doi:10.1144/0016-764920-082 2006; v. 163; p. 707-721 Journal of the Geological Society

J.R. Franzese, G.D. Veiga, E. Schwarz and I. Gmez-Prez

depocentre, southern Neuqun Basin, ArgentinaTectonostratigraphic evolution of a Mesozoic graben border system: the Chachil

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Journal of the Geological Society, London, Vol. 163, 2006, pp. 707721. Printed in Great Britain.

707

Tectonostratigraphic evolution of a Mesozoic graben border system: the Chachil

depocentre, southern Neuquen Basin, Argentina

J. R. FRANZESE 1, G. D. VEIGA 1, E . SCHWARZ 1 & I . GOMEZ-PEREZ 2

1Centro de Investigaciones Geologicas, Universidad Nacional de La PlataCONICET, Calle 1 #644, B1900TAC La Plata,

Argentina (e-mail: franzese@cig.museo.unlp.edu.ar)2Cambridge 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 Neuquen 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 coarse-

grained non-marine deposition. The late-rift stage is dominated by fine-grained turbidites and pyroclastic falls

related to the first marine sedimentation in the Neuquen Basin.

From Late Triassic to Early Jurassic time, part of the proto-

Pacific margin of Gondwana was affected by continental ex-

tension 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 Neuquen Basin)

that was active during the Mesozoic and Cenozoic at latitudes

between 308S and 408S. The initial (synrift) configuration of thebasin was characterized by the development of isolated deep

depressions bounded by normal faults and filled with volcano-

sedimentary successions (Vergani et al. 1995). Although there

have been a number of studies on the later evolution and fill of

the Neuquen Basin (Veiga et al. 2005, and papers therein) studies

of the early grabens of the Neuquen 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

are rare (Gulisano & Pando 1981; Alvarez & Ramos 1999).

Mesozoic and Cenozoic inversion in the central and southern

Neuquen Basin produced uplift and exposure of the fault-

bounded margins of the depocentres, providing good exposures

of the synrift succession. This study focuses on the tectonostrati-

graphic evolution of one of these early depocentres in the

southern part of the Neuquen Basin, located in the Cerro Chachil

area (central Neuquen 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

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 sedimentologi-

cal 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 se-

quences in continental rifts strongly dominated by volcanic or

volcaniclastic input. Recently the early rift basins of the Neuquen

Basin have been the focus of hydrocarbon exploration (Pangaro

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.

Geological setting

The Neuquen Basin is located in westcentral 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 (Legarre-

ta & Uliana 1991; Vergani et al. 1995; Franzese & Spalletti

2001; Howell et al. 2005).

The synrift depocentres of the Neuquen 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).

The study area is located in the eastern margin of the Andean

Cordillera in central Neuquen province, where the synrift units

are particularly well exposed (Fig. 1). In this area, the lithostrati-

graphic units involved in the evolution of the Chachil depocentre

can be divided into pre-rift, synrift and early post-rift units.

Pre-rift units

The basement of the Neuquen Basin is composed of meta-

morphic 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 greenschist-

facies conditions (low to intermediate PT ) during the Carboni-

ferous (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 (RbSr and

whole-rock KAr) 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.

Synrift units

The synrift fill (RhaetianEarly Pliensbachian) is composed of

volcanic, pyroclastic and siliciclastic rocks collectively termed

the Precuyano Cycle (Gulisano 1981), Precuyo Mesosequence

(Legarreta & Gulisano 1989) or Sanico 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

PermianEarly Triassic pre-rift units cropping out in other areas

of the Neuquen Basin (Choiyoi Formation, Leanza 1990; Gulisa-

no & Gutierrez-Pleimling 1995). However, as the distribution of

this complex is restricted to rift depressions and shows signifi-

cant 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) (Pangaro et al. 2002a).

