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Magnetostratigraphy of the Upper Jurassic–Lower Cretaceous from Argentina: Implications for the J-K boundary in the Neuquén Basin

Article (PDF Available) inCretaceous Research 70(2):189-208 · February 2017
DOI: 10.1016/j.cretres.2016.10.011
Maria Paula Iglesia Llanos at National Scientific and Technical Research Council
  • 22.56
  • National Scientific and Technical Research Council
Diego Kietzmann at University of Buenos Aires
  • 26.56
  • University of Buenos Aires
Melisa Kohan Martinez at National Scientific and Technical Research Council
  • 4.7
  • National Scientific and Technical Research Council
Ricardo Palma at University of Buenos Aires
  • 26.95
  • University of Buenos Aires
Abstract
A systematic sedimentologic and paleomagnetic study was carried out in the Vaca Muerta Formation, cropping out in the northern Neuquén Basin, west-central Argentina. The studied section is c.280 m-thick and represents a carbonate ramp system bearing ammonites that indicate Late Jurassic–Early Cretaceous ages. The Vaca Muerta Formation is one of the most important unconventional hydrocarbon reservoirs in the world and its thorough study has become a relevant target in Argentina. The J-K boundary is comprised within this unit, and although it is well-dated through biostratigraphy -mainly ammonites-, the position of particularly the boundary is yet a matter of hot debate. Therefore, the systematic paleomagnetic and cyclostratigraphic study in the Vaca Muerta Formation was considered relevant in order to obtain the first Upper Jurassic–Lower Cretaceous magnetostratigraphy of the southern hemisphere on the first place and to precise the position of the J-K boundary in the Neuquén Basin, on the other. Biostratigraphy is well studied in the area, so that paleomagnetic sampling horizons were reliably tied, particularly through ammonites. Almost 450 standard specimens have been processed for this study distributed along 56 paleomagnetic sampling horizons that were dated using ammonites. Paleomagnetic behaviours showed to be very stable, and their quality and primary origin have been proved through several paleomagnetic field tests The resultant magnetostratigraphic scale is made up of 11 reverse and 10 normal polarity zones, spanning the Andean Virgatosphinctes mendozanus (lower Tithonian) to Spiticeras damesi Zones (upper Berriasian). These polarity zones were correlated with those of the International Geomagnetic Polarity Time Scale 2012 and 2016 through the correlation between Andean and Tethyan ammonite zones. Cyclostratigraphy on the other hand, proved to be quite consistent with the magnetostratigraphy. Through the correlation of the resultant paleomagnetic and cyclostratigraphic data, it was possible to date the section with unprecedented precision, and therefore, to establish the position of the Jurassic-Cretaceous boundary. The paleomagnetic pole calculated from the primary magnetization is located at: Lon= 191.6°E, Lat= 76.2°S, A95= 3.5°, indicating a c. 24° clockwise rotation for the studied section, which is consistent with structural data of the region.
Figures
Magnetostratigraphy of the Upper JurassiceLower Cretaceous from
Argentina: Implications for the J-K boundary in the Neuqu
en Basin
María Paula Iglesia Llanos
a
, Diego A. Kietzmann
b
,
*
, Melisa Kohan Martinez
a
,
Ricardo M. Palma
b
a
Instituto de Geociencias B
asicas, Ambientales y Aplicadas, Departamento de Ciencias Geol
ogicas, Facultad de Ciencias Exactas y Naturales, Universidad de
Buenos Aires eConsejo Nacional de Investigaciones Cientícas y T
ecnicas, Argentina
b
Instituto de Estudios Andinos Don Pablo Groeber, Departamento de Ciencias Geol
ogicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos
Aires eConsejo Nacional de Investigaciones Cientícas y T
ecnicas, Argentina
article info
Article history:
Received 1 September 2016
Accepted in revised form 22 October 2016
Available online 28 October 2016
Keywords:
Jurassic-Cretaceous boundary
Paleomagnetism
Andean ammonite zones
Vaca Muerta Formation
abstract
A systematic sedimentologic and paleomagnetic study was carried out in the Vaca Muerta Formation,
cropping out in the northern Neuqu
en Basin, west-central Argentina. The studied section is c. 280 m-
thick and represents a carbonate ramp system bearing ammonites that indicate Late JurassiceEarly
Cretaceous ages. The Vaca Muerta Formation is one of the most important unconventional hydrocarbon
reservoirs in the world and its thorough study has become a relevant target in Argentina. The J-K
boundary is comprised within this unit, and although it is well-dated through biostratigraphy (mainly
ammonites), the position of particularly the boundary is yet a matter of hot debate. Therefore, the
systematic paleomagnetic and cyclostratigraphic study in the Vaca Muerta Formation was considered
relevant in order to obtain the rst Upper JurassiceLower Cretaceous magnetostratigraphy of the
southern hemisphere on the rst place and to precise the position of the J-K boundary in the Neuqu
en
Basin, on the other. Biostratigraphy is well studied in the area, so that paleomagnetic sampling horizons
were reliably tied, particularly through ammonites. Almost 450 standard specimens have been processed
for this study distributed along 56 paleomagnetic sampling horizons that were dated using ammonites.
Paleomagnetic behaviours showed to be very stable, and their quality and primary origin have been
proved through several paleomagnetic eld tests The resultant magnetostratigraphic scale is made up of
11 reverse and 10 normal polarity zones, spanning the Andean Virgatosphinctes mendozanus (lower
Tithonian) to Spiticeras damesi Zones (upper Berriasian). These polarity zones were correlated with those
of the International Geomagnetic Polarity Time Scale 2012 and 2016 through the correlation between
Andean and Tethyan ammonite zones. Cyclostratigraphy on the other hand, proved to be quite consistent
with the magnetostratigraphy. Through the correlation of the resultant paleomagnetic and cyclostrati-
graphic data, it was possible to date the section with unprecedented precision, and therefore, to establish
the position of the Jurassic-Cretaceous boundary. The paleomagnetic pole calculated from the primary
magnetization is located at: Lon ¼191.6
E, Lat ¼76.2
S, A
95
¼3.5
, indicating a c. 24
clockwise rotation
for the studied section, which is consistent with structural data of the region.
©2016 Elsevier Ltd. All rights reserved.
1. Introduction
The Vaca Muerta Formation is a thick rhythmic alternation of
dark bituminous shales, marlstones and limestones deposited as
result of a rapid and widespread Paleo-pacic early Tithonian to
early Valanginian marine transgression in the Neuqu
en Basin
(Fig. 1a), west-central Argentina (Legarreta and Uliana, 1991). It is
famous for its important oil and gas resources, as well as its
abundant fossil content and temporal continuity along several
hundreds of meters thick that comprise the Jurassic/Cretaceous
boundary (Leanza, 1981; Mitchum and Uliana, 1985; Leanza and
Zeiss, 1992).
In contrast to most geological systems, the Jurassic/Cretaceous
transition is characterized by the absence of a signicant faunal
*Corresponding author.
E-mail addresses: mpiglesia@gl.fcen.uba.ar (M.P. Iglesia Llanos),
diegokietzmann@gl.fcen.uba.ar (D.A. Kietzmann), melisa.kohan@gmail.com
(M.K. Martinez), palma@gl.fcen.uba.ar (R.M. Palma).
Contents lists available at ScienceDirect
Cretaceous Research
journal homepage: www.elsevier.com/locate/CretRes
http://dx.doi.org/10.1016/j.cretres.2016.10.011
0195-6671/©2016 Elsevier Ltd. All rights reserved.
Cretaceous Research 70 (2017) 189e208
turnover, as well as by the remarkable increase of faunal pro-
vinciality, especially in ammonites (e.g., Remane, 1991;
Wimbledon, 2008; Michalík and Reh
akov
a, 2011; Wimbledon
et al., 2011, 2013; Ogg and Hinnov, 2012, and references cited
there). The uncertainties in the inter-regional correlation constitute
a major classic problem for this time interval across the world. The
reliability and accuracy of chronocorrelation tools can be consid-
erably improved however, by the combination of different methods
of quantitative stratigraphy, among which magnetostratigraphy
proved to yield most interesting results (Ogg and Lowrie, 1986;Ogg
et al., 1991; Remane, 1991; Grabowski, 2011).
The geomagnetic polarity time scale (GPTS) spanning the Mid-
dle Jurassic and Cretaceous, is derived from paleomagnetic studies
in the continents with detailed biostratigraphy as well as Deep Sea
Drilling Project and Oceanic Drilling Program cores, which are
correlated with the marine M-sequence magnetic anomalies. Since
polarity reversals are recorded simultaneously in all type of rocks
all over the world, they provide a distinctive pattern or nger-print
for a certain time interval. Marine magnetic anomalies and their
calibrations to biostratigraphy make up the reference against which
magnetostratigraphic sequences, either on land or in deep-sea
cores, are correlated (Ogg and Hinnov, 2012). Thus, one funda-
mental requisite to attempt a non-ambiguous paleomagnetic cor-
relation between a section on-land and the GPTS is a good
biostratigraphic denition (e.g. Ogg and Hinnov, 2012). In the
Jurassic, biostratigraphic, magnetostratigraphic, chemostrati-
graphic and other events are calibrated typically to the standard
ammonite zones in Europe, although during the Oxfordian and
Fig. 1. A) Sketch map of the Neuqu
en Basin with detail of the studied locality. B) Stratigraphic chart for the Neuqu
en Basin showing Groeber's cycles and sequence stratigraphic
subdivision. C) Lithostratigraphic subdivision of the Lower Mendoza Mesosequence (¼lower Mendoza Group) in Southern Mendoza. Modied from Kietzmann et al. (2014). Ki:
Kimmeridgian, Ti: Tithonian, Be: Berriasian, Va: Valanginian.
