Journal of the Geological Society, London, Vol. 163, 2006, pp. 707–721. Printed in Great Britain.
Tectonostratigraphic evolution of a Mesozoic graben border system: the Chachil depocentre, southern Neuque´n Basin, Argentina 1
´ M E Z - P E´ R E Z 2 J. R . F R A N Z E S E 1 , G . D. V E I G A 1 , E . S C H WA R Z 1 & I . G O Centro de Investigaciones Geolo´gicas, Universidad Nacional de La Plata–CONICET, Calle 1 #644, B1900TAC La Plata, Argentina (e-mail:
[email protected]) 2 Cambridge Arctic Shelf Programme (CASP), West Building, 181A Huntingdon Road, Cambridge CB3 0DH, UK Abstract: The Chachil depocentre is one of a number of early Mesozoic extensional basins that form the early depocentres of the southern Neuque´n Basin in Argentina. The synrift volcanic fill is composed of andesites, rhyolites and volcaniclastic deposits. Coarse-grained, non-marine facies dominate the sedimentary fill, mainly in the form of sediment gravity flow deposits. Stream flow deposits and minor non-marine carbonates are also locally present. The evolution of the graben border system was mainly controlled by subsidence along the main boundary fault (the Chihuido Bayo fault system) and recurrent volcanic activity. Marked changes in the thickness of the synrift megasequence indicate that episodic normal faulting in the hanging wall was also important. The integration of structural, magmatic and sedimentary data from the study area has led to the definition of three stages in the evolution of the synrift succession. The early rift stage is defined by the interplay between bimodal volcanism and gravity-driven sedimentation. The mid-rift stage is marked by the transition to acidic magmatism (rhyolitic and pyroclastic flows), also associated with coarsegrained non-marine deposition. The late-rift stage is dominated by fine-grained turbidites and pyroclastic falls related to the first marine sedimentation in the Neuque´n Basin.
fig. 4), a ‘rift border system’. The basin-scale concept of the interplay of faulting and sedimentation in a rift border system is here applied to a single depocentre within a more complex rift system. The aims of this study are: (1) to establish the sedimentological and stratigraphic characteristics of the fill near the faulted margin of the depocentre; (2) to analyse the influence of volcanism and tectonics in the development of sedimentary sequences in such an environment; (3) to reconstruct the tectonosedimentary evolution of the depocentre. The results provide new details on the development of sedimentary sequences in continental rifts strongly dominated by volcanic or volcaniclastic input. Recently the early rift basins of the Neuque´n Basin have been the focus of hydrocarbon exploration (Pa´ngaro et al. 2002a) and the results of the present study also provide important information that will aid in understanding of the petroleum systems of the region.
From Late Triassic to Early Jurassic time, part of the protoPacific margin of Gondwana was affected by continental extension driven by the thermomechanical collapse of a Late Palaeozoic thickened crust (Franzese & Spalletti 2001). This led to the creation of an ensialic back-arc basin (the Neuque´n Basin) that was active during the Mesozoic and Cenozoic at latitudes between 308S and 408S. The initial (synrift) configuration of the basin was characterized by the development of isolated deep depressions bounded by normal faults and filled with volcanosedimentary successions (Vergani et al. 1995). Although there have been a number of studies on the later evolution and fill of the Neuque´n Basin (Veiga et al. 2005, and papers therein) studies of the early grabens of the Neuque´n Basin have concentrated on the limited well log and core data from oil fields in the east of the area. The outcrops in the western portion have been largely ignored. Previous studies on the stratigraphic development of these depocentres have focused on their shared regional evolution (Gulisano 1981; Gulisano et al. 1984; Legarreta & Gulisano 1989; Riccardi & Gulisano 1990), and detailed studies of particular depocentres within the basin or in neighbouring areas ´ lvarez & Ramos 1999). are rare (Gulisano & Pando 1981; A Mesozoic and Cenozoic inversion in the central and southern Neuque´n Basin produced uplift and exposure of the faultbounded margins of the depocentres, providing good exposures of the synrift succession. This study focuses on the tectonostratigraphic evolution of one of these early depocentres in the southern part of the Neuque´n Basin, located in the Cerro Chachil area (central Neuque´n Province, Fig. 1). Detailed logging along the strike of an inverted hanging-wall section, combined with the interpretation of aerial photographs, was performed to investigate the structural styles and depositional sequences from the tip to the centre of the boundary fault. The system is described as a ‘graben border system’, after Magnavita & da Silva (1995), who termed the architecture of a rift border, characterized by a main boundary fault, adjacent step blocks and a clastic wedge (their
Geological setting The Neuque´n Basin is located in west–central Argentina and central Chile and was active from Late Triassic to Early Tertiary time (Yrigoyen 1979; Legarreta & Uliana 1991). The basin has a multiphase tectonic history that includes an initial extensional phase, the development of the Andean magmatic arc during its post-rift stage and several inversion periods related to active tectonic movements during the Mesozoic and Cenozoic (Legarreta & Uliana 1991; Vergani et al. 1995; Franzese & Spalletti 2001; Howell et al. 2005). The synrift depocentres of the Neuque´n Basin were generated during the Late Triassic to Pliensbachian as isolated troughs following a complex multidirectional pattern, mainly subparallel to the basin margins (Franzese & Spalletti 2001) (Fig. 1). Subsurface studies show them as a series of half-grabens with variable polarity, intersected by en echelon transfer faults (Vergani et al. 1995). 707
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N
Pre-rift units
OCEA N
South America
?