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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 accumula-

tion 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 Neuquen Basin. These

units contain marine invertebrates assigned to the Early Pliensba-

chian (Chachil Formation) and Early to Late Pliensbachian

(Sierra Chacaico Formation) (Volkheimer 1973; Leanza 1990).

The Chachil Formation is composed of shallow-marine siliciclas-

tic deposits that pass upward into spiculitic limestones deposited

on a low-energy carbonate ramp (Gomez Perez & Franzese

1999a). The Sierra Chacaico Formation is composed of volcani-

clastic 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 deposi-

tion during the later post-rift phase throughout the Neuquen

Basin. The Los Molles Formation was deposited in a relatively

deep, open marine environment, under restricted sub-oxic condi-

tions (Poire & del Valle 1992; Burgess et al. 2000).

The Chachil depocentre

Synrift depocentres are clearly identifiable in the southern

Neuquen 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 igneousmetamorphic

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 NNW-

oriented southern segment and a NNE-oriented northern seg-

ment. 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; Pangaro et al. 2002b;

Fig. 4).

Close to the faulted boundary of the depocentre, minor

NNWSSE faults cut the synrift sequence. The tectonic inver-

sion of these faults has created a series of NNW-oriented folds

(Fig. 3). These structures may reflect the presence of down-

stepping blocks towards the east, where the synrift succession

shows an incremental increase in thickness. Part of the sedimen-

tary 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

Fig. 3. Geological map of the study area and location of logged sections.

AB, location of cross-section shown in Figure 4a.

EVOLUTION OF A MESOZOIC GRABEN BORDER SYSTEM 709

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 fewmetres 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, autobrec-

ciated 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

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.

Pyroclastic lithofacies. The detailed description and identifica-

tion 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 clast-

supported 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 air-

fall tuffs that settled from suspension in a low-energy environ-

ment.

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 mthick. 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 matrix-

supported deposits are rhyolitic ignimbrites laid down by pyro-

clastic 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 un-

modified 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 1050 cm thick,

mainly massive (*Tm), graded (*Tg) or thinly laminated (*Tl).

Fig. 4. Cross-sections showing synrift depocentres in the Neuquen 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

Pangaro et al. (2002b).

J. R. FRANZESE ET AL.710

Graded facies, which show normal grading of lithic fragments,

inverse grading of pumice fragments, erosive bases and biotur-

bated tops (Fig. 6d), are interbedded with fine-grained thinly

laminated deposits.

The massive deposits are interpreted as the result of accumula-

tion 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 character-

istics: (1) breccias; (2) interbedded fine-grained breccias and

sandstones; (3) sandstones and conglomerates with minor mud-

stones (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. Matrix-

supported 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 inter-

preted 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).

#"

Table 1. Facies description and interpretation

Code Lithology Structural features Texture Geometry Thickness Interpretation

VolcanicA Andesite Massive, hyaloclastic Porphyritic Tabular Flows

These are the coarsest deposits of the synrift sequence, with

clasts up to 2 m in diameter. This facies is interpreted as the

result of cohesive debris flows (Smith & Lowe 1991).

Matrix-supported breccias with moderate sorting and very

diffuse trough cross-bedding in sets up to 80 cm thick with a

upward-fining trend are present (*Bx, Table 1, Fig. 7d). These

contain very angular clasts up to 40 cm in diameter and have a

coarse-grained sandy matrix. These deposits may represent the

result of partial reworking (probably by ephemeral stream flows)

of the original sediment gravity flow deposits. This reworking is

interpreted to have improved the sorting and organization while

preserving much of the original texture of the gravity flow

deposits (Smith & Lowe 1991).

All of the breccia lithofacies are here interpreted as sediment

gravity flow deposits that accumulated in the proximal areas of a

volcanic setting. The abundance of lithofacies related to non-

cohesive flows is a common situation in proximal volcanic

environments, compared with alluvial settings where deposits

derived from cohesive debris flows are more common (Fisher &

Schmincke 1984; Smith 1986).