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208190
Fig. 2. Stratigraphic distribution of ammonite species and ammonite biozones in Arroyo Loncoche section (Riccardi pers. comm.).
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 191
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208192
Tithonian other paleogeographic realms take place such as the
Boreal (Arctic and northernmost Europe), sub-Boreal (northern
Europe), sub-Mediterranean (southern Europe) and Tethyan
(southernmost Europe). In fact, all chronostratigraphic data shown
in the GPTS for this time come from basins located in these regions,
and these Upper Jurassic-Lower Cretaceous ammonite zones from
Europe have been directly calibrated to the M-sequence chrons.
The denition of the boundary and base of the Cretaceous Sys-
tem is still controversial. Historically, at least three denitions are
considered (Remane, 1991; Wimbledon, 2008; Grabowski, 2011;
Gradstein et al., 2012): 1) One is the base of the Grandis
ammonite Subzone dened in the Colloque sur la Cr
etac
e inferieur
(1963), that corresponds to the lower part of the Calpionella Zone,
almost coinciding with the base of magnetozone M18r. 2) The
second is the base of the Jacobi ammonite Zone, dened in the
Colloque sur la limite Jurassique-Cr
etac
e (1973), which is often
regarded as approximately equivalent to the base of the Calpionella
Zone and correlated with the upper part of magnetosubzone
M19n.2n. 3) The last denition corresponds to the base of the
Occitanica ammonite Zone, correlated with the middle part of the
Calpionella Zone and the lower part of magnetozone M17r
(Hoedemaeker, 1990).
According to the magnetostratigraphic revision of Grabowski
(2011), the integration of calpionellid and magnetic stratigraphy
is fairly robust, although calcareous nannofossils still needs
renement and the correlation with ammonite stratigraphy is still
poorly constrained. Recently, the Berriasian Working Group has
dened the Jurassic/Cretaceous boundary at the base of the Cal-
pionella alpina Zone in the middle part of magnetosubzone M19n2n
(Leanza pers. comm. 2016).
In the Neuqu
en Basin, ammonites have an excellent biostrati-
graphic resolution, but their autochthony prevents a straightfor-
ward correlation with the Tethys, and prompts the occurrence of
different correlation schemes between Andean and Tethyan
ammonite Zones (e.g. Leanza, 1981; Riccardi, 2008, 2015; Vennari
et al., 2014). The aim of this paper is to contribute to the calibra-
tion of such biostratigraphic correlations using magneto-
stratigraphy and previous cyclostratigraphic data.
2. Geological framework
The Neuqu
en Basin comprises a Mesozoic-Cenozoic sedimen-
tary succession of at least 7 km thick covering an area of
c.120,000 km
2
that extends between 32
and 40
S in west central
Argentina (Fig. 1a,b). It is a back-arc basin that was originated in the
Triassic due to the break-up of Gondwana (Uliana and Biddle, 1988).
Subsidence in the Neuqu
en Basin lasted at least 220 my, and started
in the Late Triassic-Early Jurassic as the result of the extensional
collapse of the Permian-Triassic orogenic belt. Sedimentation
began in the Triassic as mainly volcanic and coarse-grained conti-
nental deposits along long, narrow half-grabens. Subsequently,
during the Early to end of the Middle Jurassic, the Cuyo Group was
deposited. From this time and until the Paleogene, the process was
replaced by regional subsidence (e.g. Legarreta and Uliana, 1991).
During the Late Jurassic, the onset of the Araucanian diastro-
phism (Stipanicic and Rodrigo, 1970) and inversion prompted a
signicant change in the stress elds: during the Late Jurassic-Early
Cretaceous extension prevailed, and subsidence in the basin was
reestablished (Vergani et al., 1995) producing the deposition of the
Mendoza Group. Later on, during the rest of the Cretaceous and
Cenozoic, denite continental conditions with a large supply of
coarse clastic deposits from the west were established, and the nal
isolation of the basin from the Pacic Ocean took place. As a result,
the Bajada del Agrio, Neuqu
en and Malargüe Groups were depos-
ited. During this time span, the stress elds changed once more into
a compressive regime, which resulted in the development of the
Andean fold and thrust belt and the reactivation of older exten-
sional faults in the opposite sense.
3. The Vaca Muerta Formation at the Arroyo Loncoche section
The Vaca Muerta Formation is part of the Mendoza Group or
Mendoza Mesosequence, that is made up of three main shallowing
upward sedimentary cycles: Lower Mendoza (upper Kimmerd-
gianelower Valanginian), Middle Mendoza (lower Valanginian),
and Upper Mendoza Mesosequence (lower Valanginianelower
Barremian). The Lower Mendoza Mesosequence begins with con-
tinental deposits (alluvial, uvial and aeolian) from the Tordillo
Formation (Kimmeridgian-lower Tithonian?), and is overlaid by
mainly marine deposits of the Vaca Muerta and Chachao Forma-
tions (Fig. 1) containing ammonites of Tithonian-Valanginian ages
(e.g., Legarreta and Uliana, 1991).
The study section crops out along the Loncoche creek (Arroyo
Loncoche) located in southern Mendoza province (Fig. 1a), and is
made up of c. 280 m ethick ammonite-bearing rocks (Fig. 2). In a
decimetre-scale, the unit shows rhythmic alternations of marl-
stones, shales and limestones that were intruded in the lower part,
by a c. 20 ethick andesitic sill (Fig. 2).
3.1. Sedimentology
A detailed facies analysis of the Vaca Muerta Formation in the
studied section was published by Kietzmann et al. (2008, 2011a,
2014), and interpreted as basinal to middle carbonate ramp de-
posits (Fig. 3). In order to establish the sedimentological features of
cyclic deposits described in this paper, four facies associations
characterized in this areas (FA-1 to FA-4) are briey described here,
which correspond to basinal to middle carbonate ramp deposits.
3.1.1. Facies association 1 (distal outer ramp to basin)
Facies association 1 consists of dark grey to black well-
laminated marlstones, and subordinately intraclastic packstones,
ripple-laminated packstones, radiolaritic wackestones, and micro-
bialites. Radiolarians are abundant, but disarticulated thin-shelled
oysters are also common. These facies are particularly rich in
shes, turtles, ichthyosaurs and crocodiles remains. Trace fossils are
represented by Chondrites, but also evidences of cryptobioturbation
are found in microbialite laminae.
Facies association 1 was deposited in a poorly-oxygenated distal
outer ramp to basin. Sedimentation took place mainly from sus-
pension in a restricted marine environment with suboxic bottom
waters, as indicates the absence of wave-induced structures, and
the passive accumulation of organic materials and pelagic micro-
fossils. However, the presence of ne-grained ripple laminated
carbonate, as well as bioclastic remains derived from shallow-
water are probably associated with storm-generated turbidity
Fig. 3. Stratigraphic column of the Vaca Muerta Formation at the Arroyo Loncoche section. Data show (from left to right): Andean ammonite zones, low (E) and high (e) frequency
eccentricity cycles (modied from Kietzmann et al., 2015), facies associations, sequence stratigraphy (Kietzmann et al., 2014), paleomagnetic horizons, polarities sequence and VGP
latitudes. In facies associations 1: distal outer ramp to basin, 2: bioclastic outer ramp, 3: bioclastic middle ramp to proximal outer ramp, 4: Oyster autoparabiostrome dominated
middle ramp. HFS ¼high frequency sequence: grey-shaded bars are possible stratigraphic intervals where there may be condensation and omission of orbital cycles. Red in the
stratigraphic column: sill. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 193
ows. Sedimentation rates in this facies association are to vary
between 10 and 30 m/Myr.
3.1.2. Facies association 2 (bioclastic outer ramp)
Facies association 2 is represented by an alternation of peloidal
and intraclastic packstones, radiolaritic wackestones and thin
gradded bioclastic rudstones, rhythmically interbedded with
marlstones. They contain abundant crustacean pellets, ne to me-
dium sand-size micritic intraclasts, ammonites, bivalves, gastro-
pods and foraminifera. Bioturbation is rare, although there may be
some Planolites,Palaeophycus and Thalassinoides tubes.
Facies association 2 was deposited in a poorly-oxygenated bio-
clastic outer ramp. Sedimentation during fair-weather periods is
represented by marlstones and laminated radiolaritic wackestones.
Pelloidal accumulations probably come from the unroong of
crustacean galleries (Kietzmann and Palma, 2014). The abundance
of intraclasts suggests intermittent erosion of wackestones facies
and transport from bottom currents associated with storm events.