Pacific Ocean
Atuel Rift
PACIF IC
34° S
Valparaiso
Hualane Curepto
36°
Sierra Azul Concepción
38°
Sierra Cara Cura
Neuquén Basin boundary
Sierra de Reyes
Andacollo
CHILE
Atlantic Ocean
Malargüe rift
Study Area Lonquimay
Entre Lomas
Neuquén embayment
Synrift units
Co. Chachil
Catan Lil Aluminé
Approximate location of cross section in figure 4b
40° Piedra del Aguila
Valdivia
72° W
Rift depocentres
Bariloche
68° W
50 km
Fig. 1. Distribution of rift depocentres in the Neuque´n Basin and Chilean Coastal Cordillera with location of the study area. Modified from Franzese & Spalletti (2001).
The study area is located in the eastern margin of the Andean Cordillera in central Neuque´n province, where the synrift units are particularly well exposed (Fig. 1). In this area, the lithostratigraphic units involved in the evolution of the Chachil depocentre can be divided into pre-rift, synrift and early post-rift units.
AGE
Orchuela & Ploszkiewicz (1984) (Subsurface)
Toarcian
Early Pliensbachian Jurassic Sinemurian Hettangian Late Triassic
Late Carboniferous
Devonian?
Punta Rosada Fm
Puesto Kauffman Fm
The basement of the Neuque´n Basin is composed of metamorphic and igneous rocks linked to the evolution of a Late Palaeozoic orogenic belt (Fig. 2). The Piedra Santa Complex is a polydeformed metasedimentary unit that reached greenschistfacies conditions (low to intermediate P–T ) during the Carboniferous (Franzese 1995). This complex has been correlated with the eastern series of the Coastal Cordillera in Chile (Kato 1985; Herve´ 1988; Franzese 1995). The Chachil Plutonic Complex (Leanza 1990) comprises a series of calc-alkaline plutons, varying from gabbros to pegmatitic granitoids that intrude the Piedra Santa Complex. According to isotopic data (Rb–Sr and whole-rock K–Ar) the age of these plutons ranges between 300 and 281 Ma (Sillitoe 1977; Varela et al. 1994; Franzese 1995). Pre-rift regoliths are also present and locally preserved as granitic conglomerates.
Leanza (1990)
The synrift fill (Rhaetian–Early Pliensbachian) is composed of volcanic, pyroclastic and siliciclastic rocks collectively termed the Precuyano Cycle (Gulisano 1981), Precuyo Mesosequence (Legarreta & Gulisano 1989) or San˜ico ‘Subsynthem’ (Riccardi & Gulisano 1990). In the study area these deposits constitute the Lapa Formation (Fig. 2; Leanza 1990). A volcano-sedimentary complex that overlies the igneous– metamorphic basement was previously correlated with Late Permian–Early Triassic pre-rift units cropping out in other areas of the Neuque´n Basin (Choiyoi Formation, Leanza 1990; Gulisano & Gutierrez-Pleimling 1995). However, as the distribution of this complex is restricted to rift depressions and shows significant thickness variations associated with the main rift structures it is considered an integral part of the synrift succession and is therefore included in the Lapa Formation (Fig. 2) in this study. Although there are no absolute indicators of the age of the synrift sequence in the study area, ignimbrites of the Lapa Formation in neighbouring areas have been dated as Late Triassic (219 Ma) to Early Jurassic (182 Ma) (Pa´ngaro et al. 2002a).