Interbedded fine-grained breccias and sandstones are charac-

terized by the close association of fine-grained sandy matrix-

supported breccias (with clasts no more than 2 cm in diameter)

and porphyritic, crystal-rich, coarse-grained sandstones. They

form tabular bodies 0.301 m thick with a sharp and occasion-

ally erosional lower boundary. Two lithofacies have been

identified according to their primary sedimentary structures:

graded (BSg) and horizontally laminated (BSh) interbedded

breccias and sandstones (Table 1). The graded lithofacies (Fig.

8a) shows normal grading and occasionally low-angle cross-

stratification or horizontal lamination. The BSh lithofacies (Fig.

8b) is characterized by the interstratifications of millimetre-

scale laminae with contrasting grain sizes (granule to medium-

grained sand).

These deposits may represent the accumulation of hypercon-

centrated flow deposits as a result of rapid deposition from

sediment-laden currents (Smith 1987). The different sedimentary

structures present in these deposits may be related to variations

in the sediment content of the original flow that modified its

rheological behaviour (Smith & Lowe 1991). These facies could

have originated from relatively dilute flows with enough turbu-

lence to erode the underlying sediments and to facilitate the

separation of particles, generating erosively based deposits.

Graded deposits with low-angle cross-stratification could be

related to the development of shallow bedforms in upper-flow

regime conditions (Smith 1986).

Sandstones, mudstones and conglomerates are infrequent and

localized. Conglomerates are relatively well sorted and coarse

grained with a clast-supported texture. Massive conglomerates

(Gm, Table 1) form tabular bodies, up to 2 m thick, with an

erosive, horizontal lower boundary. They usually stack vertically

to form sequences up to 4 m thick. They represent the accumula-

Fig. 6. Volcanic and pyroclastic lithofacies. (a) Hyaloclastic andesite (facies A); lens cap is 52 mm in diameter. (b) Rhyolite with foliated structure (facies

R); pen is 11 cm long. (c) Ignimbrite with coarse-grained deformed pumice fragments (facies I); hammer is 33 cm long. (d) Bioturbation at the top of

graded reworked pyroclastic deposits (facies *Tg); lens cap is 52 mm in diameter.

EVOLUTION OF A MESOZOIC GRABEN BORDER SYSTEM 713

tion of gravel longitudinal bars in well-developed, high-energy

fluvial channels (Hein & Walker 1977).

Cross-bedded conglomerates (Gx, Table 1) are finer grained

(maximum clast size 10 cm) and show large-scale trough cross-

bedding. This facies usually occurs as lenticular bodies, up to

4 m thick and 20 m wide (Fig. 8c), with a conspicuous erosive,

concave lower boundary. These bodies may represent coarse-

grained channel fill deposits formed by the migration of sinuous-

crested gravel dunes.

Coarse-grained, well-sorted, trough cross-bedded sandstones

(Sx, Table 1) are also present associated with the cross-bedded

conglomerates, and form lenticular bodies with erosive, concave

bases up to 1.5 m thick. These bodies may represent waning-

stage, channel-fill deposits (Miall 1996) associated with the

gravel bars and channels.

Poorly sorted fine- to coarse-grained sandstones with horizon-

tal to low-angle cross-bedding (Sh, Table 1) form lenticular

bodies with flat bases and convex tops (Fig. 8d) up to 1 m thick.

These deposits are closely associated with massive to horizon-

tally laminated red mudstones (F, Table 1) in tabular bodies up to

8 m thick (Fig. 8d). The Sh lithofacies represents accumulation

from ephemeral (unconfined) stream flows with variable dis-

charge in the intermediate to distal parts of the fluvial systems

(Smith 1987). The lithofacies F, on the other hand, may represent

settling from suspensions in more distal parts of the volcano-

sedimentary environment (Smith 1987). Therefore, conglomer-

ates, sandstones and mudstones represent stream flow deposits

associated with more diluted flows than those responsible for the

accumulation of the breccias and intercalated breccias and

porphyritic sandstones.