Thin massive or gradded bioclastic rudstones were originated from
storm-generated turbidity-like ows. Trace fossils are related to the
Cruziana ichnofacies, which characterizes low-energy environ-
ments (MacEachern et al., 2008). Sedimentation rates in this facies
association range between 30 and 40 m/Myr.
3.1.3. Facies association 3 (bioclastic middle ramp to proximal outer
ramp)
Facies association 3 consists of gradded bioclastic oatstones/
rudstones, bioturbated wackestones, laminated packstones and
wackestones, and marlstones. Allochems are dominated by bi-
valves, rynchonellids, gastropods, serpulids, ammonites, aptychi,
nautiloids, belemnites, echinoderms, and abundant epistominid
foraminifera and pellets, which are chaotically distributed. Bio-
turbation is represented by Thalassinoides,Diplocaterion and
Rhizocorallium.
Facies association 3 was deposited in a moderate well-
oxygenated bioclastic middle ramp to proximal outer ramp. Taph-
onomic features of skeletal particles, suggest reworking by storm-
wave action (Kietzmann and Palma, 2009a). Occurrence of lami-
nated wackestones and marls suggest deposition by settling of ne-
grained suspended material, during fair-weather stages, as well as
sedimentary particles transported during storm events. Trace fos-
sils also indicate alternating energy regimes and a relatively well
oxygenated substrate. Sedimentation rates in this facies association
increase between 40 and 60 m/Myr.
3.1.4. Facies association 4 (Oyster autoparabiostrome dominated
middle ramp)
Facies association 4 consists of oyster accumulations, which
form up to several meters thick internally bedded biostromes,
alternating with poorly stratied, massive, or low-angle cross-
stratied bioclastic rudstones/oatstones, marlstones and lami-
nated wackestones/packstones. Fossil content includes abundant
oysters (Aetostreon sp.), ammonites, serpulids (Rotularia sp.) and
rynchonellid brachiopods, as well as ostracods, ophiuroid ossicles,
dasycladacean algae, crustacean microcoprolites, and epistominid
and textularid foraminifera. Beds are commonly bioturbated by
Taenidium,Diplocaterion,Rhizocorallium and/or Thalassinoides.
Facies association 4 is interpreted as deposits of a well-
oxygenated middle ramp. Laminated wackestones and marls
represent fair-weather periods. Low-angle cross-stratied deposits
are interpreted as accretionary bioclastic bars located probably
within the upper part of the middle ramp, similar to those
described by B
adenas and Aurell (2001). Trace fossil assemblage is
indicative of the Cruziana ichnofacies, which characterizes low-
energy environments (MacEachern et al., 2008). Sedimentation
rates in this facies association vary between 10 and 20 m/Myr.
3.2. Biostratigraphy
JurassiceCretaceous Andean biostratigraphy is well dened by
ammonites (Fig. 4)(Riccardi, 2008, 2015), and to a lesser extent, by
microfossils such as calcareous nannofossils (Bown and Concheyro,
2004; Ballent et al., 2011). Ammonite data from the studied section
(Figs. 2 and 4) restrain the deposition of the Vaca Muerta Formation
to the early Tithonian (Virgatosphinctes mendozanus Zone) elate
Berriasian (Spiticeras damesi Zone) (Leanza et al., 1977; Mitchum
and Uliana, 1985; Kietzmann et al., 2011a, 2014, 2015). The
boundary between ammonite zones in Arroyo Loncoche was placed
according to the rst occurrence of the index species (Riccardi, pers.
comm.). Nannofossil events at the Arroyo Loncoche section include
the rst occurrences of Polycostella beckmanii,Eiffellithus primus,
and Umbria granulosa located within the Pseudolissoceras zitelli
ammonite Zone, indicate a middle Tithonian age, while Polycostella
senaria and Raghodiscus asper occurring within the Wind-
haunseniceras internispinousm ammonite Zone, point to an upper
Tithonian age (Lescano and Kietzmann, 2010; Kietzmann et al.,
2011b). The presence of Cruciellipsis cuvillieri and Micrantholithus
hozchulzi was recognized in the upper part of the Substeueroceras
koeneni Zone to the Spiticeras damesi Zone, suggesting a Berriasian
age. Finally, the rst occurrence of Eiffellithus windii indicating an
early Valanginian age, coincides with the Neocomites wichmanni
ammonite Zone (Lescano and Kietzmann, 2010). Recently, Vennari
et al. (2014) mentioned the presence of the nannofossil Nannoconus
steinmannii minor at Las Loicas section, whose rst occurrence in
the Tethys is located close to the Jurassic/Cretaceous boundary,
albeit with substantial controversy (in Riccardi, 2015).
More recently, rst occurrences of calcareous dinoagellate
cysts of signicant stratigraphic importance were described by
Ivanova and Kietzmann (2016) for the studied section, which
include Parastomiosphaera malmica,Colomisphaera nagyi and
Committosphaera pulla for the lower Tithonian, Colomisphaera
tenuis,Colomisphaera fortis and Stomiosphaerina proxima for up-
permost Tithonian eBerriasian, Colomisphaera conferta in the up-
permost part of upper Berriasian, and Carpistomiosphaera
valanginiana -Colomisphaera vogleri for the Lower Valanginian.
These associations show a good biostratigraphic consistency be-
tween the Tethyan and Andean realms.
Calpionellids in the Neuqu
en Basin are generally poorly pre-
served due to recrystallization of carbonate mud. Chittinoidella
boneti is reported for the Windhauseniceras internispinosum Zone in
Sierra de la Cara Cura (Fern
andez- Carmona and Riccardi, 1998;
Kietzmann et al., 2011b). Tintinnopsella sp., Crassicollaria sp. and
Calpionella sp. have been found within the Corongoceras alternans,
the lower part of the Substeueroceras koeneni, and the Spiticeras
damesi ammonite Zones (Fern
andez-Carmona et al., 1996;
Fern
andez Carmona and Riccardi, 1999), which is consistent with
a late Tithonian age for these levels. However, only chitinoidellids
were reported for the moment in the Arroyo Loncoche section
(Kietzmann et al., 2011b).
3.3. Cyclostratigraphy
The Vaca Muerta Formation is characterized by decimetre-scale
rhythmic alternations of marlstones, shales and limestones,
showing a well-ordered hierarchy of cycles within the Milankovitch
frequency band that comprise ~21 ky elementary cycles (preces-
sion), modulated by ~90e120 ky and ~400 ky, Earth's orbital ec-
centricity cycles (Kietzmann et al., 2011a, 2015). A oating orbital
scale was constructed by Kietzmann et al. (2015) from 4
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208194
stratigraphic sections in the southern Mendoza area of the Neu-
qu
en Basin, including the presented Arroyo Loncoche section. This
oating orbital scale was constructed by high and low frequency
eccentricity cycles, and tied to the international time scale at the
base of the Windhauseniceras internispinosum and Neocomites
wichmanni Zones, where there is full agreement with respect to the
correlation. Thus, ammonite zones are interpreted to be distributed
as follows: 1) The Virgatosphinctes mendozanus Zone is probably
located at the base of the middle Tithonian (upper part of the
Semiforme to Fallauxi Standard Zone), but the inconsistency with
paleomagnetic data suggest an older age. 2) The Pesudolissoceras
zittelli and Aulacosphinctes proximus Zones correlates with the
middle Tithonian Fallauxi to Ponti Standard Zones. 3) The Wind-
hauseniceras internispinosum Zone would be located at the base of
the upper Tithonian Microcanthum Standard Zone. 4) The Coro-
ngoceras alternans Zone coincides with the upper Tithonian
(Microcanthum Standard Zone and lower part of the Durangites
Standard Zone). 5) The Substeueroceras koeneni Zone would corre-
late with the Durangites Standard Zone and the lower part of the
Occitanica Standard Zone. 6) The Argentiniceras noduliferum Zone
correlates with the upper part of the Occitanica Standard Zone, and
the Spiticeras damesi Zone correlates with the Boissieri Standard
Zone. It is relevant to keep in mind that the duration of the
Tithonian stage is still under debate due to the low precision of
available absolute data. The oating orbital scale by Kietzmann
et al. (2015) is in agreement with the duration of 5.3 my pro-
posed by Ogg (2004). However, if we take into consideration that
the Tithonian spans 7.1 my (Ogg and Hinnov, 2012), it would imply
that c. ve low frequency eccentricity cycles (~400 ky) are missing
either by omission or condensation in the Arroyo Loncoche section.
In Fig. 3 we show in grey the possible condensation stratigraphic
intervals.
4. Paleomagnetic methodology
For this study, 56 sampling sites or horizons were distributed
along the 280 m-thick section. The average distance between them
was c. 5 m, except in the upper part where they are approximately
2 m apart or less, due to condensation. At each sampling horizon,
four oriented cores were drilled with a portable core drill, from
which at least, two standard specimens were cut, making a total of
8 specimens per horizon. Altogether, c. 450 altogether have been
obtained and processed.
