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Jurassic Uplift Along the Huincul Arch and Its Consequences in the Stratigraphy of the Cuyo and Lotena Groups. Neuquén Basin, Argentina

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Jurassic Uplift Along the Huincul Arch and Its Consequences in the Stratigraphy of the Cuyo and Lotena Groups. Neuquén Basin, Argentina

Abstract and Figures

Marine sedimentation in the Neuquén Basin started in the Early Jurassic with the accumulation of a progradational clastic system: the Cuyo Group. This sedimentary cycle begins with offshore shales of the Los Molles Formation, transitionally overlaid by shelfal to littoral sandstones and conglomerates of the Lajas Formation. According to the current stratigraphic schema, in the southern Neuquén Basin the Cuyo Group ends with continental red beds of the Challacó Formation, a unit interpreted as being the proximal equivalent of the Lajas Formation. Nevertheless, recent studies performed in the Picún Leufú area, complemented with regional stratigraphic evidence, suggest that the accumulation of these stratigraphic units is more likely related to a complex depositional scenario that involves tectonic activity during the Jurassic along the Huincul Arch. The uplift and exposure of Jurassic rocks along the Huincul Arch divided the basin into two sub-basins, each one having a different stratigraphy for the Middle-Late Jurassic time span. The stratigraphic analysis revealed that the red beds traditionally located at the top of the Cuyo Group in fact represent two different stratigraphic units separated by a regional unconformity. The lower unit (1) sharply overlies the Lajas Formation, corresponding to the Challacó Formation s.s. This unit was accumulated in a brackish lacustrine basin developed southward of the Huincul Arch (Picún Leufú Sub-basin). The upper unit (2) corresponds to the Bosque Petrificado Formation and is stratigraphically located at the base of the Lotena Group. The Bosque Petrificado Formation was accumulated in a fluvial to marine environment developed in the southern Neuquén Basin. Paleocurrent analysis suggests a sediment supply from the northeast, evidencing a local source of sediments related to an uplifted area. Recent biostratigraphic studies performed in the Bosque Petrificado and Lotena formations revealed an Early Callovian-Early Oxfordian age for these deposits.
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Jurassic Uplift Along the Huincul Arch
and Its Consequences in the Stratigraphy
of the Cuyo and Lotena Groups.
Neuquén Basin, Argentina
Carlos Zavala, Mariano Arcuri, Mariano Di Meglio, Agustín Zorzano
and Germán Otharán
Abstract Marine sedimentation in the Neuquén Basin started in the Early Jurassic
with the accumulation of a progradational clastic system: the Cuyo Group. This
sedimentary cycle begins with offshore shales of the Los Molles Formation, tran-
sitionally overlaid by shelfal to littoral sandstones and conglomerates of the Lajas
Formation. According to the current stratigraphic schema, in the southern Neuquén
Basin the Cuyo Group ends with continental red beds of the Challacó Formation, a
unit interpreted as being the proximal equivalent of the Lajas Formation. Neverthe-
less, recent studies performed in the Picún Leufú area, complemented with regional
stratigraphic evidence, suggest that the accumulation of these stratigraphic units is
more likely related to a complex depositional scenario that involves tectonic activity
during the Jurassic along the Huincul Arch. The uplift and exposure of Jurassic rocks
along the Huincul Arch divided the basin into two sub-basins, each one having a dif-
ferent stratigraphy for the Middle-Late Jurassic time span. The stratigraphic analysis
revealed that the red beds traditionally located at the top of the Cuyo Group in fact
represent two different stratigraphic units separated by a regional unconformity. The
lower unit (1) sharply overlies the Lajas Formation, corresponding to the Challacó
Formation s.s. This unit was accumulated in a brackish lacustrine basin developed
southward of the Huincul Arch (Picún Leufú Sub-basin). The upper unit (2) corre-
sponds to the Bosque Petrificado Formation and is stratigraphically located at the
base of the Lotena Group. The Bosque Petrificado Formation was accumulated in a
fluvial to marine environment developed in the southern Neuquén Basin. Paleocur-
rent analysis suggests a sediment supply from the northeast, evidencing a local source
of sediments related to an uplifted area. Recent biostratigraphic studies performed
in the Bosque Petrificado and Lotena formations revealed an Early Callovian-Early
Oxfordian age for these deposits.
C. Zavala (B
)·M. Arcuri ·M. Di Meglio ·A. Zorzano ·G. Otharán
GCS Argentina SRL, Molina Campos 150, Bahía Blanca 8000, Argentina
e-mail: czavala@gcsargentina.com
C. Zavala ·M. Arcuri ·M. Di Meglio ·G. Otharán
Departamento de Geología, Universidad Nacional del Sur, San Juan 670, Bahía Blanca 8000,
Buenos Aires, Argentina
© Springer Nature Switzerland AG 2020
D. Kietzmann and A. Folguera (eds.), Opening and Closure of the Neuquén
Basin in the Southern Andes, Springer Earth System Sciences,
https://doi.org/10.1007/978-3-030-29680-3_3
53
54 C. Zavala et al.
Keywords Cuyo Group ·Lotena Group ·Challacó Formation ·Synsedimentary
tectonics ·Huincul Arch
1 Introduction
The Cuyo Group (Groeber 1946; Stipanicic 1969) represents the first fully
marine transgressive-regressive depositional cycle recognized in the Neuquén Basin.
Depending on the position within the basin, the accumulation of this unit started in
the Hettangian–Pliensbachian and extended up to the Middle Callovian.
The internal organization of the Cuyo Group has been described, discussed and
analyzed in detail during more than a century. According to the conventional stratig-
raphy of the Neuquén Basin, the most accepted depositional model suggest that the
accumulation of the Cuyo Group started with offshore to prodelta marine shales
(Los Molles Formation) transitionally overlaid by shelfal to littoral clastic deposits
(Lajas Formation), ending up with continental (fluvial) red beds (Challacó Forma-
tion). This classic prograding clastic system has been integrated and adapted to the
former sequence stratigraphic framework of the Neuquén Basin.
In recent years, new high-resolution seismic surveys and extensive fieldworks
suggested that the stratigraphy of these units is largely more complex and was
strongly controlled by synsedimentary tectonics.
This paper provides new stratigraphic evidence for the regional understanding of
the origin and evolution of the Jurassic Cuyo and Lotena groups.
2 Geological Framework
Located in west central Argentina, the Neuquén Basin is one of the main oil-
producing basins of South America (Fig. 1). The origin of the Neuquén Basin is
closely associated with the Mesozoic evolution of the western margin of Gondwana.
Basin configuration took place during Late Triassic, over a highly heterogeneous
Paleozoic basement resulting from the collision of the Patagonia terrane with the
southern margin of Gondwana (Mosquera et al. 2011). The stratigraphic column of
the Neuquén Basin comprises a thick (> 7,000 m) sedimentary succession mainly
integrated by marine and continental siliciclastic deposits, and minor carbonate and
evaporitic units (Fig. 1). Sedimentological and stratigraphic evidences suggest that
the Neuquén Basin was a satellite basin with a nearly continuous connection with
the Pacific Ocean during the Jurassic and Early Cretaceous.
Sedimentation in the Neuquén Basin started in the Late Triassic as a conse-
quence of a regional rift phase. During this interval, volcanic and volcaniclastic
deposits accumulated in irregular and structurally controlled depocenters (Precuyo
Jurassic Uplift Along the Huincul Arch and Its Consequences … 55
Fig. 1 Location map and generalized stratigraphic column of the Neuquén Basin. Main outcrops
of basement, Jurassic and Cretaceous rocks are indicated. The box in the map shows the location
of the Jurassic outcrops discussed in this study (modified after Zavala et al. 2006)
Group, Gulisano et al. 1984) (see Chap. “The Syn-Rift of the Neuquén Basin-Pre-
cuyano and Lower Cuyano Cycle: Review of Structure, Volcanism, Tectono-Stratig-
raphy and Depositional Scenarios”). Afterwards, during the Jurassic and Creta-
ceous the Neuquén Basin was periodically flooded from the paleo-Pacific, resulting
in the accumulation of deposits related to several transgressive-regressive cycles.
The first two cycles, corresponding to the Cuyo and Lotena groups, comprise the
lapse Hettangian-Oxfordian.
The Cuyo Group starts with offshore to prodeltaic marine shales and turbidites of
the Los Molles Formation (Weaver 1931). This unit represents a fine-grained clastic
succession up to 1000 m thick. Regional data and thickness maps suggest that this
unit was accumulated over a tectonically controlled relief. Sediment gravity-flow
deposits (muddy, sandy and gravelly) are common in areas associated with lower
submarine landscapes, where they form a very thick succession of sandstones and
shales related to intrabasinal and extrabasinal turbidity currents (Paim et al. 2011;
Zavala and Arcuri 2016). The Los Molles Formation is transitionally overlaid by
56 C. Zavala et al.
a thick (up to 750 m) succession of sandstones and conglomerates of the Lajas
Formation (Weaver 1931). This unit was accumulated in a complex (littoral to sub-
marine) deltaic environment highly influenced by sandy hyperpycnal flows locally
modified by tidal action (Zavala 1993;1996b; Zavala and Gonzalez 2001;McIlroy
et al. 2005; Canale et al. 2014; Gugliotta et al. 2016; Steel et al. 2018). Field evi-
dence suggest a syntectonic accumulation of this unit during Middle Jurassic. One of
the most important unconformities recognized in the southern part of the basin is the
Intra-Bajocian Unconformity (Zavala 1996a). According to its position within the
basin, the Lajas Formation is sharply overlaid by red beds of the Challacó Formation
(southern Neuquén Basin) or evaporites of the Tábanos Formation (central-northern
Neuquén Basin).