Gulisano & GutierrezPleimling (1995)
Los Molles Fm
This study
Los Molles Fm Los Molles Fm
Chachil Fm/ Sierra Chacaico Fm
Lapa Fm
Chachil Fm/ Sierra Chacaico Fm Lapa Fm
Lapa Fm Choiyoi Fm
Choiyoi Fm
Choiyoi Fm
Huechulafquen Fm
Chachil Plutonic Complex
Huechulafquen Fm
Chachil Plutonic Complex
Colohuincul Fm
Piedra Santa Complex
Colohuincul Fm
Piedra Santa Complex
Fig. 2. Stratigraphy of the pre-rift, synrift and early post-rift units of the southern Neuque´n Basin.
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Plant remains collected in the Lapa Formation towards the south of the study area were also identified as Late Triassic in age (Spalletti et al. 1999).
Early post-rift units Coeval siliciclastic and carbonate deposits of the Sierra Chacaico and Chachil Formations represent the early post-rift accumulation in the study area (Fig. 2). They overlie both the synrift succession and the pre-rift basement and are related to a widespread marine transgression of the Neuque´n Basin. These units contain marine invertebrates assigned to the Early Pliensbachian (Chachil Formation) and Early to Late Pliensbachian (Sierra Chacaico Formation) (Volkheimer 1973; Leanza 1990). The Chachil Formation is composed of shallow-marine siliciclastic deposits that pass upward into spiculitic limestones deposited on a low-energy carbonate ramp (Go´mez Pe´rez & Franzese 1999a). The Sierra Chacaico Formation is composed of volcaniclastic turbidites. Both units grade upward into a thick succession of deep-marine black shales and turbidites of the Los Molles Formation (Fig. 2). The Los Molles Formation represents deposition during the later post-rift phase throughout the Neuque´n Basin. The Los Molles Formation was deposited in a relatively deep, open marine environment, under restricted sub-oxic conditions (Poire´ & del Valle 1992; Burgess et al. 2000).
The Chachil depocentre Synrift depocentres are clearly identifiable in the southern Neuque´n Basin as many of their faulted margins (where synrift and pre-rift units are in contact) have been inverted and uplifted, initially during the Late Jurassic and subsequently since the Late Cretaceous, in the Andean orogenic cycle (for a review, see Howell et al. 2006). Locally younger volcanic and sedimentary rocks overlie the synrift deposits, making it difficult to trace the margins laterally and to define the complete geometry of the grabens. However, it is possible to estimate the geometry and dimension of the depocentres on the basis of the distribution and stratigraphic relationship between the igneous–metamorphic basement and the early post-rift units. The faulted margin of the Chachil depocentre is at present a partially inverted fault system (the Chihuido Bayo fault system, Fig. 3). Only part of the fault is exposed, consisting of a NNWoriented southern segment and a NNE-oriented northern segment. This structural configuration seems to be inherited from the grain of pre-rift rocks in which microstructures and fold axis show the same structural attitude (Dalla Salda et al. 1994). The Chihuido Bayo fault system separates the synrift succession in the hanging wall from the granitic pre-rift basement in the footwall. The pre-rift block to the west and NW of the fault shows no record of synrift succession, and early post-rift deposits directly overlie the pre-rift granites. This suggests that this block acted as a topographic high throughout the evolution of the trough. The synrift interval is also absent south of the southern end of the Chihuido Bayo fault system, supporting the evidence that this is the actual limit of the depocentre (Fig. 3). The sedimentary fill of the graben can be traced for 25 km parallel to and 10 km perpendicular to the boundary fault. Beyond this to the north and west it is covered by younger volcanic units (Fig. 3). Although the exact northern and western limits of the depocentre are not exposed, these dimensions are comparable with those of the synrift depocentres documented in the subsurface (Vergani et al. 1995; Pa´ngaro et al. 2002b; Fig. 4).
Fig. 3. Geological map of the study area and location of logged sections. A–B, location of cross-section shown in Figure 4a.