Demagnetization procedures and analysis of specimens were
performed at the Laboratorio de Paleomagnetismo Daniel Valen-
cio, IGEBA, Universidad de Buenos Aires-CONICET. Four out of the
8 specimens per horizon were demagnetized using alternating
elds (AF) with a 2G device, whereas the rest were subjected to the
high temperatures (TH) demagnetizing method using a Schoen-
stedt furnace. Residual magnetizations were measured in a 2G DC
Fig. 4. Biostratigraphic subdivision of the Tithonian-Valanginian interval in the Neuqu
en Basin, showing correlation between Andean and Tehtyan Ammonite Zones after Riccardi
(2015) and Vennari et al. (2014).
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 195
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208196
SQUID magnetometer. Demagnetization procedures involved
approximately 15 steps, usually up to 50, occasionally 110 mT in the
case of AF, and up to 450e500
C, sometimes 610
C for TH. After
each thermal demagnetisation step, bulk magnetic susceptibility
(X) of specimens was measured, in order to detect the formation of
new magnetic minerals. In general, Xis between 10
5
and 10
6
SI
for. Above 450e500
C, Xincreased signicantly in most samples
and magnetic behaviours became erratic.
Two magnetic components were systematically recognized
along the section, and in the lower part, a third component
appeared. One of the components yields almost northern declina-
tions with negative inclinations, and is removed at <300
Cor
15 mT (Fig. 5b,c). The mean direction of this soft component in-
cludes that of the local dipolar eld, and therefore we interpret it as
a remagnetization (Fig. 6b). The second widespread component
shows NE (SW) declinations with negative (positive) inclinations
when tectonic corrected and was generally removed between 450
and 500
C, or 40 and 60 mT (Figs. 5a, b, 6a). This component which
is interpreted as the characteristic (ChRM), showed two funda-
mental types: i) those with straight trajectories to the origin
Fig. 6. Stereoplots showing magnetic directions isolated in the Vaca Muerta Fm. A) bedding-corrected mean site directions of the ChRM showing opposite polarities and antiparallel
directions. Inset: bedding-corrected means of reverse and normal polarity populations (here shown in reverse polarity) with corresponding semi-angles of condence. Note that
both means are overlapped, indicating that they are statistically undistinguishable. These magnetizations are interpreted to be primary. B) in situ directions from the soft component
bearing northern declinations and negative inclinations yield. The mean coincides with the local dipolar eld (¼55), and is thus interpreted as a secondary magnetization
produced by the local dipolar eld. Symbols: full (open) symbols ¼lower (upper) hemisphere, grey circle ¼semi-angle of condence. C) The bootstrap reversal test (Tauxe et al.,
2010) shows that condence bounds from the two data sets overlap in all three components which implies that the means of the reverse and normal modes cannotbe distinguished
at the 95% level of condence, passing the test. The bootstrap test was performed through the Paleomagnetism.org website (Koymans et al., 2016).
Fig. 5. Representative Zijderveld plots and stereoplots (bedding-corrected) from the Vaca Muerta Formation showing two magnetic components. The soft component yields
northern declinations and negative inclinations, and is removed at c. 200 C/15 mT. The hard component shows two different behaviours. A) Component with trajectory to the
origin, with SW declinations and positive inclinations, that is removed between 450 and 500 Cor40e60 mT. B) Component with trajectory to the origin, with NE declinations and
negative inclinations, that is removed between 450 and 500 Cor40e60 mT. C) Component with curved trajectory ending at the reference direction (A). Together with the soft
component, they dene a remagnetization circles. Symbols: full dot ¼declination; open dot ¼apparent inclination. Jr ¼NRM intensity in Ampers/meter.
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 197
(Fig. 5a, b) and, ii) those with curved trajectories resulting from the
partial overlap of coercivities and/or blocking temperatures
(Fig. 5c). In the rst case, calculations were performed using prin-
cipal component analysis (Kirschvink, 1980) which functioned as
reference for the second type of behaviour. Calculations for be-
haviours type ii) was carried out with remagnetization circles
(Halls, 1976; McFadden and McElhinny, 1988).
Therefore, calculation of site mean directions with samples type
i) (Fig. 5a,b), was achieved using sherian statistics (Fisher,1953)by
means of the Paldir program (University of Utrecht) shown in
Table 1. On the other hand, the analysis of site mean directions with
at least three samples type ii) (Fig. 5c), was approached by means of
the combined analysis of direct observations (directions) and
remagnetization circles (McFadden and McElhinny, 1988) using the
Palt software (University of Utrecht).
The ChRM component was subjected to eld tests for the
paleomagnetic stability, i.e. reversal and baked contact tests. When
plotting the means of the reverse and normal populations (Fig. 6a;
Table 2), the overlap of the condence intervals (Fig. 6a in reverse
polarity) reveals that they are statistically undistinguishable. In
order to assess the statistics of the data, we applied two thoroughly
different reversal tests. On the one hand, we used the bootstrap test
Table 1
Palaeomagnetic data of sampling horizons at Arroyo Loncoche with distances from the base and ammonite zones. n/N ¼total/selected samples; ChRM ¼characteristic
remanent magnetization; D
t
,I
t
¼declination, inclination after bedding
a
95
¼semi-angle of condence; k¼Fisher's precision parameter; VGP ¼virtual geomagnetic pole,
Paleolatitudes (
S); Bedding correction ¼Strike follows the right-hand rule.
Site Distance from base (m) n/N k ChRM
a
95
kVGP Paleolat Tilt cor.
SI D
t
,I
t
Lat
S, Long
E(
S) Strike, dip
AL20 1.0 V. mendozanus 4/4 2.2E-05 203.5, 49.2 4.6
231.5 69.6, 191.4 30.1 173, 27
AL08 7.4 V. mendozanus 5/5 e183.8, 62.0 3.9
257.7 82.0, 274.4 43.2 178, 24
AL21 10.6 V. mendozanus 5/3 4.4E-05 186.4, 52.2 7.2
294.9 84.0, 174.4 32.8 175, 8
AL22 13.6 V. mendozanus 7/7 8.4E-05 24.7, 54.8 9.2
182.5 69.9, 206.9 35.3 176, 22
AL09 15.8 V. mendozanus 6/5 e204.7, 43.9 5.0
234.6 66.6, 182.1 25.7 178, 24
AL23 22.1 V. mendozanus 7/5 8.3E-05 23.2, 46.1 6.4
95.7 68.7, 184.4 27.5 193, 26
*AL24/AL10 32.1 P. zitteli 12/5 4.3E-03 180.3, 67.8 8.4
64.7 73.8, 289.7 e193, 26
AL25 40.0 P. zitteli 8/4 7.3E-05 19.6, 52.8 8.4
121.9 73.7, 198.2 33.4 188, 24
AL26 44.1 P. zitteli 8/7 3.9E-05 37.7, 60.4 8.3
40.6 59.4, 224.4 42.1 188, 24
AL11 46.5 P. zitteli 8/8 e178.1, 66.4 3.6
232.5 76.7, 295.8 48.9 178, 25
AL27 48.9 P. zitteli 8/5 4.7E-05 190.6, 62.7 8.4
83.1 78.3, 249.9 44.1 178, 27
AL12 51.9 P. zitteli 6/4 e217.4, 44.0 11.8
61.3 56.6, 193.8 25.8 178, 29
AL28 55.8 P. zitteli 8/6 4.5E-05 351.4, 48.4 7.2
63.0 80.5, 58.6 29.4 188, 26
AL29 63.5 A. proximus 8/3 1.6E-05 11.1, 40.0 9.3
175.6 73.9, 150.3 22.8 193, 39
AL30 75.8 W. internispinosum 8/6 3.6E-05 218.6, 38.7 8.6
44.2 53.7, 188.7 21.8 178, 71
AL13 87.3 W. internispinosum 6/3 e356.4, 59.8 15.0
63.0 84.2, 318.5 40.7 203, 27
AL31 97.2 W. internispinosum 8/3 6.5E-06 12.8, 44.9 9.3
176.3 75.8, 164.1 26.5 183, 25
AL32 105.2 W. internispinosum 7/5 5.0E-05 19.8, 55.6 13.9
20.3 74.0, 208.1 36.1 158, 28
AL33 115.8 C. alternans 8/5 4.8E-05 200.8, 45.9 6.6
90.0 70.5, 181.0 27.3 158, 28
AL14 118.6 C. alternans 8/6 e161.8, 50.6 6.7
72.8 74.3, 30.9 31.3 233, 29
AL34 124.2 C. alternans 8/5 1.4E-05 6.8, 37.9 8.9
49.5 74.5, 134.7 21.3 173, 29
AL35 124.4 C. alternans 8/5 1.6E-05 35.4, 52.0 9.8
40.8 60.7, 204.9 32.6 165, 60
AL15 124.6 C. alternans 7/7 e352.9, 47.3 6.9
77.3 80.7, 68.3 28.5 168, 42