The Lotena Group conforms a clastic-carbonate system accumulated over a
regional unconformity (Intra-Callovian Unconformity). This unit starts with con-
glomerates, marine shales and sandstone lobes of the Lotena Formation. In central
and northern basin areas, the Lotena Formation is transitionally followed by shallow-
water limestones of the La Manga Formation and evaporites of the Auquilco Forma-
tion. In the southern area, the Lotena Group concludes with red beds of the Fortín
1° de Mayo Formation (Gulisano et al. 1984).
After the accumulation of the Cuyo and Lotena groups, the basin was tem-
porarily disconnected from the Pacific Ocean, resulting in the accumulation of
fluvial, lacustrine and eolian deposits of the Tordillo Formation and equivalent
units. These continental deposits laid over a regional unconformity (Intramalm
Unconformity) that defines the boundary between the Lotena and Mendoza groups.
Marine sedimentation was reestablished in the Early Tithonian after a catas-
trophic marine flooding coming from the Pacific Ocean, resulting in the accumu-
lation of offshore black shales of the Vaca Muerta Formation. During the Late
Jurassic–Early Cretaceous, the basin was mainly controlled by thermal subsi-
dence (SAG phase). Consequently, the marine deposits of the Mendoza Group and
the overlying Huitrín Group display a more regular distribution with an inter-
nal stratigraphy controlled by eustatic sea level fluctuations and climatic cycles.
Finally, the early uplift of the Andean foothills during the Late Cretaceous (see
Chap. “The Late Cretaceous Orogenic System: Early Inversion of the Neuquén
Basin and Associated Synorogenic Deposits 35°–38°S”) caused the final discon-
nexion of the Neuquén Basin with the Pacific Ocean. The last results in the final
basin infill by a thick interval of continental red beds in the eastern boundary of the
basin (Neuquén Group). A simplified schematic stratigraphic column for the central
area of the Neuquén Basin is shown in Fig. 1.
3 Materials and Methods
This chapter summarizes more than 30 years of research focused on the under-
standing of the stratigraphy of the Cuyo Group. During these years, an extensive
fieldwork was carried out along the entire basin. More than 15,000 m of stratigraphic
Jurassic Uplift Along the Huincul Arch and Its Consequences … 57
sections were described and analyzed in detail along different localities and outcrops
of the Early-Middle Jurassic succession of the Neuquén Basin. The stratigraphic sec-
tions were measured with a Jacob Staff and positioned by GPS. Over the years, the
annotation procedure migrated from the conventional field book towards the digital
record using our in-house “LithoHero” software (Iparraguirre et al. 2018). A free
version of this software is currently available in Google Play for Android devices.
Also, a IOS version will be shortly available. The digital record of field annotations
is very important since it supports data integrity, save time, and allows to export
all field observations as a .las file, enable data integration with conventional soft-
wares used in the oil industry. Consequently, apart from saving time drawing strati-
graphic sections, LithoHero provides a very useful software to integrate outcrop and
subsurface data.
Stratigraphic sections were described in detail “bed by bed”, with a careful char-
acterization of lithology, sedimentary structures, type of bounding surfaces, geome-
try, trace fossils and paleocurrent directions. As a complement, stratigraphic sec-
tions were conveniently sampled for different paleontologic, palynostratigraphic
and petrographic studies. Field observations and interpretations were applied to the
construction of a number of cross sections, geologic and paleogeographic maps.
4 The Jurassic of the Neuquén Basin and the Cuyo Group
The current stratigraphic framework for the Jurassic in the Neuquén Basin comes
out from the seminal contributions of Weaver (1931), Groeber (1946), Groeber et al.
(1953) and Herrero Ducloux (1946). Groeber (1946)termedas“Jurásico”tothe
lower stratigraphic interval of the Neuquén Basin, extended from the Rhaetian up
to the Kimmeridgian. This stratigraphic interval comprises three main depositional
cycles known as Cuyano,Loteniano and Chacayano. These cycles where later rede-
fined according to the lithostratigraphic nomenclature as the Precuyo, Cuyo and
Lotena groups (Stipanicic 1969; Gulisano et al. 1984). These groups have a chronos-
tratigraphic significance, since they represent lithogenetic units bounded by regional
unconformities (Gulisano et al. 1984).
The base of the Cuyo Group is defined by the Intraliasic Unconformity, which is
characterized by a sharp contact between shelfal and shallow marine clastic/carbonate
deposits of the Los Molles Formation on top of different units of the basement
(plutonic, volcanic and metamorphic) and the synrift volcanic and volcaniclastic
deposits of the Precuyo Cycle (Upper Triassic). According to regional data, the Early
Jurassic transgression advanced from north to south during the lapse Hettangian–
Pliensbachian (Riccardi et al. 1988).
The Cuyo Group upper boundary was largely more controversial, specially in the
southern part of the Neuquén Basin (Picún Leufú sub-basin, Hogg 1993), due to
the poor definition of the overlying Lotena Group. Originally, the term Lotena (or
“Loteno”) Formation was introduced by Weaver (1931) to name the thick-bedded
conglomerates and red beds that crop out in the surroundings of the Cerro Lotena
58 C. Zavala et al.
locality. These conglomerates are actually considered as the proximal deposits of the
Cuyo Group (Lajas Formation, Gulisano et al. 1984). Nevertheless, the term Lotena
Formation was widely applied in the past to designate the red beds with some marine
intercalations recognized in between the Cuyo and Mendoza groups in the southern
part of the Neuquén Basin.
Dellapé et al. (1979) provided an outstanding contribution about the origin and
the significance of the red beds attributed to the Lotena Formation. These authors
analyzed the classic section outcropped near the bridge of the Ruta Nacional 40 over
the Picún Leufú River. According to Dellapé et al. (1979), the red beds recognized
on top of the Lajas Formation should be considered as part of the Cuyo Group,
while the term “Lotena Formation” should be restricted to the fine-grained marine
succession having ammonoids and marine microfossils typical of the Middle Juras-
sic (Callovian). According to these authors, at this locality the base of the Lotena
Formation is defined by a conglomerate that lay on top of a regional unconformity
(Intra-Callovian Unconformity, Dellapé et al. 1979), and extends up to the Tithonian
shales of the Vaca Muerta Formation. Gulisano et al. (1984) redefined the upper
boundary as corresponding to the Intramalm Unconformity (Stipanicic and Rodrigo
1970), located below the fluvial/eolian deposits of the Tordillo/Quebrada del Sapo
formations.
A great number of studies have demonstrated that, during the Jurassic, the sed-
imentation in the southern part of the Neuquén Basin was deeply controlled by
coetaneous tectonic activity along the Huincul arch (Vergani et al. 1995; Zavala and
Gonzalez 2001; Silvestro and Zubiri 2008; Mosquera et al. 2011; Naipauer et al.
2012). This complex structure (Fig. 1) constitutes an East-West oriented intraplate
deformation belt developed along the Permian suture of the Patagonia terrane with
Western Gondwana (Mosquera et al. 2011).
Tectonic activity along the Huincul arch during the Middle-Late Jurassic resulted
in the growth of localized structural highs that completely disconnected the southern
sector of the basin, (known as Picún Leufú sub-basin) from the northern Neuquén
embayment.
The stratigraphic consequences of these temporary uplifts are of such magnitude
that the Jurassic column at both sides of the Huincul arch is markedly different
(Fig. 2). As an example, the Challacó, Fortín 1° de Mayo, Bosque Petrificado and
Quebrada del Sapo formations were developed exclusively in the southern area.
These units are partially equivalent to the Lajas, Tábanos, La Manga, Barda Negra,
Auquilco and Catriel/Sierras Blancas Formations recognized in the central/northern
Neuquén Basin. Although in most stratigraphic schemas (e.g., Gugliotta et al. 2016)
the Challacó Formation is considered as the fluvial progradational equivalent of the
Lajas Formation, field data support a tectonostratigraphic origin for this unit.