Carbonate lithofacies. The carbonate lithofacies is more re-

stricted and localized than the other lithofacies. The carbonates

are always extremely silicified and show a very low preservation

of their primary sedimentary features (Table 1). They are

typically thinly laminated (C), ,20 cm thick, with an irregulargeometry. They show a corrugated structure and are intercalated

with fine-grained siliciclastic deposits (F). They may represent

the non-marine precipitation of carbonate in shallow, ephemeral

water bodies (Ridding 2000).

Spatial and stratigraphic distribution of the lithofacies

Distribution of volcanic and pyroclastic lithofacies

The large volume of volcanic and volcaniclastic material within

the graben border system indicates that there was a close

interplay between magmatism and tectonism during the synrift

evolution. Systematic variations in the distribution of the volca-

nic lithofacies have been documented throughout the synrift

Fig. 7. Siliciclastic lithofacies related to sediment gravity flow processes. (a) Coarse-grained breccia with sandy matrix (facies Bm(a)); hammer is 33 cm

long. (b) Fine-grained breccia with sandy matrix (facies Bm(a)); lens cap is 52 mm in diameter. (c) Coarse-grained breccia with muddy matrix (facies

Bm(b)); geologist for scale. (d) Cross-bedded breccia with matrix-supported texture and poor sorting (facies *Bx); hammer is 33 cm long.

J. R. FRANZESE ET AL.714

sequence. Volcanic rocks are volumetrically the most important

in the lower part of the section and are less frequent in the

middle and upper parts of the synrift succession (Figs 5 and 9).

Deposits of acidic pyroclastic flows (I) and tephra derived from

pyroclastic falls (T) replace the volcanic lithofacies in the

uppermost part of the rift section.

There is a clear compositional trend within the magmatic-

related lithofacies from intermediate to acidic. In the lower

portion of the Nireco section (Fig. 5), successive deposits show

contrasting composition (andesite and rhyolite). Andesitic lavas

form most of the lower part of the succession, reaching a

thickness of .100 m in the Nireco and Piletas sections, andthinning towards the southern margin of the depocentre (with a

thickness of 50 m in the Picun Leufu section). Rhyolites

associated with these lower andesites occur as thin intercalations.

The andesitic rocks do not occur in the middle and upper parts

of the succession, where the increasingly acidic magmatic

composition resulted in the development of rhyolitic domes, lava

flows and explosive volcanism. Whereas evidence for andesitic

volcanism is present in all of the studied sections, rhyolite lava

flow deposits are restricted to the thickest section (Nireco). In

this area, rhyolitic flows and domes compose up to 140 m of the

section. This restricted distribution could reflect a limited disper-

sion of acidic flows away from localized eruptive centres in the

deepest section of the graben border system.

Pyroclastic flows dominate the magmatic-related lithofacies

from the middle part of the succession to the top. The most

remarkable pyroclastic event is recorded by a locally thick

(.50 m) ignimbrite that can be correlated throughout the studyarea (Fig. 9). Ignimbrites and rhyolites (facies I and R) are

closely associated in the middle portion of the Nireco and Piletas

sections (Fig. 5). The rhyolitic lava domes observed in the Nireco

section may be the potential source areas for the pyroclastic

density currents that led to the accumulation of the ignimbrite

layers.

Distribution of siliciclastic and carbonate lithofacies

The sedimentary record of the synrift succession is mainly

dominated by the alternation of sediment gravity flow deposits

and hyperconcentrated flow deposits (Table 1, Fig. 5) accumu-

lated in a proximal continental environment. Although the lateral

correlation of individual beds between sections is difficult

because their distribution is controlled by internal volcanic and

structural highs, the overall vertical facies trend in every studied

locality is persistent. Towards the top of the succession resedi-

Fig. 8. Siliciclastic lithofacies related to hyperconcentrated (a, b) and stream flow processes (c, d). (a) Normally graded fine-grained breccias and

sandstones (facies BSg); hammer is 30 cm long. (b) Intercalation of millimetre-thick fine-grained breccias and sandstones (facies BSh); lens cap is 52 mm

in diameter. (c) Lenticular cross-bedded conglomerates and sandstones (facies Gx and Sx); erosive lower boundary should be noted; geologist for scale.