AL36 131.2 C. alternans 7/7 6.7E-06 202.9, 45.9 8.5
38.8 68.8, 183.7 27.3 163, 37
AL37 140.1 C. alternans 8/6 2.3E-05 202.4, 44.2 8.2
48.6 68.5, 179.8 25.9 165, 35
AL38 146.5 C. alternans 8/5 1.6E-05 38.4, 47.5 10.7
34.2 57, 199.2 28.6 173, 37
AL39 153.2 S. koeneni 8/6 2.1E-05 18.4, 53.5 13.2
18.8 74.8, 199.8 34.0 173, 30
AL16 156.6 S. koeneni 8/3 e1.7, 45.6 14.3
75.4 81.3, 120.5 27.0 173, 32
AL40 160.9 S. koeneni 8/5 2.4E-05 196.6, 42.0 8.5
54.3 71.7, 166.5 24.2 173, 30
AL43 164.5 S. koeneni 8/5 2.3E-05 205.2, 41.1 10.3
36.9 65.1, 178.4 23.6 173, 30
AL41 169.9 S. koeneni 8/6 1.8E-05 204.5, 47.8 6.5
77.3 68.3, 189.3 28.9 173, 38
AL42 174.4 S. koeneni 8/5 2.0E-05 209.8, 50.5 9.7
41.7 64.8, 199.0 31.2 173, 38
AL44 176.3 S. koeneni 8/6 1.8E-05 338.9, 48.4 9.4
51.9 71.2, 33.7 29.4 173, 25
AL45 177.9 S. koeneni 8/4 1.8E-05 17.2, 53.7 13.4
27.3 75.8, 199.9 34.2 173, 25
AL46 181.3 S. koeneni 6/5 1.1E-05 201.6, 41.5 10.7
34.2 68, 174.1 23.9 173, 36
AL47 184.7 S. koeneni 8/4 2.5E-05 204.8, 51.4 4.6
231.6 69.2, 197.8 32.1 178, 28
AL48 186.9 S. koeneni 8/3 3.1E-06 210.6, 44.4 8.8
84.4 62.2, 188.9 26.1 178, 30
AL49 189.4 S. koeneni 7/4 8.4E-06 32.3, 59.8 12.5
31.4 64.2, 221.5 40.7 178, 30
AL50 194.9 S. koeneni 8/5 1.1E-05 181.3, 56.1 3.0
435.6 88.5, 245.8 36.7 188, 25
AL51 197.1 S. koeneni 8/5 2.0E-05 16.7, 60.4 10.5
54.5 75.7, 229.2 41.4 188, 32
AL52 200.0 S. koeneni 8/4 1.1E-05 21.0, 52.3 9.1
103.8 72.5, 197.6 32.9 188, 52
AL53 205.3 S. koeneni 8/7 2.3E-05 21.0, 59.3 13.1
16.3 72.9, 221.9 40.1 188, 30
AL54 207.5 S. koeneni 8/6 1.5E-05 347.6, 52.8 3.6
339 72.5, 29.1 33.4 178, 28
AL55 209.4 A. noduliferum 8/5 2.2E-05 190.5, 45.6 10.0
39.2 77.6, 159.6 27.0 178, 30
AL56 213.3 A. noduliferum 6/5 3.1E-05 202.3, 44.2 8.8
50.6 68.6, 179.7 25.9 178, 30
AL57 215.6 A. noduliferum 8/5 4.6E-06 207.4, 43.7 10.9
33.0 64.5, 184.8 25.5 178, 30
AL58 216.3 A. noduliferum 8/5 1.3E-05 30.8, 43.7 12.5
25.1 61.8, 188.1 25.5 178, 30
AL17 218.4 S. damesi 6/6 e207.4, 46.3 7.8
53.7 65.4, 189.1 27.6 173, 35
AL59 223.4 S. damesi 8/4 5.0E-06 206.1, 40.8 11.7
35.8 64.3, 179 23.3 178, 30
AL60 230.1 S. damesi 8/5 1.2E-05 352.1, 59.8 9.0
72.9 82.0, 338.8 40.7 178, 30
AL61 232.5 S. damesi 7/6 1.0E-05 22.3, 57.5 8.1
49.8 72.0, 215.1 38.1 178, 30
AL62 239.0 S. damesi 8/5 8.8E-06 27.7, 56.8 8.3
56.9 67.7, 213.2 37.4 178, 30
AL63 251.9 S. damesi 8/4 7.3E-06 200.9, 41.3 11.1
39.8 68.4, 172.8 23.7 178, 30
AL64 257.6 S. damesi 8/6 1.8E-05 204.9, 49.6 11.0
27.0 68.6, 193.6 30.4 178, 30
AL18 262.2 S. damesi 8/7 e166.7, 56.4 7.8
46.0 79.2, 9.2 37.0 178, 30
* Indicates sampling horizons in the Cenozoic sill.
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208198
for common mean (Tauxe et al., 2010) which works with actual site
directions. Results show (Fig. 6c) that condence bounds from the
two data sets overlap in all three components, indicating that the
means of the reverse and normal modes cannot be distinguished at
the 95% level of condence and so, that the bootstrap reversal test is
positive. This statistical test was used through the Paleomagnetism.
org website (Koymans et al., 2016). On the other hand, we applied
the reversal test from McFadden and McElhinny (1990) which
resulted class B (angle
g
between mean normal and reverse ¼5.2
;
critical
g
¼5.6
), proving that the ChRM has been properly isolated
and that both samples belong to the same population. The baked
contact test was carried out where the sill intruded the host rock in
the lower part of the section, both in the baked contact and far away
from the intrusion (Fig. 7). At the baked contact, the thermal
demagnetization shows two components (Fig. 7b), one soft which
is removed at <300
C carrying the sill's identical direction with
reverse polarity (Fig. 7a), and another harder non-antipodal normal
polarity component that is removed at c. 450
C. Such component is
found in the host rock, c. 30 m above the intrusion and corresponds
to the characteristic of the section (Fig. 7c), thus passing the baked
contact test. Therefore, the characteristic magnetizations isolated at
Arroyo Loncoche are interpreted as primary related to the time of
deposition of the sediments during the Jurassic-Cretaceous.
The third component that was found in the lower part of the
studied section, yields corrected S-SE declinations with positive
high inclinations and it was eliminated at c. 300
C and 110 mT,
sometimes higher elds (Fig. 7a, b). This component is remarkably
different in terms of both directions and coercive forces which are
distinctively higher, and belong to the component found in the sill
and in the host rock several meters away from the intrusion. It is
interpreted to represent a secondary component of Tertiary age,
corresponding to the time of the sill's intrusion.
The mean average direction of the soft component in situ is:
Decl ¼4.5
, Incl. ¼51.2
,
a
95
¼6.5
,k¼7.0, N ¼78. The primary
component, acquired during or shortly after deposition of the Vaca
Muerta Fm.ereverse polarity-, corrected to the horizontal, is:
Decl ¼197.2
, Incl. ¼50.9
,
a
95
¼2.8
,k¼47.5, N ¼54.
5. Anisotropy of magnetic susceptibility (AMS)
The magnetic anisotropy of a rock depends on the anisotropy of
individual grains of its minerals and their spatial arrangement. By
knowing the magnetic anisotropy characteristics of the rock-
forming minerals, it is possible to determine the spatial distribu-
tion of the grains, which results from the various forces acting
when the rock was formed. Magnetic properties vary according to
direction and therefore there are various types of anisotropy (Lanza
and Meloni, 2006). A few magnetic grains can yield a reproducible
petrofabric if they possess an unusually strong preferred orienta-
tion; for example, caused by stress-controlled crystallization. AMS
registers the preferred orientation of anisotropic magnetic min-
erals, i.e. the magnetic fabric derived from the average contribu-
tions of nonmagnetic (paramagnetic and diamagnetic) and
ferromagnetic minerals present in a rock. The susceptibility of
anisotropic samples is a second-order tensor which is represented
in terms of the three mean susceptibility axes (k
1
maximum, k
2
intermediate, k
3
minimum).
In the case of non-deformed sedimentary rocks, the suscepti-
bility ellipsoid is strongly oblate, with the axis k3 orthogonal to the
bedding (normalsedimentary fabric), well developed magnetic
foliation and poorly dened or undened lineation. Imbrication of
the magnetic foliation gives the absolute paleocurrent direction
(Lanza and Meloni, 2006).
In Arroyo Loncoche, AMS studies were generally performed in
all 8 specimens from each horizon, using an AGICO MFK1A Kap-
pabridge equipped with an automatic spinner that allowed 64
measurements on each of the three axes. The anisotropy magnitude
and orientation of the magnetic susceptibility ellipsoid were
calculated according to Jelinek (1978).
The average magnetic susceptibility from stratigraphic horizons
(K
m
)rangesfrom8.410
5
to 3.1 10
6
SI, except in the sill where it
amounts up to 4.3 10
3
SI. This variability of K
m
would suggest that
the distribution of magnetite grains is heterogeneous at that scale.
The degree of anisotropy, P
j
, is low and ranges from 1.156 (2 horizons)
to 1.004. Parameters F(foliation) range from 1.102 to 1.002 whereas L
(lineation) ranges from 1.038 to 1.001. Shapes of the AMS ellipsoids
are primarily oblate (AL20 to AL27 in the lower part of the section) to
prolate-oblate with minor prolate or oblate shapes for the rest of the
section (Fig. 8a,b). AMS fabrics vary from dominantly normal(Fig. 8a)
eminimum K
3
axes perpendicular to bedding plane ein the lower
part of the section, followed by inverse (Fig. 8b) emaximum K
1
axes
perpendicular to bedding plane e,intermediateeintermediate K
2
axes perpendicular to bedding plane e, and normal.