Jurassic Uplift Along the Huincul Arch and Its Consequences … 59
Fig. 2 Chronostratigraphic chart of the Jurassic in the Neuquén Basin. The uplift and local erosion
along the Huincul arch occurred during the Late Bajocian-Tithonian temporarily segmented the
Neuquén Basin into two different sub-basins, the main Neuquén embayment to the north, and the
Picún Leufú sub-basin to the south. Note that the stratigraphy during the Callovian–Kimmeridgian
is different at both sides of the Huincul arch
60 C. Zavala et al.
5 The Problem of the Challacó Red Beds
The lower red beds of the original Lotena Formation (re-located in to the Cuyo
Group by Dellapé et al. 1979) were later assigned to the Challacó Formation (De
Ferrariis 1947; Gulisano et al. 1984). Consequently, the Challacó Formation (Fig. 3)
was traditionally considered as the continental equivalent of the prograding deltaic
deposits of the Lajas Formation (Gulisano et al. 1984). Most stratigraphic schemas
illustrate a lateral transition between the red beds of the Challacó Formation and
the shallow marine sandstones and conglomerates of the Lajas Formation (see, e.g.,
Fig. 3in Gugliotta et al. 2016). Nevertheless, this lateral transition has never been
physically documented in the field. Main problems related to this interpretation are
listed below:
The boundary between the Lajas and the Challacó formations is always a sharp
surface commonly located on top of a coarse-grained bed (sometimes displaying
a coarsening upward trend). Zavala (1993) recognized a sequence boundary at this
level (boundary between sequences JC5 and JC6, Zavala 1993). Veiga (1998)also
describes a sequence boundary between the Lajas and Challacó formations.
At the locality of Bosque Petrificado, a well defined angular unconformity (>40°)
is recognized at the boundary between the Lajas and Challacó formations (Zavala
1993,Fig.4a). At this locality, these red beds were later assigned to the Bosque
Fig. 3 Red beds of the Challacó Formation at the Picún Leufú anticline area. These deposits were
traditionally considered as the proximal equivalent of the shallow marine deposits of the Lajas
Formation
Jurassic Uplift Along the Huincul Arch and Its Consequences … 61
Fig. 4 a Angular unconformity between the Lajas and Bosque Petrificado formations at the Bosque
Petrificado locality. bDetail of the angular unconformity recognized at Barda Colorada, where Early
Jurassic shales of the Los Molles Formation are unconformably overlaid by eolian sandstones of the
Neuquén Group. cand dComposed satellite image of the Cerro Lotena area. The Lajas Formation
is composed of near-vertical thick-bedded conglomerates, which are unconformably covered by red
beds of the Bosque Petrificado Formation (BP Fm) and offshore marine shales of the Vaca Muerta
Formation. Note that the Vaca Muerta shales passively infill a very complex tectonic relief carved
by erosion
Petrificado Formation (Zavala and Freije 2002) and were temporarily located in
the Lotena Group.
The red beds of the Challacó Formation (interpreted as accumulated in a fluvial
environment, Veiga 1998) are finer-grained than its conceptually related shallow
marine equivalent outcropped in the north of Huincul arch. As an example, at
the Sierra de la Vaca Muerta locality the Lajas Formation is mainly composed of
sandstones and conglomerates, while its apparently equivalent Challacó Formation
is dominated by gray to reddish shales with minor channel-fill sandstone deposits.
The proximal fluvial deposits of the Cuyo Group are well exposed at the Cerro
Lotena/Cerro Granito/Bosque Petrificado area, consisting of thick-bedded con-
glomerates assigned to the Lajas Formation. These deposits are largely coarser-
grained than the “proximal” red beds of the Challacó Formation, also outcropped
at the same locality.
62 C. Zavala et al.
The palynological content of the Challacó Formation is commonly characterized
by Botryococcus, thus suggesting a lacustrine environment for the accumulation
of these red beds instead of a fluvial depositional system (Garcia et al. 2006).
The palynology of the upper part of these “red beds” in the Quebrada del Sapo
area indicates marine brackish conditions, exhibiting a palynological association
that suggest a Late Callovian age (Olivera et al. 2018).
These observations suggest that the origin of the Challacó Formation is largely
more complex than the result of a simple progradation. Zavala and Gonzalez (2001)
proposed a tecnostratigraphic significance for this unit. Following these observations,
the Challacó Formation was interpreted as accumulated in a lacustrine (endorheic)
environment generated as a consequence of an early uplift of the Huincul arch. The
early tectonic uplift along this tectonic unit is known for the Late Jurassic–Early Cre-
taceous, and deeply controlled the stratigraphy of the Neuquén Basin. According to
Zavala and Freije (2002), the red beds located between the Lajas and Lotena Forma-
tions in reality correspond to two different (partially isopic) stratigraphic units, sep-
arated by the Intra-Callovian Unconformity: The Challacó Formation (s.s.) located
in the Cuyo Group, and the Bosque Petrificado Formation, located at the base of the
Lotena Group. In this chapter, new data are provided focused in the understanding
of the physical stratigraphy and significance of these stratigraphic units.
6 Evidence of Early Uplift and Erosion at the Huincul Arch
and Their Consequences in the Stratigraphy of the Cuyo
Group
Evidence of tectonic uplift and erosion along the Huincul arch during the Mid-
dle Jurassic are clearly visible at different outcrops in the Cerro Lotena area. At
Barda Colorada locality (Fig. 4b), an angular unconformity is recognized between
the Los Molles Formation and the overlying Neuquén Group, evidencing an impor-
tant uplift and erosion during the lapse Late Jurassic–Early Cretaceous. At a more
local scale, in the Cerro Lotena locality (Fig. 4c, d), Bajocian conglomerates of the
Lajas Formation are unconformably overlaid by Tithonian shales of the Vaca Muerta
Formation (also infilling a structurally controlled relief), thus evidencing a local
non-deposition/erosion during the Middle to Late Jurassic (lasting about 20 million
years).
Since the subsidence in the Neuquén Basin was continuous at both sides of the
Huincul arch during the entire Jurassic, a stratigraphic consequence of this localized
uplift and erosion should be a coarse-grained related depositional unit with sediments
supplied from the eroded landscapes. The last contrasts with erosional areas, where a
time gap is recognized at the unconformity surface. The related depositional units
are very important since they can display a complete and detailed record of the
time of erosion and rate of uplift (Riba 1976). Recent studies conducted in the
Picun Leufú area allowed a clear documentation of the magnitude and timing of
Jurassic Uplift Along the Huincul Arch and Its Consequences … 63
these localized tectonic movements. The most interesting stratigraphic interval to
characterize this tectonic activity is that corresponding to the time gap registered at
main unconformities, represented by the depositional units accumulated between the
Bajocian (top of the Lajas Formation) and the Late Callovian (base of the Lotena
Formation). This stratigraphic interval is characterized by red beds corresponding to
the Challacó and Bosque Petrificado Formations.
7 Stratigraphic Analysis of the Challacó and Bosque
Petrificado Formations
A detailed field survey has been conducted in the southern part of the Neuquén
Basin trying to unravel the stratigraphic significance of the Challacó and Bosque
Petrificado formations. Eight stratigraphic sections were measured in the interval
comprised between the Lajas and the Quebrada del Sapo formations. These sec-
tions have been described, measured, and sampled in detail, also registering several
paleocurrent measurements from tractive sedimentary structures (tabular and trough
cross-bedding, ripples and imbrication). The measured sections are distributed along
a distance of 12.64 km, and record the thickness and stratigraphic variations recog-
nized along the southern flank of the Picun Leufú anticline. The detailed correlation
panel is shown in Fig. 5a. The location of these sections and measured paleocurrent
directions in the Bosque Petrificado Formation is shown in Fig. 5b.
In all the analyzed sections, the boundary between the Lajas and Challacó forma-
tions is sharp, often placed over a coarsening upward bed recognized at the top of the
Lajas Formation. It is considered that this contact represents a sequence boundary
having an important tectonostratigraphic significance. The thickness of the Challacó
Formation is highly variable (from 10 up to 120 m), since it is often deeply truncated
by the Intra-Callovian Unconformity (ICU).
The Challacó Formation is dominantly composed of gray to greenish shales
interbedded with lenticular to tabular sandstone and pebbly sandstone bodies. Lentic-
ular sandstone bodies represent channel-fill deposits up to 4 m thick, which can be
traced laterally for about 30–50 m, evidencing a ribbon-like geometry. These beds lay
on top of a basal erosional surface and internally show large-scale tabular and trough
cross-bedding, sigmoidal cross-bedding, parallel lamination and massive bedding.
It is common in the presence of fractured rounded clasts. Tabular sandstone bodies
are up to 2 m thick and internally show massive bedding, parallel lamination and
climbing ripples. Remnants of silicified wood and plant remains are common. These
sedimentological evidence suggest that the Challacó Formation was accumulated,
in a shallow brackish lake partially affected by tidal influence. Sandstones and peb-
bly sandstones bodies are here interpreted as sublacustrine hyperpycnal channels
and lobe complexes. At present, no relevant biostratigraphic elements have been
64 C. Zavala et al.
Fig. 5 a Stratigraphic cross section along the Picún Leufú anticline. The new proposed boundary
between the Cuyo and Lotena Groups in the Picún Leufú area is indicated. The base of the Bosque
Petrificado Formation is a regional unconformity, and in places an angular unconformity. bPale-
ocurrent directions measured in the Bosque Petrificado Formation (N: 64). Note that sediments
were mainly supplied from tectonically exhumed areas located at the E-NE. D1: Destacamento 1;
PA: Puesto Antileo; LR: La Ruta; QS: Quebrada Seca; QA: Quebrada Alvarez; ES: Escuela; QP:
Quebrada Painemilla; QI: Quebrada del Indio
recognized in the studied palynological samples of the Challacó Formation. Conse-
quently, according to its stratigraphic position, the Challacó Formation is interpreted
as accumulated during the Late Bajocian–Early Callovian.