(d) Intercalation of massive and laminated mudstones (facies F) with horizontally stratified sandstones (facies Sh) related to distal stream flows; geologist

for scale.

EVOLUTION OF A MESOZOIC GRABEN BORDER SYSTEM 715

mented volcaniclastic deposits appear and are more common

than the other sedimentary lithofacies. Also towards the upper-

most part of the synrift succession, an abrupt change from

continental to marine sedimentation is recorded. Stream flow

deposits (see Table 1) are present throughout the succession with

no discernible vertical trend (Fig. 5).

Although the intercalation of gravity flow deposits, hypercon-

centrated flow deposits and stream flow deposits does not show

an observable organization at the scale of the whole synrift

succession, it is possible to define sequences of c. 30 m thickness

with both fining- and coarsening-upward grain-size trends that

can be related to gradual changes in the depositional processes

(Fig. 5). Fining-upward intervals are dominated by coarse-

grained, gravity flow-related facies at the base (Bm(a), Bm(b)

and BSg) that are transitional to finer-grained hyperconcentrated

flow deposits towards the top (BSh) (Fig. 10a). Horizontally

stratified sandstones and conglomerates (Sh) or massive mud-

stones (F) sometimes typify the upper portions of these intervals,

and represent more diluted conditions. In contrast, coarsening-

upward intervals show an inverse array of the same lithofacies

(Fig. 10b).

As all these processes may be laterally linked and related to

the downstream transformation of original sediment gravity

flows, the general fining-upward pattern could represent the

gradual contraction and retrogradation of the facies belts towards

the active margin of the depocentre. The profusion of gravity

flow deposits at the top of these intervals suggests the prograda-

tion of the depositional system from the active margin towards

the depocentre.

Eleven of these sequences were recognized in the Nireco

section, six in the Piletas section and two in the Picun Leufu

section (Fig. 5). These sequences are separated by lava or

ignimbrite deposits, or by intervals not showing any conspicuous

trend in grain size (Fig. 5). Fining-upward sequences are

Fig. 9. General view of the Piletas section

(a) and interpretation (b), showing the

overall trend in lithologies. At the base

hyperconcentrated flow deposits (HFD) and

gravity flow deposits (GFD) are intercalated

with andesitic lavas and stream flow

deposits (SFD). At the top, an ignimbrite

flow and pyroclastic deposits are

intercalated with reworked pyroclastic

deposits and minor gravity flow deposits.

J. R. FRANZESE ET AL.716

common on top of major andesitic deposits or form minor clastic

wedges near volcanicpyroclastic highs in the Nireco and Picun

Leufu areas. Coarsening-upward intervals typically occur above

fining-upward units or below main lava flow events (e.g. the

basal and middle portions of the Nireco section, and the base of

the Picun Leufu section, Fig. 5).

Discussion: evolution of the synrift megasequence

The graben border system of the Chachil depocentre includes a

boundary fault system (the Chihuido Bayo fault system), the

associated downstepping blocks and the volcano-sedimentary

wedge that accumulated in the hanging wall of the main

boundary fault. The systematic variation in thickness of the

volcano-sedimentary infill, the change of magmatic composition,

the development of upward-fining and upward-thickening clastic

sequences, and the structural reactivation of the graben border

system are the key elements of the tectonostratigraphic evolution

of the synrift succession.

Significant and progressive increases in the thickness of the

volcano-sedimentary wedge towards the centre of the graben

border system could be a response to along-strike variations in

fault displacement that produced a lateral increase of subsidence

from the tip to the centre of the main boundary fault (Schlische

1992; Gawthorpe et al. 1994; Howell & Flint 1996). Minor

normal faults may control the accumulation locally through the

development of small troughs, although this does not affect the

general pattern of thickness increase along the Chihuido Bayo

fault system.