The normal fabric is foliated and shows (Fig. 8a) when corrected
to the horizontal, the K
3
axes are very well grouped around the
vertical, while the K
1
and K
2
axes are more or less dispersed along a
girdle within the horizontal foliation plane. Inverse fabric on the
other hand, yield when corrected to the horizontal (Fig. 8b), well-
grouped vertical K
1
axes with a strong lineation. Finally in the in-
termediate fabric, the K2 axes are clustered in the sub-vertical
either forming a girdle with the K3 axis or separately grouped
from the K1 and K3 axes. Inverse AMS fabrics are rather common in
limestones, and are suggestive of the occurrence of single or pseudo
single-domain magnetite (Rochette, 1988; Rochette et al., 1992).
6. Magnetic mineralogy
Rock magnetic and petrographic studies were carried out only in
basinal and outer ramp facies of the Vaca Muerta Formation. Rock
magnetic studies were mostly performed in well cores from El
Trapial block in northern Neuqu
en province, and they cannot be
published. Yet, we can say that thermomagnetic curves show Curie
temperatures between 570 and 580
C, which is a robust evidence
of the occurrence of titanomagnetite as the main carrier. In addi-
tion, these curves show the occurrence of pyrrhotite always asso-
ciated with the sills.
On the other hand, a thorough petrographical study was carried
out in the type section of the Vaca Muerta Formation at Puerta
Curacao Neuqu
en province, where facies are similar to those
cropping out at the base of the Loncoche section. Here, optical
observations under transmitted and reected light were performed
in representative lithologies, in order to study the mineralogical
composition and in particular to identify magnetic minerals. Tita-
nomagnetite was clearly identied, bearing anhedral, multi-
domain (MD) esized crystals (Fig. 8c) which support the occur-
rence of single or pseudo single-domain crystals. Additionally,
pyrrhotite grains were identied (Fig. 8d) (Amigo, 2016). Therefore,
we interpret that the ChRM is carried, according to paleomagnetic
diagrams (antipodal mean directions, removed between 450
and
500
Cor40e60 mT, passing several paleomagnetic eld tests),
rock magnetic and petrographic studies, by detrital pseudo-single-
domain (PSD) or single domain (SD) titanomagnetite grains.
Table 2
Mean directions of reverse and normal sites. N ¼number of sites, D
t
,I
t
¼declination,
inclination after bedding, k¼Fisher's precision parameter,
a
95
¼semi-angle of
condence.
Average N D
t
,i
t
k
a
95
Normal 26 14.2, 53.1 46.0 4.2
Reverse 28 199.7, 48.9 52.5 3.8
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 199
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208200
Fig. 7. The ChRM passes a baked contact test performed in the sill intruding the lower part of the section and the host rock. A) Paleomagnetic behaviour of the component isolated
in the sill, showing steep inclinations and reverse polarity. B) In the host rock at the baked contact, there are two components, a soft with reverse polarity bearing steep inclinations
that is removed at 250 C acquired most likely during the intrusion of the sill, and another component normal polarity with higher unblocking temperatures that coincides with the
primary reference direction (Fig. 5b), C) 30 m above from the contact, the host rock carries the same reference direction (Fig. 5b). Symbols as in Fig. 5.
Fig. 8. Anisotropy of magnetic susceptibility (AMS) and magnetic minerals in the Vaca Muerta Fm. A, B) Shapes of the AMS ellipsoids are primarily oblate (A) to prolate-oblate with
minor prolate or oblate shapes for the rest of the section (B). AMS fabrics vary from dominantly normal (A) in the lower part of the section, followed by inverse (B) and intermediate.
Inverse AMS fabrics are rather common in limestones and could point out the occurrence of SD or PSD titanomagnetite. C, D) Magnetic minerals observed under reected light
found in another part of the basin, proving that magnetic carriers could be titanomagnetite and pyrrhotite. Anhedral isotropic crystal of titanomagnetite (mg) MD-sized (C) and
cubic yellowish iron sulphide interpreted as pyrrhotite is clearly secondary (D) (in Amigo, 2016).
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 201
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208202
Additionally, the secondary component bearing northerndirections
that is removed at <300
C or 15 mT and has been interpreted as a
remagnetization produced by the local dipolar eld, is likely carried
by multi-domain titanomagnetite grains, that have been clearly
identied in polished sections. We discarded the effects of
inclination-shallowing in the calculated magnetic components
because sampling was carried out mainly in limestones, which
show hardly any evidence of compaction.
Finally, the third component that has been found in the lower
part of the studied section is, according to paleomagnetic (reverse
polarity with steep inclinations, removed at c. 350
C or 110 mT,
found in the vicinity of or in the sill), petrographic and rock mag-
netic studies, carried by pyrrhotite (Fig. 8d) and resulted from the
intrusion of the sill during the Cenozoic.
7. Magnetostratigraphy
In the Arroyo Loncoche section, Virtual Geomagnetic Pole (VGP)
were calculated from site mean directions, yielding 11 reverse and
10 normal polarity zones (Figs. 3 and 9). One interval at c. 30 m from
the base bears no polarity, and corresponds to a Cenozoic sill.
Based on the correlation between ammonite zones from the
Andean and Tethys Regions, these polarities were calibrated ac-
cording to the last Geomagnetic Polarity Time Scale (GPTS)
compiled by Ogg and Hinov (2012). Results show a good correlation
between both magnetostratigraphic scales, and also, the changes in
the sedimentation rates.
In order to tie the paleomagnetic oating scale to the GPTS, we
followed the criteria chosen by Kietzmann et al. (2015), using the
base of the Windhauseniceras internispinosum Zone as the primary
datum. This level indicates the base of the upper Tithonian since: 1)
it coincides with a change from a reverse to a normal polarity zone;
2) it includes the ammonite genus Simplisphinctes (Zeiss and
Leanza, 2010) (base of Microcanthum Zone), 3) it contains the
rst occurrence of the nannofossil Polycostella senaria (Lescano and
Kietzmann, 2010; Kietzmann et al., 2011b)aswellastherst
occurrence of the preacalpionellid Chitinoidella boneti (Fernandez-
Carmona and Riccardi, 1999; Kietzmann et al., 2011b).
Using this primary level we obtained the following paleomag-
netic correlation (Fig. 9): from base to top, the V. mendozanus Zone
comprises a set of reverse, normal, reverse and normal polarities,
which we interpret to span the M22r.2r to M22n Subchrons in the
GPTS. According to the biostratigraphic proposals (Fig. 4), this An-
dean Zone is equivalent to the S. semiforme and S. darwini Tethyan
zones. Would that be the case we should nd solely normal po-
larity. Instead, the pattern of polarities isolated in this interval
rather correlates to the upper part of H. hybonotum Zone. The cor-
relation between V. mendozanus and H. hybonotum zones has been
in fact, already suggested on the one hand, by Zeiss and Leanza
(2010) on the basis of ammonites.
On the other hand, it is consistent with the cyclostratigraphy
obtained by Kietzmann et al. (2015) who suggested that V. men-
dozanus could be younger than the top of S. darwini in the Tethys
(due to the reverse polarity at the base), but would be older than
the top of S. darwini in terms of the number of cycles recognized.
The following P. zitteli zone bears normal, reverse and normal
polarities that would correspond to M22n to the base of M21n
Subchrons. A. proximus comprises a set of normal and reverse po-
larities that are correlated with M21n to M20r Subchrons.
W. internispinosum bears only normal polarity that is correlated
with the M20n.2n Subchron. Above, C. alternans includes normal,
reverse, normal, reverse and normal polarities which are inter-
preted to correspond to the upper M20n.2n to M19n Subchrons.
S. koeneni comprises normal, reverse, normal, reverse, normal,
reverse, normal and reverse polarities that are correlated with M19n
to M16r Subchrons. The position of the Jurassic-Cretaceous bound-
ary based on ammonites remains very controversial, although the
two main proposals agree that it is located within the S. koeneni
ammonite Zone (Fig. 4). One proposal places the boundary in the
lower part of the S. koeneni (Riccardi, 2015) whereas the other as-
signs it to the very top of the ammonite Zone (Vennari et al., 2014).
Such discrepancy in the biostratigraphy makes paleomagnetism a
relevant tool to place the J-K boundary in the studied section by
correlating the polarity zones with the reference GPTS. Indeed, in
the GPTS from Ogg and Hinov (2012), the J-K boundary is located in
the boundary between M19n.1n and M18r Subchrons. Such time-
line at Arroyo Loncoche, is traced to the narrow interval between
paleomagnetic horizons AL16 and AL40, at c. 160 m from the base, in
the lower third of the S. koeneni Zone (J/K-2012,Fig. 9). The other
alternative came up very recently (Ogg et al., 2016) and places the
boundary within the M19n.2n Subchron (J/K-2016,Fig. 9). Would
that be the case, the boundary in Arroyo Loncoche is limited to the
interval between paleomagnetic horizons AL38 and AL39, at c. 15 0 m
from the base, in the lower S. koeneni Zone. The position of the J-K
boundary is in very good agreement on the one hand, with Riccardi's
biostratigraphic proposal (Fig. 4). In addition, it matches precisely
the location assigned by cyclostratigraphy (Kietzmann et al., 2015).