The contact between the Challacó and Bosque Petrificado formations represents
a regional sequence boundary recognized all along the study area, in some places
with associated evidence of erosion, angular relationships (Zavala 1993; Zavala and
Gonzalez 2001; Zavala and Freije 2002) and tectonic overprint. This unconformity
is here correlated with the Intra-Callovian Unconformity (Gulisano et al. 1984),
located at the base of the Lotena Group. At different positions along the study area,
this unconformity is sealing normal faults affecting the underlying Cuyo Group
(Figs. 6and 7). The existence of a distensive tectonics below the Intra-Callovian
Unconformity has been also recognized northward from the Huincul Arch in the Los
Catutos and Mallin del Rubio localities (Zavala 2002).
In the study area, the Bosque Petrificado Formation is up to 200 m thick and starts
with a regionally continuous conglomerate, pebbly sandstone and sandstone body up
to 8 m thick. This basal bed decreases in grain size from east to west, which is coin-
cident with the paleocurrent directions (Fig. 5). Internally, the Bosque Petrificado
Formation is composed of red, greenish and dark gray shales interbedded with sand-
stones, pebbly sandstones and conglomerate beds accumulated in a fluvial to shallow
Jurassic Uplift Along the Huincul Arch and Its Consequences … 65
Fig. 6 Linedrawing and interpretation of the Jurassic succession outcropping in the vicinity of
the Ruta Nacional 40 bridge over the Picún Leufú River. Note the existence of a series of normal
faults affecting the Cuyo Group, which are passively sealed by the Intra-Callovian Unconformity.
Stratigraphic evidence supports a genetic relationship between the Bosque Petrificado and Lotena
formations. Mo: Los Molles Formation; Lj1: Lajas Formation (1); Lj2: Lajas Formation (2); Ch:
Challacó Formation; Bp: Bosque Petrificado Formation; Lo: Lotena Formation; Qs: Quebrada del
Sapo Formation; Vm: Vaca Muerta Formation: IBU: Intra-Bajocian Unconformity; ICU: Intra-
Callovian Unconformity. PA, LR and QS refer to the sedimentary logs shown in the correlation of
Fig. 5(see Fig. 5for location)
marine environment. In general, the grain size of these beds is coarser respect to the
underlying Challacó Formation. Sandstone and conglomerate beds are tabular to
lenticular in shape and internally show large-scale asymptotic cross-bedding. Mas-
sive and laminated beds are also common. Dark gray shales are massive or laminated,
often showing plant remains and entire leaves with exceptional preservation. In the
Picun Leufú area, the upper red beds of the Bosque Petrificado Formation laterally
grade up into gray mudstones with ammonoids and other marine fauna that belong
66 C. Zavala et al.
Fig. 7 Linedrawing and interpretation of the Jurassic succession outcropped at the south of the N°
293 School in Picún Leufú. Note that normal faults only affect the Cuyo Group and are sealed by
the Intra-Callovian Unconformity. Mo: Los Molles Formation; Lj1: Lajas Formation (1); Lj2: Lajas
Formation (2); Ch: Challacó Formation; Bp: Bosque Petrificado Formation; Lo: Lotena Formation;
Qs: Quebrada del Sapo Formation; Vm: Vaca Muerta Formation: IBU: Intra-Bajocian Unconfor-
mity; ICU: Intra-Callovian Unconformity. ES refers to the sedimentary log shown in the correlation
of Fig. 5(see Fig. 5for location)
to the Lotena Formation (Fig. 5a). According to their ammonoid fauna, these marine
deposits were assigned to the Late Callovian–Early Oxfordian (Garrido and Parent
2013). Recent biostratigraphic studies performed in the basal levels of the Bosque
Petrificado Formation at the Quebrada del Sapo locality suggest an age younger than
Early Callovian for this unit (Olivera et al. 2018). This age results coherent with the
proposed stratigraphic position.
Toward the south, the thickness of the Bosque Petrificado Formation decreases
and progressively pinches out, with associated facies change (Fig. 8). At the Estancia
Charahuilla section (ECH), this unit is composed of a monotonous succession of red-
dish shales. The unit is absent at the estancia María Juana section (EMJ), where
Jurassic Uplift Along the Huincul Arch and Its Consequences … 67
Fig. 8 Regional cross section between the Estancia Maria Juana and the northern part of the Sierra
de la Vaca Muerta range (127.2 km). Note that the early uplift and erosion of Lower to Middle
Jurassic deposits at the Huincul arch produced the basin segmentation. As a consequence, the
southern Picun Leufú sub-basin evolved into a restricted lake (Challacó Formation) followed by a
clastic wedge deposit (Bosque Petrificado Formation) resultant from the supply of clastic sediments
from the uplifted region. Northward from the Huincul Arch (Sierra de la Vaca Muerta area), the
marine sedimentation was not interrupted during the Bathonian–Early Callovian, and sandstone
deposits of the Lajas Formation were supplied from the southwest
marine deposits of the Lotena Formation unconformably overlay coarse-grained
fluvial deposits of the Challacó Formation (Fig. 8).
A total of 67 paleocurrent measurements were performed in deposits of the Bosque
Petrificado Formation. These paleocurrents clearly suggest that clastic materials were
supplied from the east and northeast (Fig. 5). This direction is anomalous compared
with the main prograding direction from S-SE to N-NW registered for the Cuyo
Group in the study area (Zavala 1996a). Consequently, it is interpreted that the
conglomerates, sandstones and shales belonging to the Bosque Petrificado Forma-
tion represent recycled sedimentary deposits previously accumulated in the Lajas
and Los Molles formations. These materials were eroded from tectonically uplifted
areas located along the Huincul arch (Fig. 8) and transferred towards the flanks of
the structure.
8 Regional Stratigraphic Consequences of the Early Uplift
Along the Huincul Arch
The tectonic uplift during the late Jurassic along the Huincul Arch had important
consequences for the stratigraphy of the related deposits, since it created a physical
barrier that limited the connection between southern and central basin areas. Figure 8
shows a roughly north-south regional cross section along 127.2 km (between the north
of the Sierra de la Vaca Muerta and Estancia Maria Juana), flattened at the base of
the Upper Jurassic Tordillo Formation. In general, the correlation is relatively simple
up to the Bajocian, including the Los Molles, and Lajas (LJ1 and LJ2) Formations
68 C. Zavala et al.
(Fig. 9a). Nevertheless, during the Bathonian-Early Callovian the situation dramati-
cally changed (Fig. 9b), since the basin was partially fragmented into two different
depocenters resultant from an early tectonic uplift along the Huincul Arch. Due to
this confinement, the southern area evolves into a brackish lacustrine environment
(Challacó Formation), while open marine sedimentation continues at the north of the
Huincul Arch with the accumulation of shelfal deposits of the Lajas Formation (Lj
3toLj6)(Fig.9b).
A detailed cross section of the Cuyo Group in the Sierra de la Vaca Muerta is
showninFig.10. The sedimentation of the Lajas Formation is characterized by a
series of low-angle wedges which are progressively younger toward the north. As
documented from paleocurrent analysis, the sediments in the lower sequence (Lajas
2) show a clear provenance from the south-southwest and are mainly composed of
Fig. 9 Chronostratigraphic evolution of the Huincul Arch and its consequences in the stratigra-
phy. aDuring the Pliensbachian–Bajocian, the Cuyo Group prograded from the southeast. bAs
a consequence of an early tectonic uplift during the Bathonian-Early Callovian, a brackish lake
developed in the southern Neuquén Basin, resulting in the accumulation of red beds assigned to the
Challacó Formation. Northward from the Huincul Arch (Sierra de la Vaca Muerta) marine sedimen-
tation prevailed, with the accumulation of lithic materials sourced from the southwest. cDuring
the Middle-Late Callovian, uplifted and eroded sediments of the Cuyo Group were accumulated in
the Bosque Petrificado Formation. These continental deposits laterally grade into marine shales of
the Lotena Formation. Eroded sediments accumulated also in the northern flank of the structure,
composing coarse grained deposits of the Lotena Group recognized in the subsurface.
Jurassic Uplift Along the Huincul Arch and Its Consequences … 69
Fig. 10 A 32 km long detailed cross section of the Cuyo Group in Sierra de la Vaca Muerta area
(between Los Catutos and Cañadón del Indio). Note that the Lajas Formation is composed of
different low-angle wedges that become progressively younger northward. Sequence 2 is quartz-
rich and was supplied from the south-southwest. Source area change to the southwest for sequences
3–6. These sequences are coarse-grained and composed of lithic sediments. It is interpreted herein
that this change in the source area was a consequence of an early uplift along the Huincul Arch
quartz-rich sandstones. During the next sequence (Lajas 3), the supply area progres-
sively rotates toward the southwest. According to paleocurrent data (Fig. 10), the
sediment provenance from the southwest is clear for the younger sequences (Lajas
4–6), which are dominantly composed of lithic elements with an increasing grain size
for the younger sequences. This cross section also documents the “out of sequence”
accumulation of the Tábanos Formation, since this unit developed above different
depositional sequences of the Lajas Formation, evidencing a clear transgressive ten-
dency. For this reason, it is considered that the Tábanos evaporites probably belong
to the overlying Lotena Group (see Figs. 2and 8).