The clear trend in magmatic composition throughout the

synrift megasequence is interpreted to relate to the tectonic

evolution of the basin. The initial bimodal volcanism requires a

mixed mantlecrust magmatic source. This type of source can

be related to mantle upwelling and thermal perturbation of the

crust during the early stages of gravitational collapse (Liu 2001),

which caused extension and rifting in the Neuquen Basin. The

ascent of intermediate to basic magma from deep sources ended

in the middle part of the synrift sequence. The acidic explosive

volcanism that dominated the upper section implies the emplace-

ment of shallow silicic magmatic chambers closely associated

with normal faulting at upper crustal levels.

The fining- and coarsening-upward sequences identified in the

synrift megasequence may also record episodic tectonomagmatic

activity along the main boundary fault. Differential subsidence

rates and the development of topographic highs can alter the

relationship between accommodation and sediment supply over

time, generating a stratigraphic response that can be distin-

guished by the analysis of the depositional sequences recorded

(Blair & Bilodeau 1988; Paola et al. 1992; Gordon & Heller

1993). The fining-upward sequences may result from an increase

in the rate of accommodation creation compared with the rate of

sediment supply. During rapid, episodic, fault-generated subsi-

dence sediment entering the basin would have been trapped near

the basin margin, resulting in aggradation or retrogradation of

the sedimentary systems (Blair & Bilodeau 1988; Gordon &

Heller 1993; Marr et al. 2000). In contrast, the upward-coarsen-

ing units suggest progradation of the depositional system,

potentially during periods of tectonic quiescence, when the

subsidence rate was reduced or even suppressed (Morley 1999).

The development of thick sequences without a conspicuous

change in grain size suggests a continuous aggradation of the

system. Such a trend may be the result of equilibrium between

sediment supply and accommodation creation during limited

periods of the synrift evolution. Although active normal faulting

may have increased the topographic relief within and at the

margins of the basin, the association of the fining-upward

successions with minor normal faults indicates that subsidence

rather than localized sediment supply was the main control on

the migration of the facies belt.

The upward change in magmatic composition associated with

the record of tectonically controlled clastic sequences and the

change in sedimentary conditions from continental to marine

document a significant trend in the evolution of the synrift

megasequence. Therefore, the synrift megasequence in the

Chachil area can be divided into three major evolutionary stages,

termed here the early rift, mid-rift and late-rift stages (Figs 11

and 12). The boundaries between these evolutionary stages are

locally associated with structural reactivation, generation of

minor normal faulting and changes in subsidence along the

Fig. 10. Example of fining-upward and

coarsening-upward sequences with their

facies distribution and relation to normal

fault activity (a, accommodation; s,

sediment supply). The fining-upward

sequence is from the Piletas section and the

coarsening-upward sequence is from the

Nireco section.

EVOLUTION OF A MESOZOIC GRABEN BORDER SYSTEM 717

graben border system. Therefore these boundaries could be

related to major tectonic reorganizations of the entire depocentre.

The early rift stage is characterized by the interplay between

bimodal volcanism (facies A and R) and gravity-driven sedimen-

tation (gravity flow deposits and hyperconcentrated flow depos-

its). The base of the mid-rift stage is represented by the change

from andesitic volcanism to rhyolitic lava flows and pyroclastic

flows, which record a transition to more acidic magmatism. The

late-rift stage is characterized by resedimented pyroclastic depos-

its in continental to marine environments, which are associated

with protracted explosive volcanism.