Further above, A. noduliferum includes a dominant reverse with
a minor opposite polarity, which is correlated with M16r Subchron.
Finally, S. damesi comprises reverse, normal and reverse polarities
that are interpreted to correspond to M16r to M15r Subchrons.
Therefore, deposition of the Vaca Muerta Fm. in the locality of
Arroyo Loncoche took place during M22r.2r to M15r Subchrons.
Sedimentation rates resulting from the magnetostratigraphic
correlation (Fig. 9) range between 40 and 20 m/Myr, which are
consistent with rates estimated by facies analysis (in Section 3.1).
8. Paleomagnetic pole and tectonics
The paleomagnetic pole calculated from the primary component
is located (Table 3) at: Lon ¼191.6
E, Lat ¼76.2
S, A
95
¼3.5
,k¼33,
N¼54. There exist only a few reliable reference PP for this age for
South America,and one of them has been obtained recently precisely
from the typesection of the Vaca Muerta Formationat Puerta Curacao
(Table 3). This PP is located at: Lon ¼66.5
E, Lat ¼79.8
S, A
95
¼3.6
and coincides with reliable Early Cretaceous age paleopoles (Fig. 10).
Considering that pole as reference the PP obtained at Arroyo Lon-
coche indicates that the area has been subjected to rotation around
vertical axis (R), estimated as 24.3 ±4.6
clockwise (Beck, 1980).
In fact, the Jurassic deposits in the area of Arroyo Loncoche have
a general structural NNE trend. Such structure is remarkably
different from the NNW-oriented Malargüe anticline, which is the
result of tectonic inversion of a pre-existing NNW normal fault
(Giambiagi, 2016, pers. comm.). The rotation we found in the
Jurassic synrift/sag strata with respect to the Jurassic master fault
evidences a sinistral/normal movement of this fault, as already
suggested for the master fault in La Manga, close to the study area
(Bechis et al., 2014).
Fig. 9. Magnetostratigraphic correlation between the Andean and Tethyan regions. Data show (from left to right): Andean ammonite zones, stratigraphic column of the Vaca Muerta
Formation at the Arroyo Loncoche section, sampling horizons, polarities sequence, interpreted Subchrons and correlation with the International Geomagnetic Polarity Time Scale.
The two accepted proposals for the position of the J/K boundary in the literature are shown: J/K-2012 places it between M19n.1n and M18r (Ogg and Hinov (2012) and J/K-2016 in the
middle of M19n.2n (Ogg et al. (2016).
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 203
9. Discussion
Results from the study at Arroyo Loncoche provide a detailed
magnetostratigraphy, which can help to solve many questions
regarding the Upper Jurassic eLower Cretaceous stratigraphy of
the Neuqu
en Basin that are still unanswered. In particular, the
magnetostratigraphy calibration of the TithonianeBerriasian An-
dean succession brings in two key points: 1) the age of the
Tithonian transgression (V. mendozanus Zone), and 2) position of
the JurassiceCretaceous boundary.
9.1. Age of the Tithonian transgression (V. mendozanus Zone)
Only a few elements can be used to establish with some degree
of accuracy the age of the base of the Vaca Muerta Formation: (1)
ammonites, (2) calcareous dinoagellates cysts, (3) cyclostrati-
Fig. 10. APW path showing Late Triassic to Lower Cretaceous reference paleomagnetic poles (Table 3). There are two PP from the Vaca Muerta Formation (145 Ma), with orange A
95
.
The one marked with * and the number in italics, corresponds to the paleopole calculated in this study (c. 190ºE) that proved to be rotated c. 24in a clockwise sense with respect to
the reference paleopole (c. 60). The magnitude and sense of rotation is fully consistent with the regional structure (modied from Iglesia Llanos et al., 2006). (For interpretation of
the references to colour in this gure legend, the reader is referred to the web version of this article.)
Table 3
Selected palaeopoles from the GPDB v. 4.6 (http://www.tsrc.uwa.edu.au/data_bases) and more recent South American poles. A95: 95% condence interval (modied from
Iglesia Llanos et al., 2016).
Code/Rock Unit Mean age (Ma) Lat
S Long
EA
95
Reference
Los Colorados Fm. 210 76.0 280.0 8.0
Viz
an et al. (2004)
Anari-Tapirapua Fm. 196.6 ±0.4 65.5 250.0 3.5
Montes Lauar et al. (1994)
Neuqu
en basin 1 197 51.0 223.0 6.0
Iglesia Llanos et al. (2006)
Lep
a-Osta Arena Fm. 180e186 75.5 129.5 6.0
Viz
an (1998)
Neuqu
en basin 2 185 74.0 67.0 5.0
Iglesia Llanos et al. (2006)
Maril Complex 168e178 83.0 138.0 9.0
Iglesia Llanos et al. (2003)
El Quemado Complex 153e157 81.0 172.0 5.5
Iglesia Llanos et al. (2003)
Serra Geral 1 133 85.0 115.5 3.7
Pacca and Hiodo (1976)
Serra Geral 2 135 85.0 108.0 1.0
Ernesto et al. (1990)
Parana basalts 132 86.0 198.0 2.5
Alva-Valdivia et al. (2003)
Puerta Curacao 145 79.8 66.5 3.6 Amigo (2016)
PP-Kl 97e146 85.0 74.0 2.5
McElhinny and McFadden (2000)
Loncoche 145 76.2 191.6 3.5
This study
SA-Kl 135e130 85.0 76.5 2.0
Somoza and Zaffarana (2008)
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208204
graphy, and (4) magnetostratigraphy. Other elements, such as
organic dinoagellates, radiolarian and calcareous nannofossils are,
so far, not quite resolutive or their zones are tied to Andean
ammonite Zones.
Ammonites show the best zoning resolution, but their provin-
cialism prevents a straightforward inter-regional correlation. The
V. mendozanus Zone is traditionally correlated with the Darwini and
Semiforme Zones (Riccardi, 2008, 2015; Vennari et al., 2014).
However, it is clear that this Zone in our studied section, as well as
in others that are being studied in the basin, include a polarity
pattern that comprises more than just normal polarity. Moreover,
the distinctive pattern we obtained allows us to correlate this in-
terval with the uppermost part of Hybonotum Zone. Zeiss and
Leanza (2010) divided the V. mendozanus Zone in two subzones,
which were referred to the Hybonotum and Darwini Zones,
respectively, and correlated this zone with the Mazapilites beds of
Mexico, as suggested by a genus that in Mexico also ranges from the
Hybonotum to the Semiforme Zones. According to Leanza (pers.
comm.), such correlation was more an intuition of an expert eye
rather than a biostratigraphic conclusion, since they never found
elements indicative of the Hybonotum Zone among the ammonites
in the Andean region. In fact, Riccardi (2015) stresses that due to
lack of typical species of the Hybonotum Zone, the best correlation
should be restricted to the upper part of the Mexican Zone.
Therefore, it seems unlikely that the rst reverse polarity zone in
Arroyo Loncoche reaches down further below M22r.2r in the GPTS
which is assigned to the uppermost part of the Hybonotum Zone
(Fig. 9).
Recently, Ivanova and Kietzmann (2016) reported in the
V. mendozanus Zone the presence of two typical early Tithonian
calcareous dinocysts: Committosphaera pulla and Para-
stomiosphaera malmica. Both species are components of the Mal-
mica Zone (upper part of the Hybonotum Zone to Semiforme Zone;
see Benzaggagh and Atrops, 1996; Ivanova in Lakova et al., 1999). In
addition, Kietzmann and Palma (2009b) found remains of the
miocrocrinoid Crassicoma between V. mendozanus to P. zitteli Zone.
Crassicoma has a time range from late Oxfordian to late Kimmer-
idgian, although some ossicles that can be attributed to Crassicoma
were also reported in the lower Tithonian (Tithonica and Malmica
Zones). Although these facts are not fully conclusive for the
Hybonotum Zone, they do not contradict our interpretation of the
magnetostratigraphic data.
The oating orbital scale provided by Kietzmann et al. (2015)
show that the V. mendozanus Zone can be correlated (in time)
with the upper part of the Semiforme Zone (following ICS 2004) or
the base of the Fallauxi Zone (following ICS 2012). The reverse
polarity at the base of the Vaca Muerta Formation restricts these
two options to the Fallauxi Zone, since it would not be consistent
with the polarities pattern obtained. In fact, Kietzmann et al. (2015)
emphasize that basal biozones at the study section are found in
distal facies of carbonate ramp system and may contain minor
discontinuities that are not detectable by biostratigraphic methods
or eld evidences, and thus they may have longer durations. The
calibration of the Andean ammonite Zones by magnetostratigraphy
allow us to quantify the time that is not represented in the cyclo-
stratigraphic analysis, which a priori could be related to the stages
of low stand of sea level (Fig. 11). In summary, at the moment the
most conclusive data for this time interval in the Neuqu
en Basin are
provided by magnetostratigraphy. Although they are not fully
supported by biostratigraphy, no biostratigraphic data contradict
these results.