Maximum uplift along the Huincul Arch apparently took place during the Early
Callovian, since the main erosional angular unconformity was passively filled
with Early (?)–Middle Callovian deposits of the Lotena Group, often laying over
a normal-fault distensive system (Figs. 6and 7). According to these evidences, it
is herein interpreted that the Bosque Petrificado, Lotena and Fortin 1º de Mayo
Formations recognized in the Picún Leufú Sub-basin are temporarily equivalent to
the Tábanos, Lotena, La Manga and Auquilco formations recognized in the central
Neuquén Basin (Figs. 2,8and 9c). Sediments eroded from exhumed areas accumu-
lated also northward from the Huincul Arch, and are composed of coarse-grained
deposits of the Lotena Group recognized in different oil fields (e.g. Aguada Toledo).
70 C. Zavala et al.
9 Consequences for the Stratigraphy of the Picun Leufú
Area
Figure 11 shows a conceptual stratigraphic column for the Middle–Upper Juras-
sic at the Picún Leufú area, with indication of main paleocurrent directions and
compressive-distensive tectonic events according to field observations and evi-
dences that emerged from this study. In the study area, the Los Molles, Lajas (Lj
1 and Lj2) and Challacó Formations are bounded by unconformities. Nevertheless,
the unconformity between the Los Molles and Lajas Formations was only recognized
in certain areas around the southern flank of the Picún Leufú anticline (Freije et al.
2002), being the boundary between these two stratigraphic units generally transi-
tional. On the contrary, the Intra-Bajocian Unconformity (IBU), which establish the
boundary betweeen the Lajas 1 and Lajas 2 sequences, is a regional unconformity that
locally truncates hundreds of meters of the underlying succession (Zavala 1996a).
Compressive tectonic activity progressively increases towards the top of the Lajas 2
sequence, with a maximum activity during the accumulation of the Challacó Forma-
tion. Although the main paleocurrents measured in the Challacó Formation suggest
a source area located at the southeast, in those areas located closer to the Huincul
Arch (Picun Leufú Bridge over Ruta Nacional 40) a local supply from the northeast
is also recognized. The accumulation of the Lotena Group started after a distensive
tectonic event with a coarse-grained clastic succession supplied from the northeast
Fig. 11 Simplified stratigraphic column in the Picun Leufú Sub-basin, with indication of the main
compressive–distensive tectonic movements registered in the area. The uplift and erosion of Jurassic
strata are evidenced by a drastic change in paleocurrents direction (modified from Freije et al. 2002)
Jurassic Uplift Along the Huincul Arch and Its Consequences … 71
(Bosque Petrificado Formation). Compressive tectonic activity also continued dur-
ing the Middle-Late Callovian and Early Kimmeridgian (Intramalm Unconformity,
IMU) since an angular unconformity is commonly recognized at the base of the
Quebrada del Sapo Formation.
Finally, it is herein interpreted that the rapid marine flooding registered in the Early
Tithonian, which defines the base of the Vaca Muerta Formation, was also tectonically
enhanced, since normal fault activity was recognized as well at this boundary (Freije
et al. 2002, their Fig. 11a).
10 Conclusions
Stratigraphic and sedimentological evidence suggests a complex scenario for the
accumulation of the Jurassic units in the southern part of the Neuquén Basin. Synsed-
imentary tectonics along the Huincul Arch played a fundamental role in controlling
the stratigraphy at both margins of this tectonostratigraphic element. According to
these evidences, the Challacó Formation should no longer be considered the conti-
nental equivalent of the Lajas Formation, since it corresponds to a depositional unit
having a tectonostratigraphic significance, and exclusively developed at the south of
the Huincul Arch.
Like in other transgressive depositional cycles in the Neuquén Basin (Tordillo-
Vaca Muerta; Mulichinco-Pilmatué; Avilé-Agua de la Mula), the marine flooding
of the Lotena Formation in the southern basin is preceded by an initial continen-
tal/brackish interval, corresponding to the Bosque Petrificado Formation. A suc-
cession equivalent to this basal coarse-grained interval is recognized in the Lon-
copué area, integrated by conglomerates and coarse-grained sandstones up to 70 m
thick (Zavala et al. 2002).
Although recent zircon-based studies on the Quebrada del Sapo Formation support
the uplift of the Huincul Arch during the Late Jurassic (Naipauer et al. 2012; Naipauer
and Ramos 2016), new studies will be necessary to analyze and discuss these new
evidences of an early uplift and erosion during the Late Bajocian-Oxfordian. In the
study area, the Challacó and Bosque Petrificado formations can be clearly differenti-
ated because of the existence of an important tectonically enhanced unconformity at
their boundary. Nevertheless, in some places these units display isopic facies asso-
ciations that turn challenging their differentiation. In this way, new regional studies
will be required aimed in performing a detailed mapping of these units showing their
distribution, facies changes, paleocurrents and biostratigraphy along the Picún Leufú
Sub-Basin.
This study is mainly based in outcrops located along the southern flank of the
Huincul arch. In this area, the evidence clearly supports a sediment supply from the
northeast associated to an early uplift of Jurassic and older deposits. Nevertheless,
this ancient uplifted area have probably supplied sediments not only to the south,
but also to the depocenters located towards the north. It is probable that part of
these recycled coarse-grained sediments were included in the lower section of the
72 C. Zavala et al.
Lotena Formation recognized in the Agua Del Cajón, Loma de la Lata, Aguada
Toledo and Sierra Barrosa localities (Zavala et al. 2002). At these positions, the
Lotena Formation includes lithic clasts with angular fragments of rounded pebbles
that suggest a recycling of Middle Jurassic conglomerates belonging to the Lajas
Formation.
Acknowledgements The authors deeply acknowledge to Marcelo Martinez, Daniela Olivera and
Ainara Irastorza for their help, support and interesting discussions during fieldwork. We alsoappre-
ciate the support of the Departamento de Geología (Universidad Nacional del Sur), GCS Argentina
SRL and the CONICET for the completion of field activities.
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... Subsequently, by means of structural and stratigraphic studies, as well as U-Pb detrital zircon ages, it has been demonstrated that the early uplift of the Huincul High probably began in the Early Jurassic and lasted almost until the Cretaceous (Naipauer et al., 2012). Recently, Zavala et al. (2020) based on stratigraphic relationships mainly observed in the southern part of the Neuquén Basin, suggested that the uplift of the Huincul High began in the Middle Jurassic. ...
... It is overlain by the evaporites of the Tábanos Formation. Based on sedimentologic and stratigraphic evidence, Zavala and González (2001), Zavala (2005) and Zavala et al. (2020) included the Tábanos Formation in the basal part of the Lotena Group suggesting this unit unconformably overlies the Cuyo Group with a transgressive onlap. However, this hypothesis does not coincide with observations carried out by Leanza (2009) in Chacay Melehue where an erosive unconformity exists between the basal conglomerate of the Lotena Formation and the underlying Tábanos or Los Molles formations. ...
... deformation in Patagonia that produced tectonic inversion in numerous places, including the Huincul High, several times along the Jurassic (Mosquera and Ramos, 2006;Naipauer et al., 2012;Navarrete et al., 2016). Recently, stratigraphic studies based on regional analysis evidenced an early uplift and exhumation of the Huincul High since the Bathonian− early Callovian (Zavala et al., 2020). These authors have suggested that maximum tectonic activity along this morphostructural element occurred during the early Callovian causing significant consequences in the sedimentary supply and deposition of the Lajas Formation (see Zavala et al., 2020, Fig. 9). ...
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The Lajas Formation of the Neuquén Basin (Argentina) was deposited mainly during the Middle Jurassic but with a highly diachronic pattern. The exact time elapsed in the deposition of this unit remains controversial and depends on the location within the basin. Particularly in the Portada Covunco locality, north of the Huincul High, the debate on the age of the Lajas Formation persists. The age of the unit has been assigned by sequence stratigraphy, palynological, and magnetostratigraphic studies. Moreover, previous studies suggest that the Lajas Formation in this locality is composed of two depositional sequences (Sq1 and Sq2) with a change in the composition of the sandstone, indicating different areas of sedimentary supply. The new U–Pb data (LA-ICP-MS) in the detrital zircons allows us to define a maximum depositional age of approximately 163 Ma for Sq1 and 162 Ma for Sq2, suggesting a Callovian age for the Lajas Formation in the studied area. Furthermore, we propose that the provenance pattern in Sq1 was dominated by U–Pb ages of detrital zircons from the Permian–Triassic (peak at 264, 244 and 215 Ma), indicating derivation predominantly from the North Patagonian Massif magmatic units; while Sq2 shows a provenance pattern of detrital zircons associated with the uplift and exhumation of units located to the southwest margin of the basin during the end of the Middle Jurassic. Interestingly, it is not possible to rule out that the Huincul High may be related to the change in the sedimentary supply area; the probable uplift of the Huincul High has resulted in an effective physical barrier that limited sediment input from the extreme southeast during the deposition of Sq2.