Close to the faulted boundary of the depocentre, minor NNW–SSE faults cut the synrift sequence. The tectonic inversion of these faults has created a series of NNW-oriented folds (Fig. 3). These structures may reflect the presence of downstepping blocks towards the east, where the synrift succession shows an incremental increase in thickness. Part of the sedimentary infill is located in structurally controlled minor depressions created by normal faulting of major lava flow deposits. Also, minor high-angle normal faults are present, bounding smaller troughs within the rift border system and affecting the upper portion of the synrift succession.
Stratigraphy of the Chachil depocentre To analyse the volcano-sedimentary fill of the Chachil border system, five detailed sedimentary logs were measured along the faulted margin (Figs 3 and 5). The lithofacies documented in these logged sections are strongly associated with significant
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thickness. In contrast to the widespread andesites, the rhyolites commonly show a limited distribution, with a lateral continuity of only a few hundred metres. Most of these andesites were deposited as subaerial lava flows although the hyaloclastic texture suggests that some may have been extruded under subaqueous conditions. The homogeneous nature of the deposits makes it difficult to determine whether the thick successions represent a very thick single flow event or multiple, superimposed events. The lateral restriction of the rhyolitic bodies indicates that they might have been emplaced as shallow intrusions (sills?). The volcanic rocks of the Chachil border system could have originated both from fissural events fed through fracture systems and from isolated effusive centres, although no evidence of volcanic cones was encountered in the study area.
Fig. 4. Cross-sections showing synrift depocentres in the Neuque´n Basin. (a) Simplified cross-section of the Chachil depocentre. (See Fig. 3 for location.) (b) Seismic reflection section and schematic interpretation of a subsurface depocentre (Cerro Bandera; see Fig. 1 for location) after Pa´ngaro et al. (2002b).
magmatic activity, which occurred throughout the evolution of the rift. Significant variations in the thickness of the synrift fill were recorded, from .1000 m in the central part to only a few metres towards the southwestern tip of the Chihuido Bayo Fault. The proportion of volcanic lithofacies also increases towards the central areas of the border system, with volcanic deposits being less common towards the tip of the fault (Fig. 5). Five informal groups of lithofacies have been identified in the Chachil synrift succession. Two of these groups are a product of volcanic processes (volcanic and pyroclastic) whereas the other three are related to sedimentary processes (resedimented volcaniclastic, siliciclastic and carbonate lithofacies). The main characteristics of these lithofacies and an interpretation of their origin are given in Table 1, where the facies code is modified from Smith (1986). Volcanic lithofacies. Volcanic lithofacies locally form up to 40% of the total thickness of the synrift succession. Two lithofacies have been defined according to their composition: andesites and rhyolites. The andesites (A) comprise thick lava successions up to 50 m thick, which are commonly homogeneous. The andesites show massive, brecciated (Fig. 6a) or flow-laminated fabrics, and are red to purple in outcrop. Hyaloclastic textures were also observed locally. The rhyolites (R) also show foliated, autobrecciated or massive structure (Fig. 6b), and are ochre to white in outcrop. The superimposition of several flows with different structures is common, leading to sequences of over 80 m
Pyroclastic lithofacies. The detailed description and identification of the complex processes related to explosive volcanism is not the main purpose of this study and, consequently, the pyroclastic lithofacies has been subdivided only into clastsupported and matrix-supported deposits, which are related to primary fall and flow processes, respectively (Table 1). The pyroclastic lithofacies is usually altered and silicified, making the interpretation of detailed processes problematic. Clast-supported deposits (facies T, Table 1) are locally very common. They show a well-sorted tuffaceous texture (fine- to coarse-grained tuff) and form massive, laterally continuous, tabular bodies up to 1 m thick. They are light grey to greenish in outcrop and occasionally show bioturbation (horizontal tubes). This facies is strongly altered and silicified, but locally retains the ghosts of glass shards. These deposits are classified as ash tuffs (sensu Fisher & Schmincke 1984). The lateral homogeneity, sorting and lack of structures indicate mechanical