9.2. Position of the JurassiceCretaceous boundary
The lack of faunal changes in the Jurassic/Cretaceous transition
forces to establish this boundary by convention, at a stratigraphic
level constrained by the maximum number of correlatable datums.
Wimbledon et al. (2013) claim that the primary markers corre-
spond to the base of Calpionella Zone, the FAD of Nannoconus
steinmanni minor and N. kampteri minor, as well as the base of
Fig. 11. Calibration of Andean ammonite Zones by magnetostratigraphy (this paper), cyclostratigraphy based on low eccentricity cycles (Kietzmann et al., 2015), high frequency
(HFS) and composite (CS) depositional sequences (Kietzmann et al., 2014), calcareous dinocyst zones (Ivanova and Kietzmann, 2016), and calpionellid zones (Gonz
alez Tomassini
et al., 2015). Grey-shaded bars indicatethe remaining time with respect to the magnetostratigraphic calibration. Their position is an interpretation assuming that most of the time is
not represented during low-stand stage. However, the remaining time could be related to minor discontinuities.
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208 205
magnetozone M18r (J/K-2012,Fig. 9). Secondary markers include
the base of M19n.1n or M19n.1r, the FADs of Nannoconus wintereri
and Cruciellipsis cuvillieri, the base of Berriasella Jacobi Subzone, and
the LAD of Dichadogonyaulax pannea, among others. The last
available revision of these markers (see Ogg et al., 2016) point out
that the base of Calpionella Zone, as well as the FAD of Nannoconus
steinmanni minor and N. kampteri minor, are placed in the middle
part of the M19n.2n Subchron (J/K-2016,Fig. 9).
The Jurassic/Cretaceous transition in the Neuqu
en Basin pre-
sents a greater amount of elements that can be used to establish
with some degree of accuracy its position, i.e.: ammonites, nan-
nofossils, calpionellids, cyclostratigraphy, and magneto-
stratigraphy. Organic and calcareous dinoagellate cysts and
saccocomid microcronoids can be used as auxiliary markers as well.
At the present time, there is agreement that the position of the
Jurassic-Cretaceous boundary based on ammonites (Riccardi, 2008,
2015; Vennari et al., 2014) is located within the Substeueroceras
koeneni Zone (Fig. 4). This zone was referred to the uppermost
Tithonian (equivalent to the Jacobi Zone) (e.g., Leanza, 1981).
Salazar Soto (2012) on the basis of a study on material mostly from
the Lo Valdes Formation, Chile, has recognized the presence of
some diagnostic Tethyan species, and referred the S. koeneni Zone
to the Berriasian. All these considerations allowed Riccardi (2015)
to correlate the S. koeneni Zone from the Jacobi to the lower part
of the Occitanica Zone.
Nannofossil primary markers for the Jurassic/Cretaceous
boundary have not been recognized in the studied sections so far.
Vennari et al. (2014) found a few levels with Nannoconus stein-
mannii minor and N. kampteri minor in a limited interval from the
uppermost part of the Substeueroceras koeneni Zone at Las Loicas
section, not far from Arroyo Loncoche. However, some authors nd
such bioevents are somewhat questionable, since ammonite
biostratigraphy is not detailed enough, and nannofossils are too
patchy (Riccardi, 2015). In fact, both species span the entire Ber-
riasian and reach at least the Hauterivian, whereas those from Las
Loicas appear only in a 5 m ethick interval (one level in the case of
N. steinmannii minor and four levels in the case of N. kampteri mi-
nor), but do not reappear in the rest of the section (at least 40 m).
The reason may be a high degree of diagenesis in this strongly
tectonized area, so calcareous microfossils could be recrystallized,
and these markers could have their rst occurrence in older strat-
igraphic levels.
Calpionellids are also present in Argentina, but need detailed
study. Fern
andez-Carmona and Riccardi (1998) reported Chittinoi-
della boneti within the Windhauseniceras internispinosum Zone, and
Fern
andez-Carmona et al. (1996) found Tintinnopsella sp., Crassi-
collaria sp. and Calpionella alpina within the Corongoceras alternans
and Substeueroceras koeneni Zones, and Fern
andez-Carmona and
Riccardi (1999) reported the presence of Calpionella alpina and
Tintinnopsella sp. in the Spiticeras damesi Zone. More detailed
studies carried out in well cores from El Trapial block, allowed to
place the base of Calpionella Zone at the lowermost part of the
Substeueroceras koeneni Zone (Kietzmann in Gonz
alez Tomassini
et al., 2015). However, this bioevent needs to be supported
through more detailed studies in different sections of the basin.
There are also some auxiliary markers indicating the proximity
of the J/K boundary. There need to be found calcareous dinoa-
gellate cysts that are considered resolutive in the Tithonian, for
example. Placed in the middle of the Corongoceras alternans Zone,
there appears the rst occurrence of Stomiosphaerina proxima
(Ivanova and Kietzmann, 2016). This bioevent denes the Proxima
Zone that correlates to the Durangites elowermost Boissieri Zones
(
Reh
anek, 1992; Benzaggagh and Atrops, 1996; Lakova et al., 1999).
On the other hand, Kietzmann and Palma (2009b) show that Sac-
cocoma disappear towards the top of the Corongoceras alternans
Zone. According to the review by Hess (2002) Saccocoma has a time
range from the late Kimmeridgian to late Tithonian.
The cyclostratigraphic scale provided by Kietzmann et al. (2015)
allows to place (in time) the Jurassic/Cretaceous boundary within
the lower third of the S. koeneni Zone (Fig. 11). These data fully
coincide with our magnetostratigraphic correlation, since both the
base of magnetozone M18r and the middle part of magnetozone
M19n.2n are located within this same interval.
10. Concluding remarks
Paleomagnetic data indicate the occurrence of three magnetic
components: i) a secondary component that was removed at c.
300
C/15 mT bearing northern declinations with negative in-
clinations that is interpreted to result from the remagnetization of
the local dipolar eld, and carried by MD titanomagnetite. The
second component has both positive and negative inclinations, is
removed between 450
C and 500
C/40 and 60 mT, passes several
paleomagnetic eld tests and is carried by SD or PSD titano-
magnetite. The latter is interpreted to represent the primary
magnetization acquired during or soon after the deposition of the
Vaca Muerta Formation in the Late Jurassic-Early Cretaceous. The
last component is found in or the vicinity of the sill in the lower part
of the section, bears a reverse polarity with steep inclinations, is
removed at c. 300
C/110 mT, and represents most likely, a
remagnetization of Cenozoic age produced by the intrusion.
The resultant magnetostratigraphic scale comprises 11 reverse
and 10 normal polarity zones, spanning the Andean Virgatos-
phinctes mendozanus (lower Tithonian) to Spiticeras damesi Zones
(upper Berriasian). These polarity zones were correlated in turn,
with those of the International Geomagnetic Polarity Time Scale
using the correlation between Andean and Tethyan ammonite
zones. The correlation of the paleomagnetic and cyclostratigraphic
data allowed to date the section with unprecedented precision. This
has turned particularly relevant in the case of the lower
V. mendozanus Zone whose equivalence in the Tethyan Realm had
not been well established yet, and also in the position of the
Jurassic-Cretaceous boundary, that in the Neuqu
en Basin had been
hot matter of debate. In this regard, according to the recent pro-
posals in Ogg et al. (2016), the boundary should be placed in the
studied section, at the base of the S. koeneni Zone, either at the
M19n.1n-M18r Subchrons boundary (c. 160 m from the base) or in
the middle of M19n.2n Subchron (c. 150 m from the base). Both
alternatives shown in Ogg et al. (2016) which are concordant with
the biostratigraphic criteria from Riccardi (2015).
A paleomagnetic pole was calculated from the primary magne-
tization that is located at: Lon ¼191.6
E, Lat ¼76.2
S, A
95
¼3.5
.
Such position is indicative of a c. 24
clockwise rotation for the
studied section that is consistent with structural data of the region.
Acknowledgements
This research has been done under the framework of the UBA-
CyT X-801 (Universidad de Buenos Aires) projects. We are espe-
cially indebted to Dr. A.C. Riccardi (Universidad Nacional de La Plata
y Museo, Argentina) who not only provided us with the biostra-
tigraphy but also shared his expertise. Also, we thank Dr. H.A.
Leanza (Museo de Ciencias Naturales Bernandino Rivadavía,
Argentina) for his helpful comments regarding the V. mendozanus
Zone. We were honored with the valuable reviews by James Ogg
and Cor Langerais as well as an anonymous reviewer which allowed
to improve the original manuscript notoriously.
We are grateful to Alan Seint Buchanan, Graciela S. Bressan and
Martín Hoqui for their valuable assistance in the eld, as well as the
Pacheco family for allowing us to work in their property.
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208206
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.
1016/j.cretres.2016.10.011
M.P. Iglesia Llanos et al. / Cretaceous Research 70 (2017) 189e208208
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