... According to Zavala et al. (2006), periods of marine disconnection are usually associated with regional unconformities defined by sharp lithologic contacts between continental and marine deposits. Locally, angular unconformities can be recognized at the boundary between major sequences, thus suggesting some tectonic overprint (Zavala et al. 2020). Groeber (1946) recognized three main transgressive-regressive sedimentary cycles: Jurásico, Ándico, and Riográndico. ...
... The Cuyo Group starts with offshore to prodeltaic marine shales and turbidites of the Los Molles Formation (Weaver 1931), a thick, organic-rich unit (1-5% TOC, Chebli et al. 2011) originally interpreted as deposited in a deep-marine environment characterized by euxinic bottom waters. More recently, Zavala et al. (2014Zavala et al. ( , 2020, based on facies analysis, reinterpreted these deposits as a complete suite of offshore to prodelta deposits, which are in part the result of muddy hyperpycnal flows. The Los Molles Formation is transitionally overlain by a thick succession of shelfal to shallow-marine sandstones and conglomerates of the Lajas Formation (Weaver 1931). ...
... The abundance of benthic palynoforaminifera, previously observed in the Neuquén Basin only in the Lotena Formation (Martínez and Quattrocchio 2004), confirms the interpretation of a palaeoenvironment characterized by normal marine salinities. The Fig. 11 Paleogeographic reconstruction of the Neuquén Basin during Early-Middle Jurassic based on the main palynological results emerged from this study and regional data from previous works (e.g., Uliana and Legarreta 1993;Legarreta and Uliana 1996;Zavala 1996a, b;Zavala et al. 2020) a Pliensbachian-Bajocian; b Bathonian-Early Callovian high maturity and poor preservation of the organic matter in this section could be related to the emplacement of several Cenozoic intrusions in the Cordillera del Viento area (Turienzo et al. 2018). ...
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Being the main oil-bearing basin of Argentina, the Neuquén Basin contains a well-documented stratigraphic record of continental and marine sedimentation during the Jurassic and Cretaceous in the western margin of Gondwana. Marine sedimentation started in the Early Jurassic with the deposition of the offshore to prodelta shales of the Los Molles Formation, the basal unit of the Cuyo Group. A palynological study of outcrop samples of the Los Molles Formation at two localities, Puente Picún Leufú, southern Neuquén Basin, and Cordillera del Viento, central basin area, is presented. The palynological evidence allows inferring two different palaeoceanographic contexts during the deposition of the Los Molles Fm. At Puente Picún Leufú and the lower part of the Cordillera del Viento localities, the record of acritarchs and prasinophytes suggests a stratified water column, suboxic-to-anoxic bottom conditions, and a reduced salinity within the photic zone, associated with a marginal marine environment under restricted oceanic circulation. These conditions would have last at least until the Early Bajocian. Conversely, at the middle and mainly the upper part of Cordillera del Viento locality, the predominance of dinocysts in the assemblages indicates a hydrographically unstable shelf (non-stratified water mass column) with well-oxygenated bottom waters developed under open-marine settings with non-restricted oceanic circulation. The abundance and diversity of dinocyst assemblages are comparable with those observed in the Late Callovian Lotena Formation. These evidences suggest an open oceanic circulation due to the establishment of different seaways in the Neuquén Basin, during the final accumulation of the Los Molles Formation (Early Callovian).
... This unit has an estimated thickness of 500-750 m and was deposited primarily by submarine fans within a prograding volcaniclastic apron (Davidson, 1971(Davidson, , 1988Davidson and Vicente, 1973). In the Argentinean side, this unit is equivalent to the Cuyo and Lotena Groups (Gulisano et al., 1984b;Vergani et al., 1995;Palma et al., 2009;Zavala et al., 2020) (Fig. 2). ...
Article
This work presents new biostratigraphic data based on calpionellids and calcareous dinoflagellate cysts for the Tithonian in Central Chile. This stratigraphic interval is mostly represented by a thick marine succession of limestone and sandstone known as Baños del Flaco Formation. This formation originated in the shallow-water setting of a carbonate ramp and correlates with other well-studied units in the Neuquén basin of Argentina. Calpionellids and calcareous dinogflagellate cysts are studied on the Chilean side of the Andes for the first time. The recognition of several calpionellids and calcareous dinogflagellate cyst species within the lower Baños del Flaco Formation allowed the identification of a preliminary microbiostratigraphic zonation. Calcareous dinoflagellate cysts include eleven species that defines the Colomisphaera tenuis, and Colomisphaera fortis zones, whereas six chiti-noidellid species allowed the identification of the Chitinoidella and Crassicollaria zones, with the Slovenica, Boneti and Remanei subzones. These new microbiostratigraphic data suggest a latest early to earliest late Tithonian age for the studied interval.
... The earliest tectonic regime exerted an overall compression during the Late Triassic-Early Jurassic, bringing about a set of narrow, isolated depocenters bounded by large strike-slip fault systems and filled with continental to marine deposits of the Precuyano Cycle (D'Elia et al. 2012, Gulisano et al. 1984a, Vergani et al. 1995, D'Elia et al. 2020. Deposition during the Early Jurassic to Late Cretaceous was controlled by thermal subsidence accompanied by local tectonic events, making up the continental and marine siliciclastic, carbonate and evaporitic sediments of the Cuyo, Lotena, and Mendoza Groups (Gulisano et al. 1984b, Mitchum and Uliana 1985, Vergani et al. 1995, Palma et al. 2009, Zavala et al. 2020. From the Late Cretaceous to the Cenozoic, the basin was affected by a generalized compression, which resulted in the formation of an extensive fold and thrust belt (Ramos 2010). ...
... During Pliensbachian times (~186 Ma), the regional unconformable deposition of the limestones of the Chachil Formation over the Precuyano and the Chachil Plutonic Complex represents both the first Pacific marine transgression and the transition to a post-rift stage associated with an abrupt decrease of the magmatic activity and an expansion of the subsiding area. In the eastern and southern areas of the Chachil depocenter, these deposits are conformably overlain by the offshore to pro-deltaic marine shales of the Los Molles Formation, which integrates the Lower to Middle Jurassic Cuyo Group (Legarreta and Gulisano, 1989;Arregui et al., 2011b;Zavala et al., 2020) (Fig. 2). Since the middle-late Toarcian, basin-wide generalized subsidence marked the onset of the post-rift phase, with the deposition of shallow marine sequences of the Lajas Formation and continental fluvial deposits of the Challacó Formation east of the Chachil depocenter (Vergani et al., 1995;Veiga, 2002;Arregui et al., 2011a). ...
Article
The Chachil depocenter is located in the Andean foreland, at ∼39°S. Its origin relates to the syn-rift phase of the Neuquén Basin, a large marine embayment developed along the southwestern margin of Gondwana (30–40°S). In this contribution, we characterize the Late Triassic-Early Jurassic extensional architecture of the Chachil depocenter, based on kinematic data from outcrop-scale normal faults, and a tectonostratigraphic analysis of the syn-rift succession. Our results indicate that the extension and mechanical subsidence were controlled by two major NNW-striking normal faults of opposite polarity, while two NE to ENE-striking faults limited the depocenter to the north and south. Second-order NNW-striking normal faults in the inner sector of the depocenter are related to minor changes in the syn-rift deposits thicknesses. Kinematic data from fourth-order normal faults allow us to characterize the Chachil depocenter as an orthogonal NNW-trending rift developed under ENE extension. These results suggest that the architecture of the rift is related to the reactivation of Paleozoic discrete basement anisotropies. We argue that the local ENE extension in the southwestern area of the Neuquén Basin results from the refraction of the NNE to NE-oriented regional extension obtained for the basin further north. This could be related to the proximity to the N–S southwestern border of Gondwana, where a higher influence of incipient subduction-related processes is expected.
... The earliest tectonic regime exerted an overall compression during the Late TriassiceEarly Jurassic, bringing about a set of narrow, isolated depocenters bounded by large strike-slip fault systems and filled with continental to marine deposits of the Precuyano Cycle (D'Elia et al., 2012(D'Elia et al., , 2020Gulisano, 1981;Gulisano et al., 1984a;Vergani et al., 1995). Deposition during the Early Jurassic to Late Cretaceous was controlled by thermal subsidence accompanied by local tectonic events, making up the continental and marine siliciclastic, carbonate and evaporitic sediments of the Cuyo, Lotena, and Mendoza Groups (Gulisano et al., 1984b;Mitchum and Uliana, 1985;Vergani et al., 1995;Zavala et al., 2020). From the Late Cretaceous to the Cenozoic, the basin was affected by a generalized compression, which resulted in the formation of an extensive fold and thrust belt (Ramos, 2010). ...