The early rift stage

Volcanic rocks form half of the stratigraphic thickness of the

early rift stage. The sedimentary fill is composed mainly of

continental gravity flow deposits and hyperconcentrated flow

deposits. Localized stream flow deposits occur in the Piletas

section (Fig. 5), where a NE-oriented drainage system may

reflect the activity of minor faults oblique to the main boundary

fault. Thin calcareous deposits (C) and associated mudstones (F)

in the Piletas section indicate subaqueous accumulation in

ephemeral water bodies. This feature contrasts with other synrift

depocentres in the Neuquen Basin, where thick lacustrine succes-

sions related to the development of deep, perennial and stratified

water bodies occupy most of the sedimentary record (Legarreta

et al. 1999).

The initial sedimentary fill of this stage is composed of a

coarsening-upward succession of gravity flow deposit and hyper-

concentrated flow deposit units (Figs 5 and 11). This upward

coarsening suggests the progradation of coarse-grained, proximal

fluvialalluvial systems over the pre-rift topography during a

time when the displacement rates of the main faults were low.

Such conditions are characteristic of the early stages of rift

evolution (Cowie et al. 2000; Gawthorpe & Leeder 2000).

The volcaniclastic nature of the early rift infill indicates that

volcanic activity was associated with the normal faulting from

the onset of basin evolution. Minor reworking of pre-rift deposits

is limited to the Picun Leufu section (Fig. 5), where coarse-

grained, gravity-driven deposits containing abundant granitic

fragments occur.

The thickness of early rift deposits is very variable, reaching

250 m in the central part of the graben border system and no

more than a couple of metres near the tip of the Chihuido Bayo

boundary fault (Fig. 11). Volcanic andesite lavas, a key lithology

within the early rift stage, are present in all the studied sections,

suggesting that the rift system had reached its full length by the

end of the early stage. Variations in thickness along the Chihuido

Bayo fault system may have resulted from the infill of small

isolated depocentres during the early evolution of the basin (Fig.

12). This is common in the early phase of rift evolution, in which

a number of distinct minor depocentres would be expected

(Cowie et al. 2000; Gawthorpe & Leeder 2000).

The mid-rift stage

During the mid-rift stage the Chachil border system was fully

developed, possibly as a result of the linkage of the initially

isolated fault segments (Fig. 12). Up to 500 m of gravity flow

deposits, hyperconcentrated flow deposits, pyroclastic flow and

Fig. 11. Evolution of the synrift megasequence. Sequence distribution, volcanic evolution and correlation of major units. (See Fig. 3 for location of

sections.)

J. R. FRANZESE ET AL.718

acidic lava flow deposits accumulated in the northern part of the

study area, where subsidence was highest (Fig. 11). The transi-

tion from the early rift to mid-rift stage is marked by the base of

an upward-fining unit. This stacking pattern may result from the

increase in accommodation associated with increased activity on

the main faults and fault linkage. The presence of some

coarsening-upward sequences within the mid-rift succession

(Nireco section, Fig. 11) suggests that volcaniclastic supply

locally outpaced subsidence rates. The composition of the

volcanic rocks becomes more acidic upward as the intermediate

magmatic deposits disappear, and rhyolitic and explosive volca-

nic rocks dominate in the mid-rift stage. Rhyolites are restricted

to the central and northern portion of the study area, suggesting

a dramatic reduction in the area covered by lava flows (Figs 4,

11 and 12).

Major variations in thickness occur within this stage; however,

the thickest sections in the mid-rift stage do not coincide with

those in the early rift stage (compare the Nireco and Piletas

sections, Fig. 12). This may be the response to changes in

subsidence pattern triggered by fault linkage during the early

evolution of the rift sequence. However, the development of

minor faults in the hanging wall of the Chihuido Bayo fault

system locally modified the distribution of highs and lows within

the graben border system and the general subsidence trend of the

graben border system (Picun Leufu section, Fig. 11).

The extrusion of thick acidic pyroclastic flows that covered

almost all of the Chachil and neighbouring depocentres during

this stage (Fig.11) suggests that these flows were part of a main

explosive event, probably the collapse of a caldera located

beneath one of the principal depocentres.