reworking, and these deposits are interpreted as airfall tuffs that settled from suspension in a low-energy environment. Matrix-supported deposits (I) are composed of thick, light brown to green beds, which are very conspicuous in outcrop. The deposits contain a high proportion of fine-grained matrix with abundant quartz and K-feldspar, and a significant amount of lithic fragments (mainly volcanic rocks) and pumiceous clasts up to 10 cm in diameter (Fig. 6c). From a compositional point of view they can be classified as rhyolitic ignimbrites. These deposits are mainly massive but locally may show inverse grading of lithic clasts. The thicknesses of individual beds vary from 1 m to .50 m, and they stack into successions over 100 m thick. Beds show great lateral extent, covering the whole outcrop area of the depocentre. However, dramatic changes in thickness occur over very short distances (up to 20 m vertical variation over 100 m lateral extent), especially toward topographic highs. The characteristics described above suggest that these matrixsupported deposits are rhyolitic ignimbrites laid down by pyroclastic density currents; probably as topographically confined ignimbrite aprons (Branney & Kokelaar 2002). Resedimented volcaniclastic lithofacies. These deposits have similar lithological characteristics to the pyroclastic material and are represented by volcaniclastic aggregates of texturally unmodified pyroclastic particles (McPhie et al. 1993). In the Chachil depocentre they comprise tuffaceous sandstones and fine-grained breccias with abundant pumice and lithic clasts closely associated with the primary flow and fall deposits (Table 1). The deposits are composed of tabular bodies 10–50 cm thick, mainly massive (*Tm), graded (*Tg) or thinly laminated (*Tl).
E VO L U T I O N O F A M E S O Z O I C G R A B E N B O R D E R S Y S T E M
North
1
Ñireco
2
Piletas
3
Picún Leufú
Resedimented pyroclastic deposits
Post-rift marine deposits
Pyroclastic fall deposits
Fine-grained stream flow deposits
Ignimbrites
Coarse-grained stream flow deposits
Rhyolites
Reworked gravity flow deposits
Andesites
Hyperconcentrated flow deposits
Granites
Gravity flow deposits
Fining-upward sequences
Coarsening-upward sequences
4
Puesto Alfaro
711
5
South
Route 46
200m
0
Fig. 5. Sedimentological logs of Lapa Formation along the Chachil depocentre margin. (See Fig. 3 for location.) No horizontal scale is implied.
Graded facies, which show normal grading of lithic fragments, inverse grading of pumice fragments, erosive bases and bioturbated tops (Fig. 6d), are interbedded with fine-grained thinly laminated deposits. The massive deposits are interpreted as the result of accumulation from subaerial and subaqueous gravity flows, which may have been derived directly from the primary pyroclastic flows (McPhie et al. 1993). The graded beds are interpreted to represent accumulation as a result of low-density turbidity flows intercalated with suspension fall-out deposits, laid down under subaqueous conditions in a low-energy environment. Siliciclastic lithofacies. The siliciclastic lithofacies is the most common sedimentary facies within the rift succession. This facies contains rhyolitic and andesitic clasts reworked from the volcanic sequences and clasts of older, pre-rift rocks. Three subgroups have been defined in terms of their textural characteristics: (1) breccias; (2) interbedded fine-grained breccias and
sandstones; (3) sandstones and conglomerates with minor mudstones (Table 1). Breccias are characterized by a wide range of textures and grain sizes. Although all of them have a matrix-supported texture, the grain size of the matrix differs considerably. Matrixsupported breccias with coarse- to fine-grained sandy matrix (Bm(a), Table 1) are the most common and form tabular deposits up to 12 m thick (Fig. 7a and b). They range from fine- to coarse-grained breccias with very angular clasts of varied composition (volcanic, granitic and metamorphic) up to 40 cm in diameter. Internally, these deposits are massive. They are interpreted as derived from non-cohesive gravity flows. The low to zero content of pelitic material within the matrix suggests that these deposits may have accumulated as a result of saturated grain flows (Smith & Lowe 1991). Matrix-supported breccias with fine-grained (muddy) matrix (Bm(b), Table 1) are less frequent but they also occur as thick tabular packages, up to 5 m thick, with a chaotic fabric (Fig. 7c).
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Table 1. Facies description and interpretation Code Lithology
Structural features
Texture
Geometry
Thickness
Interpretation
Volcanic A Andesite
Massive, hyaloclastic
Porphyritic
Tabular
Intermediate lava flows
Massive, foliated, and brecciated horizons
Porphyritic
Tabular, domal
Flows