Article
This work presents the first regional biostratigraphic study on the calpionellid zones and assemblages of the Tithonian–lower Valanginian interval in the Neuquén Basin, Southern Andes, Western Argentina. The lower Tithonian–lower Valanginian in the Neuquén Basin is mainly represented by the Vaca Muerta Formation, which is a thick sucession (100-1250 m thick) of rhythmic marlstone and limestone alternations corresponding to the distal hemipelagic facies of a carbonate ramp. This formation is one of the most important unconventional hydrocarbon reservoirs in the world and has become a relevant target in Argentina during the last decade. The Vaca Muerta Formation is characterized by an abundant fossil content and a remarkable stratigraphic continuity along several hundred meters, encompassing the Jurassic/Cretaceous boundary. The detailed study of seven outcrop and well sections (three of them studied for the first time herein), allowed the elaboration of a reliable stratigraphic scheme based on the correlation of ammonites, microfossils, magnetostratigraphy and cyclostratigraphy. The Vaca Muerta Formation contains moderate to poorly preserved calpionellids. Despite that, twenty-five calpionellid species and five calpionellid biozones known in the Tethyan regions have been identified: Chitinoidella, Crassicollaria, Calpionella, Calpionellopsis and Calpionellites. Additionally, nine subzones were recognized: Slovenica, Boneti, Remanei, Massutiniana, Alpina, Elliptica, Simplex, Oblonga, and Darderi. These results allow chronostratigraphic correlations between the Tethys and the Southeastern Pacific domains.
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The basal succession of the Lajas Formation at Trasandino section, Arroyo Covunco area, constitutes an excellent example of proximal prodelta facies cut by sediment-waves formed during a complete acceleration-deceleration hyperpycnal discharge cycle. The main outcome of this contribution was to present a new studied locality in which the Lajas Formation outcrops. Nine mudstones and fine-grained heterolites levels deposited during the waning stage, were sampled to palynological analysis. The recovered assemblages are dominated by sporomorphs, which are characterized by a great diversity of the trilete spores. Among them, is interesting to highlight the first mention of Manumia variverrucata and the extension of the last occurrence of Striatella seebergensis until to Callovian in Argentina. Based on selected key taxa a Late Bathonian–Callovian age is proposed for the Lajas Formation at the Trasandino section in this area. The Trasandino section spore assemblages show the greatest similarity with the Arroyo Covunco section spore associations when they are compared with the Lajas Formation of other studied localities. The endemic development of certain types of bryophyte (sensu lato) spores, e.g. Taurocusporites quattrocchiensis, was favored by locally humid conditions inferred at the Arroyo Covunco area. Large abundance of phytoclasts, sporomorphs and fresh-water algae characterize the recovered organic matter suggesting a high input of continental organic particles to the basin, as the result of fluvial-derived density discharges. Three palynofacies type (PT) had been identified which allow to evaluate the hydrodynamic characteristics of the flow, taking into account the differential buoyancy of the opaque particles. The large amount of equidimensional opaque particles recognized at the PT-B, characterizes the beginning of the deceleration phase of the flow and the abundance of blade-shape opaque particles, identified in the PT-A, point out a deposition from the final buoyant plume of hyperpycnal flow. The PT-C shows transitional features between these two conditions.
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The Vaca Muerta Formation is a fine-grained marine stratigraphic unit accumulated during the Late Jurassic-Early Cretaceous in the Neuquén Basin, Argentina. This contribution presents a sedimentological and stratigraphic analysis of Vaca Muerta´s shales emerged from the integration of regional and detailed outcrop research and subsurface data gathered from the study of different well cores. The fieldwork comprised, on the one hand, the description of 7 stratigraphic sections of the Early Tithonian-Early Valanginian interval widely distributed across the western region of the Neuquén province and southern Mendoza province. The sedimentological and sequence-stratigraphic analysis of the different stratigraphic sections allowed integrating them into a regional N-S oriented correlation panel covering an area of 340 km. Five composite depositional sequences were recognized (GS1-GS5). They represent the evolution of a complex mixed shelf/ramp depositional system comprising from basinal/slope facies (central area) to ramp (northern area) and mixed-shelf to continental deposits (southern area). Depositional sequences are represented by thin, organic-rich (up to 10 % TOC), mudstone dominated transgressive cycles, whereas regressive cycles are commonly thicker and mainly composed of organic-lean, carbonate and mixed (carbonate/siliciclastic) facies. On the other hand, aiming at studying the transport and accumulation processes of Vaca Muerta´s organic-rich shales, hand specimen samples of early-diagenetic carbonate concretions were collected from the organic-rich basal condensed section deposited in basinal settings. Concretion samples were thoroughly examined (mm- μm scale) integrating the observation of macroscopic polished samples, thin sections and scanning-electron microscope analyses. The evidence found inside concretions suggest an origin related to muddy underflows, possibly triggered by slope failures on the western margin of the basin. The bedload and suspended-load transport of mud within these muddy underflows would have resulted in the accumulation of graded event mudstone beds exhibiting ripple lamination. Petrographic analysis demonstrates that muddy underflows would have been important mechanisms for reworking the seafloor and redistributing mud in basinal settings. Furthermore, muddy underflows would have been effective processes for organic matter concentration in organic-rich mudstone strata (up to ≈ 6 % TOC). The subsurface studies comprised the sedimentological analysis of seven well cores of the Vaca Muerta Formation, representing a total core data of 387 m. The descriptive facies analysis at centimeter scale allowed the recognition of a distally steepened mixed ramp system. Sediment delivery to basinal settings was mainly controlled by muddy underflows triggered by different mechanisms. Deposition from muddy underflows would have interacted with fallout processes from buoyant plumes and marine snow from the water column. The stacking pattern of the studied core deposits shows ≈ 0.5 to 3 m-thick parasequences building up prograding/retrograding parasequence sets of high-order depositional cycles (10-20 m thick).
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The representation of core and outcrop data in on-scale sedimentary columns is a common practice in modern Geology. Either core or outcrop description involves the observation of a human and the related annotations. The traditional practice consist on sketching at the field or at core facilities and afterwards redrawing the chart at the lab. Conventional methodology is not only demanding in terms of time, but also extremely sensible to human errors. In this work a successful case of consistent core and outcrops representation is presented. The differential factor was the use of LithoHero software. The use of mobile devices and the instant graphical representation of the annotations reduces drastically the potential mistakes, minimizes the required time, and simplifies the process. A practical example on Vaca Muerta Formation is presented. Finally, a successful import to Petrel Software is showcased.
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En el presente trabajo se analizan la estratigrafía y los paleoambientes deposicionales de la Formación Lajas (Jurásico medio) a partir del estudio de los afloramientos de esta unidad localizados en el suroeste neuquino, y relacionados a la Dorsal de Charahuilla-Plottier, en un área de 2800 Km2. La Formación Lajas, unidad eminentemente arenosa de ambiente de plataforma, litoral a continental, se dispone mediante un contacto diacrónico sobre pelitas de plataforma de la Formación Los Molles. A su vez es cubierta, mediante un contacto neto, por pelitas rojas de la Formación Challacó. A partir del levantamiento de 12 secciones estratigráficas de detalle se realizó primeramente un análisis de facies sedimentarias y posteriormente un análisis estratigráfico y estratigráfico secuencial de estos depósitos. Esto permitió diferenciar 19 facies sedimentarias, las que mediante su agrupamiento natural definen 13 asociaciones de facies, correspondientes a los siguientes ambientes y subambientes deposicionales: Plataforma costa afuera - prodelta; Barra de desembocadura dominada por ola/Playa; Barra mareal de plataforma a estuarina; Barra de desembocadura fluvio-dominada; Barra de desembocadura de "braid-deltas"; Llanura mareal; Llanura deltaica inferior, pantano interdistributario a bahía interdistributaria; Canal estuarino; Sistema fluvial meandriforme; Sistema fluvial entrelazado arenoso; Sistema fluvial anastomosado; Sistema fluvial entrelazado gravoso; Barreal. Mediante el análisis estratigráfico secuencial de las secciones relevadas se discriminaron 4 secuencias deposicionales de tercer orden (en el sentido de Vail et al., 1977), las que se denominan JC4, JC5, JC6 y JC7. La secuencia JC4 (Toarciano superior alto - Bajociano inferior alto) se reconoce como integrada por 5 secuencias deposicionales de un orden menor, denominadas JC4.1, JC4.2, JC4.3, JC4.4 y JC4.5. La secuencia JC4.1 es la más extendida, y corresponde principalmente a la Fm. Los Molles, en tanto que las secuencias restantes pertenecen a la Fm. Lajas. Las secuencias superiores (JC4.3, JC4.4 y JC4.5) se reconocen únicamente en el sector occidental del área de trabajo debido a problemas tectosedimentarios que se detallan más abajo. La secuencia JC5 (Bajociano inferior alto - Bajociano superior alto) muestra una amplia distribución en la zona de trabajo, correspondiendo al sector medio y superior de la Fm. Lajas. El hallazgo de Stephanoceras (Stemmatoceras) sp. (Zona de Humphriesianum a Rotundum) y de Teloceras aff. crickmayi chacayi (West. & Ricc., Zona de Rotundum) permitió datar bioestatigráficamente por primera vez a la Fm. Lajas en esta región. Las secuencias JC6 (Bajociano superior alto - Bathoniano inferior) y JC7 (Bathoniano inferior alto - Bathoniano superior alto) se integran con facies propias de la Fm. Challacó. La secuencia JC6 es la que presenta la mayor distribución regional. En base a evidencias estratigráficas se reconoce una importante actividad tectónica en la dorsal de Charahuilla-Plottier, la que habría enfatizado la discordancia estratigráfica que separa a las secuencias JC4 y JC5. En el sector occidental del área de estudio (Perfiles Lohan Mahuida y Puesto Pichonleo) esta discordancia está poco desarrollada, en tanto que en el sector centro-oriental se reconoce a partir de esta discordancia la pre¬sencia casi continua de un cuerpo sedimentario de hasta 35 metros de potencia, de la base de la secuencia JC5, el que forma la conspicua cornisa notada por varios autores en el flanco este de la Sierra de Chacaico. Se estima que dicha discordancia habría producido una truncación de unos 200 metros estratigráficos de la Fm. Lajas en este sector, ya que se encuentran ausentes las secuencias JC4.4 y JC4.5. Existen asimismo evidencias de actividad tectónica entre las secuencias JC6 y JC7, reconocida localmente en el Perfil Bosque Petrificado, y expresada mediante el desarrollo de una discordancia angular de unos 40-50º entre ambas secuencias. Cabe destacar que la secuencia JC7 se encuentra a su vez en discordancia angular por debajo de niveles conglomerádicos de la Fm. Quebrada del Sapo (= Fm. Tordillo) o cuando esta está ausente, de pelitas marinas de la Fm. Vaca Muerta.