The late-rift stage

The last evolutionary stage of the synrift sequence is character-

ized by a significant change in environmental conditions and the

accumulation of marine deposits (*Tr; *Tg) in a moderately deep

environment dominated by turbidity currents, which provides the

first record of marine sedimentation in the evolution of the

Neuquen Basin (Figs 11 and 12). Outside of the graben border

system, in areas where subsidence rates were lower, coeval

deposits attributed to a restricted shallow marine environment

were described by Gomez Perez & Franzese (1999b). Volcanism

occurred as ash falls and thin ignimbritic units.

The late-rift stage started with the reactivation of normal

faulting, which produced narrow troughs filled with fine-grained

successions c. 50 m thick, particularly evident toward the south-

ern margin of the Chachil depocentre (Fig. 11). Rapid subsidence

of these newly formed sub-basins is marked by the strong

divergent stratal pattern of the deep-marine sedimentation.

The general stratigraphic pattern of upward-fining successions

and the transition from continental to deep-marine sedimentation

reflect the interplay between high rates of accommodation

creation and a major transgressive event that affected the Chachil

depocentre as a whole at the end of the synrift stage. In the

graben border system this tectonically enhanced transgression

generated the starved conditions that dominated the uppermost

section of the synrift megasequence. The decay of normal fault

activity and persistence of marine flooding provided the tectonos-

tratigraphic setting for the transition to the Lower Pliensbachian

post-rift sequence.

Conclusions

(1) The Chachil depocentre is one of the small extensional half-

grabens that were active during early rifting of the Neuquen

Basin. The faulted border of the trough was uplifted and exposed

during Andean inversion. The graben border system of the

Chachil half-graben was controlled by the Chihuido Bayo fault

system and associated structures.

(2) Subsidence along the Chihuido Bayo fault system was

variable, allowing the accumulation of .1000 m of synrift fill in

*$"

"*$"

,2

2

-&

"

2" $B $"

2" $B $"

""

7"""

"

*$"

2

"2" $ $"

"2" $ $"

2

F 90 1

Fig. 12. Schematic 3D blocks showing the main features of the three

synrift stages (see text for explanation).

EVOLUTION OF A MESOZOIC GRABEN BORDER SYSTEM 719

the central part of the system. Close to the southern tip of the

half-graben no record of synrift rocks was found.

(3) Volcanic lava flows (andesites and rhyolites), primary and

resedimented pyroclastic flows and falls, and epiclastic and

carbonate rocks form the fill of the synrift megasequence. Most

of the sedimentary record was derived from transverse flux as

gravity driven deposits.

(4) Volcanism shows a compositional trend from bimodal to

acidicexplosive types during the evolution of the synrift succes-

sion.

(5) The Chachil synrift megasequence evolved through three

stages: the early rift, mid-rift and late-rift stages. Major bimodal

volcanism and the accumulation of gravity-driven deposits

dominated the early rift stage. The final extent of the depocentre

was probably reached during this stage. The mid-rift stage saw

the transition from bimodal volcanism to more acidic events and

the accumulation of the first important pyroclastic flow deposits.

Gravity driven deposits and hyperconcentrated flow deposits were

still important throughout this stage. The late-rift stage was also

marked by newly formed faults that generated a completely new

pattern of palaeohighs and grabens. A more generalized sub-

sidence pattern combined with a transgressive event continued

through the post-rift stage and led to the accumulation of

subaqueous deposits in a relatively deep marine environment.

These are the oldest marine deposits in the Neuquen Basin.

This research was carried out with the financial support of CONICET

(PIP 02148; PEI 0495/97), ANPCYT (PICT 07-08467) and CASP

(Cambridge Arctic Shelf ProgrammeSouth Atlantic Project). Our thanks

go to S. Gupta and an anonymous reviewer for their constructive and

helpful reviews. We are grateful to the Societys editor J. Howell for his

significant contribution to the final version.

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Received 21 May 2003; revised typescript accepted 15 February 2006.

Scientific editing by John Howe

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