordani, U. 2007. Ver- structural traps. des basin, southern Patagonian Andes Late There are also other antecedents within The authors want to thanks Epsur S.A. Jurassic bimodal magmatism in the northern the study zone such as the Paso Fuhr.es- and Misahar Argentina S.A for allowing sea-floor remnant of the Rocas. Journal of 1 well from the Paso Fuhr block, where diffusion of data presented in this study. the Geological Society 164: 1011-1022. the presence of gas have been tested and The authors are also grateful to Parques Caminos, R., Haller, M., Lapido, O., Lizuain, A., dry-gas was found. Other neighboring Nacionales de Argentina for permitting Page, R. y Ramos, V.A. 1981 Reconocimiento wells have had manifestations of gas in access to Los Glaciares National Park. geológico de los Andes Fueguinos, Territorio Tertiary and Cretaceous reservoir, al- Special thanks go to Park Ranger Fabian Nacional de Tierra del Fuego. 8º Congreso though technical difficulties due to inex- from Lago Roca Section for kind logisti- Geológico Argentino (San Luis), Actas 3: 759- perience resulted in abandonment of cal support. Reviewers Martín Turienzo 786. wells without further testing. and José Silvestro are also acknowledged Coutand, I., Diraison, M., Cobbold, P.R., Gapais, Exploratory efforts are now being con- for constructive criticisms that lead to an D., Rossello, E.A., Miller, M. 1999. Structure centrated in the potential presence of improved manuscript. and kinematics of a foot-hills transect, Lago late Cretaceous-Tertiary reservoirs in Viedma, southern Andes (49°30S). Journal of structural traps along the external fold- WORKS CITED IN TEXT South American Earth Science 12: 1-15. thrust belt (Figs. 2 and 4). The petroleum Dalziel, I.W.D. 1981. Back arc extension in the systems to be evaluated could be similar Ambrosio, A. 2003. Estratigrafía, Sedimentología southern Andes: a review and critical reaprai- to other known late Cretaceous-Tertiary y Paleontología de las Adyacencias de Valle sal. 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