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MARINE HETTANGIAN AND SINEMURIAN IN ARGENTINA On the norlhern bank of the Atuel river, Mendoza province, Argentina, the occurrence of Hettangian and Lower Sinemurian marine fossiliferous levels has been documented for the first time. Up to now, it was usually accepted that the marine Jurassic sequence in the area started with Upper Sinemurian strata (Epophioceras Faunule). Representatives of the genera Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Bayle, Sulciferites Spath, Badouxia Guex y Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassiceras Spath and Arnioceras Hyatt, thus indícate the presence of beds equivalent to tho Planorbis, Liasicus, Angulata, Bucklandi, Semicostatum, (and Turneri) Zones of tho Standard International Chronostratigraphic Scale. Hence, in Argentina, the marine Hettangian, Sinemurían and lowermost Pliensbachian have only been proved in the neighborhoods of the Atuel river region, vvhere marine Triassic could also be represented. Elsewhere in Argentina the oldest Jurassic ammonite bearing Ievels belong to the upper Lower and Upper Pliensbachian. Regionally, these findings imply a substantial rnodification to previous paleogeographic reconstructions for the Early Jurassic of South America.
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Clastic Facies of the Lotena Formation (Middle Jurassic) in the Loncopué and Loma la Lata areas. Neuquén Basin. Argentina. The increasing interest in the petroleum potential of the Lotena Group (middle Jurassic) specially in the Dorsal area has required new studies to properly asses its origin, significance and stratigraphic relationships. In this way, this paper introduces preliminary results of an integrated approach using field and subsurface data. The research strategy encompassed a core analysis from Loma La Lata, Sierra Barrosa and Aguada Toledo fields, and a detailed stratigraphic and sedimentologic research on equivalent facies in the Loncopué area. Field and subsurface analysis allowed the discrimination of 18 facies of clastic and carbonate origin. Only the clastic facies are considered in the present contribution. Broadly speaking, the Lotena Group in the Loncopué area is internally composed of 5 depositional sequences. Sequence 1 starts over a regional unconformity, and it is integrated by anhydrites, carbonates (stromatolites and others) and mudstones deposited in an hypersaline lacustrine environment (Tábanos Formation). Sequence 2 is composed of conglomerates and mudstones accumulated in a dilute lacustrine environment. Finally, sequences 3 to 5 register the permanent connection with the sea, and they are composed of sandstones, black shales and minor carbonates deposited in a shelfal environment. Sedimentology indicates that main coarse-grained intervals were accumulated by long-life high-density underflows. The existence of transitional passages between different coarse-grained facies suggests fluctuating flows related to variations in the flood discharge. These clastic intervals display multiple amalgamation surfaces, with a lateral continuity that exceeds 15 kilometers. Hydrocarbon evidences in some clastics and high TOC of black shales provide new oil perspectives for the unit.
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Jurassic synsedimentary tectonics in the southern part of the Neuquén Basin. Detailed field studies in the Quebrada del Sapo – Picún Leufú area allowed the recognition of nine unconformity-bounded stratigraphic units, which are partially equivalent to Los Molles, Lajas, Challacó, Lotena, Tordillo, Vaca Muerta and Quintuco formations. These units, ranging in age from Early Bajocian to Late Tithonian, often display folding, truncation and onlap relationships at their boundaries. These characteristics strongly suggest a tectonic control in their origin. The number and significance of different tectonic episodes is consistent with a complex deformational history mainly acting along two different structural axes, which were roughly oriented east-west and north-south. The east-west axis provides the best resolution due to the number of units involved. According to the stratigraphy, this last structure shows along time evidences of both compression and extension, that could be related to the shallow expression of tectonic movements of a deep high-angle (basement-involved?) fault system. Main compressive activity aparently occurred during the Bathonian, which is substantially older that previously considered for the area (Intramalm unconformity). Broadly speaking, compressive activity appears to be occurred during the deposition of overall regressive units (Lajas, Challacó and Tordillo formations), while major transgressions (Lotena and Vaca Muerta formations) apparently coincided with extensional events. These observations provide new insights to discuss the eustasy vs. tectonics imprints in the stratigraphic record of the Neuquén Basin. On the other hand, if the main buried fault responses to regional stresses, then the recorded activity could be used to analyze the progressive geodynamic evolution of the basin during the studied interval.
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El grupo Cuyo (Jurásico inferior a medio) constituye el primer evento de sedimentación marina generalizada en la Cuenca Neuquina. Integra una sección clástica progradante, en la que depósitos peliticos de offshore (Fm Los Molles) con ocasionales niveles de turbiditas, son sucedidos por acumulaciones de areniscas a conglomerados marinos de plataforma a continentales (Fm Lajas). En este trabajo se presenta un esquema estratigráfico secuencial para el Grupo Cuyo basado en estudios de afloramientos localizados en los alrededores de la Sierra de la Vaca Muerta. El relevamiento de ocho secciones estratigráficas de detalle complementado con un estudio fotoestratlgráfico ha permitido identificar cinco secuenoas deposicionales, las que muestran evidencias de actividad tectónica sindeposioonal. La secuencia 1 se reconoce con base cubierta en el sector sur de la zona de estudio, y se integra por areniscas de plataforma a litorales de naturaleza cuarzo-feldespática, con zonas de aporte localizadas principalmente en el sureste Las secuencias 2 a 5 se integran por areniscas y conglomerados de plataforma a litorales de naturaleza lítica, con aportes elásticos desde el suroeste. Dichas secuencias evidencian una progresiva migración hacia el norte del depocentro, el que es acompañado por un aumento general de la textura. El análisis estratigráfico permite aseverar que las secuencias 2 a 5 se habrían acumulado en depocentros elongados, con geometrías y facies controladas por actividad tectonica jurásica. Dicha actividad podría corresponder a un reflejo superficial de la inversión parcial de estructuras distensivas basamentales, como consecuencia de pulsos tectónicos compresivos desde el sureste (Macizo Nordpatagónico) durante el Bathoniano / Caloviano temprano. Los cambios en la zona de aporte permiten especular sobre el creo miento de una barrera física en la zona de la Dorsal de Huíncul, la que habría limitado progresivamente los aportes elásticos desde el sureste. En base a este estudio y anteriores en la subcuenca de Picún Leufú se discute la posición estratigráfica y significado de la Fm Challacó, y la posible pertenencia de la Formación Tábanos (evaporitas) al Grupo Lotena.
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Hyperpycnal currents are river-derived turbidity currents capable of transporting significant volumes of sediment from the shoreline onto the shelf and potentially further to deep ocean basins. However, their capacity to deposit sand bodies on the continental shelf is poorly understood. Shelf hyperpycnites remain an overlooked depositional element in source to sink systems, primarily due to their limited recognition in the rock record. Recent discoveries of modern shelf hyperpycnites, and previous work describing hyperpycnites deposited in slope or deepwater settings, provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. This article describes well-sorted lobate sand bodies on the continental shelf of the Neuquén Basin, Argentina interpreted to have been deposited by hyperpycnal currents. These hyperpycnites of the Jurassic Lajas Formation are characterized by well-sorted, medium-grained, parallel-laminated sandstones with hundreds of metre extensive, decimetre thick beds encased by organic-rich, thinly laminated sandstone and siltstone. These deposits represent slightly obliquely-migrating sand lobes fed by small rivers and deposited on the continental shelf. Hyperpycnites of the Lajas Formation highlight several unique characteristics of hyperpycnal deposits, including their distinctively thick horizontal laminae attributed to pulsing of the hyperpycnal currents, the extraction of coarse gravel due to low flow competence, and the extraction of mud due to lofting of light interstitial fluid. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing facies models of hyperpycnites. Recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve current understanding of sediment transfer from rivers to deeper water, will improve palaeoenvironmental interpretations of sediment gravity-flow deposits, and has implications for modelling potentially high-quality hydrocarbon reservoirs. This article is protected by copyright. All rights reserved.